The bacteriophagous nematode Caenorhabditis elegans has been recognized as a surrogate host for human pathogens. The aim of this study was to examine whether food-borne pathogens are pathogenic in nematodes. Young adult worms were allocated onto peptone-free medium covered with a bacterial suspension of each pathogen. The plates were incubated and as the number of live and dead worms scored at least every 24 h. Twelve of the 14 pathogenic strains, namely Aeromonas sobria, the diarrheagenic Escherichia coli strains (enteroaggregative E. coli, enterohemorrhagic E. coli, enteropathogenic E. coli, enterotoxigenic E. coli, enteroaggregative E. coli heat-stable enterotoxin 1 gene-possessing E. coli, and diffusely adherent E. coli), Listeria monocytogenes, Salmonella Enteritidis, Staphylococcus aureus, Vibrio parahaemolyticus, and Yersinia enterocolitica reduced the survival rate of worms to varying degrees. The remaining 2 strains, Bacillus cereus and enteroinvasive E. coli, did not. Thus, food-borne pathogens can infect and proliferate within bacteriophagous nematodes on peptone-free medium to the same extent as reported with conventional peptone-containing medium. However, the non-enteropathogenic E. coli strain HS and deletion mutants of L. monocytogenes, which have lost their virulence in the murine model, were also still nematocidal. Although this nematode could be an alternative host for these pathogens, the nematocidal activity of these pathogens may not necessarily reflect enteropathogenicity in humans. Pathogens and the virulence genes to be analyzed must be carefully selected before using this alternative host.
The microcolony method is based on microscopic observation of the early stage of colony formation on laboratory media, which allows rapid and accurate enumeration of bacteria with growth ability via epifluorescence microscopy or microcolony automated counting system. In this study, the microcolony method was used to estimate the number of viable bacteria in various foods. The optimized protocol generated results on number of bacteria with growth ability in ground meat, precut vegetables, frozen spinach, precut melon, soybean curd, Chinese noodles, bean cake, white sauce, steamed cake within 24 hours. The accuracy of estimates by the microcolony method could be obtained by comparing the conventional plate counts. The results of this study showed that the microcolony method provides a useful tool to enumerate bacteria with growth ability in foods more rapidly and accurately than conventional plate counting.