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Genomic Study Suggests Overlap Between Chlamydia in Eye, Genital Infections in Australia

NEW YORK (GenomeWeb) – Chlamydia bacteria behind the infectious blindness disease trachoma can swap genes with sexually transmitted Chlamydia strains, leading to future trachoma cases involving slightly modified urogenital strains, according to a study published online today in Nature Communications.

Researchers from the UK, Australia, and Switzerland did whole-genome sequencing on a dozen Chlamydia trachomatis isolates from children in northern Australia who appeared to show signs of trachoma. By comparing the C. trachomatis genomes with sequences from Chlamydia reference strains, they found that the trachoma-causing bacteria from Australia clustered in lineages related to strains behind urogenital infections.

C. trachomatis strains involved in eye and urogenital infections typically differ in genotypes for major outer membrane protein-coding genes such as OmpA gene, the team explained. But while the Australian isolates appear to have borrowed sequences such as OmpA from 'ocular' C. trachomatis strains before infecting the eye, their broader genomes suggest they began as urogenital strains.

"[R]esidual ocular strains provide potential for gene variants specifying ocular tropism to be transferred to prevalent and successful urogenital strain, and so facilitate trachoma re-emergence," the Wellcome Trust Sanger Institute's Nick Thomson and co-senior author Philip Giffard, with Menzies School of Health Research, and their colleagues wrote.

"The evidence that this has happened multiple times in Australia suggest that this is not a vanishingly rare event," they added. "This provides additional justification for reducing the burden of all C. trachomatis in disadvantaged communities where trachoma re-emergence is possible, as well as continuing to address the social conditions that maintain trachoma endemicity."

Trachoma is endemic in some remote parts of Australia, the team noted, with infection rates in some Aboriginal Australian communities ranging from 12 percent to 28 percent in children under 14 years old.

To retrace the roots of such infections, the researchers sequenced 12 isolates that had been collected in Australia's Northern Territory between 1985 and 1991.

When they put together a phylogenetic tree for C. trachomatis — analyzing the newly sequenced strains alongside reference strains previously isolated from ocular, urogenital, or rare, invasive lymphogranuloma venereum infections — the researchers identified three clonal groups of Australian trachoma isolates.

Nearly all of these isolates had OmpA genotypes associated with ocular C. trachomatis strains. But rather than clustering into the known lineage of eye-infecting C. trachomatis, the Australian isolates made up two broader urogenital-like lineages — arguing against the notion that C. trachomatis-related eye infections stem from a derived lineage with OmpA genotypes that don't exist in urogenital strains.

In an attempt to reconcile these unexpected findings with what was know about C. trachomatis from prior studies, the researchers used a sliding window approach to scrutinize sequences across the new C. trachomatis genomes.

Across their genomes, the Australian trachoma isolates consistently shared sequences with several urogenital isolates, they reported, diverging only at a few sites to more closely match OmpA and another surface protein-coding gene from classical ocular isolates.