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Chapter: Microbiology

Virulence Factors

Pili are useful for the attachement of the organisms on the epithe-lial cells.

Virulence Factors

1. Pili

Pili are useful for the attachement of the organisms on the epithe-lial cells.

2. Capsule

Capsules down regulate the secretion of cytokine. They inhibit leukocyte accumulation. They also induce the suppressor T cells and inhibit lymphoproliferation

3. Intracellular residence

The following microorganisms reside intracellularly and try to avoid host defense mechanisms. They are M.tuberculosis, M.leprae, S.typhi, T.gondii, L.donovani, H.capsulatum.

4. Production of enzymes

Some enzymes like proteases, DNAses, and phospholipases are produced and they help in disruption of cell structures and to hydrolyse host tissues. In Aspergillus species proteases help in invasion.


Bacteria produce both exotoxins and endotoxins which play and important role in the pathogenesis of disease

Exotoxins are produced by some organisms like C.diphtheriae, C.tetani, C.botulinum. The exotoxin produced by V.cholerae acts on the intestine and is called enterotoxin. The toxin produced by one type of Escherichia coli causes acute gastroenteritis.

Endotoxins are lipopolysaccharide cell wall of gram negative bacteria. They induce production of cytokines by different cells of im-mune system. Coagulation system and complement system are acti-vated. They also affect various organs like kidney, heart and lungs leading to organ failure.

6. Antigenic variation

Microorganisms evade the host immune responses by changing their surface antigens. N.gonorrhoeae very often changes its outer mem-brane protein. Antigenic drift and shift are common in influenza viruses. Trypanosoma brucei are covered with thick protein coats which un-dergo antigenic change during infection. Some organisms produce sur-face proteins that are similar to host proteins or coat themselves with host proteins that they are mistaken for part of the host itself

The distinction between the commensal and the organisms asso-ciated with disease is subtle. The definition of normal flora or pathogen is derived from the resultant complex interaction between the organism and its host.

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