Predicting the aerobic growth of Y. enterocolitica O:3 at different pH-values, temperatures and L-lactate concentrations using conductance measurements.

The effect of the inoculum level and growth conditions on conductance response was studied for Yersinia enterocolitica O:3. The Gompertz equation, y = A+C exp (-exp(-B(time(-)-M))) was used in the fitting of conductance response curves. Inoculum levels between 3 and 7 log cfu/ml did not affect the B or C parameters. The M parameter was affected; the lower the inoculum level, the higher the M value. Conductance response was attained at 7.5 log cfu Y. enterocolitica/ml skim milk-SPYE medium used. Polynomial models for log B and log C were developed for Y. enterocolitica O:3 describing the effect of temperature (7-23 degrees C), pH (5.4-6.5) and L-lactate (0-1.2%), and combinations thereof, under aerobic conditions. Conductance response was attained in all combinations of L-lactate concentrations, pH levels and temperatures. The conductance rate (B.C/e) was of the same magnitude at 23 degrees C, pH 5.4 and 1.2% L-lactate as at 7 degrees C, pH 6.5 and 0% L-lactate. A high correlation was found between the conductance rates predicted from conductance polynomial models and rates predicted from an absorbance model taken from the literature. A large number of combinations of factors affecting the growth/activity of bacteria could be studied simultaneously, due to the large instrumental capacity of the Malthus 2000.