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Tag words: Bordetella pertussis, B. pertussis, pertussis, whooping cough, pertussis vaccine, acellular pertussis.

Bordetella pertussis

Kingdom: Bacteria
Phylum: Proteobacteria
Class: Beta Proteobacteria
Order: Burkholderiales
Family: Alcaligenaceae
Genus: Bordetella
Species: B. pertussis








Kenneth Todar currently teaches Microbiology 100 at the University of Wisconsin-Madison.  His main teaching interest include general microbiology, bacterial diversity, microbial ecology and pathogenic bacteriology.

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Bordetella pertussis and Whooping Cough (page 1)

(This chapter has 3 pages)

© Kenneth Todar, PhD


Bordetella pertussis, the agent of pertussis or whooping cough. Gram stain. (CDC)

Bordetella pertussis

Whooping cough (pertussis) is caused by the bacterium Bordetella pertussis. B. pertussis  is a very small Gram-negative aerobic coccobacillus that appears singly or in pairs. Its metabolism is respiratory, never fermentative, and taxonomically, Bordetella is placed among the "Gram-negative Aerobic Rods and Cocci" in Bergey's Manual. Bordetella is not assigned to any family. The bacteria are nutritionally fastidious and are usually cultivated on rich media supplemented with blood. They can be grown in synthetic medium, however, which contains buffer, salts, an amino acid energy source, and growth factors such as nicotinamide (for which there is a strict requirement). Even on blood agar the organism grows slowly and requires 3-6 days to form pinpoint colonies.

Bordetella pertussis colonizes the cilia of the mammalian respiratory epithelium (Figure 1). Generally, it is thought that B. pertussis does not invade the tissues, but some recent work has shown the bacterium in alveolar macrophages. The bacterium is a pathogen for humans and possibly for higher primates, and no other reservoir is known. Whooping cough is a relatively mild disease in adults but has a significant mortality rate in infants. Until immunization was introduced in the 1930s, whooping cough was one of the most frequent and severe diseases of infants in the United States.

Pathogenesis

The disease pertussis has two stages. The first stage, colonization, is an upper respiratory disease with fever, malaise and coughing, which increases in intensity over about a 10-day period. During this stage the organism can be recovered in large numbers from pharyngeal cultures, and the severity and duration of the disease can be reduced by antimicrobial treatment. Adherence mechanisms of B. pertussis involve a "filamentous hemagglutinin" (FHA), which is a fimbrial-like structure on the bacterial surface, and cell-bound pertussis toxin (PTx). Short range effects of soluble toxins play a role as well in invasion during the colonization stage.


Figure 1. Colonization of tracheal epithelial cells by Bordetella pertussis

The second or toxemic stage of pertussis follows relatively nonspecific symptoms of the colonizaton stage. It begins gradually with prolonged and paroxysmal coughing that often ends in a characteristic inspiratory gasp (whoop). To hear the characteristic sound of whooping cough click whoop.wav (whoop.wav is copyright of Dr Doug Jenkinson, Nottingham, England. www.whoopingcough.net). During the second stage, B. pertussis can rarely be recovered, and antimicrobial agents have no effect on the progress of the disease. As described below, this stage is mediated by a variety of soluble toxins.

Colonization

Studies of B. pertussis and its adhesins have focused on cultured mammalian cells that lack most of the features of ciliated epithelial cells. However, some generalities have been drawn. The two most important colonization factors are the filamentous hemagglutinin (FHA) and the pertussis toxin (PTx). Filamentous hemagglutinin is a large (220 kDa) protein that forms filamentous structures on the cell surface. FHA binds to galactose residues on a sulfated glycolipid called sulfatide which is very common on the surface of ciliated cells. Mutations in the FHA structural gene reduce the ability of the organism to colonize, and antibodies against FHA provide protection against infection. However, it is unlikely that FHA is the only adhesin involved in colonization. The structural gene for FHA has been cloned and expressed in E. coli, raising the possibility of its production for use in a component vaccine.

One of the toxins of B. pertussis, the pertussis toxin (PTx), is also involved in adherence to the tracheal epithelium. Pertussis toxin is a 105 kDa protein composed of six subunits: S1, S2, S3, (2)S4, and S5. The toxin is both secreted into the extracellular fluid and cell bound. Some components of the cell-bound toxin (S2 and S3) function as adhesins, and appear to bind the bacteria to host cells. S2 and S3 utilize different receptors on host cells. S2 binds specifically to a glycolipid called lactosylceramide, which is found primarily on the ciliated epithelial cells. S3 binds to a glycoprotein found mainly on phagocytic cells.

The S1 subunit of pertussis toxin is the A component with ADP ribosylating activity, and the function of S2 and S3 is presumed to be involved in binding the intact (extracellular) toxin to its target cell surface. Antibodies against PTx components prevent colonization of ciliated cells by the bacteria and provide effective protection against infection. Thus, pertussis toxin is clearly an important virulence factor in the initial colonization stage of the infection.

Since the S3 subunit of pertussis toxin is able to bind to the surface of phagocytes, and since FHA will attach to integrin CR3 on phagocyte surfaces (the receptor for complement C3b), it has been speculated that the bacterium might bind preferentially to phagocytes in order to facilitate its own engulfment. The role of such self-initiated phagocytosis is not clear. Bacteria taken up by this abnormal route may avoid stimulating the oxidative burst that normally accompanies phagocytic uptake of bacterial cells which are opsonized by antibodies or complement C3b. Once inside of cells the bacteria might utilize other toxins (i.e. adenylate cyclase toxin) to compromise the bactericidal activities of phagocytes. In any case, there is some evidence that Bordetella pertussis can use this mechanism to get into and to persist in phagocytes as an intracellular parasite. If B. pertussis is an intracellular parasite it would explain why immunity to pertussis correlates better with the presence of specific cytotoxic T cells than it does with the presence of antibodies to bacterial products.

B. pertussis produces at least two other types of adhesins, two types of fimbriae and a nonfimbrial surface protein called pertactin, but their role in adherence and pathogenesis is not well established.


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