Distribution of genes encoding resistance to streptogramin A and related compounds among staphylococci resistant to these antibiotics

Abstract

The levels of resistance to pristinamycin (Pt) and to its major constituents, pristinamycin IIA and IB (PIIA and PIB, respectively; classified as streptogramins A and B, respectively) were determined for 126 staphylococcal isolates. The results suggest tentative susceptibility breakpoints of ≤2, ≤8, and ≤0.5 μg/ml for PIIA, PIB, and Pt, respectively. Fifty-six isolates that were inhibited by ≤4 μg of PIIA per ml were investigated for the presence of staphylococcal genes encoding resistance to PIIA (vga, vat, and vatB) and PIB (vgb). None of these genes was found in the 4 isolates inhibited by 4 μg of PIIA per ml or in 4 of the other 52 isolates tested. The remaining 48 isolates harbored plasmids carrying vatB and vga or combinations of genes (vga-vat-vgb or vga-vat). The absence of any known PIIA resistance gene from the four Staphylococcus aureus isolates inhibited by ≤8 μg of PIIA per ml suggests that there is at least one PIIA resistance mechanism in staphylococci that has not yet been characterized.