Moraxella catarrhalis is a component of the normal bacterial flora of the upper airways and has been increasingly recognized as a cause of otitis media, sinusitis, and bronchopulmonary infection. Over the past several decades, this organism has been variously designated as Micrococcus catarrhalis, Neisseria catarrhalis, and Branhamella catarrhalis. Bacteriology and Immunity On Gram's staining, M. catarrhalis organisms appear as gram-negative cocci, sometimes occurring in pairs and having the side-by-side kidneybean configuration of Neisseria (Fig. 138-1). These cocci tend to retain crystal violet during the decolorizing step and may be confused with Staphylococcus aureus. Moraxella colonies grow well on blood.
