1. Microbial toxin There are several virulence factors which help to establish disease The virulence of some bacteria is thought to be aided by the production of extracellular enzymes (exoenzymes) and related substances. These chemicals can break cells open, dissolve materials between cells, and form or dissolve blood clots, among other functions. Substances called leukocidins, produced by some bacteria, can destroy neutrophils, leukocytes (white blood cells) that are very active in phagocytosis. Leukocidins are also active against macrophages (phagocytic cells) present in tissues. Among the bacteria that secrete leukocidins are staphylococci and streptococci. Leukocidins produced by streptococci degrade lysosomes within leukocytes, thereby causing the death of the white blood cell.

2. Hemolysins are bacterial enzymes that cause the lysis of erythrocytes (red blood cells). Bacteria produce hemolysins that differ in their ability to lyse different kinds of red blood cells (humans, sheep, and rabbits, for example) and the type of lysis they cause. Important producers of hemolysins are staphylococci; Clostridium pefringens, the most common causative agent of gas gangrene; and streptococci. Streptolysins are hemolysins produced by streptococci. Both streptolysins can cause the lysis of not only red blood cells, but also white blood cells (whose function is to kill the streptococci) and other body cells.

3. Coagulases are bacterial enzymes that coagulate (clot) the fibrinogen in blood. Fibrinogen, a plasma protein produced by the liver, is converted by coagulases into fibrin, the threads that form a blood clot. The fibrin clot may protect the bacterium from phagocytosis and isolate it from other defenses of the host. Coagulases are produced by some members of the genus Staphylococcus

4. Capsule Some strains of this organism have capsules, and others do not. One bacterium that owes its virulence to the presence of a polysaccharide capsule is Streptococcus pneumoniae, the causative agent of pneumococcal pneumonia. Some strains of this organism have capsules, and others do not. Strains with capsules are virulent, but strains without capsules are avirulent because they are susceptible to phagocytosis. Other bacteria that produce capsules related to virulence are Kiebsiella pneumoniae, a causative agent of bacterial pneumonia; Haemophilus influenzae, a cause of pneumonia and meningitis in children; Bacillus anthracis, the cause of anthrax: and Yersinia pestis, the causative agent of bubonic plague.

5. Components of the Cell Wall The cell walls of certain bacteria contain chemical substances that contribute to virulence. For example, Streptococcus pyogenes produces a heat—resistant and acid—resistant protein called M protein. This protein is found on both the cell surface and fimbriae. The M protein mediates attachment of the bacterium to epithelial cells of the host and helps the bacterium resist phagocytosis by white blood cells. The protein thereby increases the virulence of the microorganism. Immunity to S. pyogenes depends on the body’s production of an antibody specific to M protein.

6. Toxins Toxins are poisonous substances that are produced by certain microorganisms.. They are often the primary factor contributing to the pathogenic properties of those microbes. The capacity of microorganisms to produce toxins is called toxigenicity. Toxins transported by the blood or lymph can cause serious, and sometimes fatal, effects. Some toxins produce fever, cardiovascular disturbances, diarrhea, and shock. Toxins can also inhibit protein synthesis, destroy blood cells and blood vessels, and disrupt the nervous system by causing spasms. Of the 220 or so known bacterial toxins, nearly 40% cause disease by damaging eukaryotic cell membranes.

7. Toxins are of two types: exotoxins and endotoxins. Exotoxins may be grouped into three principal types, based on their mode of action: (1) cytotoxins, which kill host cells or affect their functions; (2) neurotoxins, which interfere with normal nerve impulse transmission; and (3) enterotoxins, which affect cells lining the gastrointestinal tract. Exotoxins are among the most lethal substances known. Only 1 mg of the botulinum exotoxin is enough to kill 1 million guinea pigs. Fortunately, only a few bacterial species produce such potent exotoxins

8. C. botulinum produces several diffe rent types of botulinum toxin, and each possesses a á&ent potency. ‘Tetanus Toxin: Ciostridium tetani produces tetanus neurotoxin, also known as tetanospasmin. This toxin reaches central nervous system and binds to nerve cells that wzxirol the contraction of various skeletal muscles. These w cells normally send inhibiting impulses that prevent idom contractions and terminate completed contrac