Microbial pathogenicity has been defined as the structural and biochemical mechanisms whereby microorganisms cause disease. Pathogenicity in bacteria may be associated with unique structural components of the cells (e.g. capsules, fimbriae, LPS or other cell wall components) or active secretion of substances that either damage host tissues or protect the bacteria against host defenses. Hence, there are two broad qualities of pathogenic bacteria that underlie the means by which they cause disease: invasiveness and toxigenesis.
Toxigenesis is the ability to produce toxins. Toxic substances produced by bacteria, both soluble and cell-associated, may be transported by blood and lymph and cause cytotoxic effects at tissue sites remote from the original point of invasion or growth.
Invasiveness is the ability of a pathogen to invade tissues. Invasiveness encompasses (1) mechanisms for colonization (adherence and initial multiplication), (2) production of extracellular substances (“invasins”), that promote the immediate invasion of tissues and (3) ability to bypass or overcome host defense mechanisms which facilitate the actual invasive process.