Occurrence, Antibiotic Resistance and Molecular Characterization of Listeria monocytogenes in the Beef Chain in the Republic of Ireland

Abstract

This study investigated the occurrence, concentration and key characteristics of Listeria monocytogenes in beef chain samples (n=1100) over a 2-year period (July 2007-June 2009). Listeria monocytogenes was isolated from bovine hides (27%), pre-chill carcasses (14%) and ground beef (29%), but not from ready-to-eat (RTE) beef. The concentration of the pathogen in the majority (95%) of contaminated samples was low and detected by enrichment only. The highest concentrations recovered (100-200 CFU/g) were in ground beef samples. The most commonly isolated serotype group was 1/2a (58%) followed by 4b (12%), 1/2b (10%) and 1/2c (6%). A small portion (<5%) isolates had demonstrated resistance to key anti-microbials including ampicillin, vancomycin and gentamycin which are recommended treatment options for listeriosis. Pulsed-field gel electrophoresis showed indistinguishable profiles for a number of isolates recovered from the hide and carcass (after slaughter and dressing) of the same animals, highlighting the role of hides as a source of contamination. Equally, indistinguishable pulsotypes for isolates recovered at different stages and time points (up to 6 months apart) in the beef chain demonstrated the persistence of specific clones in the factory, process and distribution environments. Overall, the study demonstrated a high prevalence of clinically significant L. monocytogenes entering and progressing along the beef chain and highlights the needs to control cross-contamination during beef processing and distribution and the need for thorough cooking of raw beef products.