A previously healthy 18-month-old girl is brought to the office with 2 days of irritability, poor appetite, and pulling at her left ear. She has no known allergies, and her temperature is 102.8 F. She is easily consoled by the mother and moves her neck spontaneously without discomfo. There is a clear discharge from the nares. The left tympanic membrane is erythematous, dull, and bulging. Which of the following virulence factors is generally absent in the strains of the causative organism that produce otitis media, compared with those that produce epiglottitis or meningitis?

This is most likely a case of Haemophilus influenzae otitis media. 95% of all cases of invasive disease (epiglottitis, meningitis) due to H. influenzae are caused by type b organisms that possess a polyribitol phosphate capsule. Otitis media is generally not caused by type b organisms. Beta-Lactamase is an impoant pathogenic feature of Moraxella catarrhalis, which is another impoant cause of otitis media, but would not be an agent of epiglottitis or meningitis. IgA protease is produced by Streptococcus pneumoniae and Neisseria meningitidis. Both of these cause meningitis, but not as commonly in this age group, and would not be the most common causes of otitis media in this case. Lipopolysaccharide (choice C) is present in all gram-negative bacteria and would not be a distinguishing feature between those that cause otitis media and epiglottitis. Ref: Brooks G.F. (2013). Chapter 18. Haemophilus, Bordetella, Brucella, and Francisella. In G.F. Brooks (Ed), Jawetz, Melnick, & Adelberg's Medical Microbiology, 26e.