research: turncoat sequenced

4 March 2005

The genome of a common species of gut bacterium – Bacteroides fragilis – has been sequenced by a team of researchers led by Dr Julian Parkhill from the Wellcome Trust Sanger Institute and Dr Sheila Patrick at Queen's University Belfast.

We have more bacterial cells in our body than human cells, the majority of them inside our gut. Most, including B. fragilis, do us no harm, and may even be beneficial; however, if B. fragilis escapes from the gut or find its way into our bloodstream (as a result of surgery, a burst appendix, childbirth or wound damage), it becomes an 'opportunistic' pathogen, causing abscesses and inflammation that can attack the main organ systems. If untreated, more than a third of infections are fatal. Worryingly, some antibiotic-resistant strains of B. fragilis have appeared in recent years.

Analysis of the B. fragilis genome has revealed the remarkable mechanism by which it manages to survive inside the body. Like many bacteria, B. fragilis evades immune responses by varying the molecules on its surface coat. Unusually, though, it does this by periodically flipping short fragments of its DNA through 180°, thereby switching surface coat genes on and off.

B. fragilis seems to make extensive use of this mechanism – far more so than any other bacterium. It is likely be related to its ability to survive in the gut and to survive in new environments when opportunities arise.

External links

• Extensive DNA inversions in the B. fragilis genome control variable gene expression (Research paper, Cerdeño-Tárraga AM et al.)
• The Pathogen Sequencing Unit (Wellcome Trust Sanger Institute)