Variable in vitro activity of cefaclor, cephalothin and cefadroxil against Escherichia coli, Klebsiella pneumoniae and Proteus mirabilis.

The in vitro activity of cefaclor, cephalothin and cefadroxil to 50 Escherichia coli, 23 Klebsiella pneumoniae and 20 Proteus mirabilis strains was tested with the disc diffusion and microdilution plate techniques. Cefaclor was more active than cefadroxil, while cephalothin was intermediate. The differences seriously affected the classification into the routine SIR system. For susceptibility testing we question the use of cephalothin as a class representative of oral cephalosporins.