WORLD OF MICROBIOLOGY AND IMMUNOLOGY VOL 2 - PART 6 pot

Staphylococci and staphylococci infections WORLD OF MICROBIOLOGY AND IMMUNOLOGY 529 • • molecules. In 1955 Heinz Fraenkel-Conrat, a protein chemist, and R. C. Williams, an electron microscopist, took TMV apart and reassembled the viral RNA, thus proving that RNA was the infectious component. In addition, their work indicated that the protein component of TMV served only as a protective cover. Other workers in the virus laboratory succeeded in isolating and crystallizing the virus responsible for polio, and in 1960, Stanley led a group that determined the complete amino acid sequence of TMV protein. In the early 1960s, Stanley became interested in a possible link between viruses and cancer. Stanley was an advocate of academic freedom. In the 1950s, when his university was embroiled in the politics of McCarthyism, members of the faculty were asked to sign oaths of loyalty to the United States. Although Stanley signed the oath of loyalty, he publicly defended those who chose not to, and his actions led to court decisions which eventually invalidated the requirement. Stanley received many awards, including the Alder Prize from Harvard University in 1938, the Nichols Medal of the American Chemical Society in 1946, and the Scientific Achievement Award of the American Medical Association in 1966. He held honorary doctorates from many colleges and universities. He was a prolific author of more than 150 publi- cations and he co-edited a three volume compendium entitled The Viruses. By lecturing, writing, and appearing on television he helped bring important scientific issues before the public. He served on many boards and commissions, including the National Institute of Health, the World Health Organization, and the National Cancer Institute. Stanley married Marian Staples Jay on June 25, 1929. The two met at the University of Illinois, when they both were graduate students in chemistry. They co-authored a scientific paper together with Adams, which was published the same year they were married. The Stanleys had three daughters and one son. While attending a conference on biochemistry in Spain, Stanley died from a heart attack at the age of 66. See also History of immunology; History of microbiology; Viral genetics; Viral vectors in gene therapy; Virology; Virus replication; Viruses and responses to viral infection STAPHYLOCOCCI AND STAPHYLOCOCCI INFECTIONS Staphylococci and staphylococci infections Staphylococci are a group of Gram-positive bacteria that are members of the genus Staphylococcus. Several infections are caused by staphylococci. In particular, infections associated with methicillin-resistant Staphylococcus aureus are an increasing problem in hospitals. The name staphyloccus is derived from Greek (staphyle—a bunch of grapes). The designation describes the typical grape-like clustered arrangement of staphylococci viewed under a light microscope. Staphylococci are divided into two groups based on the presence or absence of the plasma-clotting enzyme called coagulase. The coagulase-pos- itive staphylococci consist mainly of Staphylococcus aureus and the coagulase-negative group consists primarily of Staphylococcus epidermidis and Staphylococcus saprophyti- cus. Because the treatment of infections caused by these bac- teria can be different, the coagulase test provides a rapid means of indicating the identity of the bacteria of concern. Staphylococci are not capable of movement and do not form spores. They are capable of growth in the presence and absence of oxygen. Furthermore, staphylococci are hardy bac- teria, capable of withstanding elevated conditions of tempera- ture, salt concentration, and a wide pH range. This hardiness allows them to colonize the surface of the skin and the mucous membranes of many mammals including humans. Staphylococcus aureus is the cause of a variety of infec- tions in humans. Many are more of an inconvenience than a threat (e.g., skin infection, infection of hair follicles, etc.). However, other infections are serious. One example is a skin infection known as scalded skin syndrome. In newborns and burn victims, scalded skin syndrome can be fatal. Another example is toxic shock syndrome that results from the infec- tion of a tampon with a toxin-producing strain (other mecha- nisms also cause toxic shock syndrome). The latter syndrome can overwhelm the body’s defenses, due to the production by the bacteria of what is called a superantigen. This superantigen causes a large proportion of a certain type of immune cells to release a chemical that causes dramatic