[Subinhibitory activity of cefaclor and cephalexin (author's transl)]

The determination of the minimal inhibitory concentration (MIC), which is usually performed after 18 to 24 hours of incubation, results in the case of cefaclor in a false picture of the actual activity. Cefaclor is chemically so unstable that after 24 hours only 2-5% of the substance is microbiologically active. In order to compare the activity of cefaclor and cephalexin, the effect of subinhibitory concentrations of both antibiotics against Escherichia coli and Klebsiella pneumoniae strains was studied by means of turbidimetry. After eight hours the absolute inhibitory concentration of cefaclor was lower than that of cephalexin with both methods. At the same time the effect of subinhibitory concentrations of both antibiotics was equal. Automatic measurement over 20 hours showed at the end of the experiment a higher MIC, and also a higher subinhibitory range, for cefaclor than for cephalexin. In our opinion the absolute inhibitory concentration after eight hours should be the criterion used to evaluate the effectiveness of cefaclor and cephalexin, and not the MIC which is usually used. The eight-hour criterion should also be applied in clinical use.