Characterization of recurrent and sporadic Listeria monocytogenes isolatesfrom raw milk and nondairy foods by pulsed-field gel electrophoresis, monocin typing, plasmid profiling, and cadmium and antibiotic resistance determination

Following previous surveys to assess the incidence of Listeria. monocytogen es in raw milk and nondairy foods processed in Northern Ireland, isolates w ere characterized as recurrent or sporadic on the basis of multilocus enzym e electrophoresis (MEE) analysis and restriction fragment length polymorphi sm typing. Tn the present study, 45 representative recurrent and sporadic e lectrophoretic types (ETs) previously identified by MEE were subjected to p ulsed-field gel electrophoresis (PFGE) of genomic DNA macrorestriction frag ments, monocin typing, plasmid profiling, and an examination of resistance to cadmium and nine different antibiotics. Although PFGE proved to be capab le of subdividing a number of recurrent and sporadic ETs, the grouping of s trains arrived at by PFGE and MEE were in broad agreement, and previous con clusions regarding the designation of L. monocytogenes strains as recurrent or sporadic remained unaltered, It is considered that PFGE was able to det ect minor genetic changes in recurrent ETs which occurred during the time p eriod in which food surveys were carried out, Production of type E monocin (Types A to E were found among the 45 strains), plasmid carriage, and resis tance to cadmium occurred more frequently in recurrent than in sporadic str ains and may he important with regard to the ability of L. monocytogenes to persist in food and food processing environments. Only 2 of 45 strains sho wed resistance to any of the nine antibiotics tested: two sporadic strains were resistant to tetracycline (MIC, 64 mug ml(-1)).