A number of bacteria produce extracellular proteins, which break down host tissues, encourage the spread of the organism and aid the establishment and maintenance of disease. These proteins, which are mostly enzymes, are called virulence factors.
For example, streptococci, staphylococci and pneumococci produce hyaluronidase, an enzyme that breaks down hyaluronic acid, a host tissue cement. They also produce proteases, nucleases and lipases that depolymerize host proteins, nucleic acids and fats. Clostridia that cause gas gangrene produce collagenase, and -toxin, which breaks down the collagen network supporting the tissues.
The ways in which pathogens bring about damage to the host are diverse. Only rarely are symptoms of a disease due simply to the presence of a large number of microorganisms, although a large mass of bacterial cells can block vessels or heart valves or clog the air passages of the lungs. In many cases, pathogenic bacteria produce toxins that are responsible for host damage. Toxins released extracellularly are called exotoxins, and these may travel from the focus of infection to distant parts of the body and cause damage in regions far removed from the site of microbial growth.
The first example of an exotoxin to be discovered was the diphtheria toxin produced by Corynebacterium diphtheriae. Some Gram negative bacteria produce lipopolysaccharides as part of their cell walls, which under some conditions can be toxic. These are called endotoxins and have been studied primarily in the genera Escherichia, Shigella, and Salmonella.
Cambodia, Phnom Penh,
Russian Federation, Moscow City,
Peoria, Arizona, USA
Comoros, Moroni (on Grande Comoro)