M catarrhalis is a gram-negative aerobic coccus morphologically and biochemically similar to Neisseria. It causes sinusitis, bronchitis, and pneumonia. Bacteremia and meningitis have also been reported in immunocompromised patients. The organism frequently colonizes the respiratory tract, making differentiation of colonization from infection difficult. If M catarrhalis is the predominant isolate, therapy is directed against it. M catarrhalis typically produces beta-lactamase and therefore is usually resistant to ampicillin and amoxicillin. It is susceptible to amoxicillin-clavulanate, ampicillin-sulbactam, trimethoprim-sulfamethoxazole, ciprofloxacin, and second- and third-generation cephalosporins.