All subtypes of the Pmp adhesin family are implicated in chlamydial virulence and show species-specific function

The Number, Organization, and Size of Polymorphic Membrane Protein Coding Sequencesas well as the Most Conserved Pmp Protein Differ within and across Species

Variation is a central trait of the polymorphic membrane protein (Pmp) family. The number of <i>pmp</i> coding sequences differs between <i>Chlamydia</i> species, but it is unknown whether the number of <i>pmp</i> coding sequences is constant within a <i>Chlamydia</i> species. The level of conservation of the Pmp proteins has previously only been determined for <i>Chlamydia trachomatis.</i> As different Pmp proteins might be indispensible for the pathogenesis of different <i>Chlamydia </i>species, this study investigated the conservation of Pmp proteins both within and across <i>C. trachomatis,</i><i>C. pneumoniae,</i><i>C. abortus,</i> and <i>C. psittaci.</i> The <i>pmp</i> coding sequences were annotated in 16 <i>C. trachomatis,</i> 6 <i>C. pneumoniae,</i> 2 <i>C. abortus,</i> and 16 <i>C. psittaci</i> genomes. The number and organization of polymorphic membrane coding sequences differed within and across the analyzed <i>Chlamydia </i>species. The length of coding sequences of <i>pmpA,</i><i>pmpB,</i> and <i>pmpH</i> was conserved among all analyzed genomes, while the length of <i>pmpE/F</i> and <i>pmpG,</i> and remarkably also of the subtype <i>pmpD,</i> differed among the analyzed genomes. PmpD, PmpA, PmpH, and PmpA were the most conserved Pmp in <i>C. trachomatis,</i><i>C. pneumoniae,</i><i>C. abortus,</i> and <i>C. psittaci</i>, respectively. PmpB was the most conserved Pmp across the 4 analyzed <i>Chlamydia</i> species.

Recombinant polymorphic membrane protein D in combination with a novel, second-generation lipid adjuvant protects against intra-vaginal Chlamydia trachomatis infection in mice

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rPmpD in combination with SLA elicits significant protection against intra-vaginal Ct challenge.

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Antibodies induced by immunisation with rPmpD recognise Ct elementary bodies.

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SLA is a novel adjuvant class that may be widely used in future preclinical Ct vaccine development.

The development of a chlamydial vaccine that elicits protective mucosal immunity is of paramount importance in combatting the global spread of sexually transmitted Chlamydia trachomatis (Ct) infections. While the identification and prioritization of chlamydial antigens is a crucial prerequisite for efficacious vaccine design, it is likely that novel adjuvant development and selection will also play a pivotal role in the translational potential of preclinical Ct vaccines. Although the molecular nature of the immuno-modulatory component is of primary importance, adjuvant formulation and delivery systems may also govern vaccine efficacy and potency. Our study provides the first preclinical evaluation of recombinant Ct polymorphic membrane protein D (rPmpD) in combination with three different formulations of a novel second-generation lipid adjuvant (SLA). SLA was rationally designed in silico by modification of glucopyranosyl lipid adjuvant (GLA), a TLR4 agonistic precursor molecule currently in Phase II clinical development. We demonstrate robust protection against intra-vaginal Ct challenge in mice, evidenced by significantly enhanced resistance to infection and reduction in mean bacterial load. Strikingly, protection was found to correlate with the presence of robust anti-rPmpD serum and cervico-vaginal IgG titres, even in the absence of adjuvant-induced Th1-type cellular immune responses elicited by each SLA formulation, and we further show that anti-rPmpD antibodies recognize Ct EBs. These findings highlight the utility of SLA and rational molecular design of adjuvants in preclinical Ct vaccine development, but also suggest an important role for anti-rPmpD antibodies in protection against urogenital Ct infection.

Chlamydia cell biology and pathogenesis

Chlamydia spp. are important causes of human disease for which no effective vaccine exists. These obligate intracellular pathogens replicate in a specialized membrane compartment and use a large arsenal of secreted effectors to survive in the hostile intracellular environment of the host. In this Review, we summarize the progress in decoding the interactions between Chlamydia spp. and their hosts that has been made possible by recent technological advances in chlamydial proteomics and genetics. The field is now poised to decipher the molecular mechanisms that underlie the intimate interactions between Chlamydia spp. and their hosts, which will open up many exciting avenues of research for these medically important pathogens.

