INFECTION AND IMMUNITY, Apr. 2004, p. 1843–1855
0019-9567/04/$08.00ϩ0 DOI: 10.1128/IAI.72.4.1843–1855.2004
Copyright 2004, American Society for Microbiology. All Rights Reserved.
Chlamydial Persistence: beyond the Biphasic Paradigm
Richard J. Hogan,1,2 Sarah A. Mathews,1 Sanghamitra Mukhopadhyay,3
James T. Summersgill,3 and Peter Timms1,2*
Infectious Diseases Program
1 and Cooperative Research Centre for Diagnostics,
2 School of
Life Sciences, Queensland University of Technology, Brisbane, Australia, and Division of
Infectious Diseases, Department of Medicine, University of Louisville, Louisville,
The chlamydiae are an evolutionarily distinct group of eu-
that 146 million people have trachoma due to ocular infection
bacteria sharing an obligate intracellular lifestyle and a unique
by C. trachomatis
serovars A to C and that 4.9 million of these
developmental cycle that has been well characterized under
are totally blind (121). Ascending infection by serovars D to K
favorable cell culture conditions. This cycle begins when infec-
of the female upper genital tract, known as pelvic inflammatory
tious, metabolically inert elementary bodies (EB) attach to and
disease, causes salpingitis, which in turn leads to fibrosis and
stimulate uptake by the host cell. The internalized EB remains
scarring of the fallopian tubes, and eventual complications of
within a host-derived vacuole, termed an inclusion, and differ-
ectopic pregnancy and tubal infertility (22). C. trachomatis
entiates to a larger, metabolically active reticulate body (RB).
originating from the genital tract is also associated with reac-
The RB multiplies by binary fission, and after 8 to 12 rounds of
tive arthritis, which develops in 1 to 3% of patients after genital
multiplication, the RB differentiate to EB asynchronously (78).
chlamydial infection (123). C. pneumoniae
, which can also dis-
At 30 to 84 h postinfection (PI), depending primarily on the
seminate from the site of the initial infection (74), has been
infecting species, EB progeny are released from the host cell to
associated with cardiovascular disease (62, 102) and late-onset
Alzheimer’s disease (4). In addition, unresolved respiratory C.
Shortly after Moulder (79) definitively reported the bacterial
infection may contribute to the pathogenesis of
nature of chlamydiae in 1966, the genus Chlamydia
chronic inflammatory lung diseases, such as asthma (43) and
lished (86) and divided into two species, Chlamydia trachomatis
chronic obstructive pulmonary disease (15). Similar Chlamy-
and Chlamydia psittaci
(85). Chlamydia pneumoniae
-associated chronic diseases and their sequelae occur in
(34), formerly known as strains of C.
many animals, for example, trachoma-like blindness (25) or
, were designated as distinct species in 1989 and 1992,
infertility (71) in koalas and polyarthritis in sheep (117).
respectively. More recently, a new taxonomy was proposed that
Recurrent chlamydial disease may result from either re-
increases the number of species in the family Chlamydiaceae
peated infections or persistence of the organism after unre-
nine and groups these species into two genera (31). However,
solved infections. Indeed, the high incidence of chlamydial
this review will not use the emended taxonomy, since much
infections and transient immunity typically observed after in-
debate continues on the issue (106).
fection (90) present difficulties in differentiating between per-
Chlamydial species cause widespread infections in humans.
sistent infection and reinfection. Nonetheless, characterization
serovars D to K are considered the world’s most
of the in vitro persistent phase of chlamydiae and multiple lines
common sexually transmitted bacterial pathogens (40) and,
of in vivo evidence suggest that chlamydiae persist in an altered
following vertical transmission through an infected birth canal,
form during chronic disease. This review will provide an up-
cause neonatal conjunctivitis (105) and pneumonia (12). Re-
date on chlamydial persistence, focusing on recent insights that
spiratory infection with C. pneumoniae
occurs in almost every-
have been obtained into the molecular basis of this important
one during his lifetime (120). C. pneumoniae
is estimated to
cause an average of 10% of community-acquired pneumonia
cases and 5% of bronchitis and sinusitis cases (61). In addition,
avian strains of C. psittaci
have long been known to cause
CHLAMYDIAL PERSISTENCE IN VITRO
Chlamydial persistence has been described as a viable but
In addition to these acute chlamydial infections, chlamydiae
noncultivable growth stage resulting in a long-term relation-
are associated with a range of chronic diseases that are char-
ship with the infected host cell (9). Such relationships have
acterized by inflammation and scarring and result in significant
been established in vitro, usually through deviations from con-
damage to the host. The World Health Organization estimates
ventional cell culture conditions for productive chlamydial de-
velopment. The different in vitro persistence systems often
share altered chlamydial growth characteristics, for example, a
* Corresponding author. Mailing address: School of Life Sciences,
loss of infectivity and the development of relatively small in-
Queensland University of Technology, GPO Box 2434, Brisbane 4001,
Australia. Phone: 61 7 38642120. Fax: 61 7 38641534. E-mail:
clusions containing fewer chlamydiae. In addition, these sys-
tems often produce common ultrastructural traits (Table 1).
TABLE 1. Selected in vitro ultrastructural observations of atypical persistent chlamydiae
0–12 h PI, normal EB-to-RB differentiation;
enlarged RB; 20–48 h PI, membranes within
released as membrane sheets); 36–48 h PI,
budding and internal subdivision to normal
10% amino acids (48 h PI), RB-sized swollen
containing few RB-like forms with multiple
cytoplasmic nucleoid-like masses, ring forms
24 h PI, electron-dense material surrounding
inclusions; 72 h PI, inclusions containing
normal-sized RB with dense and wavy outer
chlamydiae, which were generally enlarged
5–7 days PI, vacuolization and electron-dense
18–32 h PI, small inclusions containing few
chlamydiae, including maxi-RB 2–5 times
the size of normal RB; 32–44 h PI, lysis of
10–50% of host cells featured either (i)
protoplast-like bodies, or (iii) dense, 100-
nm-diameter oval bodies bounded by single
48 h postsplit, 10% of inclusions were either
proportion of aberrant RB 4–5 times the
5 h after transfer, mosaic-like display of RB in
Similar development of abnormal forms was reported for infections (with or without cyclohexamide) with C. trachomatis
serovar E or C. psittaci
Moulder (78) proposed that the strain had been from the lymphogranuloma venereum biovar (serovars L1 to L3).
