Moraxella (Branhamella) catarrhalis bacteria, illustration. These are aerobic, gram-negative cocci (spherical bacteria) which are commonly found in the mucous membranes of the respiratory tract of mammals, including humans. In immunosuppressed individuals, or opportunistically, they may cause respiratory tract infections. These include sinusitis, an inflammation of the sinus membranes. This may also lead to otitis media, an infection of the middle ear. Treatment is with antibiotics.