COMPARISON OF RIBOTYPING AND MULTILOCUS ENZYME ELECTROPHORESIS FOR SUBTYPING OF LISTERIA-MONOCYTOGENES ISOLATES

Ribotyping was compared with multilocus enzyme electrophoresis (MEE) f or subtyping 305 Listeria monocytogenes isolates from clinical and non clinical sources. For ribotyping, EcoRI-restricted genomic DNA fragmen ts of L. monocytogenes strains were separated by agarose gel electroph oresis, and Southern blots were probed with a cloned Escherichia coli rrnB operon (plasmid pKK3535) labeled with digoxigenin. The L. monocyt ogenes isolates were divided into 28 distinct ribotypes, while MEE ana lysis divided the same isolates into 78 electrophoretic types (ETs). O n the basis of their ribotype profiles, the strains were divided into two subgroups. The ribotype alpha (RT alpha) subgroup contained seroty pes 1/2a. 1/2c, and 3a, and the ribotype beta (RTP) subgroup contained serotypes 1/2b, 3b, 4b, and 4ab. This division is in complete agreeme nt with MEE analysis, which divides the species into two subgroups (ET groups A and B), with the same serotype distribution in each subgroup . Overall, MEE was more discriminating than ribotyping. However, in se veral instances ribotyping discriminated between isolates within the s ame ET. Ribotyping was more discriminating for serotypes 1/2a, 1/2c, a nd 3a (Simpson's Index for Diversity [DI] = 0.81) than for serotypes 1 /2b and 4b (DI = 0.76). A substantial proportion (69%) of serotype 1/2 b and 4b strains clustered in five ETs and five ribotypes. These data suggest that ribotyping and MEE do not provide adequate discrimination between strains of serotypes 1/2b and 4b. Methods such as pulsed-fiel d gel electrophoresis and random amplified polymorphic DNA analysis sh ould be explored for further discrimination of strains of these seroty pes.