Many studies have described enlarged, pleomorphic RB that
serovars had differential requirements for specific amino
are inhibited in binary fission and their differentiation to EB
acids in McCoy cell infections that correlated with their re-
but nevertheless continue to accumulate chromosomes. These
spective anatomical sites of origin. Of particular note, ocular
changes are generally reversible upon removal of the growth
(A to C) but not oculogenital (D to I) serovars required tryp-
inhibitory factor. However, despite the general similarities,
tophan for normal growth (2). However, a recent study of
significant differences in growth and ultrastructural character-
HeLa cell infections with reference strains representing all C.
istics have also been reported among different systems or
serovars failed to reproduce this finding, since the
growth of all serovars was inhibited in the absence of trypto-
Many early in vitro studies
phan (33). Instead, the tissue tropisms correlated with an in-
described abnormal chlamydial development after exposure to
dole-rescuable phenotype, since genital (D to K and L1 to L3)
antibiotics. In general, agents that target bacterial protein or
but not ocular (A to C and Ba) serovars growing in tryptophan-
RNA synthesis can inhibit chlamydial differentiation either
deficient medium were able to generate tryptophan from ex-
from EB to RB or from RB to EB, depending on when they are
ogenous indole and therefore recover their infectivities (33).
added to an in vitro infection, whereas those that target DNA
This finding was a crucial contribution towards establishing the
or peptidoglycan synthesis specifically inhibit RB-to-EB differ-
in vivo relevance of IFN-␥-mediated persistence.
entiation (78). For example, 10 g of erythromycin/ml, which
Depletion of nutrients other than amino acids from cell
reduces ribosome activity, inhibited C. trachomatis
culture medium can also induce persistence. For example, C.
EB-to-RB differentiation in McCoy cells when added within
serovar L2 in McCoy cells became reversibly per-
12 h PI (24). When the erythromycin was applied at later times
sistent in response to the removal of glucose from the cell
PI, infections were established featuring enlarged RB that
culture medium, temporarily losing infectivity and showing
could not differentiate to EB (24). In contrast, early addition of
abnormal morphology comparable to that seen during amino
200 U of penicillin/ml, which targets peptidoglycan cross-link-
acid depletion (45). However, the major focus of recent re-
ing, had no effect on C. psittaci
MN/Cal-10 development in L
search into deficiency-induced persistence has been iron de-
cells up to 12 h PI, but from that time onwards, the RB became
privation (3, 46, 95). Exposure of C. trachomatis
enlarged and progressively more aberrant (69) (Table 1).
fected polarized endometrial epithelial (HEC-1B) cells to 100
Upon transfer to penicillin-free medium, the productive devel-
M concentrations of the iron chelator deferoxamine mesylate
opmental cycle resumed and normal RB were produced by
(DAM) inhibited infectivity and caused significant morpholog-
budding from and endosporulation-like subdivision of en-
ical changes in the chlamydiae that were generally distinct
from those observed in other persistence systems (95) (Table
More recently, exposure of C. pneumoniae
1). The persistence of C. pneumoniae
TW-183 in HEp-2 cells
in HeLa cells to 50 g of ampicillin/ml led to the development
was similarly induced by exposure to 30 M DAM and main-
of aberrant, giant RB (122). Interestingly, when directly com-
tained for as long as 6 days, although not all of the morpho-
pared to C. trachomatis
infections subjected to the same con-
logical features described for the C. trachomatis
ditions, the persistent C. pneumoniae
infections were extremely
observed (3). The addition of iron-saturated transferrin led to
inefficient in reactivation after the removal of ampicillin (122).
the recovery of infectivity and productive development for
The explanation for this result is unclear at present, since
both species, thus supporting depleted host cell iron pools as
observations of C. pneumoniae
exposed to gamma interferon
the cause of the changes (3, 95). These studies support the
(IFN-␥) or deprived of tryptophan (72) (see below) argue
hypothesis that fluctuating iron levels, for example under the
against slow recovery from persistence as a distinguishing fea-
influence of estradiol in endometrial tissue (57), may contrib-
ture of this species. These examples and many other observa-
ute to the outcome of chlamydial infections in vivo.
tions of persistence induced in vitro by various antibiotics (re-
Exposure of in vitro chlamyd-
viewed in reference 9) indicate that inadequate antimicrobial
ial infections to cytokines, particularly IFN-␥, provides a sys-
therapy may allow chlamydiae to persist in vivo.
tem of indirect deficiency-induced persistence that could plau-
In contrast to persistence
sibly reflect in vivo events. In early studies, IFN-␥ was
induced by antibiotics, the depletion of essential nutrients from
identified as the active component in supernatant fluids from
cell culture medium temporarily arrests the growth of both
stimulated T cells that inhibited replication of C. psittaci
chlamydiae and their host cells until the missing nutrients are
in fibroblast (17) and macrophage (101) cultures that had been
replaced (78). In the first ultrastructural characterization of
preexposed to the supernatant. Preexposure of epithelial cells
deficiency-induced persistent chlamydiae, progressive deple-
for 24 h to high concentrations of IFN-␥ inhibited inclusion
tion of all amino acids caused increasingly abnormal develop-
formation by C. trachomatis
serovar L2 (111), C. psittaci
ment of C. trachomatis
serovar L2 in McCoy cells (Table 1),
(18), and C. pneumoniae
BAL-37 (118). However, lower IFN-␥
with partial recovery of particle size after reintroduction of
levels only partially restricted chlamydial development (18,
amino acids (26). Induction of persistence in the same Chlamy-
-host cell system by blood plasma concentrations of amino
Persistent chlamydial infections are induced by exposing cul-
acids (45) suggested that amino acid levels could directly in-
tures to moderate IFN-␥ levels, usually following infection. In
fluence chlamydial development in vivo.
this way, persistence was established for C. trachomatis
Minimum requirements for individual amino acids are likely
A in HeLa cells at IFN-␥ levels as low as 0.2 ng (2.4 U)/ml (7)
to be even more important than total amino acid concentra-
and for C. pneumoniae
A-03 in HEp-2 cells at a level of 25
tions in determining the outcome of chlamydial infections in
U/ml (87). Persistence of C. trachomatis
serovar A was main-
vivo. Allan and Pearce (2) reported that selected C. trachoma-
tained for several weeks (10). Ultrastructurally, the IFN-␥-
induced persistent chlamydiae were enlarged and aberrant (7,
a loss of infectivity (1, 59). These observations suggested that the
87) (Table 1). In C. trachomatis
serovar A, there was also
chlamydiae were surviving in a viable but culture-negative state.
evidence of budding and endopolygeny, the production of multi-
The addition of neither tryptophan nor antibodies against IFN-␥
ple progeny from a single enlarged form, during resumption of
(nor antibodies against tumor necrosis factor alpha and IFN-␣ for
productive infection after removal of IFN-␥ from the cultures
) (59) counteracted the chlamydial growth inhibi-
(10). These morphological observations were consistent with
tion (1, 59). This finding, coupled with the unique morphological
those from other persistence induction systems (26, 69). However,
characteristics of this model (Table 1), led to the suggestion that
a direct comparison of IFN-␥- and amino acid depletion-induced
a cytokine-independent mechanism is at least partially responsi-
persistent C. trachomatis
serovars E and L2 in HeLa cells revealed
ble for monocyte-induced persistence (59). The mechanism is also
different growth characteristics between the two systems, since
thought to be oxygen independent (100), but its precise nature
only IFN-␥-exposed cultures showed decreases in inclusion size
and the number of infected cells (52).
