The frequency of BRO β-lactamase and its relationship to antimicrobial susceptibility and serum resistance in Moraxella catarrhalis

Abstract We investigated the frequency of BRO β-lactamase and its relationship to antibiotic susceptibility profiles and serum susceptibility. Moraxella catarrhalis clinical isolates ( n = 197) were collected from patients with respiratory tract infections in Tokyo between November 2004 and April 2005. Phenotypic and genotypic detection of β-lactamases was performed. The MICs of 6 antibiotics were determined by Etest, and the serum bactericidal assay was conducted by using the culture-and-spot test. Nearly all (192; 97%) of the clinical isolates were β-lactamase producers; of these, 182 (95%) were bro-1 and 10 (5%) were bro-2 positive. MIC50 , MIC90 , and geometric mean MICs of penicillin, amoxicillin, cefixime, and clarithromycin for BRO-1 isolates were significantly higher than for BRO-2 isolates. The frequency of intermediate and full serum resistance was significantly higher in BRO-1 isolates than in BRO-2 isolates ( P = 0.0056), but not BRO-negative isolates ( P = 0.1333). We provide the first evidence that the presence of BRO-1 in M. catarrhalis is associated with reduced susceptibility to clarithromycin and β-lactam antibiotics, as well as serum non-sensitive (intermediate and resistant).