Antibiotic susceptibility of Moraxella catarrhalis biofilms in a continuous flow model.

Antibiotic susceptibility of Moraxella catarrhalis biofilms was assessed using a Sorbarod filter continuous flow model. Ceftriaxone, erythromycin, amoxicillin, and Augmentin produced significant decreases in both biofilm and planktonic viable cell populations collected from the effluent. Augmentin produced the greatest reduction in biofilm (2.5 orders of magnitude) and planktonic populations (4 orders of magnitude). However, the minimum biofilm eradication concentration was not reached within the concentration range tested (4-64 mg/L), despite demonstrable susceptibility in standard microdilution tests (minimum bactericidal concentrations [MBC] ≤0.06 mg/L). Antibiotic tolerance of M. catarrhalis biofilm populations was partly due to an inoculum effect and partly inherent. Amoxicillin had no effect against a β-lactamase-producing M. catarrhalis. Compared to batch-grown cells, planktonic cells recovered from the Sorbarod filter effluent were more resistant to the antibiotics tested (MBC ≤0.06 and >64 mg/L, respectively). Overall, the findings may explain the lack of response of some M. catarrhalis infections to antimicrobial therapy.