General Characteristics and Cultivation
The genus name Haemophilus derives from the Greek words haima (blood) and philos (loving). This name perfectly describes the primary biological feature of these microorganisms—a high requirement for specific blood factors to sustain life.
Morphologically, members of the genus are small Gram-negative bacteria. They are completely incapable of growing on simple nutrient media. Successful laboratory cultivation strictly requires enriched media containing whole or lysed blood, as well as its derivatives. In nature, these bacteria are widely distributed, and many species are typical inhabitants of the normal human mucous membranes.
Growth Factors and Species Differentiation
The key method for identifying different species within the genus is evaluating their requirement for specific growth factors and their ability to cause hemolysis. The primary growth factors are:
- X-factor — hemin.
- V-factor — nicotinamide adenine dinucleotide (NAD).
| Bacterial Species | X-Factor | V-Factor | Hemolysis |
|---|---|---|---|
| H. influenzae (and H. aegyptius) | + | + | Absent |
| H. parainfluenzae | - | + | Absent |
| H. ducreyi | + | - | Absent |
| H. haemolyticus | + | + | + |
| H. parahaemolyticus (non-pathogenic) | - | + | + |
Medical Significance of Key Pathogens
The medical importance of Haemophilus bacteria is determined by their capacity to cause several dangerous diseases. The primary pathogen is considered to be Haemophilus influenzae, particularly its serotype b (Hib). This agent is transmitted via the respiratory route and causes severe infections such as meningitis, bronchitis, and sinusitis.
Of particular note is H. aegyptius, historically known as the Koch-Weeks bacillus (isolated by Robert Koch in Egypt in 1883). It can cause two clinically polar presentations in terms of severity:
- Purulent conjunctivitis. Characterized by high contagiousness and a short incubation period (1–3 days). It is transmitted primarily by contact (via unwashed hands, towels, cosmetics, contact lenses) and less frequently via the respiratory route.
- Brazilian purpuric fever. A severe systemic infection in children with a respiratory mechanism of transmission. It presents with fever, shock, and purpura (widespread hemorrhages), frequently resulting in a fatal outcome.
Other Notable Haemophilus Species
Beyond the classic pathogens, the genus includes other microorganisms worthy of clinical attention:
- H. parainfluenzae — a common resident of the respiratory tract, largely similar to H. influenzae. In immunocompromised states, it can act as a causative agent of pneumonia, urethritis, and infective endocarditis.
- H. haemolyticus — a member of the normal nasopharyngeal flora that is occasionally considered a potential pathogen for upper respiratory tract infections in children.
- H. ducreyi — features a completely different pathogenicity profile. This microorganism is a sexually transmitted pathogen causing chancroid.