Cefaclor appeared to be an effective antibiotic against both beta-lactamasepositive and beta-lactamase-negative strains of Haemophilus influenzae type b when tested with 105 colony-forming units per ml. With inocula in excess of 106 colony-forming units per ml, these organisms were neither inhibited nor killed at concentrations of 400 ,ug/ml. This inoculum effect was also demonstrated in timekill curve studies.Cefaclor, an oral cephalosporin derivative with antibacterial activity against Haemophilus influenzae, has become a popular therapeutic choice for otitis media and for systemic H. influenzae infection after initial parenteral treatment with other agents in children. This study demonstrates the in vitro activity of cefaclor against H. influenzae type b, including betalactamase-producing strains, the effect of inoculum size, and the growth kinetics of these organisms in the presence of cefaclor.Thirty strains of H. influenzae type b, including fifteen beta-lactamase-positive strains, were tested for susceptibility to cefaclor. Twenty-nine of these isolates were obtained from blood or cerebrospinal fluid samples from children with infection, and one was cultured from an infected eye sample. The definitions of minimal inhibitory and bactericidal concentrations and the methodology used have been described previously (1). Minimal inhibitory and bactericidal concentrations were determined for three inoculum sizes, 104, 106, and 107 colony-forming units (CFU)/ml, prepared by diluting an overnightgrowth suspension of the organisms adjusted to a McFarland no. 0.5 turbidity standard.Time-kill curves were determined with cefaclor for five beta-lactamase-positive and five beta-lactamase-negative H. influenzae type b strains with both light (104 to i05 CFU/ml) and heavy (106 to 107 CFU/ml) inocula. The organisms were grown overnight in Mueller-Hinton broth with 4% Fildes enrichment. A concentration of 2 ,ug/ml was chosen for the cefaclor because it correlated with achievable middle-ear fluid levels obtained after oral therapy (5). Each test sample (total volume, 4 ml) was incubated at 37°C in a candle jar, and 0.5-ml portions were removed at 0, 4, 8, and 24 h. These were serially diluted and streaked onto Fildes-enriched Mueller-Hinton agar. Colonies were counted after 24 h of incubation. Specimens from antibiotic-free control tubes were sampled simultaneously, and colony counts were determined in the same manner.With the standard inoculum (104 to 105 CFU/ml), 100%o of both beta-lactamase-positive and beta-lactamase-negative strains were inhibited by 1.56 pg of cefaclor per ml, and 97% were killed with 3.12
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