Haemophilus influenzae

Haemophilus influenzae is a small non-motile gram negative bacillus most commonly found in the upper respiratory tract. Its shape on Gram stain can vary from coccobacilli to filamentous rods. The name Haemophilus translates as ‘blood-loving’ and is derived from the organism’s growth requirement for both hemin and NAD (also known as X and V factor) that are acquired from red blood cells. Colonization of the upper respiratory tract with H. influenzae occurs in early childhood.

Pathogenic Haemophilus influenzae strains are frequently mucoid & encapsulated, and can be typed as one of six serotypes (a-f) based on the polysaccharide capsule. Serious invasive infections in young children due to serotype b have been largely eliminated due to use of conjugate Hib vaccine. Other strains of H. influenzae, termed ‘nontypeable’ due to lack of a polysaccharide capsule, have emerged as major pathogens in recent years. Unlike type b strains, which enter the blood stream, nontypeable strains cause disease by local invasion of mucosal surfaces. Hence, these strains are most frequently associated with conjunctivitis, otitis media, exacerbations of COPD, community-acquired pneumonia and sinusitis. Neonatal and maternal sepsis occur less commonly but have an overall mortality rate of 50%. The causative nontypeable strain of these infections (biotype IV) is also associated with tubo-ovarian abscess and salpingitis. Nontypeable H. influenzae strains are less frequently isolated in other invasive adult infections including bacteremia and meningitis.

Overall, approximately 30% of Haemophilus influenzae strains produce beta-lactamase, so physicians treating patients empirically should consider using anti-microbials which maintain activity in the presence of this enzyme. All H. influenzae reported by Microbiology include a beta-lactamase result, and strains causing invasive infections have full susceptibility testing performed.

Written July 8, 2010

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