Detection, enumeration, and RAPD analysis of Listeria monocytogenes isolates in fish derived from retail outlets in Western Massachusetts

Fish were sampled over a 24-month period from two major supermarket retail outlets in Hadley, Massachusetts, USA, designated A and B, for the incidence of Listeria monocytogenes and numbers of the organism present per 100 g of tissue. Fifteen species of fish were represented. Seventy-four samples out of a total of 320 were confirmed by PCR as yielding L. monocytogenes. From retail source A, a total of 171 samples yielded 59 (34.5%) that were positive for the presence of L. monocytogenes. In contrast, from retail source B, a total of 149 samples yielded 15 (10.0%) that were positive. Only six samples (3.5%) from retail source A had MPN counts of L. monocytogenes in the range of 100 to 1,000 per 1,000 g. Only two samples (1.3%) from retail source B had counts of L. monocytogenes from 100 to 1,000 per 100 g. A total of 221 strains of L. monocytogenes were derived from the MPN cultures, 164 from retail source A, and 57 from retail source B. All 221 strains were subjected to RAPD analysis using three random primers. Primer LMPB1 yielded 21 RAPD profiles, primer LMPB4 yielded 19 profiles, and primer HLWL74 yielded 26 profiles. A total of 55 composite profiles were identified by combining the profiles derived from the three primers. Source A yielded 50 composite RAPD profiles, whereas source B yielded only ten composite profiles. In addition, 27 of the 55 composite profiles were derived from individual isolates and RAPD types 11 and 18 included 49 and 27 isolates respectively. Fish from retail source A clearly harbored far more RAPD types than did source B. The results clearly indicated that two major retail sources in close geographic proximity can vary considerably with respect to the incidence and numbers of L. monocytogenes present on the fish tissue. It was not possible to determine whether the processors furnishing fish to retail outlet A or the supermarket itself was responsible for the notably higher incidence and numbers of L. monocytogenes on fish from retail source A compared to fish from retail source B.