Phage infection of chlamydiae.
Observations that lytic infec-
The most important mechanism underlying the effects of
tion by naturally occurring bacteriophages can alter chlamydial
IFN-␥ on chlamydial growth in cultured human cells is trypto-
development in vitro suggest yet another persistence induction
phan depletion through activation of the host tryptophan-de-
mechanism. An early report of Chp1 infection of C. psittaci
grading enzyme indoleamine 2,3-dioxygenase (IDO). Byrne et
N352 described the formation of enlarged, distended RB (98).
al. (18) showed that growth restriction of C. psittaci
More recently, Hsia et al. (49) thoroughly characterized the
human epithelial cells preexposed for 24 h to 20 ng (240 U) of
ultrastructural effects of ⌽CPG1 infection on C. psittaci
IFN-␥/ml could be reversed by the addition of tryptophan
The maxi-RB observed had the basic characteristics of aber-
following infection. IDO induction was later confirmed to be
rant bodies seen in other cell culture models of persistence and
the major mechanism of IFN-␥-mediated persistence for C.
were of a similar size to the distended RB induced by Chp1
serovar A in HeLa cells (6) and C. pneumoniae
A-03 in aortic smooth muscle cells (88). Recently, tryptophan
If phage-induced chlamydial persistence represents an in
depletion provided an important link between IFN-␥ and dif-
vivo reality, it will only be relevant to certain chlamydial
ferential tissue tropisms among C. trachomatis
strains, since infecting phage was evident in neither the major-
well and colleagues (20) showed that, in agreement with the
ity of complete genome sequences nor an additional analysis of
previous study of direct tryptophan depletion (33) (see above),
six clinical C. pneumoniae
isolates (55). However, the C. pneu-
serovar D, I, or L2 but not serovar A in HeLa
AR-39 genome sequence revealed that this strain is
cells displayed the indole-rescuable phenotype after exposure
infected by a phage, ⌽Cpn1/⌽AR-39 (96), that is very similar
(24 h preexposure for serovar L2) to 5 ng (60 U) of IFN-␥/ml.
to ⌽CPG1 at the nucleotide level (5). Interestingly, a recent
Interplay between the IFN-␥ concentration and other fac-
seroepidemiological study showed that seropositivity to
tors may affect the outcome of a chlamydial infection in vivo.
⌽Cpn1/⌽AR-39 Vp1 antigen correlated more strongly with
Such factors could include the susceptibility of the infecting
the presence of aortic abdominal aneurysm than did seropos-
strain to IFN-␥-mediated inhibition (76) and the activity of
itivity to C. pneumoniae
AR-39 EB (55). One of the authors’
other cytokines such as tumor necrosis factor alpha (112, 118)
conclusions was that phage-bearing strains may become per-
and interleukin-1 (21) that may synergistically enhance the
sistent more readily in vivo, resulting in lower cultivability.
effects of IFN-␥ on chlamydial growth. In the case of trypto-
There is the intriguing possibility that ⌽Cpn1/⌽AR-39 itself
phan depletion as the mechanism of inhibition, additional fac-
could be at least partially responsible for an increased ten-
tors such as the availability of exogenous indole and ability of
dency of C. pneumoniae
AR-39 to become persistent. Studies
the infecting strain to use it (20, 33) and the IDO expression
are warranted to determine whether ⌽Cpn1/⌽AR-39 can af-
level of the host cell type (103) may also contribute. However,
fect C. pneumoniae
AR-39 growth in a similar way to the
despite the evident importance of tryptophan catabolism,
induction of C. psittaci
GPIC maxi-RB formation by ⌽CPG1
other mechanisms such as the inducible nitric oxide synthase
and whether some maxi-RB and inclusions are indeed able to
effector pathway and iron deprivation could also be attribut-
persist (escaping phage-induced lysis).
able to IFN-␥, thus adding to the potential complexity of the in
In contrast to all other persistence
vivo situation (50). In addition, the relative importance of
systems, continuous cultures become spontaneously persistent
these different mechanisms is likely to vary among host species.
when both chlamydiae and host cells are free to multiply in the
For example, inducible nitric oxide synthase induction seems
absence of stresses (even spontaneous monocyte persistence
to play a central role in IFN-␥-dependent inhibition of exper-
must involve unidentified stresses on the chlamydiae). Contin-
imental chlamydial infection in mice, whereas IDO induction
uous cultures are characterized by cycles consisting of several
in response to chlamydial infection is yet to be observed in
days of mostly inclusion-free host cell multiplication followed
by rapid chlamydial multiplication leading to partial (84) or
In contrast to other persistence sys-
almost complete (65, 80) host cell destruction. These cycles
tems, chlamydiae become spontaneously persistent following
continue indefinitely when the cultures are maintained by pe-
infection of monocytes. Cell culture infections of freshly iso-
riodic washing and growth medium replacement.
lated human monocytes were morphologically characterized
Some continuous cultures seem to be established by a ge-
for C. trachomatis
serovar K (59) and C. pneumoniae
netic block, as opposed to blocks caused by inhibitors or defi-
Kajaani 7 (1). In both studies, no normal RB (only aberrant
ciencies, in the progression of a productive cell culture infec-
RB) were observed at any time over the monocyte infection
tion (65, 84). For example, an early report described
period and chlamydial mRNA continued to be detected, despite
continuous C. psittaci
6BC infections of Chang’s human liver
cells that showed large fluctuations in the percentage of inclu-
tence and continuous cultures. The in vitro persistent state that
sion-positive host cells among successive passages (84). Isola-
chlamydiae enter after experiencing this broad range of hostile
tion of chlamydiae and host cells from selected passages re-
conditions may, at least in part, represent a general stress
vealed chlamydial variants that infected parent Chang’s human
response that chlamydiae have evolved to ensure their survival
liver cells with higher efficiency than wild-type C. psittaci
under harsh conditions. This hypothesis is supported by the
host cell variants that were more resistant to infection by the
observation that heat shock of C. trachomatis
serovar L2 in
chlamydial variants (84). Thus, chlamydiae, their host cells, or
BGM cells at 42°C induced a morphological response that was
both may undergo population shifts favoring genotypes that
similar to those often observed in other persistent cell culture
are more suited to a long-term persistent relationship.
