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Until about 10 years ago, the gram-negative diplococcus Moraxella catarrhalis (formerly Branhamella catarrhalis and Neisseria catarrhalis) was considered a nonpathogenic resident of the upper respiratory tract. Now the organism's ability to cause significant lung, ear, and sinus infections has become clear, and clinical laboratories routinely report its presence.
To determine when M. catarrhalis should be treated as a pathogen, Australian investigators compiled clinical data on 212 outpatients from whom M. catarrhalis was isolated by a single laboratory. They classified the isolates as pathogenically significant if they met the following criteria: positive evidence of an infection consistent with an M. catarrhalis-associated disease; isolati…