ARS | Publication request: Selective enrichment media bias the types of salmonella enterica strains isolated from mixed strain cultures and complex enrichment broths

Submitted to: PLoS One
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: March 9, 2012
Publication Date: April 4, 2012
Citation: Gorski, L.A. 2012. Selective enrichment media bias the types of salmonella enterica strains isolated from mixed strain cultures and complex enrichment broths. PLoS One.7:e34722

Interpretive Summary: Detection and isolation of a foodborne pathogen from a food or environmental sample can be a difficult process. The competitive enrichment cultures that are used require competition for nutrients and other natural bacteria in the mixture can compete with and inhibit the growth of the target organism. Sometimes more than one strain of the target organism is present in a sample, but they may not all be detected. In this study mixed cultures of Salmonella enterica were grown in standard enrichment culture protocols to see if culture bias patterns emerged. A total of nineteen strains representing four serogroups and ten serotypes were compared in four-strain mixtures in Salmonella-only and in cattle fecal culture enrichment backgrounds using Salmonella enrichment media. In each mixture one or more strain emerged as dominant. Two versions of a common enrichment medium were tested and the ratio of strains that dominated the cultures was different between the two media. No one serotype of serogroup was the most dominant. Strains of serogroup B, which includes serotypes often seen in foodborne outbreaks such as S. Typhimurium, S. Saintpaul, and S. Schwarzengrund were less likely to emerge as dominant strains, suggesting traceback investigations searching for these strains might need to use more than one enrichment protocol to ensure isolation.

Technical Abstract: For foodborne outbreak investigations it can be difficult to isolate the relevant strain from food and/or environmental sources. If the sample is contaminated by more than one strain of the organism the relevant strain might be missed. In this study mixed cultures of Salmonella enterica were grown in standard enrichment media to see if culture bias patterns emerged. Nineteen strains representing four serogroups and ten serotypes were compared in four-strain mixtures in Salmonella-only and in cattle fecal culture enrichment backgrounds using Salmonella enrichment media. One or more strain emerged as dominant in each mixture, and no serogroup or serotype emerged as most fit. Different versions of Rappaport-Vassiliadis (RV) medium gave different patterns of strain dominance in both Salmonella-only and fecal enrichment culture backgrounds. The fittest strains belonged to serogroups C1, C2, and E, and included strains of S. Infantis, S. Thompson S. Newport, S. 6,8:d:-, and S. Give. Strains of serogroup B, which included serotypes often seen in outbreaks such as S. Typhimurium, S. Saintpaul, and S. Schwarzengrund were less likely to emerge as dominant strains in the mixtures when using standard RV as part of the enrichment. Using a more nutrient-rich version of RV as part of the protocol led to a different pattern of strains emerging, however some were still present in very low numbers in the resulting population. Traceback investigations need to include more than one enrichment protocol to ensure isolation.