GROWTH VARIATION AMONG SPECIES AND STRAINS OF LISTERIA IN CULTURE BROTH

Culture suspensions of 45 species and strains of Listeria were prepared in tryptic soy broth with 0.6% yeast extract (TSBYE) for 24 h at 37 degrees C, and were then diluted with phosphate buffer solution and standardized to 0.10 +/- 0.01 absorbance at 600 nm. Spectrophotometer tubes containing 5 mi of TSBYE (pH 7.2) were inoculated with 0.1 mi of the standardized cultures and incubated at 4, 10 or 37 degrees C. Absorbance readings were taken during storage. Growth curves were fitted using the Gompertz function, and growth parameters were calculated. There were major differences in lag phase duration (LPD), generation time (GT) and exponential growth rate (EGR) among species and strains of Listeria tested. Values for LPD and GT decreased (P <0.05) with increasing temperature of incubation, while EGR and maximum population density (MPD) values increased. Lag phase duration and GT values at a given temperature were lower for Listeria monocytogenes compared to other Listeria spp. At 4 degrees C, LPDs for L. monocytogenes strains ranged from 69.8 to 270.8 h. Of the L. monocytogenes cultures tested, strain Scott A had the longest average (209.8 +/- 0.1) h LPD at 4 degrees C. At 10 degrees C, LPDs ranged from 36.5 to 68.9 h, with Scott A being again one of the strains with the longest average LPD (62.8 +/- 0.7 h). At 37 degrees C, LPDs ranged from 4.4 to 11.1 h. Variation was also observed in GT and EGR, especially at 4 degrees C. Although there were major variations in growth parameters due to strain and temperature, no significant (P >0.05) trends were observed in average values among different serotypes of L. monocytogenes tested.