Modelling growth rates of Listeria innocua as a function of lactate concentration

The effect of sodium lactate (NaL) concentration on growth of Listeria innocua in a yeast-extract/peptone broth at pH 5.5, 6.0, 6.5 and 7.0 at 4, 10, 20 and 30 degrees C was modelled with the modified Gompertz model. NaCl was used as a reference to distinguish between the water activity effect and the specific inhibitory effect of NaL. Minimum inhibitory concentrations (MIC) of NaCl appeared to be significantly higher than MIC values of NaL, indicating that NaL had a specific inhibitory effect on growth of L. innocua. The MIC values of NaL and NaCl were not much influenced by the temperature. The pH of the growth medium was shown to have influence on the MIC values of NaL but not on the MIC values of NaCl. Total growth inhibition of L. innocua at low pH (5.5) took place at lower NaL concentrations (217 mM) than at neutral pH (1071-1339 mM), indicating that the undissociated lactic acid plays a role in the mechanism of inhibition. However, MIC values for undissociated acid increased with decreasing pH from 0.8 mM at pH 7 to 5 mM at pH 5.5. It is therefore likely that besides acidification of the cytoplasm due to diffusion of undissociated acid into the cell, other mechanisms are involved. Growth rates at NaL concentrations between 0 and the MIC value decreased progressively with increasing concentrations down to 0 at the MIC value, and were strongly influenced by both temperature and pH. Growth rates in the presence of NaCl were influenced by the temperature only. It was shown that a modified Monod equation with three parameters was effective for description of growth rates of L. innocua at NaL and NaCl concentrations over the whole experimental range.