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Comparative Genomics Provides Look at Spread of Dysentery Bacteria

NEW YORK (GenomeWeb) – A genomic study of the dysentery-causing bacteria Shigella dysenteriae has retraced the pathogen's spread over more than a century, identifying lineages involved in outbreaks around the world since the late 1800s.

An international team reporting in Nature Microbiology sequenced hundreds of type 1 S. dysenteriae strains collected at sites around the world from the early 1900s until 2011. Based on relationships between the S. dysenteriae behind outbreaks during the First and Second World Wars and/or following natural disasters, the group estimated that the pathogen likely originated somewhere in Europe in the 18th century.

Using genetic patterns and historical clues in the S. dysenteriae lineages identified, the researchers mapped the pathogen's historical march from one part of the world to the next. For example, their results hint that S. dysenteriae made its way to parts of Africa, Asia, and the Americas from the late 1800s until the early 1900s alongside European groups migrating to or colonizing these regions. Dysentery dynamics subsequently varied by continent, depending on everything from population movement to situations of political unrest or natural disasters that created S. dysenteriae-friendly circumstances.

"This bacterium is still in circulation, and could be responsible for future epidemics if conditions should prove favorable — such as a large gathering of people without access to drinking water or treatment of human waste," senior author François-Xavier Weill, director of the Pasteur Institute's enteric bacterial pathogens unit, said in a statement. "This study highlights the need for an effective vaccine, which will be crucial for controlling this disease in the future in view of the reduced efficacy of antibiotics."

The Shiga toxin-producing bug S. dysenteriae is believed to represent a form of Escherichia coli that became deft at infecting humans, Weill and his team noted. Since its detection in the late 1800s in Japan, it has been linked to several large outbreaks around the world, claiming the lives of tens of thousands of people and sickening hundreds of thousands more.

"[T]here are still many unanswered questions relating [to] this infamous and important bacterial pathogen," co-author Nicholas Thomson, who leads the Sanger Institute's bacterial genomics and evolution group, said in a statement.

In an effort to understand the pathogen's emergence, evolution, and spread, Weill, Thomson, and their colleagues performed whole-genome sequencing on 325 S. dysenteriae type 1 (Sd1) isolates from almost three-dozen strain collections, representing strains from 66 countries that went back to 1915, when an outbreak of Sd1 dysentery afflicted many of the World War I troops withdrawing from Gallipoli.

When they analyzed the new genomes alongside Sd1 reference strains sequenced in the past, the researchers identified four main lineages, largely coinciding with isolates from specific sites over given time periods.

In particular, their results point to S. dysenteriae movement from Europe to Central America sometime between 1889 and 1893. Starting around the same time, European strains also appeared to move into parts of Africa and Asia. Despite being re-introduced to Europe between 1910 and 1944, the team noted, S. dysenteriae infections largely petered out on that continent. In contrast, the bug has continued to spread in other parts of the world, primarily in Africa, where strains moved across the continent or were reintroduced from Asia as recently as the 1990s.

Further, the researchers found that antibiotic resistance in Sb1 started to turn up in Asia and the Americas in the 1960s, and became prominent and more extensive in Sb1 strains over the next four decades.