systems (53) (Table 1). However, one potential inconsistency
More recently, Kutlin et al. (64) established a continuous
with this hypothesis is the occasional observation of aberrantly
culture system in HEp-2 cells for C. pneumoniae
enlarged RB during productive cell culture infections (60,
TW-183. The authors suggested that population shifts analo-
122). Could such occurrences indicate localized areas of nutri-
gous to those described for C. psittaci
(84) and C. trachomatis
ent depletion in a cell culture? Further studies with more
biovar trachoma (65) occur in the C. pneumoniae
relevant systems, for example, polarized cells and cells other
though corresponding characterizations of isolated C. pneu-
than epithelial cells, are required to further validate in vitro
and host cells have not been reported. Ultrastructur-
persistence in relation to in vivo events.
ally, continuous C. pneumoniae
infections at 48 h postsplit
largely resembled a productive infection, except for a subpopu-
EVIDENCE FOR CHLAMYDIAL PERSISTENCE IN VIVO
lation of atypical inclusions that were either partially or fully
occupied by aberrantly enlarged RB resembling those de-
The recognition that chlamydiae are likely to cause persis-
scribed in other persistence systems (63) (Table 1). These
tent infections in their hosts dates back to early descriptions of
infections also showed reduced sensitivity to azithromycin or
latent psittacosis in birds (73). Since these early studies of
ofloxacin at levels up to four times the MIC, as indicated by a
natural animal infections, the body of evidence that has accumu-
significantly slower reduction of inclusion-forming units when
lated for the existence of persistence in vivo has come mostly from
compared over 6 days to primary infections similarly exposed
experimental animal infections and clinical data from human dis-
from 0 h PI onwards (64). However, this property is not re-
ease. Various characteristics link these in vivo infections to the
stricted to continuous cultures or even to established persistent
well-studied cell culture models of persistent chlamydial infection.
cultures in general, since a similar effect was observed when
Among the most convincing lines of evidence for persistence in
established (48 h PI onwards) primary C. trachomatis
vivo are observations of altered morphological forms in vivo,
infections of HEp-2 cells were exposed to ofloxacin at up to
detection of chlamydial macromolecules in diseased hosts in the
four times the MIC over 18 days (30). In that study, the antibiotic
absence of cultivability, recurrences that occur when reinfection is
itself induced persistence, which in turn was considered to be
unlikely, and clinical antibiotic resistance.
responsible for the reduced antimicrobial susceptibility (30).
Electron microscopic visualization in diseased tissues of
There is a special case of continuous culture that cannot be
morphologically aberrant chlamydial forms resembling those
explained even by the genetic block hypothesis. Moulder et al.
observed in vitro (Table 1) indicates that such forms are un-
(80) reported continuous C. psittaci
6BC infections of L cells
likely to be mere laboratory artifacts. Nanagara et al. (81)
from which all isolated chlamydiae and host cells were found to
showed that atypical, pleomorphic RB with poorly defined
be indistinguishable from their wild-type counterparts. In ad-
outer membranes dominated within infected fibroblasts and
dition, nine clones taken from cultures with 25% inclusion-
macrophages in synovial membrane samples from patients with
positive cells were initially inclusion free but subsequently all
-associated reactive arthritis or Reiter’s syn-
gave rise to persistently infected populations (80). To explain
drome, despite antibiotic therapy. More recently, C. pneu-
these observations, the authors proposed that some chlamyd-
forms of a similar size to aberrant RB seen in vitro
iae in a cryptic form survived each wipeout of host cells and
were observed within macrophages in aortic valve samples
that every host cell was always infected by either cryptic or
from patients with degenerative aortic valve stenosis (113). In
productive chlamydiae (80). Moulder (78) has since raised the
addition, miniature C. trachomatis
forms have been observed
possibility that cryptic bodies may be related to single, inclu-
in total ejaculate and expressed prostatic secretion samples
sion-bound, dense oval bodies observed in C. trachomatis
from patients with chronic chlamydial prostatitis (70) and in
cell cultures (107) (Table 1) that behaved similarly to the C.
the oviducts of mice experimentally infected with C. trachoma-
6BC-L cell cultures (80). However, the precise mor-
biovar mouse pneumonitis (92). These miniature forms may
phology of cryptic chlamydiae remains unknown.
correspond to budding forms seen in vitro (10, 69). However,
Despite their relatively protected intracellular niche, chla-
observations of ultrastructurally aberrant chlamydiae alone fall
mydiae are subject to a variety of insults, particularly in intact
short of proving that chlamydiae persist in vivo, since the
hosts with competent immune systems. Examples of naturally
viability of these particles is uncertain.
occurring insults include shortages of essential nutrients, which
The presence of viable but atypical chlamydiae in vivo is
may be brought about by host-elaborated factors such as cyto-
suggested by the detection of chlamydial macromolecules at
kines and hormones, and phage hyperinfection of certain chla-
diseased sites in the absence of cultivable organisms. Chlamyd-
mydial strains. In addition, antibiotics specifically target chla-
ial DNA and antigen are often detected in tubal biopsy spec-
mydiae in multiple ways. Apart from these known factors,
imens from culture-negative women with postinfectious tubal
there are other mechanisms that remain incompletely ex-
infertility following antibiotic treatment (91). Even more con-
plained, including those underlying monocyte-induced persis-
vincingly, persistence of noncultivable chlamydiae in the va-
gina, uterus, and oviduct of ewes experimentally infected with
cin, and ofloxacin at concentrations above 4 g/ml) (115). This
a naturally occurring strain of C. psittaci
was documented for
indicated the presence of a more global resistance mechanism
more than a year by the detection of DNA or antigen in these
such as the induction of a persistent phenotype that is refrac-
tissues (89). Although the detection of chlamydial DNA or
tory to multiple antibiotics, for example, through membrane
antigen could reflect the presence of chlamydial cell debris
alterations that affect drug intake. In some cases, the explana-
remaining from resolved infections, PCR data showing that
tion for resistance could be more complex; certain genotypes
UV-inactivated C. pneumoniae
organisms were cleared far
could confer antibiotic resistance by encouraging development
more efficiently than live organisms from inoculated mice (74)
of the persistent phenotype. Such a scenario seems to occur in
argue against long-term persistence of chlamydial macromol-
tetracycline-resistant porcine C. trachomatis
strains, which pro-
ecules. In addition, chlamydial RNA has been detected in the
duced large aberrant RB in response to the antibiotic at 2
absence of cultivability in experimental trachoma of primates
g/ml (66). Could the gyrA
mutations that developed in cell
(48) as well as in synovial biopsy samples from patients with
culture in response to fluoroquinolone exposure (29) also favor
reactive arthritis or Reiter’s syndrome (35, 93). Since RNA is
the formation of a persistent phenotype, since alterations to
highly labile, its detection in vivo strongly suggests the presence of
DNA gyrase could inhibit RB-to-EB differentiation?
