influenzae

Influenzae most commonly refers to Haemophilus influenzae, a small, Gram-negative coccobacillus in the family Pasteurellaceae. It is a human-restricted bacterium that colonizes the upper respiratory tract and can cause mucosal and invasive infections. Growth in culture requires X (hemin) and V (nicotinamide adenine dinucleotide); on chocolate agar it grows readily, whereas routine blood agar is insufficient unless supplemented.

Encapsulated strains (types a–f) include type b (Hib), historically a leading cause of meningitis, epiglottitis, and septicemia in children. Widespread Hib vaccination with conjugate vaccines has dramatically reduced invasive Hib disease. Non-typable, unencapsulated strains are common colonizers and are often implicated in otitis media, sinusitis, bronchitis, and COPD exacerbations.

Haemophilus influenzae produces virulence factors such as a polysaccharide capsule (PRP in Hib) and surface structures including lipooligosaccharide (LOS) and adhesins; IgA protease contributes to mucosal colonization. Transmission occurs via respiratory droplets; risk increases in young children and in settings with close contact. Antibiotic susceptibility varies; Hib disease is typically treated with beta-lactams or third-generation cephalosporins, depending on susceptibility, with beta-lactamase producing strains requiring alternative therapy.

Prevention relies on Hib vaccination; routine childhood immunization has markedly reduced invasive disease. Prophylaxis for close contacts of an invasive Hib case with rifampin is recommended in many guidelines.

Historically, Haemophilus influenzae was first described by Richard Pfeiffer in 1892 during an influenza pandemic; despite its name, it is not the cause of influenza.