Prevalence of Listeria spp. over Commercial Frozen and Refrigerated Foods at the Supermarket Level

As a part of investigation for listeriosis, we attempted isolation of Listeria spp. from commercial frozen and refrigerated foods at the supermarket level. Seven strains of Listeria spp. were isolated from 6 samples (7.1%) among 84 samples of frozen foods, and ten strains of Listeria spp. were also isolated from 8 samples (7.6%) among 105 samples of refrigerated foods at the supermarkets in Pusan area. From a total of 189 commercial foods, the number of isolated Listeria spp. and ratio were 6 strains (3.2%) of L. grayi, one strain (0.5%) of L. welshimeri, 6 strains (3.2%) of L. innocua and 4 strains (2.1%) of L. monocytogenes. Listeria spp. isolates except L. monocytogenes did not show β-hemolysis on blood agar and positive reaction in CAMP test with Staphyloccccus aureus. In the antibiotic susceptibility, most isolates of Listeria spp. were susceptible to 12 antibiotics such as ampicillin, cephalothin, penicillin, amikacin, gentamicin, erythromycin, kanamycin, vancomycin, tobramycin, carbenicillin, tetracycline and trimethoprim/sulfamethoxazole. Four strains of L. monocytogenes were susceptible to antibiotics used in this study except nitrofurantoin. The serotype of 3 strains and one strain of L. monocytogenes were classified into Listeria O serotype 1 and 4, respectively.