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Monitor the transmission of S. Typhimurium throughout the food system.

Continuing research on Salmonella may enable scientists to identify and track strains of antibiotic-resistant bacteria as they evolve and spread.

Tracing the transmission of individual strains from agricultural environments to humans through the food system is difficult because of the rapid evolution of resistance patterns in these bacteria, which change quickly. But now researchers at Penn State University have developed a method for identifying and tracking strains of S. enterica serological variant Typhimurium as they evolve and spread.

Annually in the United States, the strains of Salmonella are responsible for approximately 1 million illnesses, 20,000 hospitalizations and 400 deaths, at a cost exceeding $3 billion. S. Typhimurium accounts for at least 15% of clinically reported salmonellosis infections in humans. The number of antibiotic-resistant isolates identified in humans is increasing steadily, making it imperative to effectively monitor the transmission of S. Typhimurium throughout the food system and to implement control measures.

The scientists developed their new technique for identifying antibiotic-resistant strains of S. Typhimurium by focusing on virulence genes and novel regions of the bacteria's DNA known as clustered regularly interspaced short palindromic repeats (CRISPRs), which are present in many foodborne pathogens.

The researchers demonstrated that CRISPR sequences can be used to identify populations of Salmonella with common antibiotic-resistance patterns in animals and humans, by tracing the transmission of antibiotic-resistant S. Typhimurium from farm to fork. The investigators used CRISPRs to separate isolates by their propensity for resistance to seven common veterinary and human clinical antibiotics.

Several subtypes of the bacteria showed up repeatedly in the frozen collection of Salmonella samples taken from cows, pigs and chickens in Penn State's Animal Diagnostic Laboratory. Researchers looked at 84 unique S. Typhimurium isolates collected from 2008 to 2011. These strains are widely disbursed, and they all have noticeably higher levels of antibiotic resistance.

The scientists examined clinical samples of Salmonella taken from humans. The ones they saw in humans are the ones seen often in animals, which confirms that the method works. The researchers identified subsets of the overall Salmonella bacteria population that seem to be more prone to acquiring antibiotic resistance. Their challenge is to learn why some strains acquire resistance while others don't.

Further information. Edward G Dudley, Department of Food Science, Penn State University, 427 Rodney A. Erickson Food Science Building, University Park, PA 16802; phone: 814-8670439; email:

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Publication:Emerging Food R&D Report
Date:Jun 1, 2015
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