Subunit vaccines for the prevention of mucosal infection with Chlamydia trachomatis

Chlamydia trachomatis is the most common preventable cause of tubal infertility in women. In high-income countries, despite public health control efforts, C. trachomatis case rates continue to rise. Most medium and low-income countries lack any Chlamydia control program; therefore, a vaccine is essential for the control of Chlamydia infections. A rationally designed Chlamydia vaccine requires understanding of the immunological correlates of protective immunity, pathological responses to this mucosal pathogen, identification of optimal vaccine antigens and selection of suitable adjuvant delivery systems that engender protective immunity. Fortunately, Chlamydia vaccinology is facilitated by genomic knowledge and by murine models that reproduce many of the features of human C. trachomatis infection. This article reviews recent progress in these areas with a focus on subunit vaccine development.

Chlamydia trachomatis from Australian Aboriginal people with trachoma are polyphyletic composed of multiple distinctive lineages

Chlamydia trachomatis causes sexually transmitted infections and the blinding disease trachoma. Current data on C. trachomatis phylogeny show that there is only a single trachoma-causing clade, which is distinct from the lineages causing urogenital tract (UGT) and lymphogranuloma venerum diseases. Here we report the whole-genome sequences of ocular C. trachomatis isolates obtained from young children with clinical signs of trachoma in a trachoma endemic region of northern Australia. The isolates form two lineages that fall outside the classical trachoma lineage, instead being placed within UGT clades of the C. trachomatis phylogenetic tree. The Australian trachoma isolates appear to be recombinants with UGT C. trachomatis genome backbones, in which loci that encode immunodominant surface proteins (ompA and pmpEFGH) have been replaced by those characteristic of classical ocular isolates. This suggests that ocular tropism and association with trachoma are functionally associated with some sequence variants of ompA and pmpEFGH.

Chlamydia trachomatis isolates causing a blinding disease (trachoma) form a single lineage that is different from the lineages causing urogenital infections. Here, Andersson et al. show however that trachoma isolates from Australia are more closely related to urogenital strains than to other trachoma isolates.

The novel chlamydial adhesin CPn0473 mediates the lipid raft-dependent uptake of Chlamydia pneumoniae

Chlamydiae are Gram‐negative, obligate intracellular pathogens that pose a serious threat to public health worldwide. Chlamydial surface molecules are essential for host cell invasion. The first interaction with the host cell is thereby accomplished by the Outer membrane complex protein B (OmcB) binding to heparan sulfate moieties on the host cell surface, followed by the interaction of the chlamydial polymorphic membrane proteins (Pmps) with host cell receptors. Specifically, the interaction of the Pmp21 adhesin and invasin with its human interaction partner, the epidermal growth factor receptor, results in receptor activation, down‐stream signalling and finally internalization of the bacteria. Blocking both, the OmcB and Pmp21 adhesion pathways, did not completely abolish infection, suggesting the presence of additional factors relevant for host cell invasion. Here, we show that the novel surface protein CPn0473 of Chlamydia pneumoniae contributes to the binding and invasion of infectious chlamydial particles. CPn0473 is expressed late in the infection cycle and located on the infectious chlamydial cell surface. Soluble recombinant CPn0473 as well as rCPn0473‐coupled fluorescent latex beads adhere to human epithelial HEp‐2 cells. Interestingly, in classical infection blocking experiments pretreatment of HEp‐2 cells with rCPn0473 does not attenuate adhesion but promotes dose‐dependently internalization by C. pneumoniae suggesting an unusual mode of action for this adhesin. This CPn0473‐dependent promotion of infection by C. pneumoniae depends on two different domains within the protein and requires intact lipid rafts. Thus, inhibition of the interaction of CPn0473 with the host cell could provide a way to reduce the virulence of C. pneumoniae.

The Chlamydia trachomatis Ctad1 invasin exploits the human integrin β1 receptor for host cell entry

Infection of human cells by the obligate intracellular bacterium Chlamydia trachomatis requires adhesion and internalization of the infectious elementary body (EB). This highly complex process is poorly understood. Here, we characterize Ctad1 (CT017) as a new adhesin and invasin from C. trachomatis serovar E. Recombinant Ctad1 (rCtad1) binds to human cells via two bacterial SH3 domains located in its N-terminal half. Pre-incubation of host cells with rCtad1 reduces subsequent adhesion and infectivity of bacteria. Interestingly, protein-coated latex beads revealed Ctad1 being an invasin. rCtad1 interacts with the integrin β1 subunit on human epithelial cells, and induces clustering of integrins at EB attachment sites. Receptor activation induces ERK1/2 phosphorylation. Accordingly, rCtad1 binding to integrin β1-negative cells is significantly impaired, as is the chlamydial infection. Thus interaction of C. trachomatis Ctad1 with integrin β1 mediates EB adhesion and induces signaling processes that promote host-cell invasion.

