Comparative in vitro activity of newer cephalosporins against anaerobic bacteria.

The in vitro susceptibilities of 408 recent clinical isolates of anaerobic bacteria against cefaclor, cephalexin, cephalothin, cefazolin, cefamandole, and cefoxitin were compared by an agar dilution technique. Against gram-positive bacteria, especially peptococci, peptostreptococci, and propionibacteria, cephalexin and cefaclor were significantly less active than cephalothin (P < 0.05). Cephalexin was also less active than cephalothin against clostridia and lactobacillus (P < 0.05). Against gram-negative bacteria, major differences were observed primarily with Bacteroides fragilis, against which cephalexin, cefazolin and cefoxitin were all significantly more active than cephalothin (P < 0.001). At concentrations of 16 mug per ml, however, all cephalosporins showed high in vitro activity, except against Lactobacillus species and B. fragilis. Cephalothin, cefazolin and cefamandole were considerably more active against the former, whereas cefoxitin was distinctly more active against the latter.