viable organisms and correlates with similar data indicating viable
Taken together, the in vivo data suggest that chlamydiae
but culture-negative chlamydiae in vitro (1, 30, 37, 59).
persist in their hosts. However, these studies do not conclu-
Experimental and clinical data provide evidence for reacti-
sively prove that the chlamydiae persist in an altered form. For
vating persistent chlamydiae in vivo. Early reports described
example, detection of chlamydial macromolecules in the ab-
individuals who had experienced acute ocular C. trachomatis
sence of cultivability could also reflect low-grade productive
infection as children living in areas of trachoma endemicity
infections that are not detectable by the culture methods used.
and showed no more symptoms until they developed active
Similarly, although Beatty et al. (10) definitively proved that in
trachoma decades after leaving those areas (119), indicating
vitro aberrant forms can give rise to productive infections, in
that the recurrences were more likely due to reactivations of
vivo data indicating reactivation could also represent enhance-
persistent infection than to reinfections. Particularly convinc-
ment of an inapparent productive infection. A final line of
ing evidence of periodic reactivation of persistent infections
evidence for altered forms in vivo that will be discussed below
comes from long-term studies that described individuals expe-
comes from studies that have demonstrated similarities in chla-
riencing multiple recurrent infections with chlamydial isolates
mydial gene or protein expression trends between persistent cell
of the same genotype. In a study of women with genital C.
culture systems and tissue samples from sites of chronic disease.
infection, Dean et al. (28) demonstrated recur-
rences of the original ompA
genotype over 2 to 4.5 years,
MOLECULAR BASIS OF CHLAMYDIAL PERSISTENCE
despite administration of accepted treatment regimens. Al-
though this study was limited by the inability to control for
Although our understanding of the molecular basis of chla-
reexposure to untreated partners, data from another study
mydial persistence is still at an early stage, the rate of progress
showed that 10% recurrence of genital C. trachomatis
in this area has significantly increased in recent years. This has
can occur even among subjects who report either abstinence or
been made possible primarily by the completion of several
100% condom use following treatment with azithromycin (56).
genome sequencing projects (54, 96, 97, 116) and by recent
Preliminary genotyping data from Dean et al. and Hammer-
improvements in molecular methodology, in particular, quan-
schlag et al. (27, 44) suggested that C. pneumoniae
titative PCR, microarrays, and proteomics. While persistence
persist for many years after initial respiratory infection. Animal
is a simple term, there are several systems for chlamydial
infection models provide additional evidence for reactivation
persistence with a significant number of variables that make
of persistent infections in vivo. In mice infected with either C.
direct comparison of results difficult. Variables include the
(125) or C. pneumoniae
(67), infections that had
infecting chlamydial species and strain, the origin of host cells,
become asymptomatic reactivated to productive infections af-
the in vitro persistence induction, and times PI chosen for
ter treatment with cortisone. Since the reactivation occurred
analysis. Despite this complexity, some common themes are
specifically after suppression of the immune system, these ob-
emerging from the results of molecular studies undertaken to
servations supported the hypothesis that immune factors such
date (Table 2). The categories described below represent a
as IFN-␥ play a significant role in persistence in vivo.
current overview of a rapidly advancing field and are therefore
Several studies have reported resistance of chlamydial iso-
likely to be modified as further data are generated, particularly
lates to antibiotics (reviewed in reference 99). Whether these
from global microarray and proteomics studies.
data reflect direct resistance or phenotypic resistance mani-
MOMP and cHSP60.
Until the recent availability of com-
fested by altered chlamydial forms is unclear. Chlamydiae are
plete chlamydial genome sequences, most molecular studies of
capable of developing true genotypic resistance to antibiotics
persistence focused on relative levels of the major outer mem-
in vitro. For example, C. trachomatis
serovar L2 mutants iso-
brane protein (MOMP) and chlamydial heat shock protein 60
lated from cell culture after several rounds of exposure to
(cHSP60). Using immunoelectron microscopy, Beatty et al. (8)
various fluoroquinolones consistently showed a point mutation
found that exposure of C. trachomatis
serovar A infections in
(encoding DNA gyrase subunit A), suggesting that
HeLa cells to 0.5 ng (6 U) of IFN-␥/ml caused cHSP60 levels
DNA gyrase is the primary target of these antibiotics (29).
to increase slightly (1.4-fold) and MOMP levels to decrease
However, a recent study described C. trachomatis
twofold, after correcting for the decreased surface-to-volume
sociated with treatment failure that were resistant to multiple
ratio in enlarged persistent forms. Subsequent immunoblot
drugs with diverse molecular targets (doxycycline, azithromy-
analyses showed a decreased MOMP-to-cHSP60 ratio over
TABLE 2. Postgenomic molecular studies of chlamydial persistence
MOMP was only down-regulated in serovar A.
Expression of these genes and proteins was normalized in genital serovars but not in serovar A after the addition of indole.
Only genes that were confirmed by rtRT-PCR to be differentially expressed have been included.
