Chlamydia trachomatis is responsible for both trachoma and sexually transmitted infections causing substantial morbidity and economic cost globally. Despite this, our knowledge of its population and evolutionary genetics is limited. Here we present a detailed whole genome phylogeny from representative strains of both trachoma and lymphogranuloma venereum (LGV) biovars from temporally and geographically diverse sources. Our analysis demonstrates that predicting phylogenetic structure using the ompA gene, traditionally used to classify Chlamydia, is misleading because extensive recombination in this region masks true relationships. We show that in many instances ompA is a chimera that can be exchanged in part or whole, both within and between biovars. We also provide evidence for exchange of, and recombination within, the cryptic plasmid, another important diagnostic target. We have used our phylogenetic framework to show how genetic exchange has manifested itself in ocular, urogenital and LGV C. trachomatis strains, including the epidemic LGV serotype L2b.

Background: Chlamydia trachomatis is the most common cause of sexually transmitted infections globally and the leading cause of preventable blindness in the developing world. There are two biovariants of C. trachomatis: 'trachoma', causing ocular and genital tract infections, and the invasive 'lymphogranuloma venereum' strains. Recently, a new variant of the genital tract C. trachomatis emerged in Sweden. This variant escaped routine diagnostic tests because it carries a plasmid with a deletion. Failure to detect this strain has meant it has spread rapidly across the country provoking a worldwide alert. In addition to being a key diagnostic target, the plasmid has been linked to chlamydial virulence. Analysis of chlamydial plasmids and their cognate chromosomes was undertaken to provide insights into the evolutionary relationship between chromosome and plasmid. This is essential knowledge if the plasmid is to be continued to be relied on as a key diagnostic marker, and for an understanding of the evolution of Chlamydia trachomatis.

Chlamydia trachomatis is a major cause of bacterial sexually transmitted infections worldwide. In 2006, a new variant of C. trachomatis (nvCT), carrying a 377 bp deletion within the plasmid, was reported in Sweden. This deletion included the targets used by the commercial diagnostic systems from Roche and Abbott. The nvCT is clonal (serovar/genovar E) and it spread rapidly in Sweden, undiagnosed by these systems. The degree of spread may also indicate an increased biological fitness of nvCT. The aims of this study were to describe the genome of nvCT, to compare the nvCT genome to all available C. trachomatis genome sequences and to investigate the biological properties of nvCT. An early nvCT isolate (Sweden2) was analysed by genome sequencing, growth kinetics, microscopy, cell tropism assay and antimicrobial susceptibility testing. It was compared with relevant C. trachomatis isolates, including a similar serovar E C. trachomatis wild-type strain that circulated in Sweden prior to the initially undetected expansion of nvCT. The nvCT genome does not contain any major genetic polymorphisms – the genes for central metabolism, development cycle and virulence are conserved – or phenotypic characteristics that indicate any altered biological fitness. This is supported by the observations that the nvCT and wild-type C. trachomatis infections are very similar in terms of epidemiological distribution, and that differences in clinical signs are only described, in one study, in women. In conclusion, the nvCT does not appear to have any altered biological fitness. Therefore, the rapid transmission of nvCT in Sweden was due to the strong diagnostic selective advantage and its introduction into a high-frequency transmitting population.

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The prevalence and complications of Mycoplasma genitalium and Chlamydia trachomatis infections among women undergoing termination of pregnancy were studied in this nested case-control study at Malmo University Hospital, Sweden, during 2003 to 2007. The study comprised 2079 women presenting for termination of pregnancy. Forty-nine women with M. genitalium infection and 51 women with C. trachomatis infection, together with 168 negative control women, were evaluated. The prevalences of M. genitalium and C. trachomatis were 2.5% and 2.8%, respectively. The M. genitalium was strongly associated with posttermination pelvic inflammatory disease (odds ratio 6.29, 95% CI 1.56-25.2). The increased risk for pelvic inflammatory disease associated with M. genitalium infection after termination of pregnancy suggests a causal relationship.