changes in the physi- ology of the body. Staphylococci can also infect wounds. From there, the infection can spread further because some strains of staphylo- cocci produce an arsenal of enzymes that dissolve membranes, protein, and degrade both DNA and RNA. Thus, the bacteria are able to burrow deeper into tissue to evade the host’s immune response and antibacterial agents such as antibiotics. If the infection spreads to the bloodstream, a widespread contami- nation of the body can result (e.g., meningitis, endocarditis, pneumonia, bone inflammation). Because staphylococci are resident on the skin of the hands, the bacteria can be easily transferred to objects or peo- ple. Within the past few decades the extent to which staphylo- cocci infection of implanted devices is a cause of chronic diseases has become clear. For example, contamination of implanted heart valves and artificial hips joints is now recog- nized to be the cause of heart damage and infection of the bone. Additionally, the ready transfer of staphylococci from the skin is an important reason why staphylococci infections are pronounced in settings such as hospitals. Staphylococcus aureus is an immense problem as the source of hospital- acquired infections. This is especially true when the strain of bacteria is resistant to the antibiotic methicillin and other com- mon antibiotics. This resistance necessitates more elaborate treatment with more expensive antibiotics. Furthermore, the infection can be more established by the time the antibiotic resistance of the bacteria is determined. These so-called methicillin-resistant Staphylococcus aureus (MRSA) are resistant to only a few antibiotics currently available. The prevalence of MRSA among all the Staphylococcus aureus that is isolated in hospitals in the United States is about 50%. The fear is that the bacteria will acquire resistance to the remaining antibiotics that are currently effective. This fear is womi_S 5/7/03 8:20 AM Page 529 Steam pressure sterilizer WORLD OF MICROBIOLOGY AND IMMUNOLOGY 530 • • real, since the MRSA is prevalent in an environment (the hos- pital) where antibiotics are in constant use. Development of a fully resistant strain of Staphylococcus aureus would make treatment of MRSA infections extremely difficult, and would severely compromise health care. Staphylococci are also responsible for the poisoning of foods (e.g., ham, poultry, potato salad, egg salad, custards). The poisoning typically occurs if contaminated food is allowed to remain at a temperature that allows the staphylo- cocci to grow and produce a toxin. Ingestion of the toxin pro- duces an intestinal illness and can affect various organs throughout the body. The need for more effective prevention and treatment strategies for staphylococcal infections is urgent, given the wide variety of infections that are caused by staphylococci and the looming specter of a completely resistant staphylococcus. See also Bacteria and bacterial infection; Infection and resistance S TEAM PRESSURE STERILIZER Steam pressure sterilizer Steam pressure sterilization requires a combination of pres- sure, high temperatures, and moisture, and serves as one of the most widely used methods for sterilization where these func- tions will not affect a load. The simplest example of a steam pressure sterilizer is a home pressure cooker, though it is not recommended for accurate sterilization. Its main component is a chamber or vessel in which items for sterilization are sealed and subjected to high temperatures for a specified length of time, known as a cycle. Steam pressure sterilizer has replaced the term auto- clave for all practical purposes, though autoclaving is still A cluster of Staphylococcus bacteria. womi_S 5/7/03 8:20 AM Page 530 Sterilization WORLD OF MICROBIOLOGY AND IMMUNOLOGY 531 • • used to describe the process of sterilization by steam. The function of the sterilizer is to kill unwanted microorganisms on instruments, in cultures, and even in liquids, because the presence of foreign microbes might negatively affect the out- come of a test, or the purity of a sample. A sterilizer also acts as a test vehicle for industrial products such as plastics that must withstand certain pressures and temperatures. Larger chambers are typically lined with a metal jacket, creating a pocket to trap pressurized steam. This method pre- heats the chamber to reduce condensation and cycle time. Surrounding the unit with steam-heated tubes produces the same effect. Steam is then introduced by external piping or, in smaller units, by internal means, and begins to circulate within