10-day C. trachomatis
serovar K infections in either HEp-2 cell
serovar K-infected monocyte cultures (36–38).
cultures exposed to 0.5 g of ciprofloxacin/ml (30) or unex-
Finally, two-dimensional (2-D) protein gel electrophoresis
posed monocyte cultures (37). In addition, semiquantitative
data showed that exposure to 100 U of IFN-␥/ml caused down-
reverse transcriptase PCR (sqRT-PCR) analyses demonstrated
regulated MOMP expression in C. trachomatis
serovar A (108)
selectively down-regulated transcription of the ompA
but not in serovars D and L2 (108, 109) in HeLa cells. Since the
latter two serovars are among the most resistant to IFN-␥ (76),
tence systems; all genes studied were strongly expressed during
those cultures may have required preexposure at the concen-
productive infections of HEp-2 cells used as controls (19, 38).
tration used to become persistent. Indeed, preexposure of
In C. trachomatis
serovar K-infected monocytes, polA
HeLa cells to 50 U of IFN-␥/ml was sufficient to induce per-
, and parB
transcripts were produced over the whole
sistence of C. trachomatis
serovar D and down-regulation of
7-day infection period, whereas ftsK
were not de-
, as assessed by microarray and real-time RT-PCR
tected after 1 day PI (38). Expression profiles in synovial tissue
(rtRT-PCR) (13). Ge´rard et al. (39) recently used rtRT-PCR
samples from patients with C. trachomatis
to quantitate the relative transcript levels of the three groEL
arthritis reflected the in vitro results; polA
genes (encoding cHSP60 homologues) in C. trachomatis
were weakly detected but neither ftsK
var K-infected monocytes over 7 days. Interestingly, transcrip-
tected (38). In C. pneumoniae
TW-183-infected HEp-2 cell
tion of groEL
was markedly higher than that of groEL
cultures exposed to 0.5 ng (6 U) of IFN-␥/ml, polA
in monocyte cultures, a trend not seen in HEp-2 cell
, and minD
expression was steady, whereas that of ftsK
control infections (39). However, the data also seemed to sug-
was absent (19). When the C. pneumoniae
gest that groEL
expression was strongly down-
were exposed to an IFN-␥ concentration (0.15 ng [1.8 U]/ml)
regulated in monocyte persistence relative to the controls. In
that caused no significant morphological alterations to the
addition, many other recent studies, mostly involving C. pneu-
chlamydiae compared to unexposed controls, ftsK
, have reported data contrary to a decreased ompA
expression was present but significantly attenuated (19). Taken
/cHSP60 ratio, indicating that this may not be
together, these studies demonstrated that down-regulated ftsK
as universal a marker of persistence as was once thought (47,
expression occurs in different in vitro persistence
models and in vivo and that these genetic alterations can pre-
In general, the in vivo data support the decreased ompA
cede evidence of morphological alterations during the estab-
/cHSP60 ratio described in many cell culture
studies of persistent C. trachomatis
. Immunoelectron micro-
More recent cell culture data have been inconclusive regard-
scopic (81) and sqRT-PCR (35, 36, 38) analyses of synovial
ing cell division and DNA replication gene expression during
tissue samples from patients with C. trachomatis
persistence. In our own laboratory (R. J. Hogan, D. A. Good,
chronic arthritis demonstrated diminished levels of MOMP
S. A. Mathews, S. Mukhopadhyay, J. T. Summersgill, and P.
transcripts, respectively. In apparent contrast, re-
Timms, unpublished data), rtRT-PCR analysis of C. pneu-
cent data from similar samples indicate down-regulated ex-
A-03-infected HEp-2 cells exposed to 50 U of IFN-
pression of all three groEL
genes, including the virtual absence
␥/ml revealed strong down-regulation of ftsK
transcripts (39). Nonetheless, many studies have
differential expression of ftsW
. Microarray studies of persistent
reported enhanced production of cHSP60-specific antibodies
serovar D in HeLa cell cultures have also pro-
in various chronic chlamydial infections (reviewed in reference
vided mixed data; exposure of such infections to 100 U of
penicillin/ml was associated with up-regulated ftsK
Whether an altered MOMP-to-cHSP60 ratio has any signif-
expression (82), whereas exposure to IFN-␥ lead
icance to chlamydial pathogenesis beyond serving as a marker
to unchanged ftsK
and down-regulated ftsW
of persistence in vivo is a subject of much debate. Beatty et al.
These discrepancies are not surprising, since the precise func-
(7) proposed that reduced levels of MOMP, an immunopro-
tions of chlamydial FtsK and FtsW are unlikely to be directly
tective antigen, could enable chlamydiae to avoid the develop-
related. FtsW is a predicted septum-peptidoglycan biosynthetic
ment of protective immunity. At the same time, according to
protein involved in cell wall formation, whereas FtsK is re-
the traditional immunological paradigm of chlamydial patho-
quired for chromosome segregation (116). Similarly, DNA rep-
genesis, steady or increased cHSP60 levels would promote
lication gene expression profiles were varied in the microarray
immunopathology through delayed-type hypersensitivity or
study of IFN-␥-exposed C. trachomatis
, with some genes up-
cross-reactivity with HSP60 from either the human host or
, and xerC
) and others down-regulated
other pathogens. Another potential explanation for the rela-
, and ihfA
tive abundance of cHSP60 during C. trachomatis
Using a similar experimental design to
its function as a stress response chaperone (77), which is likely
that of their previous study (38) (see above), Ge´rard and col-
to be of particular importance under the conditions that induce
leagues (36) studied genes encoding enzymes predicted to be
persistence. In support of this view, the production of five
involved in energy metabolism in both cultured C. trachomatis
proteins thought to be major heat shock proteins, including
infected monocytes and synovial tissue samples from patients
cHSP60, was strongly increased as assessed by one-dimen-
with C. trachomatis
-associated reactive arthritis. Since genes
sional gel electrophoresis after heat shock of C. trachomatis
encoding enzymes belonging to glycolysis (pyk
, and pgk
and the pentose phosphate pathway (gnd
) were found
Cell division and chromosome replication and partitioning.
to be selectively down-regulated in vitro and in vivo relative to
Recent sqRT-PCR investigations have provided molecular in-
genes encoding enzymes in the tricarboxylic acid cycle (mdhC
sights into the common observation that persistent chlamydiae
), the authors concluded that persistence may be
are inhibited in cell division and yet continue to accumulate
characterized by a shift from a partial to a full reliance on the
chromosomes (Table 1). These studies analyzed expression of
host cell for ATP (36). The microarray expression data for
genes encoding products predicted to function in DNA repli-
genes encoding tricarboxylic acid cycle enzymes in IFN-␥-in-
, and mutS
), chromosome partitioning (parB
duced persistence of C. trachomatis
were mixed. Genes encod-
), and cell division (ftsK
) in various persis-
ing 2-oxoglutarate dehydrogenase (sucA
, and sucB
and succinate thiokinase (sucC
) were down-regu-
Another immunoavoidance strategy that has been
lated, whereas genes encoding other enzymes in the cycle were
identified in both C. trachomatis
(126) and C. pneumoniae
either up-regulated (fumC
) or unchanged (mdhC
is the secretion of a chlamydial protease-like activity factor
, and sdhC
). There was little evidence of down-regulation
(CPAF) into the host cell cytoplasm that cleaves eukaryotic
in glycolysis or the pentose phosphate pathway (13). Also in
transcription factors required for both major histocompatibil-
apparent contrast to the results of Ge´rard et al. (36), 2-D gel
ity complex class I and II antigen expression. Recent rtRT-
analysis of C. pneumoniae
A-03 exposed to 50 U of IFN-␥/ml
PCR (R. J. Hogan, S. A. Mathews, S. Mukhopadhyay, J. T.
showed up-regulated Pgk and unchanged Pyk levels (75).
