SETS OF ECORI FRAGMENTS CONTAINING RIBOSOMAL-RNA SEQUENCES ARE CONSERVED AMONG DIFFERENT STRAINS OF LISTERIA-MONOCYTOGENES

To classify Listeria monocytogenes using taxonomic characters derived from the rRNA operons and their flanking sequences, we studied a sampl e of 1346 strains within the taxon. DNA from each strain was digested with a restriction endonuclease, EcoRI. The fragments were separated b y gel electrophoresis, immobilized on a membrane, and hybridized with a labeled rRNA operon from Escherichia coli. The pattern of bands, pos itions, and intensities of hybridized fragments were electronically ca ptured. Software was used to normalize the band positions relative to standards, scale the signal intensity, and reduce the background so th at each strain was reproducibly represented in a data base as a patter n. With these methods, L. monocytogenes was resolved into 50 pattern t ypes differing in the length of at least one polymorphic fragment. Pat tern types representing multiple strains were recorded as the mathemat ical average of the strain patterns. Pattern types were arranged by si ze polymorphisms of assigned rRNA regions into subsets, which revealed the branching genetic structure of the species. Subtracting the polym orphic variants of a specific assigned region from the pattern types a nd averaging the types within each subset resulted in reduced sets of conserved fragments that could be used to recognize strains of the spe cies. Pattern types and reduced sets of conserved fragments were conse rved among different strains of L. monocytogenes but were not observed in total among strains of other species.