In vitro activity of cefaclor against aerobic and anaerobic bacteria

Cefaclor (CCL), a new cephalosporin, was tested in vitro against 602 (271 anaerobic and 331 aerobic) clinical isolates in comparison with cephalothin, cefazolin, cephradine, and cefamandole. Sixteen micrograms of CCL per ml inhibited 68% of all aerobes tested and 80% of the 211 enteropathogenic organisms (Escherichia coli, Salmonella, and Shigella) isolated from cases of infantile diarrhoea. CCL inhibited 88% of gram-positive anaerobic cocci and 72% of Bacteroides other than B. fragilis at a concentration of 16 mug/ml. B. fragilis and Clostridia were resistant to CCL. Increased inoculum of E. coli from 10(5) to 10(9) increased the minimal inhibitory concentration of CCL and cefamandole by fourfold against 7 of the 64 strains tested. All seven were beta-lactamase negative. No antimicrobial synergism was noted between CCL and penicillin. The in vitro efficacy of CCL, an oral cephalosporin, against enteropathogenic E. coli, if proven safe, may be tested in vivo against such infections.