Moraxella catarrhalis: pathogenic significance in respiratory tract infections treated by community practitioners.

We prospectively studied the pathogenic significance of Moraxella (Branhamella) catarrhalis isolated from 212 patients of community practitioners in Australia. This organism was most commonly isolated during winter and early spring, and 92% of isolates were beta-lactamase producers. On the basis of predetermined clinical and microbiological criteria, 42% of the isolates were definitely pathogenic, 7% were probably pathogenic, 21% were of indeterminate pathogenicity, and 30% were nonpathogenic. Factors associated with pathogenic significance included pneumonia or bronchitis (87% of patients), predisposing respiratory or systemic conditions (62%), isolation from sputum, and pure isolation. Thirty-six percent of patients were < 5 years old, but only 9% of isolates from these patients were pathogenic or probably pathogenic, a finding that reflects the fact that nasal-swab and nasopharyngeal-aspirate sampling is a common practice. Isolates from older patients were more likely to be pathogenically significant. An assessment of the pathogenic significance of M. catarrhalis isolated from a patient in a community practice should take into consideration factors such as the patient's age, clinical illness, and underlying conditions; the presence of other organisms; and the source of the isolate.