- Bordetella pertussis is a small non-motile, non-sporing ovoid.
- Gram negative cocco-bacillus.
- It measures about 1-1.5µm x 0.3µm and is haemolytic.
- Capsulated (fresh isolates from patients), strict aerobes and fastidious.
- On staining with toludine blue, metachromatic granules are seen.
- The genes coding for B. pertussis virulence factors are under the control of two sets of genes, which may act in tandem.
- The genes coding for adhesins and pertussis toxin are inactive at 250C but active when the temperature is raised to 370C.
- Changes in external environment may “turn off” the genes responsible for virulence to allow survival and dissemination by carriers infected with less virulent pertussis.
- The various virulence factors are as follows:
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- Adherence of B.pertussis to ciliated epithelial cells is essential for the pathogenesis of the organisms.
- This is mediated by capsular fimbriae or agglutinogens.
- The most important is filamentous haemagglutinin which also causes agglutination of red blood cells.
- Toxins are of various types. They are:
i) Pertussis toxin(PT)
- It is a major virulent factor playing a central role in pathogenesis of whooping cough.
- It is produced only by B. pertussis.
- PT is synonymous with the earlier terms such as, histamine sensitizing factor, islet cell activating factor.
- It has a molecular weight of 117,000 which are made up of two units, A and B.
- A unit is enzymatically active.
- The B-unit binds the toxin to the target cells and helps A-unit to cross the membrane.
- The toxin causes increased concentration of cAMP within the target cell.
- Production of PT is controlled by regulator PT gene.
- This toxin increases sensitivity to histamine and serotonin resulting in the increased susceptibility to anaphylaxis.
- It raises the level of insulin causing the reduction in blood sugar and inhibits epinephrine induced hyperglycaemia.
- It causes leukocytois by altering the migration pattern of lymphocytes.
- It is a potent T cell mitogen that causes accumulation of lymphocytes in the intravascular compartment.
- It increases binding to CR3 receptor of phagocytic cells and causes phagocytosis without the involvement of the superoxide burst.
ii) Adenylate cyclase-like toxin (ACT)
- It is produced by all Bordetellae except B. avium.
- ACT enters non-infected cells and causes intracellular increase in cAMP leading to intoxication of mammalian cells.
- It also inhibits chemotaxis and superoxide burst in polymorphonuclear leucocytes, which helps in intracellular survival of B. pertussis.
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iii) Tracheal cytotoxin(TCT)
- It is a Bordetella toxin derived from the peptidoglycan of cell wall and present in all species of Bordetellae.
- TCT induces ciliostasis in the epithelial cells, followed by extrusion of these cells from the epithelium of respiratory tract.
- This makes a person more prone to secondary infection.
iv) Endotoxin LPS
- It does not have any important role in pathogenesis but anti-LPS antibody aids in the immune clearance of Bordetellae.
Virulence factors of Bordetella pertussis