The aerobic growth of Aeromonas hydrophila and Listeria monocytogenes in broths and on pork

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 Aeromonas hydrophila or a strain of Listeria monocytogenes. The broth cultures were incubated at temperatures between 0 and 25 °C, and growth rates were determined from optical density increases. The tissue samples were incubated at temperatures between − 2.4 and 25.2 °C, and growth rates were determined from viable count increases. Both organisms grew without lag in all broths at temperatures greater than 10 °C. A. hydrophila did not grow at 5 °C in TSB acidified with lactic acid to pH 5.5, and grew in other broths at that temperature after a lag of about 10 h. The organism did not grow in either broth of pH 5.5 at 2 °C, but grew in other broths at that temperature after a lag of about 40 h. A. hydrophila did not grow in any broth at 0 °C. L. monocytogenes grew in all broths at 5 °C only after a lag of about 60 h, and did not grow in any broth at 2 °C. For both organisms, the rates of growth, at any temperature, were lower in broths of pH 6.3, and lower again in broths of pH 5.5, than in the unacidified broth. Growth rates in the broths of pH 6.3 were similar, but growth rates were lower in lactic acid acidified broth of pH 5.5 than in HCl acidified broth of that pH. The data for the growth of each organism in each medium were well described by the regression line of the plot of the square roots of growth rates against temperature. A. hydrophila grew on fat tissue of pH 6.3 ± 0.3, without lag, at 1.8 °C and higher temperatures at rates greater than the rates of growth in unacidified TSB. Numbers of the organism declined on muscle tissue of pH 5.6 ± 0.2 at any temperature. L. monocytogenes grew on fat tissue without lag at − 0.3 °C and higher temperatures, at rates which at lower temperatures were greater than the rates of growth in unacidified TSB. The organism grew on muscle tissue only at temperatures greater than or equal to 15.4 °C, at rates which were less than the rates of growth in lactic acid acidified broth of pH 5.5. Models derived from the cultivation of A. hydrophila and L. monocytogenes in commercial broths appear to be highly unreliable guides to the behaviours of those organisms on pork.