Biochemical Identification of Bordetella pertussis: The Causative Agent of Whooping Cough
The genus *Bordetella* comprises several species, with *Bordetella pertussis* being the causative agent of pertussis, or whooping cough, a highly contagious respiratory infection. Due to its significant public health impact, accurate and timely identification of this bacterium is crucial. However, the identification of *B. pertussis* in a clinical laboratory is uniquely challenging compared to many other common pathogens. This difficulty stems from the organism’s fastidious nature, requiring specialized media and prolonged incubation times, and its phenotypic similarity to other members of the *Bordetella* genus, such as *B. parapertussis* and *B. bronchiseptica*.
While modern molecular techniques like Polymerase Chain Reaction (PCR) have become the primary method for screening and diagnosis due to their speed and superior sensitivity, classical biochemical testing remains an essential tool in the reference laboratory. Biochemical analysis is fundamental for confirming the species identity of isolates, particularly in outbreak settings or when molecular methods yield equivocal results. The panel of tests focuses on specific enzymatic reactions and metabolic pathways that help differentiate *B. pertussis* from its closely related cousins.
Cultural Characteristics and Unique Growth Requirements
*Bordetella pertussis* is highly fastidious, meaning it has complex nutritional requirements and selective growth patterns. It requires specific growth factors, including nicotinamide, various amino acids, and organic sulfur compounds like cysteine, for primary isolation. It is a strict aerobe and demonstrates optimal growth at temperatures between 35°C and 37°C. Colonies are typically small, pinpoint, white, and shiny on appropriate media, with a diameter of approximately 1 mm.
The choice of culture medium is a critical differentiating factor. *B. pertussis* cannot grow on standard, non-selective laboratory media such as nutrient agar or MacConkey agar. This inability to grow on MacConkey agar is a key trait that immediately distinguishes it from *Bordetella bronchiseptica* and *Bordetella holmesii*, both of which exhibit growth on this medium. For primary isolation, specialized enriched media such as Bordet-Gengou agar or Regan-Lowe selective medium (charcoal blood agar with cefalexin) are indispensable. Even on these selective media, primary isolation is slow, often requiring 5 to 7 days of incubation before colonies become visible, a factor that makes culture less rapid than PCR for initial diagnosis. Plates should, in fact, be incubated for a minimum of seven days before being discarded as negative. On blood agar, *B. pertussis* exhibits hemolytic activity.
Key Differentiating Biochemical Tests
Several fundamental biochemical tests are paramount in distinguishing *B. pertussis* from other *Bordetella* species. These tests typically include the detection of certain enzymes and the assessment of motility, which reveal crucial differences in the organism’s enzymatic profile and physical characteristics.
Oxidase Test
*B. pertussis* typically gives a **weakly positive** result in the oxidase test. The enzyme oxidase detects the presence of cytochrome c oxidase, a component of the electron transport chain involved in aerobic respiration. This differentiates it from species like *Bordetella holmesii* and *Bordetella ansorpii* which are generally oxidase negative.
Urease Test
The urease test is one of the most critical differentiating biochemical markers. *B. pertussis* is consistently **urease negative**. Urease is an enzyme that hydrolyzes urea into ammonia and carbon dioxide. The absence of this enzyme is a reliable trait for separating *B. pertussis* from *Bordetella parapertussis* and especially from *Bordetella bronchiseptica*, which is strongly urease positive, often producing a positive result within four hours.
Motility Test
*B. pertussis* is characteristically **non-motile**, lacking flagella, which is confirmed by a motility test. This contrasts sharply with *Bordetella bronchiseptica* and *Bordetella hinzii*, which are motile by means of peritrichous flagella, making motility a rapid and clear distinction between the species.
Metabolic Profile and Secondary Enzymatic Activities
The carbohydrate metabolism profile of *B. pertussis* further solidifies its identification. Unlike many bacteria that use fermentation or oxidation to derive energy from sugars, *Bordetella* species are notable for their **asaccharolytic** nature—they do not ferment carbohydrates. Instead, they oxidatively utilize various amino acids such as glutamic acid, proline, alanine, aspartic acid, and serine with the production of ammonia and CO2, a distinct metabolic process.
In a standard biochemical test panel, *B. pertussis* is consistently **Citrate negative**, meaning it cannot utilize citrate as a sole carbon source, which distinguishes it from *Bordetella hinzii*, which is citrate positive. Additional negative test results commonly associated with *B. pertussis* include the absence of activity for Malate, Maltose, Mannitol, Phenylacetate, Sucrose, and Xylose, as well as being negative for Gelatin Hydrolysis, Lysine decarboxylase, Alkaline Phosphatase, Esculin Hydrolysis, and Trypsin.
Conversely, *B. pertussis* is **Catalase positive**, an enzyme that decomposes hydrogen peroxide into water and oxygen. Other positive enzymatic reactions, though typically part of an expanded commercial identification system, include Ester C8 lipase activity and Naphthol-AS-B1 phosphohydrolase activity. The presence of these enzymatic markers is part of the comprehensive phenotypic assessment.
The Role of Biochemical Data in the Modern Laboratory
The traditional identification of *B. pertussis* is complicated by its slow growth rate and fastidious nature, which can lead to delays or the need for confirmatory testing. Due to these inherent limitations, biochemical and cultural methods are often used in conjunction with more rapid and sensitive molecular and serological techniques.
Molecular assays, particularly multiplex RT-PCR, are now the CDC-recommended and preferred tests for diagnosis in the early stages of illness (up to three to four weeks post-cough onset) because of their high sensitivity and rapid turnaround. Serological tests, such as Enzyme-Linked Immunosorbent Assay (ELISA) measuring IgG antibodies against pertussis toxin, are used later in the course of the disease (optimal between 2 to 8 weeks) to confirm diagnosis retrospectively.
Nonetheless, the biochemical profile—characterized by non-motility, urease-negativity, asaccharolytic metabolism, and poor growth on general media—remains the gold standard for pure culture confirmation. The ability to correctly identify an isolate’s biochemical fingerprint is crucial for reference laboratories to confirm an etiologic agent, perform serotyping using agglutination tests for epidemiological studies, and maintain a robust surveillance system for this vaccine-preventable disease.