What are bacterial infections?
Bacteria are very small, one-celled organisms (the cell being the smallest unit of a living organism) with an average size of thousandths of a millimeter. Based on their relatively simple structure, they are classified as prokaryotic cells. Prokaryotic cells have a rigid outer cell wall, very simply organized hereditary material (deoxyribonucleic acid, or DNA) floating free within the cell, and only a few other structures necessary for their survival, growth, and reproduction. Eukaryotic cells, such as those found in humans, plants, and other animals, have highly organized DNA and many more internal structures. Despite the fact that bacteria are relatively “simple,” they are still very complex living organisms.
The many types of bacteria can be divided into three categories based on their shape: coccus (round), bacillus (rod-shaped), or spirillum (spiral). Another major distinction between types of bacteria is based on the sugar and lipid (fat) composition of their cell walls. This difference can be identified through Gram staining, the result of the stain determining whether the organism is gram-positive or gram-negative. Various types of bacteria may have additional structures that are useful in their identification. Capsules and slime layers are water-rich sugary materials secreted by the bacteria that cling to their surfaces and form halolike structures. Flagella are long, thin, whiplike structures found in one location on the bacterium or occasionally covering its entire surface. These structures are used to enhance the motility, or movement, of the bacteria.
Some bacteria are normal, harmless inhabitants of human bodies, such as those on the surface of the skin. Others, such as those that live in the human intestinal tract, aid in digestion and are essential for good health. The warm, moist, nutrient-rich human body also provides an excellent breeding ground for numerous harmful bacterial invaders. For bacteria to cause infectious disease, several stages must occur. The bacteria must enter the person, they must survive and multiply on or in the person, they must resist the natural defenses of the human body, and they must damage the infected person. Most bacterial diseases are infectious because of the ease with which they can be transmitted from individual to individual by physical contact with the person, a contaminated object, or bacteria expelled into the air, such as by coughing or sneezing. A few bacterial diseases, such as food poisoning, are not classified as infectious.
Bacterial infections cause disease by a variety of mechanisms. Many of them produce chemical compounds that are toxic to human beings. For example, Salmonella and Staphylococcus aureus are two types of bacteria that are capable of causing food poisoning. Clostridium botulinum produces the deadly botulism toxin. In each case, ingestion of the toxin in contaminated food can lead to serious illness. Clostridium tetanii can enter the body through puncture wounds and will multiply rapidly deep in tissue where there is little exposure to the air. The toxin that it produces acts on the central nervous system and causes severe muscle spasms, which can lead to death from respiratory failure. Water that has been contaminated with raw sewage is a potent source of disease-causing bacteria. Vibrio cholera produces a potent toxin that causes severe diarrhea leading to death if it is not vigorously treated. Certain varieties of Escherichia coli and Shigella found in contaminated water can also cause severe intestinal disorders. Toxic shock syndrome is associated with the production of toxins by Staphylococcus aureus.
Another common cause of disease from bacterial infections is the result of the physical destruction of tissue by the invading organisms. Leprosy (also called Hansen’s disease), caused by Mycobacterium leprae, if left untreated, can lead to severe deterioration and disfiguration of large areas of a person’s body. If a wound interrupts the blood supply to an area of the body such as a hand or foot, the tissues begin to decay, thereby providing nutrients for many bacteria, especially Clostridium perfringens. These bacteria can greatly accelerate the destruction of the tissue, which causes the condition known as gas gangrene.
In many cases, disease results when the infecting bacteria are recognized by the body’s natural defense system (the immune system) as “nonself,” that is, as invaders. Certain cells within the body are designed to attack intruders and eliminate them. During this process, disease symptoms that are consequences of the immune system’s response may be evident: inflammation (redness and swelling), the production of pus, and fever, among other symptoms. In some cases, certain components of the bacteria, such as capsules and slime layers, may protect them from being eliminated by the immune system. The bacteria may also multiply exceedingly rapidly, producing increasing amounts of toxins that overwhelm the capacity of the immune system to eliminate them. In these cases, continued and increasingly elevated disease symptoms such as fever can cause severe, even fatal, damage unless an alternate method for eliminating the infection is found. Failure to eliminate the bacterial invaders can also lead to a long-term, chronic infection that damages body tissues.
