The selectivity parameter was utilized to clarify the ability to isolate the target species from other
contaminants by comparing the growth ability of Listeria spp. (L. innocua and L. ivanovii) to the growth
rate of non-Listeria strains (E. co li and Salmonella spp.). These primary and secondary enrichment media
also had different degree of selectivity on each selected competing microorganisms. The selectivity of the
primary and secondary enrichment broths had more significant impacts toward the non-Listeria strains.
PB provides the highest selectivity; see Fig. 3 and Fig. 4.
FB and UVM had stronger negativity toward Salmonella spp. whereas MOPS-BLEB showed the
opposite effect. In all selective enrichment media used (MOPS-BLEB, SEB, FB and UVM), they contain
a gram-negative inhibitor agent which is nalidixic acid in different concentrations. It is known that almost
absolute inhibition of gram-negative bacteria was observed when grown in the presence of nalidixic acid
[5]. In addition, MOPS-BLEB and SEB have more concentration of nalidixic acid than FB and UVM. As
a result, E. coli can withstand to higher concentration of nalidixic acid than Salmonella spp.