Overview
The genus Bordetella comprises small, Gram-negative, non-motile coccobacilli, containing three species of medical importance: B. pertussis, B. parapertussis, and B. bronchiseptica. Unlike Haemophilus, they do not require X and V factors for growth.
B. pertussis, the most fastidious of the three, is the primary cause of classical whooping cough (pertussis) — one of the most contagious diseases known and still a significant cause of childhood morbidity and mortality where vaccination coverage is incomplete.
Learning Objectives
After this lesson you will be able to…- Describe the morphological and cultural characteristics of Bordetella species.
- List the antigenically defined virulence factors of B. pertussis.
- Describe the three clinical stages of pertussis (whooping cough).
- Explain the specimen collection and laboratory diagnosis methods for pertussis.
- Discuss treatment and prophylaxis, including DPT and acellular vaccines.
Clinical Story
Why This MattersA 3-month-old, only partially vaccinated infant presents with two weeks of a mild cough that has progressively worsened into severe paroxysms of coughing followed by a characteristic inspiratory 'whoop', with cyanosis and vomiting after coughing fits. A pernasal swab is collected using a flexible calcium-alginate swab and plated on Bordet-Gengou medium. After 72 hours, small, dome-shaped, glistening colonies resembling 'bisected pearls' or mercury drops appear — confirming Bordetella pertussis infection, the causative agent of whooping cough.
Core Concepts
Bordetella was first identified by Bordet and Gengou. B. pertussis is a small, ovoid, Gram-negative coccobacillus (1–1.5 µm × 0.3 µm), non-motile and non-sporing. It is capsulated when freshly isolated from a clinical sample but rapidly loses this capsule on subculture. In smears, bacilli are arranged in loose clumps with clear spaces between them, producing a characteristic 'thumb print' pattern.
Bordetella species are aerobes and facultative anaerobes, growing best at 35–36°C. They are sensitive to various inhibitory substances, which is why Bordet and Gengou developed a special medium — glycerine potato blood agar — with a higher blood concentration used to neutralize inhibitory substances rather than to provide extra nutrition.
Plates are incubated for 48–72 hours because growth is slow. Colonies are small, dome-shaped, opaque, greyish, refractile and glistening, resembling 'bisected pearls' or mercury drops, surrounded by hazy haemolysis. Regan-Lowe medium (charcoal, cephalexin, amphotericin with horse blood) is a widely used selective alternative.
Unlike diphtheria and tetanus, which are single-toxin diseases, pertussis is a multitoxin disease. B. pertussis produces eight different antigenically defined virulence factors: agglutinogens, lipopolysaccharide, heat-labile toxin, tracheal cytotoxin, pertussis toxin, adenylate cyclase, filamentous haemagglutinin, haemolysin and pertactin. The major virulence factors with excellent immunogenicity are pertussis toxin, adenylate cyclase and filamentous haemagglutinin.
Pertussis is a childhood disease spread by the respiratory route, and is amongst the most contagious diseases known. B. pertussis causes 95% of cases, B. parapertussis 5%, and B. bronchiseptica 0.1%. Incubation is 1–2 weeks.
The disease progresses through three stages:
| Stage | Features |
|---|---|
| Catarrhal | Non-specific symptoms; maximal infectivity but diagnosis is difficult here — the ideal window for effective treatment is missed. |
| Paroxysmal | Violent coughing paroxysms deplete lung oxygen, followed by the characteristic inspiratory 'whoop'; cyanosis, vomiting, bulging eyes, drooling may occur. |
| Convalescent | Frequency and severity of coughing gradually decrease. Untreated illness can last 6–8 weeks total. |
Specimen collection: cough plate method, nasopharyngeal aspirate, pernasal swab, and West's post-pharyngeal swab. Swabs should be calcium alginate or Dacron on a nichrome wire — never cotton (cotton fibres are toxic to Bordetella).
Microscopy: smears stained with Gram's stain or, preferably, fluorescent antibody technique. Culture: plated on Bordet-Gengou or Regan-Lowe medium, incubated 48–72 hours; identification by biochemical tests and slide agglutination. Serology: a fourfold rise in antibody titre (agglutination, complement fixation, or immunofluorescence) is diagnostic.
B. parapertussis is non-fastidious, grows on nutrient agar with pigmented colonies, is urease positive, and causes whooping cough in about 5% of cases. B. bronchiseptica is unique among the three in being motile (peritrichous flagella); it also grows on nutrient agar, reduces nitrate, hydrolyses urea, and is oxidase/catalase positive, causing about 0.1% of pertussis cases.
Key Growth & Diagnostic Data
Identification Panel⚠️ Values summarised from standard microbiology references; always confirm with your laboratory's SOPs.
