Bordetella species, particularly B. pertussis, are significant pathogens responsible for whooping cough, a highly contagious respiratory disease. Understanding their characteristics, pathogenic mechanisms, and effective laboratory diagnosis is crucial for managing infections caused by Bordetella. Ongoing vaccination efforts and public health strategies are essential to control and prevent the spread of this disease.
General Character
- Genus: Bordetella
- Key Species:
- Bordetella pertussis (causes whooping cough)
- Bordetella parapertussis (causes a milder form of whooping cough)
- Bordetella bronchiseptica (associated with respiratory infections in animals and humans)
- Family: Bordetellaceae
- Gram Staining: Bordetella species are Gram-negative, appearing pink due to their thin peptidoglycan layer and outer membrane.
- Shape and Arrangement:
- Shape: Small coccobacilli (short rod-shaped).
- Arrangement: Typically found as single cells or in small clusters.
- Oxygen Requirements: Bordetella species are obligate aerobes.
Morphology
- Cell Wall Structure:
- Composed of a thin peptidoglycan layer and an outer membrane with lipopolysaccharides (LPS), contributing to its virulence.
- Non-motile: Bordetella species are non-motile and do not have flagella.
Cultural Characteristics
- Growth Media:
- Bordet-Gengou Agar: A selective medium for isolating B. pertussis, enriched with blood or glycerol.
- Regan-Lowe Medium: Another selective medium that contains charcoal and sheep blood.
- Colony Appearance:
- On Bordet-Gengou agar, colonies appear small, pearly, or silver-like.
- Temperature and pH Range:
- The optimal growth temperature is around 35-37°C, with a favourable neutral pH.
Biochemical Reactions
- Catalase Test: Bordetella species are catalase-positive.
- Oxidase Test: Bordetella species are oxidase-positive.
- Lactose Fermentation: Bordetella does not ferment lactose.
- Urease Test: Bordetella species are urease-negative.
Pathogenicity
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Virulence Factors:
- Pertussis Toxin (PT): A major virulence factor that disrupts cellular signalling and immune responses.
- Filamentous Hemagglutinin (FHA): A surface protein that facilitates adherence to respiratory epithelial cells.
- Tracheal Cytotoxin: Damages ciliated epithelial cells, contributing to the characteristic cough.
- Endotoxin: The lipopolysaccharide component can induce inflammatory responses.
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Clinical Infections:
- Whooping Cough (Pertussis): Characterized by severe coughing fits, often followed by a “whooping” sound as the person breathes in. It can lead to complications, especially in infants, such as pneumonia, seizures, and even death.
- Bordetella parapertussis: Causes a milder respiratory illness with symptoms similar to whooping cough.
- Bordetella bronchiseptica: Primarily associated with respiratory infections in animals but can also cause infections in humans, particularly those with underlying conditions.
Laboratory Diagnosis
- Specimen Collection: Clinical specimens typically include nasopharyngeal swabs or aspirates.
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Microscopic Examination:
- Gram staining may reveal small Gram-negative coccobacilli.
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Culture Techniques:
- Inoculation on selective media (e.g., Bordet-Gengou or Regan-Lowe agar) followed by incubation for several days.
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Serological Testing:
- Antibodies can be detected, though it is less commonly used than culture or PCR.
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Molecular Methods:
- PCR is often employed to detect B. pertussis DNA rapidly.
Antibiotic Resistance
- Emergence of Resistance: Generally, Bordetella species are sensitive to macrolides (like azithromycin) and other antibiotics.
- Treatment Options: Treatment typically includes macrolides (azithromycin or erythromycin) or trimethoprim-sulfamethoxazole.
Prevention
- Vaccination: The DTaP (diphtheria, tetanus, and pertussis) vaccine effectively prevents whooping cough, especially in children. The Tdap booster is recommended for adolescents and adults.
- Hygiene Practices: Good hygiene and respiratory etiquette can help reduce transmission.
- Monitoring and Outbreak Control: Public health measures are essential in managing outbreaks, particularly in communities with low vaccination rates.