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A method that combined the immunomagnetic separation (IMS) technique and the multiplex polymerase chain reaction (PCR) method (i.e., the IMS-mPCR method) was developed for simultaneous detection of Listreria monocytogenes and Salmonella spp. in food samples. When only the multiplex PCR method was used, it was found that if cell numbers of each of the two target organisms (L. monocytogenes and Salmonella spp.) were above the detection limit, but differed by more than 2 logs-e.g., n x 10(7) to n x 10(4) or n x 10(6) to n x 10(3)--the organism presenting the lower numbers might go undetected. Following the enrichment step with universal preenrichment (UP) broth, if an IMS method using equal quantities of anti-Listeria and anti-Salmonella immunomagnetic beads was performed prior to PCR, both pathogens could be detected unambiguously. Such results could be obtained for target organisms in food samples, such as milk, dairy, and meat products, if similar enrichment and IMS steps were performed prior to PCR.

Aims:  To screen from pickled vegetables the potential probiotic lactic acid bacteria (LAB) strains with antagonistic activity against Salmonella invasion in host. Methods and Results:  Probiotic properties including acid and bile tolerance as well as inhibition on pathogenic bacteria were used for screening of LAB strains from pickled vegetables. Two strains, i.e Pediococcus pentosaceus MP12 and Lactobacillus plantarum LAP6, were selected and further assayed for their activities against Salmonella invasion in mouse liver and spleen. For these two LAB strains, strain LAP6 was able to adhere to the mouse intestinal epithelium cells. Conclusions:  In screening of the probiotic strains able to inhibit the Salmonella invasion in host, factors other than the adherence to host intestinal epithelium may contribute some roles. Significance and Impact of the Study:  Probiotic LAB strains with activity against Salmonella invasion in host could be isolated from vegetable origins. These strains may be used for vegetable processing.

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Contamination of different bacterial species with great cell count difference in food samples may affect the efficacy of multiplex polymerase chain reaction (mPCR) detection of these organisms. The purpose of this study was to improve the efficiency of mPCR detection of two pathogenic bacterial species, i.e., Salmonella and Shiga‐like toxin I Escherichia coli (SLTI EC), with great cell count difference in food samples. Without the enrichment step, when the cell count ratio of these two species was higher than 103, the bacteria presenting the lower number might go undetectable. For beef and milk samples with great ratios of SLTI EC/Salmonella, such as 103–105/100–101, if gram‐negative (GN) broth was used for the pre‐enrichment step, both organisms were detectable by mPCR. However, for samples with great ratios of Salmonella/SLTI EC, after enrichment with GN broth, an immunomagnetic separation (IMS) step using equimolar mixture of anti‐Salmonella and anti‐enterohemorrhagic Escherichia coli immunobeads had to be performed prior to mPCR to allow obvious results for both organisms. Therefore, although GN is a selective medium for both Salmonella and E. coli, depending on the ratios of the two target organisms in food samples, an IMS step may be required to allow the simultaneous PCR detection of both organisms. PRACTICAL APPLICATIONS Although multiplex polymerase chain reaction (mPCR) method allows the simultaneous detection of different target organisms, if the ratios of the cell counts for these target organisms were great, the PCR results for target organisms with lower counts will be obscured. Using gram‐negative broth for pre‐enrichment followed by mPCR or pre‐enrichment and immunomagnetic separation (IMS) followed by mPCR, we were able to simultaneously detect enterohemorrhagic Escherichia coli and Salmonella in food samples even the cell count ratios of these two organisms were great. Similar approach could be used for the simultaneous detection of two or more bacterial species in different samples.