Virulence and virulence factors

Introduction

Virulence is not synonymous with pathogenicity (see also pathogenicity above), but describes the degree of damage that the pathogen has caused the host animal. A highly virulent bacterium is very contagious and/or gives severe symptoms. Thus, the infectious dose (ID) is lower for a high-virulent than for a low-virulent microorganism.

Virulence factors

Virulence factors are the components that bacteria produce in order to:

• Colonize some tissue of the host animal (adhesins).
• Invade tissues (tissue-degrading enzymes, see Toxins).
• Invade and get out the cells of the host (applies for intracellularbacteria).
• Escape the immune system of the host animal (by producing a capsule or by means of phase variation, which refers to a change in expression of surface proteins).
• Inhibit the immune system of the host animal (by digesting the antibodies of the host animal with specific proteases). Some mollicutes (mycoplasmas) have a special system, called MIB-MIP (Mycoplasma Immunoglobulin Binding - Mycoplasma Immunoglobulin Protease). MIB-MIP are surface proteins that bind to antibodies and cut off the V_H domain from the immunoglobulin.
• Utilize nutrients from the host cells (by means of e.g. siderophores).
• For some of the bacterial toxins (e.g. botulinum toxin and tetanus toxin), the functions for the bacteria are not known, i.e. which benefits the bacteria may have from them. However, these toxins are still very important virulence factors.

Virulence genes are the genes encoding for virulence factors or components required for the synthesis of virulence factors.