Selective enrichment media bias the types of Salmonella enterica strains isolated from mixed strain cultures and complex enrichment broths

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 pathogen the relevant strain might be missed. In this study mixed cultures of Salmonella enterica were grown in one set of 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(s) emerged as dominant in each mixture. No serotype was most fit, but strains of serogroups C2 and E were more likely to dominate enrichment culture mixtures than strains of serogroups B or C1. 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. These results indicate that outbreak investigations of food and/or other environmental samples should include multiple enrichment protocols to ensure isolation of target strains of Salmonella.

Figure 1. Ratio of each strain in the Salmonella enrichment to the whole population.

(A) Ratios from the TSB mixtures plated onto TSA agar; (B) Ratios from enrichments using RV and plated onto XLD; (C) Ratios from enrichments using RVS and plated onto XLD. Values are the averages of all replicates from multiple experiments, and the error bars represent standard deviation.

Figure 2. Ratio of each strain of Salmonella in the Salmonella+fecal bacteria enrichments to the whole Salmonella population.

(A) Ratios from enrichments using RV and plated onto XLD; (B) Ratios from enrichments using RVS and plated onto XLD. Values are the averages of all replicates from multiple experiments, and the error bars represent standard deviation.