Serotyping and Mouse PathogenicityAll isolates confirmed as L. monocytogenesagglutinated with serum type 1 and 4. Of23 L. monocytogenes isolates, 17 (73.9%)belonged to serotype 4b, and 6 (26.1%) toserotype 1. Of the Listeria species, only L.monocytogenes is considered to be a significant human and animal pathogen, eventhough occasional human infections causedby L. welshimrei, L. innocua, and L. seeligerihave been reported. L. monocytogenes serotypes (1/2a, 1/2b, and 4b) are associated withmost cases of human listeriosis (Perrin, Berner, & Delamare, 2003).Eight of the 115 L. monocytogenes–inoculated mice (7%) died after four days of inoculation, and 17.3% died after five days ofinoculation. Mortality increased sharply onday 6 (34.7%, 40 mice) and on day 7 (56.5%,65 mice). Serotype 1 isolates were more virulent (100% of mortality rate at 48 hours) thanserotype 4 isolates. For serotype 4 isolates,only by increasing bacterial concentrationwas mice mortality achieved. The numberof viable cells required to cause death (LD50)with all the isolates ranged from 1.4 × 106 to3.3 × 107 CFU/mg/L.