What is Haemophilus influenzae?

Updated: Jun 11, 2021
  • Author: Joseph Adrian L Buensalido, MD; Chief Editor: Pranatharthi Haran Chandrasekar, MBBS, MD  more...
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Haemophilus influenzae is a small (1 µm X 0.3 µm), pleomorphic, gram-negative coccobacillus. It is a nonmotile, non–spore-forming, fastidious, facultative anaerobe. Some strains of H influenzae possess a polysaccharide capsule. These strains are serotyped into 6 different types (a-f) based on their biochemically different capsules. Some strains have no capsule and are termed nonencapsulated H influenzae or nontypeable H influenzae (NTHi). The different strains can be identified with slide agglutination for serotyping or polymerase chain reaction (PCR) for capsular typing.

The most virulent strain is H influenzae type b (Hib), with its polyribosyl ribitol phosphate (PRP) capsule. It accounts for more than 95% of H influenzae invasive diseases in children and half of invasive diseases in adults, including bacteremia, meningitis, cellulitis, epiglottitis, septic arthritis, pneumonia, and empyema. Less-common invasive Hib infections include endophthalmitis, urinary tract infection, abscesses, cervical adenitis, glossitis, osteomyelitis, and endocarditis.

The other encapsulated strains H influenzae occasionally cause invasive disease similar to that of Hib. H influenzae type A (Hia) has been known to cause invasive disease (eg, meningitis) clinically indistinguishable from that caused by Hib. In a retrospective study in Canada conducted from 2000-2010, of the 130 H influenzae infections reported, 56% were Hia. Meningitis, bacteremia, and pneumonia were the most common clinical presentations. [2]

The nonencapsulated, or NTHi, strains cause mucosal infections, including otitis media, conjunctivitis, sinusitis, bronchitis, and pneumonia. Less commonly, these strains cause invasive disease in children but account for half of the invasive infections in adults. The population structure of NTHi demonstrates substantial genetic diversity, as opposed to the clonal nature of Hib. [3] Furthermore, the outer membrane proteins of NTHi show high strain-to-strain variability, making vaccine development a challenge. [4, 2]

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