Effect of inoculum size on results of macrotube broth dilution susceptibility tests with Branhamella catarrhalis.

The effect of inoculum size on the results of macrotube broth dilution MICs was assessed with 15 strains of Branhamella catarrhalis versus 22 antimicrobial agents. The study isolates included five beta-lactamase-negative strains, five strains that produced beta-lactamase and for which ampicillin MICs were high, and five beta-lactamase-producing strains for which ampicillin MICs were low. An inoculum effect was observed only with strains in the last group when tested versus the following antimicrobial agents: penicillin G, ampicillin, cephalothin, cefamandole, cefuroxime, and cefaclor.