STAPHYLOCOCCAL RESISTANCE TO STREPTOGRAMINS AND RELATED ANTIBIOTICS

Streptogramin and related antibiotics are mixtures of two compounds,A and B (e.g. Dalfopristin and Quinupristin), particularly against Gram- positive bacteria. Staphylococci resistant to these mixtures are alway s resistant to the A compounds but are not necessarily resistant to th e B compounds. Resistance to A compounds and to the mixtures is confer red by acetyltransferases or ATP-binding proteins via unknown mechanis ms. Several genes encoding each of the two categories of protein have been characterized and regularly detected on plasmids. Genes encoding lactonases, which inactivate B compounds,have been occasionally detect ed on these plasmids. Staphylococci which harbour plasmids conferring resistance to A compounds should not be treated with the mixtures even if they appear susceptible in vitro. Indeed, susceptibility to the mi xtures of staphylococci carrying resistance to A compounds has often b een attributed to partial loss of the plasmids conferring this resista nce. When staphylococci are constitutively resistant to B compounds, t he in vitro activities of the mixtures should be evaluated, because th ey are better correlated than MICs with their efficacy in therapy.