EFFECT OF PROTEIN ON RAMOPLANIN BROTH MICRODILUTION MINIMUM INHIBITORY CONCENTRATIONS

Ramoplanin is a glycolipodepsipeptide antibiotic active against Gram-p ositive bacteria. We observed that microdilution minimum inhibitory co ncentrations (MICs) were higher than those obtained in glass tubes or by agar dilution. Initial studies showed that these differences disapp eared when 30% bovine serum was added to the broth. Further studies sh owed that addition of 0.01% bovine serum albumin (BSA) to the broth lo wered the microdilution MICs far staphylococci, streptococci, and ente rococci by four- to 32-fold. This phenomenon occurred in several commo nly used growth media and in different types of commercially available microtiter trays. Precoating of the microtiter wells with a dilute so lution of BSA (0.02%) had the same effect. It seems likely that ramopl anin adsorbs to plastic surfaces and is lost from solution, and that p rotein masks the sites of adsorption. Ramoplanin MICs may be reliably determined by broth microdilution if a small amount of protein is adde d to the diluent.