The presence of Listeria spp. was investigated in 369 samples of cooked meat products and 52 of smoked salmon. Incidences of 17.6% for cooked meat and 38.5% for smoked salmon samples were found. All Listeria monocytogenes isolates (34 from meat products and 16 from smoked salmon) were typed serologically and by random amplified polymorphic DNA (RAPD) typing using primers HLWL74 (5'-ACGTATCTGC-3'), HLWL85 (5'-ACAACTGCTC-3'), and OMP-01 (5'-GTI'GGTGGCT-3'). Strains from cooked meat products were characterized and compared in relation to their origin. The detection of identical strains in products of different type and brand packed on the same date suggested cross-contamination, probably during the slicing process. All L monocytogenes isolates from smoked salmon were indistinguishable by serotyping and RAPD, suggesting that this strain was highly disseminated and adapted to the treatment used for the preservation of this food. RAPD subtypes were analyzed using GelCompar version 4.1 software and the unweighted pair method using arithmetic averages, and six groups with at least 78% similarity were established. Serotyping and RAPD results were in concordance, although RAPD showed a higher discriminatory power with L. monocytogenes isolates from meat products. RAPD is an easy method that could be useful to detect cross-contamination occurring during postprocessing manipulations.
The susceptibility of 440 Listeria monocytogenes strains isolated from food (n=401) and clinical cases (n=39) between 1995 and 2005 was determined by standard agar dilution and E-test methods. Antimicrobial drugs currently used in veterinary and human therapy were tested, and they included penicillin G, ampicillin, cephalothin, gentamicin, chloramphenicol, tetracycline, doxycycline, trimethoprim, erythromycin, and clindamycin. The sensitivity of strains was established using Clinical and Laboratory Standards Institute (formerly NCCLS) breakpoints and MIC50 (the MIC for 50% of the strains) to MIC90 values. In general, isolates were susceptible to the majority of the antimicrobials tested, including beta-lactamics and aminoglycosides, which are normally used in the treatment of listeriosis. Resistance to tetracycline and doxycycline was found in five strains isolated from fresh trout belonging to the same fish farm. Molecular analysis by restriction endonuclease analysis showed a similar profile, suggesting the persistence of a strain well adapted to the presence of tetracycline in the environment of a fish farm, which is frequently used in aquaculture in order to prevent infections of fish.
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