Enzymes and Toxins

Enzymes. Many pathogens produce a series of enzymes to help overcome body defenses and establish themselves in the host. One example is leukocidins, a group of enzymes that destroy white blood cells. This destruction lessens the body's ability to perform phagocytosis.

Other bacterial enzymes are hemolysins. These enzymes destroy red blood cells. Streptococci, staphylococci, and certain Clostridium species produce hemolysins.

Coagulases are bacterial enzymes that clot the blood. These enzymes convert fibrinogen into fibrin, which forms the threads of a blood clot. The clot helps staphylococci avoid the body's phagocytes and contributes to its pathogenicity.

Other important enzymes are streptokinase and hyaluronidase. Streptokinase is a streptococcal enzyme that dissolves blood clots. This activity helps the organism escape the body's attempt to wall off an infection. Hyaluronidase destroys hyaluronic acid, a polysaccharide that “cements” cells together in a tissue. Hyaluronidase thus permits organisms to spread through tissues and establish themselves at sites distant from that of the intial infection. Another enzyme, called collagenase, breaks down collagen in the connective tissues of muscles. It thereby encourages the spread of infection.

Toxins. Many bacteria are able to produce poisonous substances called toxins. Toxins act on the body's cells, tissues, and organs and interfere with important body processes, thereby interrupting normal body functions. Those microorganisms that produce toxins are said to be toxigenic. The condition in which toxins are produced is called toxemia.

Two important types of toxins are exotoxins and endotoxins. Exotoxins are proteins produced by bacteria during their growth and liberated into their surrounding environment. Exotoxins are produced chiefly by Gram‐positive bacteria, and the genes for this production are carried primarily on the plasmids.

Various types of exotoxins exist. Neurotoxins interfere with the nervous system, while enterotoxins interfere with activities of the gastrointestinal tract. In response to toxins, the body produces special antibodies called antitoxins, which unite with and neutralize the toxins, providing defense against disease.

It is possible to immunize against the effects of exotoxins by injecting toxoids into individuals. Toxoids are preparations of exotoxins chemically treated to destroy their toxigenicity but retain their ability to elicit antibody formation in the body. Toxoids are currently available to protect against diphtheria and tetanus (the DT injection).

Endotoxins are portions of the cell wall of Gram‐negative bacteria. They consist primarily of lipopolysaccharides and are released when bacteria break apart during the process of lysis. Since lysis occurs during antibiotic therapy, the effects of endotoxins can bring about a worsening of symptoms during the recovery period. This condition is called endotoxin shock. It is accompanied by fever, chills, aches, and cardiovascular collapse.