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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 2)

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Toxins Produced by B. pertussis

B. pertussis produces a variety of substances with toxic activity in the class of exotoxins and endotoxins.

It secretes its own invasive adenylate cyclase which enters mammalian cells (Bacillus anthracis produces a similar enzyme, EF). This toxin acts locally to reduce phagocytic activity and probably helps the organism initiate infection. This toxin is a 45 kDa protein that may be cell-associated or released into the environment. Mutants of B. pertussis in the adenylate cyclase gene have reduced virulence in mouse models. The organisms can still colonize but cannot produce the lethal disease. The adenylate cyclase toxin is a single polypeptide with an enzymatic domain (i.e., adenylate cyclase activity) and a binding domain that will attach to host cell surfaces. The adenylate cyclase was originally identified as a hemolysin because it will lyse red blood cells. In fact, it is responsible for hemolytic zones around colonies of Bordetella pertussis growing on blood agar. Probably it inserts into the erythrocyte membrane which causes hemolysis. An interesting feature of the adenylate cyclase toxin is that it is active only in the presence of a eukaryotic regulatory molecule called calmodulin, which up-regulates the activity of the eukaryotic adenylate cyclase. The adenylate cyclase toxin is only active in the eukaryotic cell since no similar regulatory molecule exists in procaryotes. Thus, the molecule seems to have evolved specifically to parasitize eukaryotic cells. Anthrax EF (edema factor) is also a calmodulin-dependent adenylate cyclase.

It produces a highly lethal toxin (formerly called dermonecrotic toxin) which causes inflammation and local necrosis adjacent to sites where B. pertussis is located. The lethal toxin is a 102 kDa protein composed of four subunits, two with a mw of 24kDa and two with mw of 30 kDa. It causes necrotic skin lesions when low doses are injected subcutaneosly in mice and is lethal in high doses. The role of the toxin in whooping cough is not known.

It produces a substance called the tracheal cytotoxin which is toxic for ciliated respiratory epithelium and which will stop the ciliated cells from beating. This substance is not a classic bacterial exotoxin since it is not composed of protein. The tracheal cytotoxin is a peptidoglycan fragment, which appears in the extracellular fluid where the bacteria are actively growing. The toxin kills ciliated cells and causes their extrusion from the mucosa. It also stimulates release of cytokine IL-1, and so causes fever.

It produces the pertussis toxin, PTx, a protein that mediates both the colonization and toxemic stages of the disease. PTx is a two component, A+B bacterial exotoxin. The A subunit (S1) is an ADP ribosyl transferase. The B component, composed of five polypeptide subunits (S2 through S5), binds to specific carbohydrates on cell surfaces. The role of PTx in invasion has already been discussed. PTx is transported from the site of growth of the Bordetella to various susceptible cells and tissues of the host. Following binding of the B component to host cells, the A subunit is inserted through the membrane and released into the cytoplasm in a mechanism of direct entry. The A subunit gains enzymatic activity and transfers the ADP ribosyl moiety of NAD to the membrane-bound regulatory protein Gi that normally inhibits the eukaryotic adenylate cyclase. The Gi protein is inactivated and cannot perform its normal function to inhibit adenylate cyclase. The conversion of ATP to cyclic AMP cannot be stopped and intracellular levels of cAMP increase. This has the effect to disrupt cellular function, and in the case of phagocytes, to decrease their phagocytic activities such as chemotaxis, engulfment, the oxidative burst, and bacteridcidal killing. Systemic effects of the toxin include lymphocytosis and alteration of hormonal activities that are regulated by cAMP, such as increased insulin production (resulting in hypoglycemia) and increased sensitivity to histamine (resulting in increased capillary permeability, hypotension and shock). PTx also affects the immune system in experimental animals. B cells and T cells that leave the lymphatics show an inability to return. This alters both AMI and CMI responses and may explain the high freqency of secondary infections that accompany pertussis (the most frequent secondary infections during whooping cough are pneumomia and otitis media).

Although the effects of the pertussis toxin are dependent on ADP ribosylation, it has been shown that mere binding of the B oligomer can elicit a response on the cell surface such as lymphocyte mitogenicity, platelet activation, and production of insulin effects.

The pertussis toxin gene has been cloned and sequenced and the subunits expressed in E. coli. The toxin can be inactivated and converted to toxoid for use in component vaccines.

Comparison between cholera toxin and pertussis toxin (ptx) in their ability to interfere with the regulation of the eucaryotic adenylate cyclase complex.

Normal regulation of adenylate cyclase activity in mammalian cells. Adenylate cyclase (AC) is activated normally by a stimulatory regulatory protein (Gs) and guanosine triphosphate (GTP); however the activation is normally brief because an inhibitory regulatory protein (Gi) hydrolyzes the GTP.

Adenylate cyclase activated by cholera toxin The cholera toxin A1 fragment catalyzes the attachment of ADP-Ribose (ADPR to the regulatory protein Gs, forming Gs-ADPR from which GTP cannot be hydrolyzed. Since GTP hydrolysis is the event that inactivates adenylate cyclase (AC), the enzyme remains continually activated.

Adenylate cyclase activated by pertussis toxin (The pertussis A subunit transfers the ADP ribosyl moiety of NAD to the membrane-bound regulatory protein Gi that normally inhibits the eukaryotic adenylate cyclase. The Gi protein is inactivated and cannot perform its normal function to inhibit adenylate cyclase. The conversion of ATP to cyclic AMP cannot be stopped.

Lipopolysaccharide. As a Gram-negative bacterium Bordetella pertussis possesses lipopolysaccharide (endotoxin) in its outer membrane, but its LPS is unusual. It is heterogeneous, with two major forms differing in the phosphate content of the lipid moiety. The alternative form of Lipid A is designated Lipid X. The unfractionated material elicits the usual effects of LPS (i.e., induction of IL-1, activation of complement, fever, hypotension, etc.), but the distribution of those activities is different in the two forms of LPS. For example, Lipid X, but not Lipid A, is pyrogenic, and its O-side chain is a very powerful immune adjuvant. Furthermore, Bordetella LPS is more potent in the limulus assay than LPS from other Gram-negative bacteria, so it is not reliable to apply knowledge of the biological activity of LPS in the Enterobacteriaceae to the LPS of Bordetella. The role of this unusual LPS in the pathogenesis of whooping cough has not been investigated.

Regulation of Virulence Factors in B. pertussis

The production of virulence factors in B. pertussis is regulated in several different ways. Expression of virulence factors is regulated by the bvg operon.

First, the organisms undergo an event called phase variation resulting in the loss of most virulence factors and some undefined outer membrane proteins. Phase variation has been shown to occur at a genetic frequency of 10-4 - 10-6 generations and results from a specific DNA frame shift that comes about after the insertion of a single nucleotide into the bvg (also known as vir) operon.

A similar process called phenotypic modulation, occurs in response to environmental signals such as temperature or chemical content, and is reversible. This is an adaptive process mediated by the products of the bvg operon, and is an example of a two-component environmental-sensing (regulatory) system used by other bacteria. The expression of these regulatory proteins is itself regulated by environmental signals, such that entry into a host might induce components required for survival and production of disease.

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