An elderly diabetic consults a physician because of severe earache. Examination demonstrates foul-smelling purulent otorrhea and a red mass lesion of the external ear canal. Biopsy of the mass demonstrates granulation tissue. Most likely causative organism is?

External otitis, or infection of the external ear canal, can be caused by a variety of organisms, notably including Escherichia coli, Pseudomonas aeruginosa, Proteus vulgaris, and Staphylococcus aureus. There is, however, a severe subtype of external otitis, malignant external otitis, of which you should be aware. This form is specifically caused by Pseudomonas aeruginosa, and tends to affect elderly diabetics and AIDS patients, causing the findings illustrated in the question stem. It is paicularly worrisome both because the Pseudomonas organism is so tissue destructive and because it is often highly resistant to most intravenous antibiotics. (Consult your local microbiology or pharmacology depaments for advice about local sensitivities if you encounter the condition.) Complications can be devastating, including deafness, facial nerve paralysis, osteomyelitis, and meningitis. Escherichia colic a cause both external otitis and acute otitis media, but does not usually cause malignant external otitis. Haemophilus influenzae is an impoant cause of acute otitis media. Proteus Vulgaris can cause external otitis but does not usually cause the malignant form. Staphylococcus aureus can cause external otitis but does not usually cause the malignant form.