Haemophilus species, particularly H. influenzae, are significant pathogens responsible for serious infections, especially in children. Understanding their characteristics, pathogenic mechanisms, and effective laboratory diagnosis is crucial for managing infections caused by Haemophilus. Ongoing surveillance and vaccination efforts are essential for preventing these infections
General Character
- Genus: Haemophilus
- Key Species:
- Haemophilus influenzae (causes respiratory infections and meningitis)
- Haemophilus ducreyi (causes chancroid)
- Haemophilus aegyptius (associated with conjunctivitis)
- Family: Pasteurellaceae
- Gram Staining: Haemophilus species are Gram-negative bacteria, appearing pink due to their thin peptidoglycan layer and outer membrane.
- Shape and Arrangement:
- Shape: Rod-shaped (bacilli) and sometimes coccobacilli.
- Arrangement: Typically found as single cells or in small clusters.
- Oxygen Requirements: Haemophilus species are facultative anaerobes.
Morphology
- Cell Wall Structure:
- Composed of a thin peptidoglycan layer and an outer membrane containing lipopolysaccharides (LPS).
- Capsule: Some strains of H. influenzae have a polysaccharide capsule that enhances virulence.
Cultural Characteristics
- Growth Media:
- Chocolate Agar: Enriched medium that supports the growth of Haemophilus species; the blood is lysed, providing necessary growth factors.
- Xylose Lysine Deoxycholate (XLD) Agar: Less commonly used for isolation.
- Colony Appearance:
- On chocolate agar, H. influenzae appears as small, greyish colonies that may be slightly mucoid.
- Temperature and pH Range:
- The optimal growth temperature is around 35-37°C, with a neutral pH.
Biochemical Reactions
- Catalase Test: Haemophilus species are catalase-positive.
- Oxidase Test: Haemophilus species are oxidase-positive.
- Lactose Fermentation: Haemophilus does not ferment lactose.
- Growth Factors: H. influenzae requires factors V (nicotinamide adenine dinucleotide, NAD) and X (hematin) for growth.
Pathogenicity
- Virulence Factors:
- Capsule: The polysaccharide capsule protects against phagocytosis.
- Adhesins: Facilitate attachment to respiratory epithelial cells.
- Endotoxin: The lipopolysaccharide component of the outer membrane can provoke strong immune responses.
- Clinical Infections:
- Haemophilus influenzae type b (Hib): Causes invasive diseases, particularly in children, including:
- Meningitis
- Epiglottitis
- Pneumonia
- Osteomyelitis
- H. ducreyi: Causes chancroid, a sexually transmitted infection characterized by painful ulcers.
- H. aegyptius: Associated with conjunctivitis and other infections.
- Haemophilus influenzae type b (Hib): Causes invasive diseases, particularly in children, including:
Laboratory Diagnosis
- Specimen Collection: Clinical specimens may include blood, cerebrospinal fluid (CSF), or respiratory secretions.
- Microscopic Examination:
- Gram staining reveals Gram-negative coccobacilli.
- Culture Techniques:
- Inoculation on chocolate agar or other enriched media, followed by incubation in a CO₂-enriched atmosphere.
- Biochemical Testing:
- Confirmatory tests for catalase and oxidase positivity and growth factor requirements help identify Haemophilus species.
- Serotyping: Antigenic testing can differentiate between strains, particularly for H. influenza.
Antibiotic Resistance
- Emergence of Resistance: Some strains of H. influenzae have shown resistance to beta-lactams and other antibiotics.
- Treatment Options: Depending on the resistance profile, most infections caused by H. influenzae can be treated with antibiotics such as amoxicillin-clavulanate or ceftriaxone.
Prevention
- Vaccination: The Hib vaccine effectively prevents infections caused by H. influenzae type b, significantly reducing the incidence of invasive diseases in children.
- Hygiene Practices: Good hygiene and respiratory etiquette can help reduce transmission, particularly in crowded settings.