Differentiation of Listeria monocytogenes isolates by using plasmid profiling and multilocus enzyme electrophoresis.

Three hundred and seven Listeria monocytogenes isolates from various origins (clinical sources, raw chicken, seafoods, dairy and meat products and processing environments) were screened for plasmids. The overall frequency of L. monocytogenes isolates containing plasmids was 77%. The highest percentages of plasmid positive isolates were found from meat (89%), chicken (81%) and dairy products (64%), while clinical isolates had the lowest plasmid percentage (28%). Seven sizes of plasmids (21, 24, 27, 35, 40, 47 and 52 MDa) were distinguished. All sizes were represented in the meat isolates, clinical isolates contained only two of the plasmid sizes, while several different sizes of plasmids were found in the isolates from other origins. Plasmid profiling divided the isolates into ten plasmid pattern types. Multilocus enzyme electrophoresis of 75 isolates demonstrated 12 distinctive multilocus genotypes (ETs) which clustered into two groups: cluster A including serotype 1 and 4 isolates, and isolates not typable by Difco antisera serotype 1 and 4, and cluster B containing only serotype 1 isolates. No relationship between ETs and plasmid profiles could be demonstrated.