Predicting the aerobic growth of Yersinia enterocolitica on pork fat and muscle tissues

Flasks of Tryptic Soy Broth, (TSB), unacidified (pH 7.2) or acidified with HCl or lactic acid to pH 6.3 or 5.5 and samples of sterile pork fat or muscle tissue, were inoculated with logarithmic phase cultures of a strain of Yersinia enterocolitica which had been isolated from a pork packing plant. The broth cultures were incubated at temperatures between 2 and 25 degrees C, and growth rates determined from increases in the optical densities at 600 nm. The tissue samples were incubated at temperatures between -2.4 and 20.4 degrees C, and growth rates were determined from increases in the viable counts. The organism grew without a lag phase in all broth cultures. At any temperature the rates of growth were lower in media of pH 6.3, and lower again in media of pH 5.5 than in unacidified broth; and growth rates were lower in media acidified with lactic acid than in media of the same pH acidified with HCl. The data for each medium were well described by the regression line of the plot of the square roots of growth rates against temperature. The organism grew on fat tissue, of pH 6.3+/-0.3, without a lag phase at all temperatures, at rates comparable with the rates of growth in unacidified TSB of pH 7.2; i.e. at rates faster than would be predicted for a medium of pH 6.3. The organism did not grow on muscle tissue of pH 5.6+/-0.2, at temperatures less than or equal to 6 degrees C. At higher temperatures, the organism grew on muscle tissue only after a lag of 24 h or more at rates which were less than those that would be predicted from the rates of growth in TSB acidified to pH 5.5 with lactic acid. It appears that models for the growth of Y. enterocolitica are likely to underestimate the growth on pork fat and to overestimate the growth on pork muscle. Three previously published models would generally predict much faster growth in broth cultures, under all the studied conditions of temperature and pH, than was observed for the strain used in this study.