Streptococcus pyogenes is one of the most common pathogenic bacteria of humans. This particular type of Streptococcus is most often associated with diseases in humans. Other animals may harbor the bacterium in their bodies, but whether it causes disease in these animals is unknown. Like Staphylococcus, Streptococcus has many characteristics that produce illness in its host. In contrast to Staphylococcus, Streptococcus pyogenes is a more aggressive pathogen that can cause disease under normal conditions in healthy people. Streptococcus pyogenes primarily inhabits the skin and respiratory system. It is not found in the digestive system like other types of Streptococcus. Thus, it can only enter the body through a few very specific routes. Natural pathogens such as Streptococcus pyogenes have features called virulence factors that enable them to cause disease. These factors provide a variety of characteristics that permit the bacterium to reside in the host and evade the host’s immune system and are similar to the disease-causing characteristics of Staphylococcus mentioned in Chapter 4. Some examples of virulence factors include the cell wall and cellular secretions. The secretions include antigens, enzymes, and toxins that assist with various functions needed to feed and fend off the host’s immune defenses. These defenses also give Streptococcus pyogenes its name. The word “pyogenes” comes from the Greek term “pyogenic,” meaning to produce pus. Pus is an indication of severe disease. It contains a large concentration of white blood cells, immune-system fluids, and bacteria. Do not confuse pyogens with pyrogens. Pyrogens, a term derived from the Greek word burn which means producing fire or warmth, are chemicals that create a fever. Streptococcus pyogenes gains a foothold in the body by using cell wall proteins called adhesins. Adhesins help microbes stick to the cells of other organisms. These adhesive proteins are not the same as the proteins that are normal constituents of the Gram-positive cell wall. Adhesions are solely for the purpose of attaching the bacterium to host tissues. As mentioned in Chapter 4, Staphylococcus adheres to its host using sticky components naturally found in the cell wall. Streptococcus pyogenes possesses a special adhesin called F protein. F protein ensures that Streptococcus pyogenes will bind to host proteins in a way that permits the bacterium to slip easily between the cells of the body. This enables Streptococcus pyogenes to pass through the mucous membranes and skin without the need for an opening. Staphylococcus and other bacteria usually gain entry into the body through damaged portions of the mucous membranes and skin. Another adhesin called M protein makes Streptococcus pyogenes difficult to remove from the mucous membranes and skin. M protein sticks specifically to the cells of these tissues. Scientists use M protein to detect different types of Streptococcus. Scientists have identified 80 different types of M proteins in bacteria. Other cell wall components also protect Streptococcus pyogenes from destruction by the body’s immune defenses. M protein not only allows the bacterium to stick to cells but it also inhibits digestion by macrophages. Another protein called M-like protein protects Streptococcus pyogenes in a similar manner. M-like proteins also cloak the bacterium from the host’s immune system. Streptococcus pyogenes uses the M-like proteins to glue host proteins to its surface, making the body think the bacterium is a body cell. A capsule covers the cell wall of Streptococcus pyogenes, protecting it from digestion by macrophages. The capsule may resemble a body component of animals called hyaluronic acid. Thus, the body is less likely to attack Streptococcus pyogenes because it is tricked into treating the bacterium as a normal part of the body. Another protein called C5a peptidase permits Streptococcus pyogenes to avoid immune-system chemicals called complements. Complements label invading microbes for destruction and removal by the immune system (Figure 5.2). Streptococcus pyogenes, like other Gram-positive bacteria, produce secretions that confound the body’s immune response. On the top of the list is a potent group of poisons called exotoxins. Streptococcus pyogenes secrete various pyrogenic exotoxins that induce fever in the host. Scientists call these pyrogens Streptococcus pyogenes exotoxins, which they abbreviate as SPE A, SPE B, and SPE C. These exotoxins can cause immediate death when laboratory animals receive an injection of small amounts. SPE A is usually associated with killing the organism outright, whereas SPE B induces specific death to the heart muscle cells. All three of these exotoxins will produce a rash on the mucous membranes and skin (Figure 5.3). Chemicals that produce a rash are called erythrogenic toxins. Erythrogen means to turn something red. Scientists now know that different strains of Streptococcus pyogenes produce varying amounts of the pyrogens, making some strains more dangerous. Aside from producing fever, pyrogens overstimulate the immune system. This then causes the body to destroy itself accidentally, producing an abnormal condition called autoimmunity. Much of the tissue decay of StrepTSS is due to the body unintentionally destroying itself. This condition is very difficult to treat and any treatments could encourage further spread of Streptococcus pyogenes. A variety of enzymes that destroy host cells and tissues occur in all strains of Streptococcus pyogenes. Hemolysins, similar to ones produced by Staphylococcus, destroy red blood cells, making available a rich supply of iron and protein needed for rapid bacterial growth.
Streptolysin O and S are two hemolysins unique to Streptococcus pyogenes. Injections of these enzymes cause immediate death to laboratory animals. The enzyme hyaluronidase helps Streptococcus pyogenes digest their way through the body, permitting them to invade any tissue readily. The body secretes deoxyribonuclease, or DNase, to assist with the digestion of DNA from dying cells. This provides Streptococcus pyogenes with the nutrients needed for rapid reproduction. Streptococcus pyogenes produces two unique enzymes called lipoproteinase and streptokinase. The role of lipoproteinase is unknown. However, most production occurs in strains of Streptococcus pyogenes that cause skin infections. The strain of Streptococcus pyogenes identified to cause StrepTSS does not secrete large amounts of this enzyme. Streptokinase plays a unique role by breaking down blood clots. This prevents Streptococcus pyogenes from being barricaded by blood clots that would restrict its movement throughout the body. This function is assisted by a cell-wall component called plasminogen-binding site protein. Staphylococcus does the opposite by encouraging clots that hide it from the immune system. Streptococcus pyogenes does not need that type of protection from the body. The different severities of disease caused by Streptococcus pyogenes relate directly to the combination of features found in a particular strain of the bacterium. This makes it challenging for physicians who have to diagnose diseases caused by Streptococcus pyogenes. The bacteria found in blood samples must be thoroughly tested to determine the type of damage they can produce. The tests also permit the physician to predict the outcome of the disease. It helps to know whether the bacteria are going to stay on the mucous membranes or are likely to invade the whole body. Physicians also need to know how good the bacteria are at evading the host’s immune system.
See also: The same toxins that bring about the signs and symptoms of toxic shock have beneficial value if used in creative ways. The enzyme streptokinase secreted by Streptococcus normally helps the bacterium by preventing clot formation in its host. The inability to form clots permits the bacteria better access to the blood and body organs. However, as a medicine, streptokinase can break down blood clots that complicate heart disease. Physicians learned that streptokinase could be injected into the blood in low dosages without harming people. These injections are therapeutic in people who form blood clots that block important blood vessels of the heart and brain. A host of other bacterial secretions are being employed for diverse medical uses such as helping skin to heal without a scar to selectively killing cancer cells without harming the rest of the body.
Description of the first image - Figure 5.2 The complement system (shown here) labels invading microorganisms for destruction. An antibody binds to the antigen (foreign cell). A complement protein binds to the antibody and then breaks apart. One part of the complement protein remains attached to the antibody and serves to attract other complement proteins. The other fragment detaches from the antibody and searches out other complement proteins. Eventually, many complement proteins will attach to a cell and cause it to burst.
Description of the second image - Figure 5.3 Streptococcus pyogenes secretions called pyogenes produce the characteristic skin rash of StrepTSS. These secretions contain exotoxins, which can be deadly if they occur in large amounts. One of the first signs that these exotoxins are present is the red rash, seen here on this man.
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