Some effects of plasmids coding for antibiotic resistance on the virulence of Staphylococcus aureus.

The relative virulence of pairs of staphylococci differing in resistance plasmid content has been studied using the neonatal mouse weight gain test. Both clinical and laboratory strains were used which had undergone genetic manipulation, either curing for loss of plasmids or transduction for gain of plasmids. A difference in virulence was detected between two variants of S. aureus NCTC 8325 possessing different plasmids coding for penicillinase. However in most cases any form of genetic manipulation seemed to reduce the virulence of the staphylococcus. In the case of NCTC 9789 (PS 80) which was originally an epidemic strain, curing of a plasmid coding for cadmium resistance resulted in reduced virulence but original virulence could not be restored by transduction of the plasmid into the cured derivative.