THE AEROBIC GROWTH OF AEROMONAS-HYDROPHILA AND LISTERIA-MONOCYTOGENESIN BROTHS AND ON PORK

Flasks of tryptic soy broth (TSB), unacidified (pH 7.2) or acidified w ith HCl or lactic acid to pH 6.3 or 5.5, and samples of sterile pork f at or muscle tissue, were inoculated with logarithmic phase cultures o f a strain of Aeromonas hydrophila or a strain of Listeria monocytogen es. The broth cultures were incubated at temperatures between 0 and 25 degrees C, and growth rates were determined from optical density incr eases. The tissue samples were incubated at temperatures between -2.4 and 25.2 degrees C, and growth rates were determined from viable count increases. Both organisms grew without lag in all broths at temperatu res greater than 10 degrees C. A. hydrophila did not grow at 5 degrees C in TSB acidified with lactic acid to pH 5.5, and grew in other brot hs at that temperature after a lag of about 10 h. The grow in either b roth of pH 5.5 at 2 degrees C, but grew in other broths at that temper ature after a lag of organism did not grow in either broth of pH 5.5 a t 2 degrees C, but grew in other broths at that temperature after a la g of about 40 h. A. hydrophila did not grow in any broth at 0 degrees C. L. monocytogenes grew in all broths at 5 degrees C only after a lag of about 60 h, and did not grow in any broth at 2 degrees C. For both organisms, the rates of growth, at any temperature, were lower in bro ths of pH 6.3, and lower again in broths of pH 5.5, than in the unacid ified broth. Growth rates in the broths of pH 6.3 were similar, but gr owth 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 organ ism in each medium were well described by the regression line of the p lot of the square roots of growth rates against temperature. A. hydrop hila grew on far tissue of pH 6.3 +/- 0.3, without lag, at 1.8 degrees C and higher temperatures at rates greater than the rates of growth i n unacidified TSB. Numbers of the organism declined on muscle tissue o f pH 5.6 +/- 0.2 at any temperature. L. monocytogenes grew on fat tiss ue without lag at -0.3 degrees C and higher temperatures, at rates whi ch al lower temperatures were greater than the rates of growth in unac idified TSB. The organism grew on muscle tissue only at temperatures g reater than or equal to 15.4 degrees C, at rates which were less than the rates of growth in lactic acid acidified broth of pH 5.5. Models d erived from the cultivation of A. hydrophila and L. monocytogenes in c ommercial broths appear to be highly unreliable guides to the behaviou rs of those organisms on pork. (C) 1997 Elsevier Science B.V.