A QUANTITATIVE-DETERMINATION OF INDUCIBIL ITY 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 resis tance to macrolide-lincosamide-streptogramin B (MLS) antibiotics, the extent of MLS-resistance induced by several macrolide antibiotics [ery thromycin (EM), oleandomycin (OL), or roxithromycin (RXM)] was determi ned 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) exponenti ally 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 c ells 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 b e useful for the analysis of a structure-inducibility relationship.