Many respiratory diseases are associated with the body’s immune response to bacterial invasion. Streptococcus pyogenes is the causative agent of strep throat, whose features include severe redness, inflammation, pain, and the production of pus in throat tissue. In a small percentage of cases, strep throat can also lead to an infection of and potential permanent damage to the heart valves in a disease called rheumatic fever. In tuberculosis, Mycobacterium tuberculosis enters the lungs through inhalation. The body’s defense system walls off the intruders and forms a nodule called a tubercle deep in the lung tissue. Nevertheless, the bacteria continue to multiply in the nodule and can travel to new sites in the lung. Tubercle formation occurs at these new sites. Eventually, this repeated cycle of infection and nodule formation becomes a chronic disease and leads to the destruction of lung tissue. Bacterial pneumonia can be caused by several different organisms, including Klebsiella pneumoniae and Mycobacterium pneumoniae. A pneumonia-like disease, Legionnaires’ disease, was first identified in 1976 after twenty-nine delegates to an American Legion convention died from a mysterious respiratory disorder. The lengthy process of identifying a causative agent led to the discovery of a type of bacteria not previously known, Legionella pneumophila.
The human urinary and genital tracts are also potential havens for invading bacteria. Cystitis (bladder infections) are caused by many different types of bacteria. Kidney infections can be acquired as corollaries of urinary tract infections. Sexually transmitted diseases (STDs) are contracted through sexual contact with an infected partner, and two common STDs have a bacterial origin. Gonorrhea is caused by Neisseria gonorrhoeae and leads to a severe inflammatory response and rapid spread of the organisms throughout the body. If not treated, it can lead to sterility as well as to diseases of the joints, heart, nerve coverings, eyes, and throat. Syphilis, caused by Treponema pallidum, can also have serious consequences if left untreated, including dementia and death. In addition, it can be passed to a fetus developing inside an infected mother in a condition known as congenital syphilis.
The medical management of the many bacterial infections and the diseases they cause begins with diagnosis. Diagnosis relies on a variety of biochemical tests that are analyzed in conjunction with the signs and symptoms exhibited by the infected individual. Treatment is then designed so that it not only eliminates the disease symptoms but also eradicates all invading bacterial organisms, thereby minimizing the chance of a recurrence of the disease. Prevention involves steps that the individual takes to avoid potential contact with infectious diseases, as well as the use of medical procedures that protect against specific bacterial diseases.
To treat a bacterial disease properly, the invading organism must be identified correctly. In some cases, symptomology can be specific enough to identify the offending bacterium, but since there are literally thousands of different types of disease-causing organisms, a systematic approach using a variety of tests is undertaken to make a definitive diagnosis. First, a specimen from the infected person is collected. This may be a blood or urine sample; a swab of the infected area, such as the throat or another skin surface; or a secretion, such as sputum, mucus, or pus. Since human bodies are normally inhabited by a variety of harmless bacteria, the individual types of bacteria are isolated in pure cultures, in which each bacterium present is of the same type. The pure cultures are then tested to determine the identity of the organisms. Staining procedures, such as Gram staining, and microscopic examination of the stained bacteria to determine the Gram reaction and the shape of the bacteria can narrow down the identity of the organisms considerably.
Based on these results, a standard series of tests is performed, continually narrowing down the possible identities until only one remains. One test measures the organisms’ growth requirements. Many identifications are aided by analyzing the types of sugars and proteins that the organisms can use as food sources. The by-products of their metabolism (chemical reactions occurring inside the bacteria), such as acids and gas, are identified. Oxygen requirements, motility, and the presence of a capsule are three other common characteristics that are examined. For cases in which the identification of a particular variety of one type of bacteria is necessary, more complex tests may be undertaken, such as an analysis of the particular sugars and proteins on the surface of the organism or tests for the production of specific toxins. Once the bacterium’s identity is confirmed, treatment may begin.
