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 o f Listeria innocua isolated from meat products (68%) and industrial si tes (23%), were compared in four conditions of temperature, water acti vity (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 w ith an automated turbidimeter (Bioscreen C, Labsystem). Growth curves were fitted using the Gompertz function, and growth parameters were ca lculated The differences between strains in lag phase duration were mu ch greater than in growth rate. The greatest differences occurred at 1 0 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, accordi ng to the lag time and maximal growth rate values using clustering ana lysis. 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 o bserved among strains raises questions about the consequences in quant itative risk assessment and about the construction of models in predic tive modelling. (C) 1997 Academic Press Limited.