Bacterial Virulence Factors

Bacterial Virulence Factors By Nimra Khan 2006MO48

Virulence factors refers to the properties (i.e., gene products) that enable a microorganism to establish itself on or within a host of a particular species and enhance its potential to cause disease. Virulence factors include bacterial toxins, cell surface proteins that mediate bacterial attachment, cell surface carbohydrates and proteins that protect a bacterium, and hydrolytic enzymes that may contribute to the pathogenicity of the bacterium.

Virulence is the sum total of several determinants as detailed below-

Adhesion : The initial event in the pathogenesis of many infections is the attachment of the bacteria to body surfaces. This attachment is not a chance event but a specific reaction between receptors on host cells and adhesive structures called adhesins. Adhesins serve as virulence factors and loss of adhesins often renders the strains avirulent. They are usually made up of proteins and are antigenic in nature.

Invasiveness: This refers to the ability of a pathogen to spread in the host tissues after establishing infection. Some highly invasive pathogens produce spreading or generalized lesions (streptococcal septicemia following wound infection), while other less invasive pathogens cause localized lesions (e.g. Staphylocoocal abscess.)

Toxigenicity : Bacteria produce two types of toxins- exotoxins and endotoxins.
Exotoxins are heat labile proteins which are secreted by certain species of bacteria and diffuse readily into the surrounding medium. They are highly potent in minute amounts and constitute some of the most poisonous substances known. Exotoxins are generally formed by Gram positive bacteria but may also be produced by Gram negative organisms such as vibrio cholera and E. coli
Endotoxins are heat stable lipopolysacchrides which form an integral part of the cell wall of Gram negative bacteria. Their toxicity depends on the lipid component and they are not secreted outside the cell. They are poor antigens and their toxicity is completely neutralized by the homologous antibodies. Intravenous injections of large doses of endotoxin and massive Gram negative septicemias cause endotoxic shock.

Bacteriophages: Phage directed virulence seen in diphtheria.

Other bacterial products: Some bacterial products other than toxins, though devoid of intrinsic toxicity , may contribute to virulence by inhibiting the mechanisms of host resistance. Eg. Staphylococci produce a thrombin like enzyme coagulase which prevents phagocytosis by forming a fibrin barrier around the bacteria and walling of lesion.

Infecting dose: Successful infections require that an adequate number of bacteria should gain entry into the host. The dosage may be estimated as the minimum infecting dose or minimum lethal dose, which is respectively, the minimum number of bacteria required to produce clinical evidence of infection or death, respectively in a susceptible animal under standard conditions.
Route of infection: Some bacteria can initiate infection whatever be the mode of entry. Others can survive and multiply only when introduced by optimal routes.