A 2-year-old had been treated three times with ampicillin for acute otitis media. Fluid aspirated from his middle ear during placement of tympanostomy tubes grows Moraxella catarrhalis, which is resistant to ampicillin. Which enzyme is responsible for this resistance?

Most isolates of Moraxella catarrhalis, a common cause of acute otitis media, produce b-lactamase (b) making them resistant to the penicillin. They are almost all susceptible to third-generation cephalosporins, macrolides, tetracycline (contraindicated in a 2-year-old), trimethoprim/sulfamethoxazole, and ampicillin/sulbactam. Chloramphenicol acetyltransferase (a) is responsible for naturally occurring chloramphenicol resistance in bacteria. Aminoglycoside phosphotransferases (e) confer resistance to aminoglycoside antibiotics. Catalase (c) is an enzyme involved in the detoxification of the reactive oxygen species H2 O2 ; DNase (d) is an enzyme that destroys DNA. Both are produced by M. catarrhalis, but neither is involved in antibiotic resistance.