Virulence factors aid in the bacteria’s ability to invade, cause disease, and evade host defense mechanisms. Some are listed below.
Adherence Factors: The ability to physically attach to mucosal sites can increase bacteria’s pathogenicity. Many pathogenic bacteria use pili to adhere to host cells.
Capsules: Protective capsules exist on many pathogenic bacteria, which helps to protect them against host defense mechanisms including phagocytosis.
Endotoxins and exotoxins: Lipopolysaccharide endotoxins can cause fever, blood pressure changes, and lethal shock. Exotoxins are secreted by pathogenic bacteria and include neurotoxins and cytotoxins.
Siderophores: Some pathogenic bacteria can use siderophores to compete with the host for iron, which is an essential growth factor.
Most pathogenic bacteria multiply in tissue fluids rather than host cells, but some species (for example Rickettsia) can only multiply and proliferate within eukaryotic cells. Salmonella bacteria invade cells but do not require them for growth.
Sometimes, tissue damage can result from the host’s own immune system when responding to invasion and bacterial toxins. This is the case with diseases such as tuberculosis.