One Face of Chlamydia trachomatis: The Infectious Elementary Body

The lifestyle of Chlamydiae is unique: the bacteria alternate between two morphologically distinct forms, an infectious non-replicative elementary body (EB), and a replicative, non-infectious reticulate body (RB). This review focuses on recent advances in understanding the structure and function of the infectious form of the best-studied member of the phylum, the human pathogen Chlamydia trachomatis. Once considered as an inert particle of little functional capacity, the EB is now perceived as a sophisticated entity that encounters at least three different environments during each infectious cycle. We review current knowledge on its composition and morphology, and emerging metabolic activities. These features confer resistance to the extracellular environment, the ability to penetrate a host cell and ultimately enable the EB to establish a niche enabling bacterial survival and growth. The bacterial and host molecules involved in these processes are beginning to emerge.

Delivery of a Chlamydial Adhesin N-PmpC Subunit Vaccine to the Ocular Mucosa Using Particulate Carriers

Trachoma, caused by the intracellular bacterium Chlamydia trachomatis (Ct), remains the world's leading preventable infectious cause of blindness. Recent attempts to develop effective vaccines rely on modified chlamydial antigen delivery platforms. As the mechanisms engaged in the pathology of the disease are not fully understood, designing a subunit vaccine specific to chlamydial antigens could improve safety for human use. We propose the delivery of chlamydia-specific antigens to the ocular mucosa using particulate carriers, bacterial ghosts (BGs). We therefore characterized humoral and cellular immune responses after conjunctival and subcutaneous immunization with a N-terminal portion (amino acid 1-893) of the chlamydial polymorphic membrane protein C (PmpC) of Ct serovar B, expressed in probiotic Escherichia coli Nissle 1917 bacterial ghosts (EcN BGs) in BALB/c mice. Three immunizations were performed at two-week intervals, and the immune responses were evaluated two weeks after the final immunization in mice. In a guinea pig model of ocular infection animals were immunized in the same manner as the mice, and protection against challenge was assessed two weeks after the last immunization. N-PmpC was successfully expressed within BGs and delivery to the ocular mucosa was well tolerated without signs of inflammation. N-PmpC-specific mucosal IgA levels in tears yielded significantly increased levels in the group immunized via the conjunctiva compared with the subcutaneously immunized mice. Immunization with N-PmpC EcN BGs via both immunization routes prompted the establishment of an N-PmpC-specific IFNγ immune response. Immunization via the conjunctiva resulted in a decrease in intensity of the transitional inflammatory reaction in conjunctiva of challenged guinea pigs compared with subcutaneously and non-immunized animals. The delivery of the chlamydial subunit vaccine to the ocular mucosa using a particulate carrier, such as BGs, induced both humoral and cellular immune responses. Further investigations are needed to improve the immunization scheme and dosage.

Chlamydial polymorphic membrane proteins: regulation, function and potential vaccine candidates

Pmps (Polymorphic Membrane Proteins) are a group of membrane bound surface exposed chlamydial proteins that have been characterized as autotransporter adhesins and are important in the initial phase of chlamydial infection. These proteins all contain conserved GGA (I, L, V) and FxxN tetrapeptide motifs in the N-terminal portion of each protein. All chlamydial species express Pmps. Even in the chlamydia-related bacteria Waddlia chondrophila, a Pmp-like adhesin has been identified, demonstrating the importance of Pmps in Chlamydiales biology. Chlamydial species vary in the number of pmp genes and their differentially regulated expression during the infectious cycle or in response to stress. Studies have also demonstrated that Pmps are able to induce innate immune functional responses in infected cells, including production of IL-8, IL-6 and MCP-1, by activating the transcription factor NF-κB. Human serum studies have indicated that although anti-Pmp specific antibodies are produced in response to a chlamydial infection, the response is variable depending on the Pmp protein. In C. trachomatis, PmpB, PmpC, PmpD and PmpI were the proteins eliciting the strongest immune response among adolescents with and without pelvic inflammatory disease (PID). In contrast, PmpA and PmpE elicited the weakest antibody response. Interestingly, there seems to be a gender bias for Pmp recognition with a stronger anti-Pmp reactivity in male patients. Furthermore, anti-PmpA antibodies might contribute to adverse pregnancy outcomes, at least among women with PID. In vitro studies indicated that dendritic cells infected with C. muridarum were able to present PmpG and PmpF on their MHC class II receptors and T cells were able to recognize the MHC class-II bound peptides. In addition, vaccination with PmpEFGH and Major Outer Membrane Protein (MOMP) significantly protected mice against a genital tract C. muridarum infection, suggesting that Pmps may be an important component of a multi-subunit chlamydial vaccine. Thus, Pmps might be important not only for the pathogenesis of chlamydial infection, but also as potential candidate vaccine proteins.

