GROWTH VARIATION AMONG SPECIES AND STRAINS OF LISTERIA IN CULTURE BROTH

Culture suspensions of 45 species and strains of Listeria were prepare d 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 st andardized to 0.10 +/- 0.01 absorbance at 600 nm. Spectrophotometer tu bes containing 5 mi of TSBYE (pH 7.2) were inoculated with 0.1 mi of t he standardized cultures and incubated at 4, 10 or 37 degrees C. Absor bance readings were taken during storage. Growth curves were fitted us ing the Gompertz function, and growth parameters were calculated. Ther e were major differences in lag phase duration (LPD), generation time (GT) and exponential growth rate (EGR) among species and strains of Li steria tested. Values for LPD and GT decreased (P <0.05) with increasi ng temperature of incubation, while EGR and maximum population density (MPD) values increased. Lag phase duration and GT values at a given t emperature were lower for Listeria monocytogenes compared to other Lis teria spp. At 4 degrees C, LPDs for L. monocytogenes strains ranged fr om 69.8 to 270.8 h. Of the L. monocytogenes cultures tested, strain Sc ott 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 aga in one of the strains with the longest average LPD (62.8 +/- 0.7 h). A t 37 degrees C, LPDs ranged from 4.4 to 11.1 h. Variation was also obs erved in GT and EGR, especially at 4 degrees C. Although there were ma jor variations in growth parameters due to strain and temperature, no significant (P >0.05) trends were observed in average values among dif ferent serotypes of L. monocytogenes tested.