Identification of a single selective enrichment media for the simultaneous recovery of Salmonella and E. coli O157 on beef hide swabs and fecal samples

Salmonella and Escherichia coli O157 (E. coli O157) are major food-borne pathogens in the United States, responsible for outbreaks and recalls linked to a variety of foods including beef products. It is essential to detect the presence of these pathogens in the food or in food contamination environments in which they may be present in low concentrations or in injured conditions. Thus, an enrichment step is required to resuscitate any injured cells and increase bacterial numbers to facilitate recovery when low concentrations of microorganisms are present. Selective enrichment is particularly important when high concentrations of background micro-flora are expected in the sample. Numerous enrichment media have been researched, proposed and are commercially available for the detection or isolation of E. coli O157 and Salmonella; however, conventional techniques are required to use a different broth for each microorganism in most situations. This study aims to identify one single enrichment media suitable for recovery of both E. coli O157 and Salmonella from cattle fecal and hide samples, especially when very low concentrations of pathogens are expected in naturally contaminated samples. Rifampicin resistant E. coli O157 and Salmonella cocktails were used to inoculate hide swabs and fecal samples with 1 Log10 CFU/ml concentration. Each set of inoculated samples was enriched in buffered peptone water with vancomycin, cefixime, and cefsulodin (BPW-VCC), modified trypticase soy broth with novobiocin (mTSB+n), and modified trypticase soy broth with an acid digest of casein (mTSB) for 6, 18, and 24 hours at 42°C. After enrichment, samples were plated on Xylose-Lysine-Tergitol 4 (XLT4) and Sorbitol MacConkey agar (SMAC) for enumeration of Salmonella and E. coli O157 respectively. Analysis of the enrichment time showed that no significant difference was detected between 18 and 24 h (p > 0.05), but a 6 h enrichment showed a lower recovery rate in terms of colony forming units (CFU) obtained in hide swabs and fecal samples. After 6 h of enrichment, mTSB and mTSB+n had a significantly better recovery (p<0.05) than BPW-VCC in recovery of E. coli O157 and Salmonella from fecal samples and BPW-VCC had significantly better recovery (P<0.05) than mTSB and mTSB+n for recovery of E. coli O157 from hide swabs. In fecal samples inoculated with E. coli O157, mTSB and mTSB+n showed a higher recovery after 18h with 6.67 Log10 CFU/g and 6.78 Log10 CFU/g respectively (p < 0.05), relative to BPW-VCC (5.60 Log10 CFU/g). The results were consistent for Salmonella in fecal samples in which mTSB+n and mTSB showed increased bacterial counts, obtaining 6.47 Log10 CFU/g and 6.40 Log10 CFU/g, respectively after 18 h, being significantly higher (p < 0.05) than BPW-VCC (5.71 Log10 CFU/g). In the case of hide swabs inoculated with E. coli O157, using BPW-VCC (7.00 Log10 CFU/ml) had significantly better recovery (p < 0.05) than mTSB (5.40 Log10 CFU/ml) and mTSB+n (5.76 Log10 CFU/ml) after 18 h enrichment. There was no significant difference (p > 0.05) in recovery of the pathogens in the hide swabs inoculated with Salmonella strains among the three-enrichment media used: BPW-VCC (6.10 Log10 CFU/ml), mTSB+n (6.21 Log10 CFU/ml), mTSB (6.13 Log10 CFU/ml) after 18 h enrichment. In conclusion, mTSB+n may be used for isolation of Salmonella and E. coli O157 in fecal samples and BPW-VCC may be used for isolation of E. coli O157 from hide swabs for 18 h enrichment based on recovery of E. coli O157 and Salmonella. An enrichment of 18 hours may be sufficient for detection of E. coli and Salmonella in hide and fecal samples for all the three-enrichment media used.