Pseudomonas aeruginosa in Otitis Externa: A Particular Variety of the Bacteria?

BACKGROUND: Pseudomonas aeruginosa rarely affects the epithelium in healthy persons except for the external ear canal, raising the possibility that P aeruginosa in otitis externa is a specific variety that displays particular characteristics. DESIGN: A cohort study was designed to outline distinct characteristics of P aeruginosa in otitis externa compared with P aeruginosa in other infections. The study period was October 1, 1994, to March 27, 1995. PATIENTS: Isolates of P aeruginosa from nonhospitalized patients were collected at the bacteriological laboratory at Karolinska Hospital, Stockholm, Sweden; there were 53 strains of P aeruginosa isolated from otitis externa and 59 strains of P aeruginosa from varicose ulcers and urinary tract infections. METHODS: Pseudomonas aeruginosa was characterized by pigmentation, growth habits, production of mucoid, and biochemical characteristics. RESULTS: Pseudomonas aeruginosa in otitis externa produced less pyocyanin and less urease and exhibited no mucoid-producing strains. CONCLUSIONS: Pseudomonas aeruginosa in otitis externa displayed fewer of the usual biochemical features of the species than did the strains isolated from other infections. Some of these features, such as the production of pyocyanin, are influenced by nutritional factors; strains found in otitis externa probably represent the type of strains present in the natural habitat in water, as opposed to the strains that have adapted to the environment of other human infections. Increased knowledge of the characteristics of the strains found in otitis externa is important in understanding the pathogenesis of the disease and why P aeruginosa is the dominant infectious agent in otitis externa.