The most common type of treatment for bacterial infection is antibiotic therapy. Antibiotics are chemical compounds that kill bacteria. Originally discovered as antibacterial compounds produced by bacteria, molds, and fungi (such as penicillin from bread mold), many more are synthetically produced. Antibiotics work in a variety of fashions. Some, such as penicillin and the cephalosporins, interfere with the synthesis of cell walls by bacteria, thus preventing the organisms from multiplying. Other commonly used antibiotics prevent the bacteria from synthesizing the proteins that they need to survive and multiply. These include the tetracyclines (a class of antibiotics that act against a large range of bacteria), erythromycin, and streptomycin. A host of other antibiotics target a variety of bacterial functions, including specific chemical reactions and the propagation of genetic material, and the structural components of the bacteria. Each class of antibiotics works best on certain types of bacteria. For example, penicillin is most efficient in killing cocci (such as Streptococcus and Staphylococcus) and Gram-positive bacilli.
Frequently, the symptoms of a bacterial disease may disappear rapidly after the beginning of antibiotic therapy. This reaction is attributable to the inhibition of bacterial multiplication and the destruction of most of the microorganisms. A small number of the bacteria may not be killed during this initial exposure to antibiotics, however, and if antibiotic therapy is ended before all are killed, a recurrence of the disease is likely. A full prescription of antibiotics should be taken to avoid this situation. For example, effective treatment and eradication of all bacteria in tuberculosis may take six months to a year or more of antibiotic treatment, despite the fact that the symptoms are alleviated in a few weeks.
Upon repeated exposure to a type of antibiotic, some bacteria develop the capacity to degrade or inactivate the antibiotic, thus rendering that drug ineffective against the resistant microorganism. In these cases, other antibiotics and newly developed ones are tested for their effectiveness against the bacteria. Such situations have arisen in the bacteria that cause gonorrhea and tuberculosis.
While antibiotics exist to combat infections of most types of bacteria, in some cases the human immune system is capable of clearing the infection without additional intervention. In these instances, the symptoms of the infection are treated until the body heals itself. This is the common treatment path in mild cases of food poisoning, such as those caused by some Salmonella and Staphylococcus varieties. Diarrhea and vomiting are treated by replacing water and salts, by drinking large volumes of fluids, and perhaps by using over-the-counter remedies to ease some of the symptoms.
Many bacterial diseases can be easily prevented through good hygiene. Foods, such as eggs and meats, that are not thoroughly cooked may become quickly contaminated by the rapid growth of food-poisoning organisms present on their surfaces. Proper cooking kills these organisms. Similarly, foods that are not properly stored but left out in warm places can also provide a potent breeding ground for toxin-producing bacteria. Picnic food not properly refrigerated is a common source of food poisoning. Similarly, questionable water sources should never be used for drinking or cooking water without proper treatment. Filtering with an ultrafine filter specifically designed to remove bacteria is one safeguard, as is boiling for the required time period based on altitude. Food that may have been washed with contaminated water sources should always be cooked or peeled before consumption.
Many diseases can be prevented with vaccinations. A bacterial vaccine is a mixture of a particular bacterium, its parts, or its inactivated toxins. When this solution is injected into an individual, it provides immunity (resistance to infection) to the particular organism contained in the vaccine. Some vaccines provide lifetime immunity when enhanced with an occasional booster shot, while some are relatively short-acting. Many types of vaccines that are directed against specific diseases are part of standard preventive care given to children. For example, the DTaP vaccine (or Tdap for adults) confers immunity to diphtheria, pertussis (whooping cough), and tetanus. Some vaccines are useful for individuals who are living, working, or traveling in areas where certain diseases are endemic, or for those who regularly come into contact with infected individuals. Examples of these sorts of vaccines include those for plague (Yersinia pestis), typhoid fever (Salmonella typhi), cholera, and tuberculosis.
Bacteria were first described as “animalcules” by the Dutch scientist Antoni van Leeuwenhoek in 1673 after he observed them in water-based mixtures with a crudely designed microscope. In 1860, Louis Pasteur recognized that bacteria could cause the spoiling of wine and beer because of the by-products of their metabolism. Pasteur’s solution to this problem was heating the beverages enough to kill the bacteria, but not change the taste of the drink—a process known as pasteurization, which is used today on milk and alcoholic beverages. In addition, Pasteur settled a long-standing debate on the origin of living things that seemed to arise spontaneously in fluids exposed to the air. He demonstrated that these life-forms were seeded by contaminating bacteria and other microorganisms found in the air, in fluids, and on solid surfaces. Pasteur’s work led to standard practices in laboratories and food processing plants to prevent unwanted bacterial contamination; these practices are referred to as aseptic techniques.
