General characteristics and cultivation
The genus name Haemophilus comes from the Greek words haima (blood) and philos (loving). This name perfectly describes the main biological feature of these microorganisms — their high demand for specific blood factors to support life.
Morphologically, representatives of the genus are small Gram-negative bacteria. They are completely incapable of growing on simple nutrient media. For their successful laboratory cultivation, enriched media containing whole or lysed blood, as well as its derivatives, are strictly required. In nature, these bacteria are widespread, and many of their species are typical members of the normal microflora of human mucous membranes.
Growth factors and species differentiation
The key method for identifying various species within the genus is assessing their requirement for specific growth factors and their ability to cause hemolysis. The main 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 (nonpathogenic) | - | + | + |
Medical significance of key pathogens
The medical significance of Haemophilus bacteria is determined by their ability to cause a number of dangerous diseases. The main pathogen is considered to be Haemophilus influenzae, particularly its serotype b (Hib). This causative agent is transmitted via the respiratory route and causes severe infections such as meningitis, bronchitis, and sinusitis.
Special attention should be paid to H. aegyptius, historically known as the Koch-Weeks bacillus (isolated by Robert Koch in Egypt in 1883). It is capable of causing two clinical conditions with polar severity:
- Purulent conjunctivitis. It is characterized by high contagiousness and a short incubation period (1–3 days). It is transmitted mainly by contact (via dirty hands, towels, cosmetics, contact lenses), and less frequently via the respiratory route.
- Brazilian purpuric fever. This is a severe systemic infection in children with a respiratory mechanism of transmission. It manifests as fever, shock, and purpura (multiple hemorrhages), frequently leading to a fatal outcome.
Other significant species of the genus Haemophilus
In addition to classical pathogens, the genus includes other microorganisms that deserve the attention of physicians:
- H. parainfluenzae is a common resident of the respiratory tract, very similar to H. influenzae. When immunity is compromised, it can act as the causative agent of pneumonia, urethritis, and infective endocarditis.
- H. haemolyticus — a member of the normal nasopharyngeal flora that is sometimes considered a potential causative agent of upper respiratory tract infections in children.
- H. ducreyi — has a completely different pathogenicity profile. This microorganism is the causative agent of a sexually transmitted infection and causes the disease known as chancroid.
Briefly: answers to topical questions
What diseases are caused by Haemophilus influenzae serotype b (Hib)?
Haemophilus influenzae serotype b (Hib) causes respiratory and generalized infections. The main diseases include:
- Respiratory tract and ENT infections — sinusitis, bronchitis, pneumonia, pharyngitis, epiglottitis, and otitis media.
- CNS disorders — purulent meningitis.
- Generalized forms — septicemia, septic arthritis, and endocarditis.
What virulence factors does Haemophilus influenzae possess?
For Haemophilus influenzae, sources indicate the capsule as a factor associated with the development of generalized forms of infection and purulent meningitis: encapsulated variants are able to cross the blood-brain barrier. In addition, the capsular antigen of the pathogen is used in rapid diagnostics and vaccination.
On which specific nutrient media are Haemophilus bacteria cultivated?
Haemophilus bacteria are cultivated on enriched nutrient media containing blood or its derivatives, as they require X (hemin) and V (NAD) growth factors.
- Chocolate agar — is the medium of choice, prepared by heating blood agar to 80 °C.
- Blood agar — growth is scant, possible with the addition of sheep blood or around colonies of other bacteria (satellite phenomenon).
- Chocolate agar with vancomycin — used for the isolation of Haemophilus ducreyi.
What is the detailed morphology of bacteria of the genus Haemophilus?
Bacteria of the genus Haemophilus are small Gram-negative bacteria. They can be coccoid or rod-shaped and are non-motile.
What specific prophylaxis is used against Haemophilus influenzae type b infection?
Specific prophylaxis of infection caused by Haemophilus influenzae type b is carried out using active and passive immunization.
- Active immunization — conjugated Hib vaccines containing the capsular antigen of the pathogen are used. They are often part of combination preparations together with DTP components, polio, and hepatitis B vaccines.
- Passive immunization — donor sera with a high concentration of IgM are used. Indicated for individuals with immunodeficiencies and children with a weak immune response.
To remember factor requirements: species with the prefix "para" (H. parainfluenzae, H. parahaemolyticus) require only one factor from the "pair" — the V factor.
Class and exam questions
Why are bacteria called "haemophilic"?
The name comes from the words haima (blood) and philos (loving). It reflects the absolute requirement of these bacteria for blood factors (X and V) for growth and reproduction.
On which growth media are members of the genus Haemophilus cultivated?
These bacteria do not grow on simple nutrient media. Their cultivation strictly requires enriched media containing whole or lysed blood and its derivatives.
What is Brazilian purpuric fever?
This is a severe systemic infection in children caused by the bacterium H. aegyptius. It is transmitted via the respiratory route, manifests with fever, shock, hemorrhages (purura), and has a very high fatality rate.
What else to study on this topic
- Pathogenicity mechanisms of Haemophilus influenzae serotype b (Hib)
- Features of the clinical course of Koch-Weeks purulent conjunctivitis
- The role of H. parainfluenzae in the development of infective endocarditis
- Clinical presentation and diagnosis of chancroid (H. ducreyi infection)
- Specifics of differential diagnosis of Haemophilus bacteria in the laboratory
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