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Genome Sequencing Sheds Light On Shigella Outbreaks In California

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Researchers from California Department of Public Health (CDPH) and UC Davis have sequenced the genome of Shigella sonnei, the bacteria that is involved in 2014 and 2015 shigellosis outbreaks in California.

Whole-Genome Analysis Of S. Sonnei

On analysis, the researchers identified how the S. sonnei strains acquired antibiotic resistance and toxin genes, which made the bacteria more virulent than ever before. The study also helped in finding the link between the Californian Shigella strains and other Shigella strains found worldwide.

The whole-genome study of S. sonnei, which is done for the first time in North America is published by American Society For Microbiology's journal mSphere.

Jonathan Eisen, professor at UC Davis said that in case of outbreak of an infection, the reason behind the virulence of the bacteria could be detected with the help of whole genomic sequencing study. The analysis would help in identifying the gene involved in causing the virulence such as presence of antibiotic resistance and toxin genes.

Shigellosis Caused By S. Sonnei In California

Eisen added that sequencing whole-genome of bacteria would eventually help in providing better intervention to patients infected with the organism. S. sonnei is one of the four Shigella species that causes major outbreaks of shigellosis in the country characterized by symptoms including diarrhea, abdominal pain, and other gastrointestinal problems. S. sonnei is estimated to be responsible for 500,000 infections and 70 deaths annually in the United States.

The investigators analyzed the genomes of 68 isolates including the strains from recent outbreak in California and other native strains from California, Asia, and so on. The researchers also analyzed antibiotic resistance exhibited by the strains.

S. Sonnei Strains Carried Toxin And Antibiotic Resistance Genes

The research team identified that San Diego and the San Joaquin Valley strain that were found in California since 2008 carried the gene specific for Shiga toxin that are usually seen in S. dysenteriae and S. flexneri. The toxin gene is found to have transferred to S. sonnei by bacteriophages, the virus that infects bacteria.

Dr. James Watt from CDPH said that S. sonnei, which usually cause less severe infection, could have got the virulent toxin gene from other Shigella and E.coli species.

However, the San Francisco strain that didn't have the Shiga toxin genes was found to have acquired genes responsible for resistance to a class of antibiotics named fluoroquinolone. These strains were similar to the ones present in Southeast Asia, which in turn provides an insight on the origin of the strain.

"We know these movements of DNA can be important for the spread of antibiotic resistance, virulence and pathogenicity factors," said Eisen, in a press release. "Having the genome data from outbreaks allows us to try to figure out what happened."

If an organism is identified to have acquired antibiotic resistance genes it would be helpful in providing appropriate treatment to the patients and also in isolating and sampling them in order to design better treatment options in case of severe infection, added the researcher.

The researchers also noted that genomic sequencing should be a routine practice in the country's pathogen related surveillance model.

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