[A quantitative determination of inducibility by 14 membered ring macrolide antibiotics in inducible resistant Staphylococcus aureus to macrolide-lincosamide-streptogramin B antibiotics].

Using Staphylococcus aureus ISP447 strain, which shows inducible resistance to macrolide-lincosamide-streptogramin B (MLS) antibiotics, the extent of MLS-resistance induced by several macrolide antibiotics [erythromycin (EM), oleandomycin (OL), or roxithromycin (RXM)] was determined in terms of a relative ratio of a growth rate of the induced cells in the presence of a challenging drug, rokitamycin (RKM), to that of uninduced cells in the absence of RKM. The ratio was referred to as a relative inducibility (%). The inducibility was obtained at an optimum-induced condition by considering the following factors: (1) exponentially growing cells, (2) the optimum concentration of an inducer drug, i.e., 50, 150, and 150 ng/ml for EM, OL, and RXM, respectively, (3) a 3-h previous incubation at 37 degrees C in the presence of the inducer, and (4) 300 ng of RKM/ml, which is found to be optimum for induced cells to challenge, because of having no inducer activity. Using these qualification methods, inducibilities of EM, OL, and RXM as an inducer were 100.4, 27.9 and 81.1%, respectively. This method is allowed to be useful for the analysis of a structure-inducibility relationship.