Feasibility of a defined microflora challenge method for evaluating the efficacy of foodborne Listeria monocytogenes selective enrichments

Comparison of isolation methods for microbial pathogens is complicated by t he variable interference caused by the competitive microflora present in te st samples such as foods. In principle, using measured amounts of a standar d competitor in a defined surrogate food matrix might control the effect of variable interference. This possibility was investigated using Listeria mo nocytogenes and enrichment broths belonging to the acriflavine-nalidixate s elective agent class. Triplicate test sample sets were prepared. Each set c onsisted of suspensions of variable levels of the standard competitor, Ente rococcus faecium strain 111 (approximate to 10 to 10(9) CFU/25 g), mixed wi th a low constant level (10 to 100 CFU/25 g) of L, monocytogenes. These tes t samples were enriched at 30 degrees C for 48 h in different selective med ia and streaked onto selective isolation agars. The input CFU ratio (E. fae cium/L. monocytogenes) that permitted a 50% end point L, monocytogenes reco very was 2,2 x 10(6) or higher for the Food and Drug Administration one-ste p enrichments and 0.8 x 10(6) for the International Standards Organization (ISO) two-step enrichment. These and other results show that this evaluatio n method is feasible with this class of enrichments. Interestingly, L. mono cytogenes could be detected in enrichment cultures at high-input E, faecium /L. monocytogenes ratios even when the enriched samples were plated onto no nselective media. The pinpoint colonies of L, monocytogenes embedded in a c onfluent lawn of E. faecium 111 were detectable by their contrasting colora tion in Henry obliquely transmitted illumination.