Requisites for Successful Growth

• Attachment

• Nutrition

• Survival from host defence

• Transmission

Virulence Factors

• Factors which promote infection and which contribute to disease

• Studied with mutants• Are multifactorial• Consist of:

Factors promoting colonization and invasion Factors which are pathogenic

Bacterial Virulence Factors I: Adherence

• Capsules

• Pili

• Adhesins

Capsules• Present in some gram negative and positive

bacteria.• May be composed of protein or polysaccharide

layers.• Is poorly antigenic and anti-phagocytic• Can act as a barrier to toxic hydrophobic

molecules such as detergents.• Can promote adherence to other bacteria or cell

surfaces

Bacterial Virulence Mechanisms

• Adherence• Invasion• By-products of growth (gas, acid)• Toxins• Superantigen• Immune evasion• Resistance to antibiotics

Pili (Fimbriae)

• Composed of subunits of pilin.• Promote adherence to other bacteria or

host.• Synonyms: adhesins, lectins, evasins,

aggressins.• Fragile, often replaced.

Adhesins

• Adherence mediated by lectin activity.• Allow attachment to vulnerable membranes

such as surfaces of the respiratory, GI and urogenitary tract as well as dentine.

• Can alter structural properties of host cell membrane by signal transduction and induction of actin rearrangement (EPEC intimin) to enhance binding.

Bacterial Virulence Factors I: Invasion

• Invasins e.g. Yersinia

• Host cell necrosis e.g. Diphtheria toxin

• Promote phagocytosis e.g. Shigella

Bacterial Virulence Factors II: Iron Adsorption

• Siderophores

Toxins

• Toxic byproducts of bacterial growthe.g. acids, gas, proteases

• Toxins Endotoxins e.g. LPS Exotoxins e.g AB toxins

• Toxins vs Toxoids• Heat labile vs Heat resistant• Ineffectiveness of antibiotics

Exotoxins

• AB. e.g. Shigella dysenteriae, C. tetani, V. cholerae.

• Cell Membrane Disruption. e.g. C. perfringens

• Superantigens. e.g. S. aureus

Exotoxins I: AB (i)

Exotoxins I:AB (ii)

Exotoxins I: AB (iii)

Toxic Shock Syndrome

Endotoxins: Lipopolysaccharide• Fever• Leukopenia, followed by

leukocytosis• Complement activation• Thrombocytopenia• Coagulation• Decreased blood

circulation• Shock• Death