In vitro activity of Ro 19-5247 (T-2525) and interpretive criteria for disk diffusion susceptibility testing.

The activity of Ro 19-5247 (the active metabolite of the oral cephalosporin Ro 19-5248 [T-2588]) was compared with that of five orally active agents against a total of 331 bacterial strains. Ro 19-5247 was more active in vitro than amoxicillin, amoxicillin-clavulanate, cefaclor, cefuroxime, and cephalexin against members of the family Enterobacteriaceae. Amoxicillin-clavulanate and amoxicillin overall were more active than the other four agents against staphylococci. Ro 19-5247, amoxicillin-clavulanate, amoxicillin, and cefuroxime were equally active against nonenterococcal streptococci and more active than cefaclor and cephalexin. All six agents showed little or no activity against nonfermentative gram-negative bacteria. Against Streptococcus (Enterococcus) faecalis, only amoxicillin and amoxicillin-clavulanate were active. The interpretive criteria for in vitro susceptibility testing with 10- and 30-micrograms Ro 19-5247 disks were established by regression analysis to correlate the inhibitory zone sizes and MICs for the bacterial isolates. The suggested tentative zone size breakpoints for the 10-micrograms disk are as follows: susceptible, greater than or equal to 22 mm (MIC, less than or equal to 2 micrograms/ml); moderately susceptible, 20 to 21 mm (MIC, 4 micrograms/ml); and resistant, less than or equal to 19 mm (MIC, greater than or equal to 8 micrograms/ml).