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Chlamydia muridarum actively grows in murine mucosae and is a representative model of human chlamydial genital tract disease. In contrast, C. trachomatis infections in mice are limited and rarely cause disease. The factors that contribute to these differences in host adaptation and specificity remain elusive. Overall genomic similarity leads to challenges in the understanding of these significant differences in tropism. A region of major genetic divergence termed the plasticity zone (PZ) has been hypothesized to contribute to the host specificity. To evaluate this hypothesis, lateral gene transfer was used to generate multiple hetero-genomic strains that are predominately C. trachomatis but have replaced regions of the PZ with those from C. muridarum. In vitro analysis of these chimeras revealed C. trachomatis-like growth as well as poor mouse infection capabilities. Growth-independent cytotoxicity phenotypes have been ascribed to three large putative cytotoxins (LCT) encoded in the C. muridarum PZ. However, analysis of PZ chimeras supported that gene products other than the LCTs are responsible for cytopathic and cytotoxic phenotypes. Growth analysis of associated chimeras also led to the discovery of an inclusion protein, CTL0402 (CT147), and homolog TC0424, which was critical for the integrity of the inclusion and preventing apoptosis.

Chlamydia trachomatis is an obligate intracellular bacterium that causes serious diseases in humans. Rectal infection and disease caused by this pathogen are important yet understudied aspects of C. trachomatis natural history. The University of Washington Chlamydia Repository has a large collection of male-rectal-sourced strains (MSM rectal strains) isolated in Seattle, USA and Lima, Peru. Initial characterization of strains collected over 30 years in both Seattle and Lima led to an association of serovars G and J with male rectal infections. Serovar D, E, and F strains were also collected from MSM patients. Genome sequence analysis of a subset of MSM rectal strains identified a clade of serovar G and J strains that had high overall genomic identity. A genome-wide association study was then used to identify genomic loci that were correlated with tissue tropism in a collection of serovar-matched male rectal and female cervical strains. The polymorphic membrane protein PmpE had the strongest correlation, and amino acid sequence alignments identified a set of PmpE variable regions (VRs) that were correlated with host or tissue tropism. Examination of the positions of VRs by the protein structure-predicting Alphafold2 algorithm demonstrated that the VRs were often present in predicted surface-exposed loops in both PmpE and PmpH protein structure. Collectively, these studies identify possible tropism-predictive loci for MSM rectal C. trachomatis infections and identify predicted surface-exposed variable regions of Pmp proteins that may function in MSM rectal versus cervical tropism differences.

We read with interest the recent paper by Somboonna and colleagues, addressing changes to tryptophan synthase in a single Chlamydia trachomatis strain that had an aberrant growth phenotype in vitro (1). This strain was isolated four times, over 4 years, from a patient who was apparently persistently infected. This strain was highly related to a serovar F strain isolated previously in their clinical setting. The authors use these strains to defend an association between in vivo persistence and a particular mutation at the 3′ end of the …

Lateral gene transfer (LGT) among Chlamydia trachomatis strains is common, in both isolates generated in the laboratory and those examined directly from patients. In contrast, there are very few examples of recent acquisition of DNA by any Chlamydia spp. from any other species. Interspecies LGT in this system was analyzed using crosses of tetracycline (Tc)-resistant C. trachomatis L2/434 and chloramphenicol (Cam)-resistant C. muridarum VR-123. Parental C. muridarum strains were created using a plasmid-based Himar transposition system, which led to integration of the Camr marker randomly across the chromosome. Fragments encompassing 79% of the C. muridarum chromosome were introduced into a C. trachomatis background, with the total coverage contained on 142 independent recombinant clones. Genome sequence analysis of progeny strains identified candidate recombination hot spots, a property not consistent with in vitroC. trachomatis × C. trachomatis (intraspecies) crosses. In both interspecies and intraspecies crosses, there were examples of duplications, mosaic recombination endpoints, and recombined sequences that were not linked to the selection marker. Quantitative analysis of the distribution and constitution of inserted sequences indicated that there are different constraints on interspecies LGT than on intraspecies crosses. These constraints may help explain why there is so little evidence of interspecies genetic exchange in this system, which is in contrast to very widespread intraspecies exchange in C. trachomatis. IMPORTANCE Genome sequence analysis has demonstrated that there is widespread lateral gene transfer among strains within the species C. trachomatis and with other closely related Chlamydia species in laboratory experiments. This is in contrast to the complete absence of foreign DNA in the genomes of sequenced clinical C. trachomatis strains. There is no understanding of any mechanisms of genetic transfer in this important group of pathogens. In this report, we demonstrate that interspecies genetic exchange can occur but that the nature of the fragments exchanged is different than those observed in intraspecies crosses. We also generated a large hybrid strain library that can be exploited to examine important aspects of chlamydial disease.

Functional genetic analysis of Chlamydia has been a challenge due to the historical genetic intractability of Chlamydia, although recent advances in chlamydial genetic manipulation have begun to remove these barriers. Here, we report the development of the Himar C9 transposon system for Chlamydia muridarum, a mouse-adapted Chlamydia species that is widely used in Chlamydia infection models. We demonstrate the generation and characterization of an initial library of 33 chloramphenicol (Cam)-resistant, green fluorescent protein (GFP)-expressing C. muridarum transposon mutants. The majority of the mutants contained single transposon insertions spread throughout the C. muridarum chromosome. In all, the library contained 31 transposon insertions in coding open reading frames (ORFs) and 7 insertions in intergenic regions. Whole-genome sequencing analysis of 17 mutant clones confirmed the chromosomal locations of the insertions. Four mutants with transposon insertions in glgB, pmpI, pmpA, and pmpD were investigated further for in vitro and in vivo phenotypes, including growth, inclusion morphology, and attachment to host cells. The glgB mutant was shown to be incapable of complete glycogen biosynthesis and accumulation in the lumen of mutant inclusions. Of the 3 pmp mutants, pmpI was shown to have the most pronounced growth attenuation defect. This initial library demonstrates the utility and efficacy of stable, isogenic transposon mutants for C. muridarum. The generation of a complete library of C. muridarum mutants will ultimately enable comprehensive identification of the functional genetic requirements for Chlamydia infection in vivo. IMPORTANCE Historical issues with genetic manipulation of Chlamydia have prevented rigorous functional genetic characterization of the ∼1,000 genes in chlamydial genomes. Here, we report the development of a transposon mutagenesis system for C. muridarum, a mouse-adapted Chlamydia species that is widely used for in vivo investigations of chlamydial pathogenesis. This advance builds on the pioneering development of this system for C. trachomatis. We demonstrate the generation of an initial library of 33 mutants containing stable single or double transposon insertions. Using these mutant clones, we characterized in vitro phenotypes associated with genetic disruptions in glycogen biosynthesis and three polymorphic outer membrane proteins.