Bordetella pertussis is the bacterium responsible for pertussis, commonly known as whooping cough, a highly contagious respiratory infection. Understanding its characteristics is important for comprehending how it causes disease.
The Distinctive Coccobacillary Shape
Bordetella pertussis exhibits a coccobacillary shape, appearing as very short rods or ovoids, intermediate between spherical cocci and rod-shaped bacilli. These bacteria are small, measuring around 0.2 to 0.5 micrometers (µm) in diameter and 0.5 to 2.0 µm in length. Their size makes them difficult to observe without high magnification.
The bacterium is also Gram-negative, referring to its cell wall structure. When subjected to a Gram stain, Bordetella pertussis will not retain the crystal violet stain, instead appearing pink or red after counterstaining. This Gram-negative classification is due to an outer membrane that prevents the crystal violet from penetrating the cell wall.
Shape’s Role in Adhesion and Virulence
The coccobacillary shape of Bordetella pertussis, combined with its surface structures, plays a role in its ability to cause infection. The bacterium possesses adhesins, such as filamentous hemagglutinin (FHA) and pertactin (Prn), located on its outer surface. These adhesins allow the bacteria to attach to the ciliated epithelial cells lining the respiratory tract.
The small, ovoid shape of Bordetella pertussis facilitates its movement and embedding within the respiratory mucosa. This adherence is a step for colonization, preventing the bacteria from being swept away by the mucociliary escalator. By attaching, Bordetella pertussis can multiply and establish the infection that leads to whooping cough.
Microscopic Identification and Diagnosis
The characteristic shape of Bordetella pertussis is utilized in laboratory diagnosis, particularly for initial assessment of samples. When a nasopharyngeal swab or aspirate is collected from a suspected patient, it can be Gram-stained and examined under a microscope. The presence of small, Gram-negative coccobacillary organisms can provide an initial indication of a Bordetella pertussis infection.
While microscopy offers a preliminary clue, it helps differentiate Bordetella pertussis from other bacteria in respiratory samples. For confirmation, samples are cultured on specific media like Bordet-Gengou agar or Regan-Lowe medium. On these media, Bordetella pertussis colonies appear as small, shiny, mercury-drop-like formations after three to five days of incubation. This characteristic colony morphology, combined with the microscopic appearance, aids in the identification and diagnosis of whooping cough.