Effect of inoculum size and beta-lactamase production on in vitro activity of new cephalosporins against Haemophilus species

Sixty-three strains of Haemophilus species, 38 of which were beta-lactamase producers (37 H. influenzae type b, 1 H. parainfluenzae) and 25 of which were beta-lactamase negative (20 H. influenzae, 5 H. parainfluenzae), were tested for susceptibility to cefoxitin, moxalactam (LY127935) (Lilly), cefsulodin (CGP 7174 E, Ciba), and cefoperazone (T 1551, Pfizer). Cefsulodin was relatively inactive at both low and high inocula. LY127935 and cefoperazone displayed inoculum-dependent bactericidal activity. Cefoxitin displayed little inoculum effect against beta-lactamase-producing strains: 8 and 16 microgram/ml killed at least 90% of those tested at 10(4) and 10(6) colony-forming units per ml, respectively.