Bordetella pertussis and Whooping Cough (page 2)
(This chapter has 3 pages)
© Kenneth Todar, PhD
Toxins Produced by B. pertussis
B. pertussis produces a variety of substances with toxic
in the class of exotoxins and endotoxins.
It secretes its own invasive adenylate cyclase which enters
cells (Bacillus anthracis produces a similar enzyme, EF). This
acts locally to reduce phagocytic activity and probably helps the
initiate infection. This toxin is a 45 kDa protein that may be
or released into the environment. Mutants of B. pertussis in
adenylate cyclase gene have reduced virulence in mouse models. The
can still colonize but cannot produce the lethal disease. The adenylate
cyclase toxin is a single polypeptide with an enzymatic domain (i.e.,
cyclase activity) and a binding domain that will attach to host cell
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
Probably it inserts into the erythrocyte membrane which causes
An interesting feature of the adenylate cyclase toxin is that it is
only in the presence of a eukaryotic regulatory molecule called
which up-regulates the activity of the eukaryotic adenylate cyclase.
adenylate cyclase toxin is only active in the eukaryotic cell since no
similar regulatory molecule exists in procaryotes. Thus, the molecule
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
toxin) which causes inflammation and local necrosis adjacent to sites
pertussis is located. The lethal toxin is a 102 kDa protein
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
toxic for ciliated respiratory epithelium and which will stop the
cells from beating. This substance is not a classic bacterial exotoxin
since it is not composed of protein. The tracheal cytotoxin is a
fragment, which appears in the extracellular fluid where the bacteria
actively growing. The toxin kills ciliated cells and causes their
from the mucosa. It also stimulates release of cytokine IL-1, and so
It produces the pertussis toxin, PTx, a protein that
both the colonization and toxemic stages of the disease. PTx is a two
A+B bacterial exotoxin. The A subunit (S1) is an ADP ribosyl
The B component, composed of five polypeptide subunits (S2 through S5),
binds to specific carbohydrates on cell surfaces. The role of PTx in
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
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
normally inhibits the eukaryotic adenylate cyclase. The Gi protein is
and cannot perform its normal function to inhibit adenylate cyclase.
conversion of ATP to cyclic AMP cannot be stopped and intracellular
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
Systemic effects of the toxin include lymphocytosis and alteration of
activities that are regulated by cAMP, such as increased insulin
(resulting in hypoglycemia) and increased sensitivity to histamine
in increased capillary permeability, hypotension and shock). PTx also
the immune system in experimental animals. B cells and T cells that
the lymphatics show an inability to return. This alters both AMI and
responses and may explain the high freqency of secondary infections
accompany pertussis (the most frequent secondary infections during
cough are pneumomia and otitis media).
Although the effects of the pertussis toxin are dependent on ADP
it has been shown that mere binding of the B oligomer can elicit a
on the cell surface such as lymphocyte mitogenicity, platelet
and production of insulin effects.
The pertussis toxin gene has been cloned and sequenced and the
expressed in E. coli. The toxin can be inactivated and
to toxoid for use in component vaccines.
cholera toxin and pertussis toxin (ptx) in their ability to interfere
the regulation of the eucaryotic adenylate cyclase complex.
Normal regulation of adenylate
cyclase activity in mammalian cells. Adenylate cyclase (AC) is
normally by a stimulatory regulatory protein (Gs) and guanosine
(GTP); however the activation is normally brief because an inhibitory
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
GTP cannot be hydrolyzed. Since GTP hydrolysis is the event that
adenylate cyclase (AC), the enzyme remains continually activated.
Adenylate cyclase activated by
pertussis toxin (The pertussis A subunit transfers the ADP ribosyl
of NAD to the membrane-bound regulatory protein Gi that normally
the eukaryotic adenylate cyclase. The Gi protein is inactivated and
perform its normal function to inhibit adenylate cyclase. The
of ATP to cyclic AMP cannot be stopped.
Lipopolysaccharide. As a Gram-negative bacterium Bordetella
pertussis possesses lipopolysaccharide (endotoxin) in its outer
but its LPS is unusual. It is heterogeneous, with two major forms
in the phosphate content of the lipid moiety. The alternative form of
A is designated Lipid X. The unfractionated material elicits the usual
effects of LPS (i.e., induction of IL-1, activation of complement,
hypotension, etc.), but the distribution of those activities is
in the two forms of LPS. For example, Lipid X, but not Lipid A, is
and its O-side chain is a very powerful immune adjuvant. Furthermore,
LPS is more potent in the limulus assay than LPS from other
bacteria, so it is not reliable to apply knowledge of the biological
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.
The production of virulence factors in B. pertussis is
in several different ways. Expression of virulence factors is regulated
by the bvg operon.
First, the organisms undergo an event called phase variation
in the loss of most virulence factors and some undefined outer membrane
proteins. Phase variation has been shown to occur at a genetic
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
(also known as vir) operon.
A similar process called phenotypic modulation, occurs in response
environmental signals such as temperature or chemical content, and is
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
is itself regulated by environmental signals, such that entry into a
might induce components required for survival and production of disease.