Genome sequencing of a global collection of Shigella sonnei, the most common cause of shigellosis, or baciliary dysentery, in developing countries is reported in a study published online this week in Nature Genetics. Their study provides insights into the course of recent shigellosis epidemics.
S. sonnei are human-adapted E. coli responsible for approximately 150 million cases of dysentery each year. S. sonnei historically were a primary cause of baciliary dysentery in developed countries, but have recently emerged also in transitional developing countries, often connected to economic developments and improvements in sanitation and water quality.
Nicholas Thomson and colleagues sequenced the whole genomes of 132 geographically and temporally distributed S. sonnei isolated over the course of 65 years. Their phylogenetic analysis provides a detailed evolutionary history, showing when epidemic strains emerged, and tracing the routes of spread. They show that current infections are caused by a small number of clones from several defined lineages, which have recently dispersed out of Europe. They also suggest that antimicrobial resistance has played a role in the dissemination. Finally, the authors suggest that the increased incidence in transitional countries with improved water quality can be attributed to reduced exposure to P. shigelloides, which provides cross-immunity and is found in some contaminated water.