the chamber. Because steam is lighter than air, it quickly builds enough mass to displace it, forcing interior air and any air-steam mixtures out of a trap or drain. Most sterilization processes require temperatures higher than that of boiling water (212°F, 100°C), which is not suffi- cient to kill microorganisms, so pressure is increased within the chamber to increase temperature. For example, at 15 psi the temperature rises to 250°F (121°C). Many clinical appli- cations require a cycle of 20 minutes at this temperature for effective sterilization. Cycle variables can be adjusted to meet the requirements of a given application. The introduction of a vacuum can further increase temperature and reduce cycle time by quickly removing air from the chamber. The process of steam sterilization is kept in check by pressure and temper- ature gauges, as well as a safety valve that automatically vents the chamber should the internal pressure build beyond the unit’s capacity. See also Infection control; Laboratory techniques in micro- biology STENTOR Stentor Stentor is a genus of protozoan that is found in slow moving or stagnant fresh water. The microorganism is named for a Greek hero in the Trojan War, who was renowned for his loud voice, in an analogous way to the sound of a trumpet rising up over the sound of other instruments. The description is fitting the microorganism because the organism is shaped somewhat like a trumpet, with small end flaring out to form a much larger opening at the other end. The narrow end can elaborate a sticky substance that aids the protozoan in adhering to plants. At the other end, fine hair-like extensions called cilia beat rhythmically to drive food into the gullet of the organism. The various species of stentor tend to be brightly colored. For example, Stentor coeruleus is blue in color. Other species are yellow, red, and brown. Stentor are one of the largest protozoa found in water. As a protozoan, Stentor is a single cell. Nonetheless, a typical organism can be 2 mm in length, making them visible to the unaided eye, and even larger than some multi-celled organ- isms such as rotifers. This large size and ubiquity in pond water has made the organism a favorite tool for school science classes, particularly as a learning tool for the use of the light microscope. In particular, the various external and internal features are very apparent under the special type of micro- scopic illumination called phase contrast. Use of other forms of microscopic illumination, such as bright field, dark field, oblique, and Rheinberg illumination, can each reveal features that together comprise a detailed informational picture of the protozoan. Thus, examination of stentor allows a student to experiment with different forms of light microscopic illumi- nation and to directly compare the effects of each type of illu- mination of the same sample. Another feature evident in Stentor is known as a con- tractile vacuole. The vacuole functions to collect and cycle back to the outside of Stentor the water that flows in to balance the higher salt concentration inside the protozoan. Careful observation of the individual protozoa usually allows detec- tion of full and collapsed vacuoles. For the student, fall is a good time to observe Stentor. Leaves that have fallen into the water decay and support the growth of large numbers of bacteria. These, in turn, support the growth of large numbers of stentor. See also Microscope and microscopy; Water pollution and purification STERILIZATION Sterilization Sterilization is a term that refers to the complete killing or elim- ination of living organisms in the sample being treated. Sterilization is absolute. After the treatment the sample is either devoid of life, or the possibility of life (as from the subsequent germination and growth of bacterial spores), or it is not. There are four widely used means of sterilization. Standard sterilization processes utilize heat, radiation, chemi- cals, or the direct removal of the microorganisms. The most widely practiced method of sterilization is the use of heat. There are a number of different means by which heat can be applied to a sample. The choice of which method of delivery depends on a number of factors including the type of sample. As an example, when bacterial spores are present the heating conditions must be sufficient to kill even these dor- mant forms of the bacteria. A common type of heat sterilization that is used many types each day in a microbiology laboratory is known as incin- eration. Microorganisms are burned by exposing them to an open flame of