Prior to the late nineteenth century, deaths from wounds and simple surgeries were quite common, but the reason for these high mortality rates was unknown. In the 1860s, Joseph Lister, an English surgeon, began soaking surgical dressings in solutions that killed bacteria, and the rate of survival in surgical and wound patients was greatly improved. In 1876, Robert Koch, a German physician, discovered rod-shaped bacteria in the blood of cattle that died from anthrax, a disease that was devastating the sheep and cattle population of Europe. When he injected healthy animals with these bacteria, they contracted anthrax, and samples of their blood showed large numbers of the same bacteria. By these and other experiments, Koch, Lister, and others proved the “germ theory of disease”—that microorganisms cause disease—and appropriate measures were instituted to protect against the transmission of bacteria to humans through medical procedures and food.
A milestone in the prevention of infectious diseases was the development of vaccinations. The first vaccine was developed in 1798, long before the germ theory of disease was proven. The British physician Edward Jenner first used vaccination as a preventive step against the contraction of deadly smallpox, a viral disease. How vaccinations work and their use as a protection against bacterial diseases were discovered around 1880 by Pasteur.
The first antibiotic, penicillin, was discovered by Alexander Fleming in 1928. Since then, scores of others, produced both naturally and synthetically, have been analyzed and used in the treatment of bacterial diseases. All these discoveries have made bacterial disease a much less deadly category of illness than it was in the late nineteenth century. Yet bacterial diseases are by no means conquered. Overuse of antibiotics in medical practice and in cattle feed results in the appearance of new varieties of bacteria that are resistant to standard antibiotic therapy. Research will continue to develop new means of controlling and destroying such infective organisms. Bacteria also play an important role in synthesizing new antibiotics and other pharmaceuticals in the laboratory through recombinant DNA technology. These organisms will continue to provide challenges and opportunities for human health in the years to come.
"Bacterial Infections." MedlinePlus, May 2, 2013.
Biddle, Wayne. A Field Guide to Germs. 3d ed. New York: Anchor Books, 2010.
Brachman, Philip S., and Elias Abrutyn. Bacterial Infections of Humans: Epidemiology and Control. 4th ed. New York: Springer Science, 2009.
Forbes, Betty A., Daniel F. Sahm, and Alice S. Weissfeld. Bailey and Scott’s Diagnostic Microbiology. 12th ed. St. Louis, Mo.: Mosby/Elsevier, 2007.
Frank, Steven A. Immunology and Evolution of Infectious Disease. Princeton, N.J.: Princeton University Press, 2002.
Hart, C. A. Microterrors: The Complete Guie to Bacterial, Viral, and Fungal Infections that Threaten Our Health. Buffalo, New York: Firefly Books, 2004.
Joklik, Wolfgang K., et al. Zinsser Microbiology. 20th ed. Norwalk, Conn.: Appleton and Lange, 1997.
US Department of Health and Human Services. National Institute of Allergy and Infectious Diseases. Understanding Microbes: In Sickness and in Health. NIH Pub. No. 09-4914, Sept. 2009.
Pelczar, Michael J., Jr., E. C. S. Chan, and Noel R. Krieg. Microbiology: Concepts and Applications. New York: McGraw-Hill, 1993.
Schlegel, Hans G. General Microbiology. 7th ed. New York: Cambridge University Press, 2003.
Shaw, Michael, ed. Everything You Need to Know About Diseases. Springhouse, Pa.: Springhouse Press, 1996.
Willey, Joanne M., et al. Microbiology. 8th ed. New York: McGraw-Hill, 2011.
Wilson, Michael, Brian Henderson, and Rod McNab. Bacterial Disease Mechanisms: An Introduction to Cellular Microbiology. New York: Cambridge University Press, 2002.