Genetic diversity in the plasticity zone and the presence of the chlamydial plasmid differentiates Chlamydia pecorum strains from pigs, sheep, cattle, and koalas

Background

Chlamydia pecorum is a globally recognised pathogen of livestock and koalas. To date, comparative genomics of C. pecorum strains from sheep, cattle and koalas has revealed that only single nucleotide polymorphisms (SNPs) and a limited number of pseudogenes appear to contribute to the genetic diversity of this pathogen. No chlamydial plasmid has been detected in these strains despite its ubiquitous presence in almost all other chlamydial species. Genomic analyses have not previously included C. pecorum from porcine hosts. We sequenced the genome of three C. pecorum isolates from pigs with differing pathologies in order to re-evaluate the genetic differences and to update the phylogenetic relationships between C. pecorum from each of the hosts.

Methods

Whole genome sequences for the three porcine C. pecorum isolates (L1, L17 and L71) were acquired using C. pecorum-specific sequence capture probes with culture-independent methods, and assembled in CLC Genomics Workbench. The pairwise comparative genomic analyses of 16 pig, sheep, cattle and koala C. pecorum genomes were performed using several bioinformatics platforms, while the phylogenetic analyses of the core C. pecorum genomes were performed with predicted recombination regions removed. Following the detection of a C. pecorum plasmid, a newly developed C. pecorum-specific plasmid PCR screening assay was used to evaluate the plasmid distribution in 227 C. pecorum samples from pig, sheep, cattle and koala hosts.

Results

Three porcine C. pecorum genomes were sequenced using C. pecorum-specific sequence capture probes with culture-independent methods. Comparative genomics of the newly sequenced porcine C. pecorum genomes revealed an increased average number of SNP differences (~11 500) between porcine and sheep, cattle, and koala C. pecorum strains, compared to previous C. pecorum genome analyses. We also identified a third copy of the chlamydial cytotoxin gene, found only in porcine C. pecorum isolates. Phylogenetic analyses clustered porcine isolates into a distinct clade, highlighting the polyphyletic origin of C. pecorum in livestock.

Most surprising, we also discovered a plasmid in the porcine C. pecorum genome. Using this novel C. pecorum plasmid (pCpec) sequence, a) we developed a pCpec screening assay to evaluate the plasmid distribution in C. pecorum from different hosts; and b) to characterise the pCpec sequences from available previously sequenced C. pecorum genome data. pCpec screening showed that the pCpec is common in all hosts of C. pecorum, however not all C. pecorum strains carry pCpec.

Conclusions

This study provides further insight into the complexity of C. pecorum epidemiology and novel genomic regions that may be linked to host specificity. C. pecorum plasmid characterisation may aid in improving our understanding of C. pecorum pathogenesis across the variety of host species this animal pathogen infects.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-2053-8) contains supplementary material, which is available to authorized users.

Intramuscular Immunisation with Chlamydial Proteins Induces Chlamydia trachomatis Specific Ocular Antibodies

BACKGROUND

Ocular infection with Chlamydia trachomatis can cause trachoma, which is the leading cause of blindness due to infection worldwide. Despite the large-scale implementation of trachoma control programmes in the majority of countries where trachoma is endemic, there remains a need for a vaccine. Since C. trachomatis infects the conjunctival epithelium and stimulates an immune response in the associated lymphoid tissue, vaccine regimens that enhance local antibody responses could be advantageous. In experimental infections of non-human primates (NHPs), antibody specificity to C. trachomatis antigens was found to change over the course of ocular infection. The appearance of major outer membrane protein (MOMP) specific antibodies correlated with a reduction in ocular chlamydial burden, while subsequent generation of antibodies specific for PmpD and Pgp3 correlated with C. trachomatis eradication.

