Bordetella pertussis: Whooping Cough, Pertussis Toxin, Diagnosis, Treatment

A 3-month-old baby is brought in after a week of what seemed like a mild cold. Now the baby has sudden fits of rapid coughing so severe that she briefly turns blue and, after one fit, makes a high-pitched gasp while breathing in. Between the fits she looks surprisingly well. She has not yet completed her vaccinations, and an older sibling has had a lingering cough for weeks.

The clue is the pattern: bursts of violent coughing with a whooping gasp and vomiting, in a baby who seems normal in between. The organism causing this stopped living in her airway weeks ago in terms of damage; a toxin it released is what is driving the cough now. This is whooping cough, caused by Bordetella pertussis.

Introduction

Bordetella pertussis is a small, gram-negative coccobacillus and the cause of whooping cough (pertussis), an acute respiratory infection that is most dangerous in young infants. In humans, pertussis is caused mainly by B. pertussis and, in a milder form, by B. parapertussis. Other species such as B. bronchiseptica, B. avium, and B. hinzii mostly cause disease in animals and are rarely found in humans.

One idea organizes the whole topic: pertussis is a toxin-mediated disease. The organism stays in the airway lining, but the toxins it releases damage the airway and disable local defenses, and this is what produces the long, violent cough. This single fact explains the clinical stages, why the cough outlasts the infection, and why the timing of antibiotics matters so much.

B. pertussis was first isolated in pure culture in 1906 by Bordet and Gengou, whose names are still attached to the classic culture medium.

General characteristics

Figure: Gram negative Bordetella pertussis (Image source: CDC)

• Bordetella pertussis is a small rod-shaped, coccoid, or ovoid Gram-negative bacterium.
• It is encapsulated and does not produce spores.
• It is a strict aerobe.
• It is arranged singly or in small groups.
• B. pertussis and B. parapertussis are non-motile.
• The bacteria are nutritionally fastidious and need enriched media for growth. They are also sensitive to substances in ordinary media (such as fatty acids), which is why special media containing charcoal or blood are used to absorb these inhibitors.

Virulence factors of Bordetella pertussis

B. pertussis does not invade deeply. It attaches to the ciliated cells lining the airway and works through toxins. The virulence factors fall into two groups: those that help it stick, and those that damage the airway and disable defenses.

Adhesins (how it sticks)

B. pertussis attaches to the ciliated respiratory epithelium using several adhesins: filamentous hemagglutinin (FHA), pertactin (PRN), and fimbriae (FIM). Firm attachment lets the organism resist being swept out by the mucociliary escalator and stay in the airway. These adhesins are also important vaccine antigens, which is why they appear in the acellular vaccine.

Pertussis toxin (PT): the central toxin

What it is: a protein exotoxin with an A-B structure. The B part (subunits S2 to S5) binds host cells and delivers the active A part (subunit S1) inside.

Why it matters: the S1 subunit is an enzyme that carries out ADP-ribosylation of a host regulatory protein called Gi. Gi normally acts as an "off switch" that keeps the cell's adenylate cyclase in check. By disabling Gi, pertussis toxin removes the brake, so adenylate cyclase runs unopposed and intracellular cAMP rises. The consequences explain several classic features of pertussis:

• Interference with immune-cell signaling, weakening the host response.
• A striking lymphocytosis: pertussis toxin prevents lymphocytes from leaving the bloodstream to enter tissues, so they pile up in the blood. This is the source of the very high lymphocyte counts seen in the paroxysmal phase.

Adenylate cyclase toxin (ACT)

What it is: a toxin that enters host cells and is switched on by the host's own calmodulin.

Why it matters: once active, it produces a massive rise in cAMP inside the cell. This cripples the killing functions of phagocytes and natural killer cells (their oxidative burst and activity), so the local immune cells cannot clear the organism. In effect, the bacterium turns the host's own signaling machinery against its defenses.

