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Cholera Genome From World War I Soldier Reveals New Features of Pathogen

NEW YORK (GenomeWeb) – A UK research team has characterized a non-pandemic Vibrio cholerae isolate collected in 1916 from a British soldier recuperating in Egypt during World War I — the oldest known publicly available strain.

The soldier's isolate "represents a significant piece of the history of cholera, a disease that remains as important today as it was in past centuries," said senior author Nicholas Thomson, a microbiology and bioinformatics researcher affiliated with the Wellcome Sanger Institute and the London School of Hygiene and Tropical Medicine (LSHTM) in a statement.

In a paper published yesterday in the Proceedings of the Royal Society B, Thomson and colleagues from the Sanger Institute and Public Health England outlined their efforts to sequence and analyze a V. cholerae isolate dubbed NCTC 30 that was initially deposited into the National Collection of Type Cultures (NCTC).

Despite a lack of antibiotic use at the time, NCTC 30 was resistant to penicillin and had diminished susceptibility to ampicillin antibiotics. Moreover, they noted that the NCTC 30 sequence data pointed to the presence of a beta-lactamase enzyme-coding gene linked to ampicillin resistance that appeared to have been obtained by the V. cholerae isolate through horizontal gene transfer.

"These data reiterate the fact that the presence of antimicrobial resistance genes in bacterial pathogens predates the introduction of antibiotic therapies," Thomson and his co-authors wrote.

The researchers used a combination of Illumina short-read and Pacific Biosciences longread technology to sequence a minimally passaged version of the 102-year-old isolate that had been cultured in 1962, assembling more than 2.9 Mb of a circular chromosome 1 and more than 1 Mb of chromosome 2.

The isolate was particularly intriguing given its age and its prior classification into the O2 serogroup, which is distinct from the O1 serogroup that was central to the so-called sixth global cholera pandemic, which began in 1899 and ended in 1923. A distinct biotype in the O1 serogroup has also been implicated in the ongoing seventh cholera pandemic.

"Studying strains from different points in time can give deep insights into the evolution of this species of bacteria and link that to historical reports of human disease," Thomson said in a statement. "Even though this isolate did not cause an outbreak, it is important to study those that do not cause disease as well as those that do."

The team's pan-genome and phylogenetic analyses — which included 197 V. cholerae genome sequences and sequences from a handful of related Vibrio species — demonstrated that NCTC 30 clustered with other V. cholerae strains, albeit at a distance from "classical" V. cholerae strains characterized previously.

"The phylogenetic separation which we observed is likely to reflect the phenotypic and molecular differences that [previously raised questions] about the classification of NCTC 30," the authors wrote. "However, our data do indicate that NCTC 30 is a V. cholerae isolate, as are its closest relatives."

Although the infected WWI soldier reportedly suffered from diarrhea symptoms, the researchers found that NCTC 30 did not contain classic cholera toxin-coding genes. Rather, the genome housed genes that may have produced an alternative, yet-unconfirmed diarrhea-causing toxin. Likewise, they found that the isolate contained a mutation predicted to alter flagellar development, corresponding to growth and flagella development defects detected in cultured NCTC 30 representatives assessed by electron microscopy. 

"Reports in the literature indicated that there was something unusual about the strain of bacteria from the WWI soldier," first author Matthew Dorman, a graduate student at the Sanger Institute, said in a statement. "It's promising to see that our genomic information aligns with those historical records."