Prevalence and typing of Listeria monocytogenes in raw catfish fillets

Raw channel catfish fillets collected from three processing plants during four time periods were tested for the presence of Listeria species. Listeria monocytogenes was the predominant Listeria species found in these catfish fillets, with 25 to 47% prevalence. Other Listeria species, such as L welshimeri, L. innocua, L ivanovii, L. grayi, and L seeligeri, were also found. L monocytogenes isolates were further fingerprinted by a repetitive element PCR. Forty distinctive electrophoretic types (ETs) and three genetic clusters were determined by Dice coefficient analysis and UPGMA (unweighted pair group method using arithmetic averages). Twenty of 40 ETs were represented by a single isolate, and the other 20 ETs were represented by 2 to I I isolates. Thirty-five ETs, represented by 76 isolates, were found in processing plant A, 13, or C and designated plant-specific types. The remaining five ETs, represented by 21 isolates, were found in multiple plants and designated nonplant-specific types. In addition, 10 ETs from 52 isolates were found repeatedly during different seasons. Plant-specific and nonplant-specific L monocytogenes coexisted in processed catfish fillets. Some isolates were persistently found in processed fillets, suggesting that either the current sanitation procedures used by these plants are inadequate or that these isolates originated from the natural habitats of the catfish. The results also suggest that the repetitive element PCR is a useful tool for differentiating L. monocytogenes subtypes and can be used for tracing the source of a contamination.