Tracheal cytotoxin and the cough

B. pertussis also releases tracheal cytotoxin, a fragment of its own cell wall (peptidoglycan). It specifically damages and destroys the ciliated cells of the airway. This is central to the disease: with the cilia destroyed, the airway cannot clear mucus and debris, and the only way left to clear the airway is violent coughing. This is why the cough is so severe and so prolonged, and why it continues even after the bacteria are gone, because the airway lining needs time to regrow its cilia.

Putting it together: the organism attaches to the ciliated airway (adhesins), disables local immune killing (adenylate cyclase toxin), disturbs immune signaling and drives lymphocytosis (pertussis toxin), and destroys the cilia (tracheal cytotoxin). The loss of cilia plus airway irritation produces the paroxysmal cough. Because the damage is toxin-driven and already done by the time the whooping starts, antibiotics given late cannot fix the cough, a point that returns in treatment.

Pathogenesis and clinical manifestations

Source and transmission: humans are the only host for B. pertussis. The organism lives in the airway, mainly the trachea and bronchi, and spreads from person to person by respiratory droplets. It is highly contagious, and the classic pattern is an older child or adult with a lingering cough passing it to an unvaccinated infant, who is at the highest risk of severe disease.

The clinical manifestation consists of 3 stages.

1. Catarrhal phase: after an incubation period of about 5 to 10 days (occasionally up to 3 weeks), pertussis begins with the catarrhal phase, lasting about 1 to 2 weeks. It looks like an ordinary cold: low-grade fever, runny nose, and a mild but progressive cough. This is the most contagious stage, and, crucially, the best time to treat, even though it is the hardest time to recognize because the symptoms are non-specific.

2. Paroxysmal phase: this phase lasts about 2 to 6 weeks (sometimes up to 10). It is defined by fits (paroxysms) of rapid, repeated coughing, often followed by a sudden deep inspiration that makes the classic high-pitched "whoop," and frequently by vomiting after the coughing fit (post-tussive vomiting). Between fits the patient can look well. A marked lymphocytosis is characteristic and is driven by pertussis toxin (as above); very high counts can occur, especially in infants, and a very high lymphocyte count is a poor prognostic sign in young infants.

   Important points on this phase: infants may not whoop at all and may instead have apnea (pauses in breathing) or turn blue, which is why pertussis is so dangerous at this age. Regarding infectivity, an untreated patient remains contagious for about 3 weeks after the paroxysmal cough begins, but 5 days of appropriate antibiotics renders the patient non-contagious.

3. Convalescent phase: the cough gradually becomes less frequent and less severe over several weeks to months. Serious complications can still occur, especially in infants, including secondary bronchopneumonia (the most common cause of death), seizures, and encephalopathy. The prolonged cough of pertussis has earned it the name "the 100-day cough.

Laboratory diagnosis

Sample: A nasopharyngeal swab or exudate. Dacron or calcium alginate swabs are preferred and inoculated immediately on a culture medium. If immediate inoculation is not possible, a sample should be placed in Reagan Lowe (RL) or Amies medium containing charcoal transport medium.

If culture cannot be performed immediately, it is possible to keep the biological samples at -80 °C and perform the culture later on.

Microscopy: On Gram staining ,Bordetella pertussis appears as small Gram-negative coccobacilli arranged singly or in small groups.

Rapid diagnosis of pertussis can also be made by examining smears of nasopharyngeal secretions using fluorescein-labeled antibodies to B. pertussis. The organism appears as an apple green fluorescent coccobacilli.

Culture:

Figure: Bordetella pertussis (A) and B. parapertussis (B) grown on Bordet and Gengos medium. (Image source: Ref-5)

Culture is highly specific and is traditionally the gold standard, but it is slow and its sensitivity falls after the catarrhal stage and after antibiotics, so in practice PCR has become the main rapid test. Specimens are plated promptly onto selective media such as Regan-Lowe agar, freshly prepared Bordet-Gengou agar, or BCYE with added cephalosporin to suppress other flora.

