Growth of Listeria monocytogenes, Aeromonas hydrophila and Yersinia enterocolitica in pâté and a comparison with predictive models.

A reference or type strain and a food derived-strain of the cold-tolerant pathogens Listeria monocytogenes, Aeromonas hydrophila and Yersinia enterocolitica were individually inoculated into samples of commercial pâté and incubated at 4 and 10 degrees C. The organisms were periodically estimated by presumptive counts, then values for the lag and generation times were calculated. Both strains of L. monocytogenes grew at both temperatures. The food strain of A. hydrophila grew only at 10 degrees C, and the type strain did not grow at either temperature. Similarly, the type strain of Y. enterocolitica did not grow at either temperature, whereas the food strain grew at both 4 and 10 degrees C. In some cases growth of non-test organisms may have inhibited the growth of these latter two species. The measured values of lag and generation times did not, in general, correlate well with those predicted by response surface models, taken from the literature and produced in this laboratory. It may be that the pâté contained an inhibitor that affected the growth of the organisms. The two strains of A. hydrophila and Y. enterocolitica showed significantly different growth characteristics, reinforcing the value of using a 'cocktail' of strains in growth experiments. Differences in predicted kinetic values from the models indicate that a model for any particular strain may not reflect the growth of naturally occurring contaminants of the same species.