[Studies on genetic differentiation of Salmonella enteritidis isolates by plasmid profile]

Since 1989, the number of salmonellosis cases caused by S. Enteritidis has increased considerably in Japan. Genetic differentiation of 385 strains isolated from January 1982 to December 1988 and January 1989 to April 1991 were used for plasmid profiles. Plasmids were found in 377 out of 385 strains; therefore, only 8 strains carried no plasmid. Among 377 strains, 15 different plasmid profile types (OP-1 to OP-15) were classified. The most common plasmid profile types from 1982 to 1988 were OP-7 (70 kbp) and OP-8 (70 kbp and 2 kbp). On the other hand, the most common plasmid profile types from 1989 to 1991 were OP-1 (60 kbp) and OP-2 (60 kbp and 54 kbp). Serovar-specific virulence 60 kbp plasmids of S. Enteritidis were identified in 7 plasmid profile types (A total of 200 strains). In the other plasmid profile types, 70 kbp plasmids were found in 5 plasmid profile types (A total of 173 strains). In restriction enzyme analysis of 70 kbp plasmid DNAs obtained from 5 plasmid profile types of S. Enteritidis, we found that these plasmid DNAs shared both 60 kbp and 10 kbp fragments. These results indicate that these plasmid profile types also carried serovar-specific virulence plasmids of S. Enteritidis. The strains of plasmid profile type OP-2 were SA (sulfisoxazole) and SM (streptomycin) resistance, and the 54 kbp plasmids in the strains of OP-2 were transferred by bacterial conjugation into the E. coli strains. All transconjugants acquired SM resistance.