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 consti tuents, pristinamycin IIA and IB (PIIA and PIE, respectively; classifi ed as streptogramins A and B, respectively) were determined for 126 st aphylococcal isolates. The results suggest tentative susceptibility br eakpoints of less than or equal to 2, less than or equal to 8, and les s than or equal to 0.5 mu g/ml for PIIA, PIE, and Pt, respectively. Fi fty-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 g enes (vga-vat-vgb or vga-vat). The absence of any known PIIA resistanc e gene from the four Staphylococcus aureus isolates inhibited by great er than or equal to 8 mu g of PIIA per mi suggests that there is at le ast one PIIA resistance mechanism in staphylococci that has not yet be en characterized.