Effect of protein on ramoplanin broth microdilution minimum inhibitory concentrations

Ramoplanin is a glycolipodepsipeptide antibiotic active against Gram-positive bacteria. We observed that microdilution minimum inhibitory concentrations (MICs) were higher than those obtained in glass tubes or by agar dilution. Initial studies showed that these differences disappeared when 30% bovine serum was added to the broth. Further studies showed that addition of 0.01% bovine serum albumin (BSA) to the broth lowered the microdilution MICs for staphylococci, streptococci, and enterococci by four- to 32-fold. This phenomenon occurred in several commonly used growth media and in different types of commercially available microtiter trays. Precoating of the microtiter wells with a dilute solution of BSA (0.02%) had the same effect. It seems likely that ramoplanin adsorbs to plastic surfaces and is lost from solution, and that protein masks the sites of adsorption. Ramoplanin MICs may be reliably determined by broth microdilution if a small amount of protein is added to the diluent.