Variability of the response of 66 Listeria monocytogenes and Listeria innocua strains to different growth conditions

The growth of 58 strains of Listeria monocytogenes and eight strains of Listeria innocua isolated from meat products (68%) and industrial sites (23%), were compared in four conditions of temperature, water activity (a(w)) and pH. Temperatures ranged from 10-37 degrees C, pH from 5.6-7.0 and a(w) from 0.96-1. Growths were performed in a meat broth with an automated turbidimeter (Bioscreen C, Labsystem). Growth curves were fitted using the Gompertz function, and growth parameters were calculated The differences between strains in lag phase duration were much greater than in growth rate. The greatest differences occurred at 10 degrees C, pH 7 and a(w) 0.96: lag time values ranged from 4 h to 4 clays. the Listeria population was separated into five groups, according to the lag time and maximal growth rate values using clustering analysis. The majority of the strains isolated from industrial sites were grouped together and showed faster growth than the others in the four conditions studied. The serotype or the nature of the meat from which the strains were isolated did not influence growth. The variability observed among strains raises questions about the consequences in quantitative risk assessment and about the construction of models in predictive modelling. (C) 1997 Academic Press Limited.