Summersgill, and P. Timms, unpublished data), 2-D gel (110),
Tryptophan metabolism in relation
and immunoblot (46) analyses of IFN-␥-exposed chlamydial
to persistence has recently become an area of intensive re-
infections, in addition to immunoblotting of iron-depleted C.
search. Using 2-D gel analysis, Shaw and colleagues (108, 109)
cultures (46), revealed little or no differential ex-
showed that exposure of C. trachomatis
serovars A, D, and L2
pression of CPAF between persistent and productive infec-
to 100 U of IFN-␥/ml caused significant tryptophan-reversible
tions. These data indicated that CPAF production is important
up-regulation of the ␣ and ␤ chains of chlamydial tryptophan
in both persistent and normal infections. Interestingly, Heuer
synthase. Induction of this enzyme is a plausible mechanism by
et al. (46) demonstrated inhibited CPAF translocation to the
which chlamydiae would counteract tryptophan deficiency in-
host (HEp-2) cell cytoplasm during both IFN-␥- and iron de-
duced by exposure to IFN-␥. However, another important find-
ficiency-induced persistence of C. pneumoniae
ing was the presence of frameshift mutations in the gene en-
These authors reasoned that if CPAF is translocated by the
coding the ␣ chain, trpA
, for serovars A and C and the resultant
same mechanism as other chlamydial proteins, and if this
synthesis of a truncated ␣ chain by these serovars (109).
mechanism is inhibited during persistence, then the resultant
More recent studies (20, 33, 124) have developed the initial
general decrease in chlamydial antigen processing and presen-
observations of Shaw and colleagues (108, 109) into a para-
tation would reduce the requirement for CPAF activity during
digm that links the tissue tropisms of C. trachomatis
this phase (46). The absence of CPAF protease activity on host
their relative abilities to synthesize tryptophan. Fehlner-
cell proteins could also reduce the availability of readily trans-
Gardiner et al. (33) cloned and sequenced a section of the
portable amino acids (including tryptophan) to chlamydiae and
plasticity zone, a highly variable region of the chlamydial ge-
therefore contribute to the maintenance of persistence.
nome that contains the trpBA
operon encoding both chains of
Down-regulated expression during persistence
tryptophan synthase, from 15 reference strains representing all
of genes and proteins that are specifically expressed late in the
human-infecting serovars of C. trachomatis
. With the exception
productive developmental cycle is a common observation most
of serovar B, which was missing the trpBA
operon, these au-
likely reflecting the inhibited RB-to-EB differentiation that
thors concluded that ocular (A to C and Ba) but not genital (D
characterizes persistence. A well-known example is the 60-kDa
to K and L1 to L3) serovars have the trpA
cysteine-rich protein (CRP), which is abundant in EB enve-
Further investigation with several experimental approaches re-
lopes. The 60-kDa CRP was either markedly diminished or
vealed that this mutation was responsible for the inability of
completely absent in early studies of C. trachomatis
ocular serovars to synthesize tryptophan from indole (33) (see
exposed to ␤-lactam antibiotics (23, 104), IFN-␥ (7, 10), or
above). Caldwell et al. (20) continued the sequencing approach
heat shock (53). Another study reported the absence of the
to confirm that this paradigm also applies to clinical isolates of
Hc-1 and Hc-2 DNA-binding proteins (involved in chromo-
and used rtRT-PCR to analyze trpBA
somal condensation) in IFN-␥-exposed C. trachomatis
sion for selected ocular (A) and genital (E) serovars in infec-
though interestingly, the 60-kDa CRP was not down-regulated
tions of HeLa cells. While both serovars strongly up-regulated
(52) (Table 2). Similarly, both persistence microarray studies
expression in response to 5 ng (60 U) of IFN-␥/ml,
carried out to date reported down-regulated hctB
only serovar E trpBA
expression returned to original levels
Hc-2) expression in C. trachomatis
, whereas omcB
following the addition of 100 M indole, providing further
the 60-kDa CRP) down-regulation was associated with expo-
support for the paradigm that ocular serovars are not indole
sure to IFN-␥ (13) but not to penicillin (82). As was the case
rescuable (20). The authors concluded that the evolution of
/MOMP, many other studies have also failed to con-
genital strains to utilize indole for tryptophan synthesis repre-
/60-kDa CRP down-regulation in persistent cultures
sents an immunoavoidance strategy, since these serovars are
(47, 52, 68, 114). Therefore, hctB
/Hc-2 down-regulation cur-
more likely to survive under IFN-␥ pressure in vivo (20).
rently appears to be a more reliable marker of persistence.
does not contain a trpBA
operon in its ge-
Nonetheless, the penicillin microarray study (82) strongly sup-
nome (54) and may therefore have an alternative strategy to
ported the late gene shut-down hypothesis, since 14 of the 15
counteract the effects of IDO activity, which this species no
most down-regulated genes in that study were subsequently
doubt encounters in vivo (103). A recent study revealed that
confirmed to be late genes (14, 83) (Table 2). Finally, this
respiratory strains of C. pneumoniae
possess multiple copies of
hypothesis was used to explain selective down-regulation of
gene, encoding a tyrosine-tryptophan permease, while
late-stage-specific type III secretion genes in an sqRT-PCR
vascular strains encode only one copy (41). Since the presence
study of C. pneumoniae
CM-1-infected HEp-2 cell cultures
of extra tyrP
copies correlated with increased mRNA levels and
exposed to 40 ng (480 U) of IFN-␥/ml (114).
higher uptake of the substrate tyrosine in respiratory strains,
Microarray analysis: new persistence gene categories iden-
the authors hypothesized that a reduced capacity for amino
The recent microarray study of IFN-␥-exposed C. tra-
acid transport may contribute to a greater tendency of vascular
by Belland et al. (13) provided the first published
strains to become persistent in vivo (41).