METHODS

We used a range of heterologous prime-boost vaccinations with DNA, Adenovirus, modified vaccinia Ankara (MVA) and protein vaccines based on the major outer membrane protein (MOMP) as an antigen, and investigated the effect of vaccine route, antigen and regimen on the induction of anti-chlamydial antibodies detectable in the ocular lavage fluid of mice.

RESULTS

Three intramuscular vaccinations with recombinant protein adjuvanted with MF59 induced significantly greater levels of anti-MOMP ocular antibodies than the other regimens tested. Intranasal delivery of vaccines induced less IgG antibody in the eye than intramuscular delivery. The inclusion of the antigens PmpD and Pgp3, singly or in combination, induced ocular antigen-specific IgG antibodies, although the anti-PmpD antibody response was consistently lower and attenuated by combination with other antigens.

CONCLUSIONS

If translatable to NHPs and/or humans, this investigation of the murine C. trachomatis specific ocular antibody response following vaccination provides a potential mouse model for the rapid and high throughput evaluation of future trachoma vaccines.

Escherichia coli Nissle 1917 bacterial ghosts retain crucial surface properties and express chlamydial antigen: an imaging study of a delivery system for the ocular surface

To target chronic inflammatory ocular surface diseases, a drug delivery platform is needed that is safe, possesses immunomodulatory properties, and can be used either for drug delivery, or as a foreign antigen carrier. A new therapeutic approach that we have previously proposed uses nonliving bacterial ghosts (BGs) as a carrier-delivery system which can be engineered to carry foreign antigens and/or be loaded with therapeutic drugs. The parent strain chosen for development of our BG delivery system is the probiotic Escherichia coli strain Nissle 1917 (EcN), whose intrinsic properties trigger the innate immune system with the flagella and fimbriae used to attach and stimulate epithelial cells. In previous studies, we have shown that EcN BGs are safe for the ocular surface route, but evidence that EcN BGs retain flagella and fimbriae after transformation, has never been visually confirmed. In this study, we used different visualization techniques to determine whether flagella and fimbriae are retained on EcN BGs engineered either for drug delivery or as a foreign antigen carrier. We have also shown by immunoelectron microscopy that EcN retains two foreign antigens after processing to become EcN BGs. Furthermore, we demonstrated that BGs derived from EcN and expressing a foreign antigen attachment to conjunctival epithelial cells in vitro without causing reduced cell viability. These results are an important step in constructing a delivery system based on a nonliving probiotic that is suitable for use in ocular surface diseases pairing immunomodulation and targeted delivery.

Bioinformatic Analysis of Chlamydia trachomatis Polymorphic Membrane Proteins PmpE, PmpF, PmpG and PmpH as Potential Vaccine Antigens

Chlamydia trachomatis is the most important infectious cause of infertility in women with important implications in public health and for which a vaccine is urgently needed. Recent immunoproteomic vaccine studies found that four polymorphic membrane proteins (PmpE, PmpF, PmpG and PmpH) are immunodominant, recognized by various MHC class II haplotypes and protective in mouse models. In the present study, we aimed to evaluate genetic and protein features of Pmps (focusing on the N-terminal 600 amino acids where MHC class II epitopes were mapped) in order to understand antigen variation that may emerge following vaccine induced immune selection. We used several bioinformatics platforms to study: i) Pmps' phylogeny and genetic polymorphism; ii) the location and distribution of protein features (GGA(I, L)/FxxN motifs and cysteine residues) that may impact pathogen-host interactions and protein conformation; and iii) the existence of phase variation mechanisms that may impact Pmps' expression. We used a well-characterized collection of 53 fully-sequenced strains that represent the C. trachomatis serovars associated with the three disease groups: ocular (N=8), epithelial-genital (N=25) and lymphogranuloma venereum (LGV) (N=20). We observed that PmpF and PmpE are highly polymorphic between LGV and epithelial-genital strains, and also within populations of the latter. We also found heterogeneous representation among strains for GGA(I, L)/FxxN motifs and cysteine residues, suggesting possible alterations in adhesion properties, tissue specificity and immunogenicity. PmpG and, to a lesser extent, PmpH revealed low polymorphism and high conservation of protein features among the genital strains (including the LGV group). Uniquely among the four Pmps, pmpG has regulatory sequences suggestive of phase variation. In aggregate, the results suggest that PmpG may be the lead vaccine candidate because of sequence conservation but may need to be paired with another protective antigen (like PmpH) in order to prevent immune selection of phase variants.

