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A genus of gram-negative, pleomorphic bacteria that are facultative anaerobes and are nonmotile and non-spore-forming.

Haemophilus influenzae was the first of the species to be isolated and is considered the type species. It was originally recovered during the influenza pandemic of 1889 and for a time was believed to be the causative agent of influenza; thus it was called the influenza bacillus. However, when this fallacy became apparent, the organism was renamed, still reflecting the historical association with influenza.

Haemophilus species are distinguished by a number of criteria. Strains of H. influenzae can be separated into encapsulated and nonencapsulated forms. Encapsulated strains express one of six biochemically and antigenically distinct capsular polysaccharides that are designated serotypes a through f. Nonencapsulated strains are referred to as nontypable. See Influenza, Meningitis

Haemophilus influenzae is a human-specific pathogen that inhabits the upper respiratory tract and is acquired by exposure to airborne droplets or contact with respiratory secretions. Nontypable strains can be isolated from the nasopharynx of up to 80% of normal children and adults at any given time, usually in association with asymptomatic colonization. Overall, these organisms are the leading cause of exacerbations of chronic bronchitis, and the second most common etiology of acute otitis media and sinusitis. On occasion, nontypable H. influenzae causes invasive disease such as meningitis, septicemia, endocarditis, epiglottitis, or septic arthritis. Invasive disease occurs most often in neonates and in patients with underlying immunodeficiency, especially when abnormalities in humoral immunity are present.

Encapsulated strains of H. influenzae are present in the nasopharynx of only 2–5% of children and an even smaller percentage of adults. Historically, H. influenzae type b strains were the primary cause of childhood bacterial meningitis and a majority of other bacteremic diseases in children. However, in recent years the incidence of disease due to H. influenzae type b has plummeted in the United States and other developed countries, reflecting the routine use of H. influenzae conjugate vaccines. These vaccines provide effective protection against disease due to H. influenzae type b but fail to protect against non-type b strains.

Haemophilus aphrophilus, H. haemolyticus, H. parahaemolyticus, H. parainfluenzae, and H. segnis are members of the normal flora in the human oral cavity and oropharynx and have low pathogenic potential. Among these species, H. parainfluenzae is the most common pathogen and has been reported in association with a variety of diseases.

Strains of H. influenzae are increasingly resistant to a wide variety of antibiotics. Accordingly, an extended-spectrum cephalosporin is generally recommended for empiric treatment of serious disease. See Antibiotic, Drug resistance, Medical bacteriology


A genus of gram-negative coccobacilli or rod-shaped bacteria of uncertain affiliation; cells may form threads and filaments and are aerobic or facultatively anaerobic; strictly blood parasites.
References in periodicals archive ?
Haemophilus influenzae Type B Infections - Therapeutics under Investigation by Universities/Institutes 12
Non-type b Haemophilus influenzae invasive infections in North Dakota and South Dakota, 2013-2015.
Coli 5 1 Micrococcus spp 5 0 Proteus mitrabilis 3 0 Pseudomonas aeruginosa 8 4 Staphyloccoccus aureus 3 2 Staphyloccoccus epidermidis 3 7 Yeast 6 4 Alloicoccus otitis 0 3 Haemophilus influenzae 0 3 Moraxella catarrhalis 0 1 Proteus mirabilis 0 1 Streptococcus pneumoniae 0 8 Note: Table made from bar graph.
The protective level of serum antibodies to the capsular polysaccharide of Haemophilus influenzae type b.
Simultaneous detection of Neisseria meningitidis, Haemophilus influenzae, and Streptococcus pneumoniae in suspected cases of meningitis and septicemia using real-time PCR.
The report reviews pipeline therapeutics for Haemophilus influenzae Type B Infections by companies and universities/research institutes based on information derived from company and industry-specific sources
Polymorphism in the hmw1A gene was found in Haemophilus influenzae strains isolated from the nasopharynx, middle ear, pleural fluid and conjunctiva in Iran.
Appelbaum, "Antimicrobial resistance in Haemophilus influenzae," Clinical Microbiology Reviews, vol.
A review of the Haemophilus Influenzae Type b Infections products under development by companies and universities/research institutes based on information derived from company and industry-specific sources
Identify and understand important and diverse types of therapeutics under development for Haemophilus Influenzae Type b Infections.
Duration of preservation of Streptococcus pneumoniae and Haemophilus influenzae in the sputum and brochial washings of patients with acute and chronic pneumonias.
Researcher Chien-Chang Lee and his colleagues at the Harvard School of Public Health also found a possible association of second-hand smoke exposure with invasive pneumococcal disease and Haemophilus influenzae type b.

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