The microbial environment of the ear canal in health and disease.

Otitis externa is an important disease of dogs and, to a lesser degree, cats. The yeast Malassezia canis is the most common organism demonstrated in ear infections. Coagulase-positive staphylococci are the most common bacteria isolated, often occurring with M. canis. Pseudomonas aeruginosa and Proteus are gram-negative bacteria that are frequently encountered. Other bacteria, such as beta-hemolytic streptococci, enterococci, Escherichia coli, and Corynebacterium are also important. Malassezia canis, coagulase-positive staphylococci, and Pseudomonas aeruginosa commonly occur as monoinfections, whereas other bacteria are typically associated with mixed infections. Smears may provide rapid diagnosis on which to base empiric treatment. Cultures should be considered in recurrent or refractory cases, especially those involving gram-negative bacteria. In practices that are not equipped to perform culture and susceptibility tests, it is advisable to seek the advice of a microbiology laboratory.