New sequencing and analysis of six strains of Chlamydia will result in improved diagnosis of the sexually transmitted infection. A study by the Wellcome Trust Sanger Institute and University of Southampton 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. 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 nondiagnosis, this version of Chlamydia spread silently. The reason was 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. 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.