propane. “Flaming” of inoculating needles and the tops of laboratory glassware before and after sampling are examples of incineration. Another form of heat sterilization is boiling. Drinking water can be sterilized with respect to potentially harmful microorganisms such as Escherichia coli by heating the water to a temperature of 212°F (100°C) for five minutes. However, the dormant cyst form of the protozoan Giardia lamblia that can be present in drinking water, can survive this period of boiling. To ensure complete sterility, the 212°F (100°C) tem- perature must be maintained for 30 minutes. Even then, some bacterial spores, such as those of Bacillus or Clostridium can survive. To guarantee sterilization, fluids must be boiled for an womi_S 5/7/03 8:20 AM Page 531 Strep throat WORLD OF MICROBIOLOGY AND IMMUNOLOGY 532 • • extended time or intermittent boiling can be done, wherein at least three—and up to 30—periods of boiling are interspersed with time to allow the fluid to cool. Steam heat (moist heat) sterilization is performed on a daily basis in the microbiology laboratory. The pressure cooker called an autoclave is the typical means of steam heat sterilization. Autoclaving for 15 minutes at 15 pounds of pres- sure produces a temperature of 250°F (121°C), sufficient to kill bacterial spores. Indeed, part of a quality control regiment for a laboratory should include a regular inclusion of com- mercially available bacterial spores with the load being steril- ized. The spores can then be added to a liquid growth medium and growth should not occur. Pasteurization is employed to sterilize fluids such as milk without compromising the nutritional or flavor qualities of the fluid. The final form of heat sterilization is known as dry heat sterilization. Essentially this involves the use of an oven to heat dry objects and materials to a temperature of 320–338°F (160–170°C) for two hours. Glassware is often sterilized in this way. Some samples cannot be sterilized by the use of heat. Devices that contain rubber gaskets and plastic surfaces are often troublesome. Heat sterilization can deform these materi- als or make them brittle. Fortunately, other means of steriliza- tion exist. Chemicals or gas can sterilize objects. Ethylene oxide gas is toxic to many microorganisms. Its use requires a special gas chamber, because the vapors are also noxious to humans. Chemicals that can be used to kill microorganisms include formaldehyde and glutaraldehyde. Ethanol is an effective ster- ilant of laboratory work surfaces. However, the exposure of the surface to ethanol must be long enough to kill the adherent microorganisms, otherwise survivors may develop resistance to the sterilant. Another means of sterilization utilizes radiation. Irradiation of foods is becoming a more acceptable means of sterilizing the surface of foods (e.g., poultry). Ultraviolet radi- ation acts by breaking up the genetic material of microorgan- isms. The damage is usually too severe to be repaired. The sole known exception is the radiation-resistant bacteria of the genus Deinococcus. The final method of sterilization involves the physical removal of microorganisms from a fluid. This is done by the use of filters that have extremely small holes in them. Fluid is pumped through the filter, and all but water molecules are excluded from passage. Filters—now in routine use in the treatment of drinking water—can be designed to filter out very small microorganisms, including many viruses. See also Bacterial growth and division; Bacteriocidal, bacte- riostatic; Laboratory techniques in microbiology STREP THROAT Strep throat Streptococcal sore throat, or strep throat as it is more com- monly called, is an infection caused by group A Streptococcus bacteria. The main target of the infection is the mucous mem- branes lining the pharynx. Sometimes the tonsils are also infected (tonsillitis). If left untreated, the infection can develop into rheumatic fever or other serious conditions. Strep throat is a common malady, accounting for 5–10% of all sore throats. Strep throat is most common in school age children. Children under age two are less likely to get the dis- ease. Adults who smoke, are fatigued, or who live in damp, crowded conditions also develop the disease at higher rates than the general population. The malady is seasonal. Strep throat occurs most fre- quently from November to April. In these winter months, the disease passes directly from person to person by coughing, sneezing, and close contact. Very occasionally the disease is passed through food, most often when a food handler infected with