London School of Hygiene & Tropical Medicine

Researchers have found that Chlamydia has evolved more actively than was previously thought, in a study published in Nature Genetics.

Using whole genome sequencing, researchers from a collaboration including the London School of Hygiene & Tropical Medicine and the Wellcome Trust Sanger Institute have found that the exchange of DNA between different strains of Chlamydia to form new strains is much more common than expected.

Chlamydia trachomatis is the most common sexually transmitted infection (STI) both in the UK and globally, with approximately 100 million new cases worldwide each year. It is also the most common cause of blindness as a result of infection, or trachoma, in the developing world. Relatively little is known about the evolution of the different strains of Chlamydia that are causing infection.

“Scientists recently discovered that if two Chlamydia strains co-infect the same person at the same time, they can swap DNA by a process called recombination.” explains Dr Simon Harris from the Wellcome Trust Sanger Institute. “This was originally thought only to affect a few ‘hotspots’ within the genome. We were very surprised to find recombination is far more widespread than previously thought.”

The team found there appeared to be no barriers to the swapping of DNA when circumstances allow, even between bacterial strains associated with infecting different parts of the body.

The study highlights that current clinical testing methods do not capture the variation between Chlamydia strains. The aim of current diagnosis is simply to return a positive or negative result, not record changes to the genome structure.

This makes it impossible to determine whether a person who tests positive again after antibiotic treatment has picked up a second infection or if their treatment has failed. This, in turn, limits the development of health policies, because it is not fully understood how Chlamydia spreads within our population.

The significance of this is that although antibiotic resistant Chlamydia has never been seen in patients who have been treated with recommended antibiotics, it can occur in the laboratory. If resistance did occur in the general population, it would not be detected by current diagnostic procedures.

The researchers are now working with hospitals to use their results to improve Chlamydia testing in terms of detecting variation between Chlamydia strains.

This study is not just important for the treatment of the sexually transmitted strains of Chlamydia but also for the treatment of African Chlamydia strains that can cause trachoma. The team sequenced strains of African Chlamydia and found that these strains are also using recombination to fool the human immune system.

"For many years various groups have observed co-circulating strains of Chlamydia causing trachoma. In our study we have shown that some strains appear to have swapped only their surface coat" says Dr Martin Holland from the London School of Hygiene & Tropical Medicine. "This provides real clues as to how this bacterium is able to avoid the human immune system and cause disease."