OmpA family proteins and Pmp-like autotransporter: new adhesins of Waddlia chondrophila

Waddlia chondrophila is a obligate intracellular bacterium belonging to the Chlamydiales order, a clade that also includes the well-known classical Chlamydia responsible for a number of severe human and animal diseases. Waddlia is an emerging pathogen associated with adverse pregnancy outcomes in humans and abortion in ruminants. Adhesion to the host cell is an essential prerequisite for survival of every strict intracellular bacteria and, in classical Chlamydia, this step is partially mediated by polymorphic outer membrane proteins (Pmps), a family of highly diverse autotransporters that represent about 15% of the bacterial coding capacity. Waddlia chondrophila genome however only encodes one putative Pmp-like protein. Using a proteomic approach, we identified several bacterial proteins potentially implicated in the adhesion process and we characterized their expression during the replication cycle of the bacteria. In addition, we demonstrated that the Waddlia Pmp-like autotransporter as well as OmpA2 and OmpA3, two members of the extended Waddlia OmpA protein family, exhibit adhesive properties on epithelial cells. We hypothesize that the large diversity of the OmpA protein family is linked to the wide host range of these bacteria that are able to enter and multiply in various host cells ranging from protozoa to mammalian and fish cells.

Deep comparative genomics among Chlamydia trachomatis lymphogranuloma venereum isolates highlights genes potentially involved in pathoadaptation

Lymphogranuloma venereum (LGV) is a human sexually transmitted disease caused by the obligate intracellular bacterium Chlamydia trachomatis (serovars L1-L3). LGV clinical manifestations range from severe ulcerative proctitis (anorectal syndrome), primarily caused by the epidemic L2b strains, to painful inguinal lymphadenopathy (the typical LGV bubonic form). Besides potential host-related factors, the differential disease severity and tissue tropism among LGV strains is likely a function of the genetic backbone of the strains. We aimed to characterize the genetic variability among LGV strains as strain- or serovar-specific mutations may underlie phenotypic signatures, and to investigate the mutational events that occurred throughout the pathoadaptation of the epidemic L2b lineage. By analyzing 20 previously published genomes from L1, L2, L2b and L3 strains and two new genomes from L2b strains, we detected 1497 variant sites and about 100 indels, affecting 453 genes and 144 intergenic regions, with 34 genes displaying a clear overrepresentation of nonsynonymous mutations. Effectors and/or type III secretion substrates (almost all of those described in the literature) and inclusion membrane proteins showed amino acid changes that were about fivefold more frequent than silent changes. More than 120 variant sites occurred in plasmid-regulated virulence genes, and 66% yielded amino acid changes. The identified serovar-specific variant sites revealed that the L2b-specific mutations are likely associated with higher fitness and pointed out potential targets for future highly discriminatory diagnostic/typing tests. By evaluating the evolutionary pathway beyond the L2b clonal radiation, we observed that 90.2% of the intra-L2b variant sites occurring in coding regions involve nonsynonymous mutations, where CT456/tarp has been the main target. Considering the progress on C. trachomatis genetic manipulation, this study may constitute an important contribution for prioritizing study targets for functional genomics aiming to dissect the impact of the identified intra-LGV polymorphisms on virulence or tropism dissimilarities among LGV strains.

In vitro passage selects for Chlamydia muridarum with enhanced infectivity in cultured cells but attenuated pathogenicity in mouse upper genital tract

Although modern Chlamydia muridarum has been passaged for decades, there are no reports on the consequences of serial passage with strong selection pressure on its fitness. In order to explore the potential for Pasteurian selection to induce genomic and phenotypic perturbations to C. muridarum, a starter population was passaged in cultured cells for 28 generations without standard infection assistance. The resultant population, designated CMG28, displays markedly reduced in vitro dependence on centrifugation for infection and low incidence and severity of upper genital tract pathology following intravaginal inoculation into mice compared to the parental C. muridarum population, CMG0. Deep sequencing of CMG0 and CMG28 revealed novel protein variants in the hypothetical genes TC0237 (Q117E) and TC0668 (G322R). In vitro attachment assays of isogenic plaque clone pairs with mutations in either TC0237 and TC0668 or only TC0237 reveal that TC0237(Q117E) is solely responsible for enhanced adherence to host cells. Paradoxically, double mutants, but not TC0237(Q117E) single mutants, display severely attenuated in vivo pathogenicity. These findings implicate TC0237 and TC0668 as novel genetic factors involved in chlamydial attachment and pathogenicity, respectively, and show that serial passage under selection pressure remains an effective tool for studying Chlamydia pathogenicity.