Plasmid-linked Resistance to Inorganic Salts in Staphylococcus aureus

Abstract

The penicillinase plasmids, a series of extrachromosomal resistance factors in S. aureus, carry determinants of resistance to a series of inorganic ions as well as resistance to penicillin and (in some cases) erythromycin. Most of the ions involved were inhibitory but not lethal to the bacteria; the resistance markers conferred an increase in resistance by comparison with susceptible organisms of between 3- and 100-fold, depending on the ion involved. Separate genetic loci for resistance to arsenate, arsenite, lead, cadmium, mercuric, and bismuth ions were demonstrated. Resistance to antimony and zinc also found were not separated genetically from resistance to arsenite and cadmium, respectively. The ion resistance markers appeared to form a cluster on the plasmid with no other known marker within it. Nautrally occurring plasmids were observed that lacked 1 or more of these ion resistance markers, and penicillinase-negative strains were resistant to 1 or more of the ions. The patterns of markers carried by these various strains may provide some understanding of the evolution of a plasmid linkage group.