Immunological and cytopathogenic properties of Listeria monocytogenes isolated from naturally contaminated meats

Meat products have been implicated as the potential source of Listeria monocytogenes infection in humans. Here, we investigated the incidence of this organism in raw beef and poultry meat products and assessed their biochemical, immunological and cytopathogenic properties. Forty meat samples (28 beef and 20 poultry) were analyzed and the isolates were tested for sugar fermentation, hemolysin production, phospholipase activity, serotype profile, abilities to react with Listeria-specific monoclonal antibodies (MAbs) EM-7G1 and C11E9, and cytotoxic effects on hybridoma Ped-2E9 cells. Thirteen (6 beef and 7 poultry) meat samples (32.5%) were positive for L. monocytogenes. A total of 276 Listeria isolates were obtained, of which 182 (66%) were confirmed to be L. monocytogenes, 80 (29%) were L. innocua, 12 (4.3%) were L. welshimeri and 2 (0.7%) were identified as L. grayi. Fifty sir percent of the L. monocytogenes isolates were serotype 4, while 42% were serotype 1, and 2% were untypeable. All but two L. monocytogenes isolates were hemolytic and phospholipase positive (99%). In the ELISA assay, MAb C11E9 showed reaction with L. monocytogenes isolates from all 13 positive meat samples (100%), while MAb EM-7G1 reacted positively with 12 of 13 positive meat samples (92.3%). Hemolysin-positive L. monocytogenes isolates were cytopathogenic to Ped-2E9 cells, while hemolysin-negative strains showed no effect. This study demonstrated that 32.5% of commercially purchased raw meat products were contaminated with cytopathogenic L. monocytogenes strains, and could be a potential source for infection in susceptible populations if these meats were not processed or cooked properly.