Colonies of B. pertussis are small, shiny, and "mercury drop-like," appearing after about 3 to 5 days at 35 to 37°C (it is a slow grower)

Suspected colonies are further identified and differentiated based on the growth rate, colony morphology, various biochemical reactions, and slide agglutination with antisera.

Serological tests: Since, the rate of isolation of Bordetella is very low in suspected individuals especially after the catarrhal stage, serological tests based on agglutination, complement fixation, and immunofluorescence are widely used.

In the absence of recent immunization, an elevated serum IgG antibody to PT after 2 weeks of cough onset is suggestive of recent B. pertussis infection.

Enzyme immunoassay (EIA) for detection of IgM, IgA, and IgG antibodies to B. pertussis and is particularly useful in diagnosis during the late stage of the disease when other tests are negative.

PCR: Results are available 1-2 days. Several polymerase chain reaction (PCR) assays have been developed for the rapid diagnosis of B. pertussis infection. Usually, two types of samples, nasopharyngeal aspirates or nasopharyngeal swabs (taken as for cultures) have been used. When effective antimicrobial therapy is started several days before the specimen is collected, the patient is likely to be PCR positive but culture-negative.

In immunized persons the laboratory confirmation of the diagnosis of pertussis is difficult, and combinations of culture, PCR, and EIA serology should be used. Epidemiological data and laboratory-confirmed contact patients often help in the diagnosis.

Serotyping of Bordetella pertussis: Serotyping i.e. the detection of the expression of the fimbriae Fim 2 and Fim 3, is performed using monoclonal antibodies.

Treatment of pertussis

The antibiotics of choice are macrolides, with azithromycin preferred for all ages. Two age-specific cautions matter: erythromycin and clarithromycin are avoided in infants under 1 month because they are linked to infantile hypertrophic pyloric stenosis (azithromycin is used instead), and trimethoprim-sulfamethoxazole is the alternative for patients over 2 months who cannot take a macrolide.

The single most important teaching point about treatment is timing, and it comes straight from the pathogenesis. Antibiotics help most when given early, in the first 1 to 2 weeks (the catarrhal stage), before the paroxysms begin. Once the paroxysmal cough is established, antibiotics no longer shorten or lessen the cough, because the toxin damage to the airway is already done. At that stage, antibiotics are still given, but for a different reason: to stop the patient spreading the organism to others (the patient becomes non-contagious after 5 days of treatment). This is the paradox of pertussis: by the time the diagnosis is obvious, the treatment can no longer fix the cough.

Close contacts, especially unvaccinated infants and households with vulnerable members, are given post-exposure antibiotic prophylaxis with the same macrolides.

Prevention and immunization

Vaccination is the most important preventive measure, but immunity from both natural infection and vaccination fades over time, which is why boosters are needed and why outbreaks still occur (including a notable resurgence in recent years). There are two vaccine types, the older whole-cell (wP) and the acellular (aP), and they are given in combination:

• DTaP (diphtheria, tetanus, acellular pertussis) for infants and children under 7 years.
• Tdap for older children, adults, and, importantly, pregnant women. Vaccinating in pregnancy passes antibodies to the newborn and protects the baby during the vulnerable first months before its own vaccination is complete.

How to Remember

Key exam facts in one table

Where Students Get Confused

References and further readings

1. Tille, P. M. (2022). Bailey & Scott's Diagnostic Microbiology (15th ed.). Elsevier.
2. Carroll, K. C., Pfaller, M. A., et al. (2020). Murray's Medical Microbiology (9th ed.). Elsevier.
3. Centers for Disease Control and Prevention. Pertussis (whooping cough): clinical overview. CDC (current version).
4. World Health Organization. (2014). Laboratory Manual for the Diagnosis of Whooping Cough Caused by Bordetella pertussis. Geneva: WHO.
5. Bouchez, V., & Guiso, N. (2015). Bordetella pertussis, B. parapertussis, vaccines and cycles of whooping cough. Pathogens and Disease, 73(7), ftv055.