Detection and prevalence of Listeria monocytogenes in the agricultural ecosystem

The sensitivity of four different enrichment procedures to detect Listeria monocytogenes in the presence of high levels of Streptococcus faecalis was investigated. Defined mixed cultures of Strep. faecalis and L. monocytogenes gave better results with one-stage enrichment techniques. For manure samples, however, two-stage enrichment techniques gave the best performance. The so-called cold enrichment techniques were found to be unsatisfactory for samples from natural environments. The following materials were examined for the presence of L. monocytogenes: fresh pig faeces (16% positive), fresh cattle faeces (20% positive), stored liquid manure (0% positive), manured soil samples (0% positive) and ground water samples (5% positive). After 3 weeks of storage L. monocytogenes could be detected in only one of the initially nine positive fresh faeces samples. Two months after inoculation of stored liquid pig manure, stored liquid cattle manure and soil with L. monocytogenes, this bacterium could not be traced in any of these materials. Radishes (Raphanus sativus) and carrots (Daucus carota), sown in soil inoculated with L. monocytogenes, were gathered after 3 months and examined for the presence of L. monocytogenes. Three of six radish samples were found to be positive. Remarkably, however, all carrot samples (six) were free of L. monocytogenes.