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

The levels of resistance to pristinamycin (Pt) and to its major constituents, pristinamycin IIA and IB (PIIA and PIE, respectively; classified as streptogramins A and B, respectively) were determined for 126 staphylococcal isolates. The results suggest tentative susceptibility breakpoints of less than or equal to 2, less than or equal to 8, and less than or equal to 0.5 mu g/ml for PIIA, PIE, and Pt, respectively. Fifty-six isolates that were inhibited by greater than or equal to 4 mu g of PIIA per mi were investigated for the presence of staphylococcal genes encoding resistance to PIIA (vga, vat, and vatB) and PTB (vgb). None of these genes was found in the 4 isolates inhibited by 4 mu g of PIIA per mi 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 greater than or equal to 8 mu g of PIIA per mi suggests that there is at least one PIIA resistance mechanism in staphylococci that has not yet been characterized.