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?

**Core Concept:** Ampicillin is a beta-lactam antibiotic that inhibits bacterial cell wall synthesis by binding to penicillin-binding proteins (PBPs). Moraxella catarrhalis, the causative organism in this scenario, can develop resistance to beta-lactam antibiotics through various mechanisms, including the production of beta-lactamases.

**Why the Correct Answer is Right:** Moraxella catarrhalis produces an enzyme called beta-lactamase, which breaks down beta-lactam antibiotics like ampicillin, rendering them inactive. In this case, the beta-lactamase is responsible for the resistance of M. catarrhalis to ampicillin.

**Why Each Wrong Option is Incorrect:**

A. Penicillinase (also known as beta-lactamase) is a type of beta-lactamase that breaks down penicillins, including ampicillin. This option is incorrect because it is a specific type of beta-lactamase and not the enzyme responsible for the resistance in this scenario.

B. Ampicillinase is not a recognized term or enzyme; it would be incorrect to choose this option.

C. The correct answer, beta-lactamase, is responsible for the resistance, making this option incorrect as well.

D. Ampicillin resistance through other mechanisms, such as efflux pumps or target modification, is not addressed by this explanation.

**Clinical Pearl:** Beta-lactamase production is a common mechanism for bacterial resistance to beta-lactam antibiotics, including ampicillin. Recognizing and understanding beta-lactamase enzymes is crucial for choosing appropriate antibiotics in clinical practice.

✓ Correct Answer: B. b-Lactamase