Haemophilus influenzae

Haemophilus influenzae
Name	Haemophilus influenzae
Domain	Bacteria
Phylum	Pseudomonadota
Class	Gammaproteobacteria
Order	Pasteurellales
Family	Pasteurellaceae
Genus	Haemophilus
Species	H. influenzae
Binomial	Haemophilus influenzae
Binomial authority	(Lehmann & Neumann 1896) Winslow et al. 1917

Haemophilus influenzae is a small, non-motile, Gram-negative bacterium first isolated during the influenza pandemic of 1892, which led to its misleading name. It is a significant human pathogen, particularly known for causing severe invasive diseases like meningitis and epiglottitis in young children. The bacterium is classified into encapsulated (typeable) and non-encapsulated (non-typeable) strains, with the former possessing a polysaccharide capsule that is a major virulence factor. Its discovery and subsequent study have been pivotal in the fields of bacteriology and vaccinology.

Microbiology and classification

H. influenzae is a fastidious, facultative anaerobe that requires specific growth factors, namely hemin (X factor) and nicotinamide adenine dinucleotide (V factor), for cultivation in the laboratory. It is a coccobacillus and is classified within the family Pasteurellaceae. The species is subdivided based on the presence and serology of its polysaccharide capsule; six serotypes (a through f) were historically defined, with serotype b (Hib) being the most clinically significant. Non-typeable strains lack this capsule entirely. Key taxonomic studies were advanced by Margaret Pittman at the National Institutes of Health, who clarified the role of the capsule in pathogenicity. The bacterium's genome was fully sequenced at The Institute for Genomic Research in Rockville, Maryland, providing insights into its metabolism and evolution.

Pathogenesis and virulence factors

The primary virulence factor for invasive disease is the polyribosylribitol phosphate (PRP) capsule of type b strains, which confers resistance to phagocytosis and complement-mediated killing. Non-typeable strains rely on other adhesins and invasins, such as pili, outer membrane proteins like P2, and proteins Hia and Hap, to colonize the respiratory tract mucosa. The bacterium can produce beta-lactamase, an enzyme that confers resistance to penicillin and ampicillin by hydrolyzing the beta-lactam ring. During infection, it induces a significant inflammatory response, contributing to tissue damage, particularly in conditions like meningitis and cellulitis. Research into its pathogenesis has been conducted at institutions like the Centers for Disease Control and Prevention and the World Health Organization.

Clinical significance and disease spectrum

H. influenzae is a major cause of both invasive and non-invasive disease, with a spectrum that varies by strain type and host age. Invasive Hib disease, now rare in vaccinated populations, classically presented as meningitis, epiglottitis, septic arthritis, and pneumonia in children under five. Non-typeable strains are common causes of localized respiratory infections, including otitis media, sinusitis, and exacerbations of chronic obstructive pulmonary disease in adults. It is also an important agent of conjunctivitis in children and can cause bacteremia, particularly in elderly or immunocompromised individuals. The clinical presentation of Hib meningitis was a major driver for vaccine development in the late 20th century.

Diagnosis and laboratory identification

Diagnosis relies on the isolation and identification of the bacterium from clinical specimens such as cerebrospinal fluid, blood, or sputum. It grows on enriched media like chocolate agar in a 5-10% carbon dioxide atmosphere, showing satelliting around colonies of Staphylococcus aureus on blood agar due to the release of V factor. Definitive identification involves demonstrating the requirement for both X and V factors, often using strips or disks. Serotyping of encapsulated strains is performed using slide agglutination tests with specific antisera. Techniques like polymerase chain reaction and latex agglutination for antigen detection in cerebrospinal fluid are also used, especially for rapid diagnosis of meningitis.

Treatment and prevention

Empiric treatment for serious invasive disease typically involves a third-generation cephalosporin such as ceftriaxone or cefotaxime, due to widespread ampicillin resistance from beta-lactamase production. For non-invasive infections like otitis media, amoxicillin-clavulanate or advanced macrolide antibiotics may be used. The cornerstone of prevention is vaccination with the Hib conjugate vaccine, which has dramatically reduced the incidence of invasive Hib disease since its introduction. This vaccine, developed through work at institutions like the National Institute of Allergy and Infectious Diseases, is part of routine childhood immunization schedules globally, as recommended by the Advisory Committee on Immunization Practices and the World Health Organization.

Epidemiology and vaccination

Prior to the introduction of effective vaccines in the late 1980s and early 1990s, Hib was a leading cause of bacterial meningitis in children under five worldwide, with high incidence in regions like Sub-Saharan Africa and among certain populations like the Navajo Nation. The Hib conjugate vaccine, introduced in the United States in 1987, has reduced the incidence of invasive Hib disease by over 99% in vaccinated populations. Current epidemiology shows a shift; non-typeable strains now cause a greater proportion of H. influenzae disease, particularly in adults. Global vaccination efforts are coordinated by entities like the World Health Organization and the Gavi, the Vaccine Alliance, though vaccine coverage remains variable in some developing countries.

Category:Bacteria Category:Pathogenic bacteria