strep throat accidentally contaminates food by coughing or sneezing. Once infected with the Streptococcus, a painful sore throat develops from one to five days later. The sore throat can be accompanied by fatigue, a fever, chills, headache, muscle aches, swollen lymph glands, and nausea. Young children may complain of abdominal pain. The tonsils look swollen and are bright red with white or yellow patches of pus on them. Sometimes the roof of the mouth is red or has small red spots. Often a person with strep throat has a characteristic odor to their breath. Others who are infected may display few symptoms. Still others may develop a fine, rough, sunburn-like rash over the face and upper body, and have a fever of 101–104ºF (38–40ºC). The tongue becomes bright red with a flecked, strawberry-like appearance. When a rash develops, this form of strep throat is called scarlet fever. The rash is a reaction to toxins released by the streptococcus bacteria. Scarlet fever is essentially treated the same way. The rash disappears in about five days. One to three weeks later, patches of skin may peel off, as might occur with a sunburn. Strep throat can be self-limiting. Symptoms often sub- side in four or five days. However, in some cases untreated strep throat can cause rheumatic fever. This is a serious illness, although it occurs rarely. The most recent outbreak appeared in the United States in the mid-1980s. Rheumatic fever occurs most often in children between the ages of five and 15, and may have a genetic component, because susceptibility seems to run in families. Although the strep throat that causes rheu- matic fever is contagious, rheumatic fever itself is not. Rheumatic fever begins one to six weeks after an untreated streptococcal infection. The joints, especially the wrists, elbows, knees, and ankles become red, sore, and swollen. The infected person develops a high fever, and possi- bly a rapid heartbeat when lying down, paleness, shortness of breath, and fluid retention. A red rash over the trunk may come and go for weeks or months. An acute attack of rheumatic fever lasts about three months. Rheumatic fever can cause per- manent damage to the heart and heart valves. It can be pre- vented by promptly treating streptococcal infections with antibiotics. It does not occur if all the Streptococcus bacteria are killed within the first 10–12 days after infection. womi_S 5/7/03 8:20 AM Page 532 Streptococci and streptococcal infections WORLD OF MICROBIOLOGY AND IMMUNOLOGY 533 • • In the 1990s, outbreaks of a virulent strain of group A Streptococcus were reported to cause a toxic-shock-like illness and a severe invasive infection called necrotizing fasciitis, which destroys skin and muscle tissue. Although these dis- eases are caused by group A Streptococcus, they rarely begin with strep throat. Usually the Streptococcus bacteria enter the body through a skin wound. These complications are rare. However, since the death rate in necrotizing fasciitis is 30–50%, prompt medical attention for any streptococcal infec- tion is prudent. The Streptococcus bacteria are susceptible to antibiotics such as penicillin. However, in some 10% of infections, peni- cillin is ineffective. Then, other antibiotics are used, including amoxicillin, clindamycin, or a cephalosporin. See also Bacteria and bacterial infection; Streptococci and streptococcal infections STREPTOCOCCAL ANTIBODY TESTS Streptococcal antibody tests Species of Gram positive bacteria from the genus Strepto- coccus are capable of causing infections in humans. There are several disease-causing strains of streptococci. These strains have been categorized into groups (A, B, C, D, and G), accord- ing to their behavior, chemistry, and appearance. Each group causes specific types of infections and symptoms. For example, group A streptococci are the most virulent species for humans and are the cause of “strep throat,” tonsillitis, wound and skin infections, blood infections (sep- ticemia), scarlet fever, pneumonia, rheumatic fever, Sydenham’s chorea (formerly called St. Vitus’ dance), and glomerulonephritis. While the symptoms affected individuals experience may be suggestive of a streptococcal infection, a diagnosis must be confirmed by testing. The most accurate common pro- cedure is to take a sample from the infected area for culture, a means whereby the bacteria of interest can be grown and iso- lated using various synthetic laboratory growth media. This process can take weeks. A more rapid indication of the pres- ence of streptococci can be obtained through the detection of antibodies that have been produced