Effect of selective enrichment media on selectivity and isolation of Listeria from non-Listeria strains in suspended cell culture

The selectivity and inhibitory effects of five standard enrichment media (i.e., SEB, UVM, FB, MOPS-BLEB, and PB) were evaluated on L. ivanovii and L. innocua. E. coli and Salmonella spp. were chosen to study the growth of competing microorganisms on these standard selective media. Pure cultures of Listeria spp., E. coli and Salmonella spp. were prepared at similar initial cell densities (approximately 109 CFU/mL) and inoculated into 96 micro-well plates containing the selected enrichment media at 37oC. The viable cell count was performed to evaluate the selectivity and inhibitory stress imposing on the chosen strains over 24 hr. For isolation purpose, PB was the most potent and effective selective substrate and provided substantial reduction of the bacterial competitors (over 9 log scale reduction) within 9 hr. This medium was also the least preferable to L. ivannovii and L. innocua producing negative growths. However, PB was able to isolate both L. ivanovii and L. innocua from other competitors within rather short incubation time and with high selectivity. The selectivity of L. ivanovii over E. coli and Salmonella spp. were as follows; PB > MOPS-BLEB > FB > SEB > UVM and PB > MOPS-BLEB > SEB > FB > UVM, respectively. To select L. innocua, the preference of potent media was PB > FB > MOPS-BLEB > UVM > SEB for both E. coli and Salmonella spp.