DNA Deletion Makes Swedish Chlamydia Invisible

Ruzanna Harutyunyan's picture

New sequencing and analysis of six strains of Chlamydia will result in improved diagnosis of the sexually transmitted infection.

This study provides remarkable insights into a new strain of Chlamydia that was identified in Sweden in 2006 after spreading rapidly across the country by evading most established diagnostic tests.

The results also reveal more about the evolution of the Chlamydia trachomatis bacterium, which is the most common cause of sexually transmitted infections (STIs) globally. The long-term effects of an undetected Chlamydia infection include infertility and ectopic pregnancy.


Long-term eye infection by Chlamydia is also the leading cause of preventable blindness in the developing world. As part of a long-standing collaboration between the Wellcome Trust Sanger Institute and University of Southampton, the team of researchers focused on six strains of Chlamydia.

Of particular interest to the team was the new Swedish strain provided by collaborators at Malmo University Hospital, Sweden. The genome of the Swedish strain features an evolutionary 'hiccup' that allowed it to go undetected in Sweden for several months. Indeed, doctors thought that the numbers of cases of Chlamydia were falling, when the opposite was true.

Through non-diagnosis, this version of Chlamydia spread silently. The reason: a deletion of the region of genetic information used to diagnose the presence of Chlamydia. The deletion 377 letters of genetic code occurred on the plasmid of the bacterium. Plasmids are small, circular molecules of DNA that are located outside the chromosome.

Chlamydial plasmids have been shown to vary little between different strains of Chlamydia, and are present in larger quantities than the chromosome. This makes them ideal candidate targets for diagnostic tools. Clinical tests have focused on one region of the bacterial plasmid - a gene of unknown function which is largely deleted in the new Swedish strain. After careful analysis of the newly sequenced plasmids of these strains, the team have identified the regions of the plasmid that vary least between strains.