Genetic Instability of R Plasmids in Relation to the Shift of Drug Resistance Patterns in Salmonella johannesburg

SUMMARY: Observation of the resistance of Salmonella johannesburg to the six drugs ampicillin (A), streptomycin (S), tetracycline (T), chloramphenicol (C), kanamycin (K) and sulphadiazine (Su) was made over the 7 years from 1973 to 1979. Strains with ASTCKSu- and ASCKSu-resistance patterns predominated in the years 1973--1975 and 1976--1979, respectively. These resistances were found to be mediated by autotransferring plasmids belonging to the incompatibility group FIme. The ASTCKSu-resistance plasmids were unstable, giving rise to deletion variants at a much higher frequency than ASCKSu-resistance plasmids either of natural origin or derived in vitro from the ASTCKSu-resistance plasmids. Thus, the ASCKSu-resistance plasmid might be a deletion variant of the ASTCKSu-resistance plasmid. This is supported by the extensive similarity of their cleavage patterns produced by specific restriction endonucleases.