A 73-year-old woman comes to the physician complaining of progressive, severe pain and discharge from her left ear for the past 2 days. She has had type 2 diabetes for many years and has been noncompliant with her medications and follow-up appointments. On examination, moving or touching the pinna produces extreme pain. Otoscopic examination shows granulation tissue in the left ear canal with a scant amount of discharge. The tympanic membrane is clear, and there is no middle ear effusion. Initial cultures from the ear show a Gram-negative rod. Which of the following microbiological characteristics best describes the infecting organism?

Question

A 73-year-old woman comes to the physician complaining of progressive, severe pain and discharge from her left ear for the past 2 days.  She has had type 2 diabetes for many years and has been noncompliant with her medications and follow-up appointments.  On examination, moving or touching the pinna produces extreme pain.  Otoscopic examination shows granulation tissue in the left ear canal with a scant amount of discharge.  The tympanic membrane is clear, and there is no middle ear effusion.  Initial cultures from the ear show a Gram-negative rod.  Which of the following microbiological characteristics best describes the infecting organism?

Quizplus · Accepted Answer

This patient's presentation is consistent with malignant otitis externa (MOE), a severe infection most commonly seen in elderly diabetic patients. It is most frequently caused by Pseudomonas aeruginosa, a nonlactose-fermenting, oxidase-positive, motile Gram-negative rod. Patients typically present with exquisite ear pain and drainage. The granulation tissue seen within the ear canal is an important characteristic finding of MOE, and the tympanic membrane is usually intact. Progression of this infection can lead to osteomyelitis of the skull base and cranial nerve damage. Treatment consists of systemic antibiotics such as ciprofloxacin that are effective against P aeruginosa. (Choice A) Vibrio cholerae is an oxidase-positive, Gram-negative, comma-shaped rod that can grow on an alkaline medium that kills most organisms. V cholerae causes a toxin-mediated (stimulates adenylyl cyclase) watery diarrhea. (Choice B) Gram-negative rods that are fast lactose fermenters include Klebsiella and Escherichia coli. They commonly cause urinary tract infections. (Choice D) Shigella is a Gram-negative, nonlactose-fermenting, nonmotile, rod-shaped bacterium. It causes shigellosis, a hemorrhagic diarrheal disease. (Choice E) Haemophilus influenza is Gram-negative, rod-shaped bacterium that requires factor X (hematin) and factor V (NAD+) for growth. Nontypable (unencapsulated) H influenzae is a frequent cause of acute otitis media in children and adults. Acute otitis media is associated with an inflamed, erythematous, bulging, immobile tympanic membrane due to the presence of infected fluid in the middle ear. Educational objective: Pseudomonas aeruginosa is a nonlactose-fermenting, oxidase-positive, motile, Gram-negative rod. It is the most common cause of malignant otitis externa (MOE), a serious infection of the ear seen in elderly diabetic patients. MOE presents with exquisite ear pain and drainage, and granulation tissue is often seen within the ear canal. __________ References: Malignant otitis externa. (http://www.ncbi.nlm.nih.gov/pubmed/18435997)

A 73-Year-Old Woman Comes to the Physician Complaining of Progressive