in response to the infecting bacteria. The antibody-based tests can alert the physician to the potential presence of living infectious streptococci. The presence of streptococci can be detected using anti- body-based assays. Three streptococcal antibody tests that are used most often are known as the antistreptolysin O titer (ASO), the antideoxyribonuclease-B titer (anti-Dnase-B, or ADB), and the streptozyme test. The antistreptolysin O titer determines whether an infection with the group A Streptococcus has precluded the development of post-infection complications. The term titer refers to the amount of antibody. Thus, this test is quantitative. That is, the amount of specific antibody in the sample can be deduced. In an infection the amount of antibody will rise, as the immune system responds to the invading bacteria. These complications include scarlet fever, rheumatic fever, or a kid- ney disease termed glomerulonephritis. The ASO titer is used to demonstrate the body’s reaction to an infection caused by group A beta-hemolytic streptococci. The beta-hemolytic designation refers to a reaction produced by the bacteria when grown in the presence of red blood cells. Bacteria of this group are particularly important in suspected cases of acute rheumatic fever (ARF) or acute glomeru- lonephritis. Group A streptococci produce the enzyme strep- tolysin O, which can destroy (lyse) red blood cells. Because streptolysin O is antigenic (contains a protein foreign to the body), the body reacts by producing antistreptolysin O (ASO), which is a neutralizing antibody. ASO appears in the blood serum one week to one month after the onset of a strep infec- tion. A high titer (high levels of ASO) is not specific for any type of poststreptococcal disease, but it does indicate if a streptococcal infection is or has been present. Tests conducted after therapy starts can reveal if an active infection was in progress. This will be evident by a decreasing antibody titer over time, as more and more of the streptococci are killed. The anti-DNase-B test likewise detects groups A beta- hemolytic Streptococcus. This test is often done at the same time as the ASO titer. This done as the Dnase-based test can produce results that are more variable than those produced by the ASO test. This blanket coverage typically detects some 95% of previous strep infections are detected. If both tests are repeatedly negative, the likelihood is that the patient’s symp- toms are not caused by a poststreptococcal disease. The final antibody-based test is a screening test. That is, the test is somewhat broader in scope than the other tests. The streptozyme test is often used as a screening test for antibod- ies to the streptococcal antigens NADase, DNase, streptoki- nase, streptolysin O, and hyaluronidase. This test is most useful in evaluating suspected poststreptococcal disease fol- lowing infection with Streptococcus pyogenes, such as rheu- matic fever. The streptozyme assay has certain advantages over the other two tests. It can detect several antibodies in a single assay, is quick and easy to perform, and is unaffected by fac- tors that can produce false-positives in the ASO test. However, the assay does have some disadvantages. While it detects dif- ferent antibodies, it does not determine which one has been detected, and it is not as sensitive in children as in adults. See also Antibody and antigen; Antibody formation and kinet- ics; Bacteria and bacterial infection STREPTOCOCCI AND STREPTOCOCCAL INFECTIONS Streptococci and streptococcal infections Streptococci are spherical, Gram positive bacteria. Commonly they are referred to as strep bacteria. Streptococci are normal residents on the skin and mucous surfaces on or inside humans. However, when strep bacteria normally found on the skin or in the intestines, mouth, nose, reproductive tract, or urinary tract invade other parts of the body—via a cut or abrasion—and contaminate blood or tissue, infection can be the result. womi_S 5/7/03 8:20 AM Page 533 . of the Rockefeller Foundation, more than 28 million 17D-strain vac- womi_T 5/7/03 11: 02 AM Page 545 Thermal death WORLD OF MICROBIOLOGY AND IMMUNOLOGY 5 46 • • cines were produced, at a cost of. School and the London School of Hygiene and Tropical Medicine, two branches of the womi_T 5/7/03 11: 02 AM Page 544 Theiler, Max WORLD OF MICROBIOLOGY AND IMMUNOLOGY 545 • • University of London 10– 12 days after infection. womi_S 5/7/03 8 :20 AM Page 5 32 Streptococci and streptococcal infections WORLD OF MICROBIOLOGY AND IMMUNOLOGY 533 • • In the 1990s, outbreaks of a virulent strain of