transcriptome-wide data set for chlamydiae in a persistence
system. While many of the differentially expressed genes fall
tryptophan residues in C. trachomatis
serovar L2 than are con-
into categories discussed above, other groups were identified
tained in C. psittaci
that have not previously been considered in the context of
chlamydial persistence. Many rl
genes (encoding ribo-
somal proteins) were markedly up-regulated, and none were
significantly down-regulated (13). Early genes that were up-
regulated in persistence included euo
and the incD
While the exact molecular mechanism by which chlamydiae
although another early gene (oppA
) was reported to be
enter and exit the persistent phase is not yet understood, there
down-regulated (13). The euo
result was consistent with data
is little doubt that this stage plays an important role in chla-
from C. trachomatis
serovar L2-infected HeLa cells cultured in
mydial development. The well-accepted biphasic paradigm of
the absence of glucose (51), conditions known to induce per-
chlamydial development warrants the addition of a persistent
sistence in this species (45). In that study, transcripts from euo
phase that represents a critical survival mechanism of this
but not from eight other genes of interest were detectable by
well-adapted intracellular pathogen. Down-regulated expres-
sqRT-PCR (51) (Table 2). Other novel observations from the
sion of the major structural protein MOMP was one of the first
microarray data included up-regulation of three members of a
markers of persistence but has not been confirmed in all sub-
gene family encoding phospholipase D-like enzymes and
sequent studies on persistence. On closer examination of these
down-regulation of four clp
and three opp
data, most C. trachomatis
studies reported down-regulation of
proteins involved in proteolysis and peptide transport, respec-
/MOMP, whereas the C. pneumoniae
up-regulation of ompA
/MOMP. Perhaps this could be ex-
Belland et al. (13) used rtRT-PCR to clarify the magnitude
plained by different roles played by MOMP in the two species.
of relative changes for 15 genes (Table 2). For example, three
In C. pneumoniae
, MOMP is apparently less important struc-
, and trpB
, with up-regulated microarray
turally, whereas its role as a porin might be more critical.
changes of a similar magnitude (approximately 6-fold) showed
Another example of a species-specific persistence marker
rtRT-PCR changes of 12.6-, 28.7-, and 458-fold, respectively
relates to the elegant work of Caldwell et al. and Fehlner-
(13). The dramatic variation in these figures emphasizes the
Gardiner et al. (20, 33), who clearly showed the key role of a
importance of quantitative PCR in verifying microarray data,
functional tryptophan synthase in the development of C. tra-
and perhaps this should include data for genes that show no
experiencing tryptophan depletion. This is also an
significant relative change by microarray analysis.
example of an expression pattern that relates to the key met-
Regulatory mechanisms in persistence.
The gene expression
abolic limitation in a particular system of persistence. Such
data described in the above categories imply that many persis-
patterns are presumably not a direct effect of persistence but
tence gene profiles are transcriptionally regulated. Based on
rather the chlamydiae trying to compensate for limitations in
their microarray data, Belland et al. (13) proposed the exis-
other parts of the same or related pathways.
tence of a chlamydial persistence stimulon that is more com-
While most persistence patterns appear complex at this
plex than a general stress response. The strongly up-regulated
stage of our understanding of chlamydial cell biology, it is
expression observed during persistence in that study may
hoped they can be unraveled by the multiple molecular ap-
provide the first insight into the molecular mechanisms of
proaches that are currently being pursued, for example RT-
regulation, since these authors proposed that Euo may con-
PCR, proteomics, and microarrays. Since these methods com-
tribute towards silencing late gene expression in persistence
plement each other, their combined use will be critical for
(13). The precise mechanisms of regulation for other differen-
answering biological questions about persistence. The estab-
tially expressed gene categories remain unknown. Another un-
lishment of gene and protein expression patterns representing
answered question is to what degree a persistence stimulon
persistence in vitro will allow the development of persistence
may vary among persistence systems resulting from different
assays to be applied to in vivo samples, resulting in the corre-
lation of model systems to clinical reality.
In addition, translational control may contribute to some
An important question that remains is whether the persis-
observed profiles, at least in IFN-␥-mediated persistence. Be-
tence phenotype is simply a survival response by chlamydiae to
atty and colleagues (6) proposed that direct control at the
a range of unfavorable environmental conditions, with the sub-
translational level may explain their observations of steady
sequent complex gene expression profiles being the uncon-
cHSP60 levels and reduced membrane antigen levels in IFN-
trolled outcome. Alternatively, have chlamydiae evolved over
␥-mediated persistence. This proposal is based on gene se-
millions of years to deliberately enter the persistent state to
quence data revealing that both MOMP and the 60-kDa CRP,
maximize evasion of host cell immune mechanisms and in-
but not cHSP60, contain significant numbers of tryptophan
crease growth and survival opportunities in different host
residues and hence require adequate tryptophan levels for
niches? Indeed, can we further document the overall preva-
their synthesis (6). Brown and Rockey (16) also suggested this
lence of this persistent state in vivo? Have different ecotypes of
mechanism to potentially explain their observation that expres-
(serovars and strains) evolved subtly different ways
sion of an unidentified antigen (termed SEP) that localizes to
of responding to environmental cues so as to enter this quies-
the septum of dividing chlamydiae was severely attenuated in
cent phase to enhance survival or transmission? As we con-
IFN-␥-exposed C. trachomatis
serovar L2 but not in IFN-␥-
tinue to better understand the molecular and cellular mecha-
exposed C. psittaci
GPIC and not after exposure of either
nisms of chlamydiae, we will also begin to understand the role
strain to ampicillin. If SEP is a peptidoglycan, the inhibition
of persistence in this uniquely adapted obligate intracellular
may have been due to a biosynthetic enzyme containing more
demiology and reproductive sequelae. Am. J. Obstet. Gynecol. 164:
R.J.H. was supported by a Queensland University of Technology
23. Cevenini, R., M. Donati, and M. La Placa.
1988. Effects of penicillin on the
and Cooperative Research Centre for Diagnostics Postgraduate Re-
synthesis of membrane proteins of Chlamydia trachomatis
search Award. Research in the Timms laboratory has been supported
FEMS Microbiol. Lett. 56:
by NHMRC and NIH grant AI51255, and work in the Summersgill
24. Clark, R. B., P. F. Schatzki, and H. P. Dalton.
1982. Ultrastructural analysis
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AIMAC 2005 8th International Conference on Arts and Cultural Management July 3-6, 2005. Montréal, Canada Sunday, July 3rd 18:00 Welcome Cocktail: Musée du Château Ramezay 280 Notre-Dame Street East Monday, July 4th Opening remarks "We Manage as We Are" by Laurent Lapierre Session 1 - Consumer Behaviour Room: Meloche Monnex Chair : Anne-Gaëlle Jol
Karolina Kucerova Charles University in Prague Evidence-Based Medicine Course The University of North Carolina at Chapel Hill Spring 2004 Introduction This work intends to concentrate on comparison of two sources of medical information included in the Medline database, PubMed and OVID. Its first part should consist of some “academic information” about these resources, their history a