Clinical Interpretation
| Finding | Possible Significance | Action / Follow-up |
|---|---|---|
| Colonies resembling bisected pearls/mercury drops on Bordet-Gengou medium | Presumptive B. pertussis | Confirm with slide agglutination and biochemical panel; correlate with clinical stage |
| Paroxysmal cough with inspiratory whoop in an unvaccinated infant | Classic pertussis presentation | Urgent specimen collection (pernasal swab) and initiate erythromycin therapy |
| Urease-positive, pigmented colonies on nutrient agar | Suggests B. parapertussis | Report as distinct species; milder pertussis-like illness |
| Motile coccobacilli with peritrichous flagella | Suggests B. bronchiseptica | Rare human cause of pertussis; consider animal exposure history |
Common Errors & How to Avoid Them
Cause: Cotton fibres and fatty acids in cotton are toxic to Bordetella, reducing recovery.
Prevention: Use calcium alginate or Dacron swabs on a flexible nichrome wire for nasopharyngeal/pernasal specimens.
Cause: Bordetella is highly fastidious and sensitive to drying and delay.
Prevention: Inoculate directly onto Bordet-Gengou or Regan-Lowe medium at the bedside where possible, or use validated transport media promptly.
Cause: Bacterial shedding is highest during the catarrhal stage and early paroxysmal stage; later sampling reduces yield.
Prevention: Collect specimens as early as possible once pertussis is suspected, ideally during the catarrhal phase.
Laboratory Tips from the Bench
Colonies resembling 'bisected pearls' or mercury drops on Bordet-Gengou medium after 48–72 hours are a highly memorable and reliable presumptive clue for B. pertussis.
Because culture sensitivity for pertussis is often disappointing (especially after antibiotic initiation or late in disease), PCR-based molecular testing is increasingly preferred where available.
Remember the three stages of pertussis with 'CPC': Catarrhal (most infectious, hardest to diagnose), Paroxysmal (the whoop), Convalescent (recovery).
Important Notes
The catarrhal stage, when treatment is most effective, is also when symptoms are least specific — a high index of suspicion in unvaccinated infants and known contacts is essential.
B. pertussis acts as an adjuvant for tetanus and diphtheria toxoids in the DPT combination vaccine, enhancing overall immunogenicity.
Interactive Quiz
Test Your KnowledgeFlashcards
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Clinical Case Study
Apply Your Knowledge10 days of mild cough progressing to severe paroxysmal coughing fits with a high-pitched inspiratory whoop, cyanosis and post-tussive vomiting. No fever.
The progression from mild catarrhal cough to paroxysmal coughing with a classic inspiratory whoop and marked lymphocytosis in an unvaccinated infant strongly suggests pertussis (whooping cough) due to Bordetella pertussis.
- →Marked lymphocytosis is a classic hematologic clue supporting pertussis in infants.
- →Pernasal swab on Bordet-Gengou or Regan-Lowe medium remains a key diagnostic method.
- →Early antibiotic treatment (erythromycin) is most effective if started during the catarrhal stage.
Frequently Asked Questions
The catarrhal stage, when the patient is most infectious and treatment is most effective, presents with non-specific cold-like symptoms, making clinical diagnosis difficult without a high index of suspicion.
Cotton fibres contain fatty acids that are toxic to the fragile Bordetella organisms, significantly reducing recovery rates; calcium alginate or Dacron swabs are required instead.
The whole-cell killed vaccine (part of DPT) is highly effective but has more side effects, while the newer acellular vaccine (containing pertussis toxin, pertactin and filamentous haemagglutinin) has fewer side effects but is costlier.
Quick Revision
10-Minute ReviewKey Takeaways
- Bordetella pertussis is a fastidious, Gram-negative coccobacillus and the primary cause of whooping cough.
- Bordet-Gengou medium remains the classic culture medium, showing pearl-like colonies after prolonged incubation.
- Pertussis is a multitoxin disease with pertussis toxin, adenylate cyclase and filamentous haemagglutinin as major virulence factors.
- The disease progresses through catarrhal, paroxysmal and convalescent stages, with the whoop characteristic of the paroxysmal phase.
- Correct specimen collection (non-cotton swabs) and prompt culture are essential for laboratory diagnosis.
- DPT vaccination remains highly effective at preventing pertussis and reducing its severity.
Competency Checklist
Track Your MasteryReferences
- National Institute of Open Schooling. Microbiology Module — Lesson 30: Bordetella.
- Collee JG, et al. Mackie & McCartney Practical Medical Microbiology, 14th ed.
- World Health Organization. Pertussis Fact Sheet.