The skin of the normal ear canal is not sterile. The normal flora consists primarily of Gram positive bacteria, but numbers are low. Yeasts are also present in small numbers. Therefore bacteria and yeasts are not primary factors in the development of otitis, since they will not cause otitis in normal ears.
However, changes in the microenvironment of the ear canal can result in overgrowth of these commensal microbes or a shift to non-commensal pathogenic Gram negative bacteria. If this happens, bacterial and yeast infections can act as secondary causes of otic inflammation.
A shift to Gram negative organisms such as Pseudomonas is most commonly associated with chronic otitis externa. Stopping acute cases from becoming chronic, via correct initial treatment and ongoing preventative management, can help to prevent infection with this difficult-to-treat bacterium.
This table outlines the prevalence of different micro-organisms isolated from over 3,200 canine ears affected by otitis externa in 15 different clinical studies.2
Staphylococci and Malassezia were the most common microbes isolated. Gram negative organisms, such as Pseudomonas, were isolated much less frequently.
2 = Harvey, R.G. et al. Ear Diseases of the Dog and Cat (2001); 37. Manson Publishing