The small one-celled microorganisms of the class Schizomycetes. Some are round (cocci), rod-shaped (bacilli), spiral (spirochetes), or comma-shaped (vibrios). The nature, severity, and outcome of any infection caused by a bacterium depends on the species.
Five major groups of microorganisms are responsible for the majority of infections. They include protozoa and helminths, or worms—both of which are considered in Parasites and Parasitology—as well as bacteria and viruses. Bacteria and viruses often are discussed, along with fungi (the fifth major group), in the context of infection and infectious diseases. In the present context, however, we limit our inquiry to viruses and bacteria.
Bacteria are very small organisms, typically consisting of one cell. They are prokaryotes, a term referring to a type of cell that has no nucleus. In eukaryotic cells, such as those of plants and animals, the nucleus controls the cell's functions and contains its genes. Genes carry deoxyribonucleic acid (DNA), which determines the characteristics that are passed on from one generation to the next. The genetic material of bacteria is contained instead within a single, circular chain of DNA.
Members of kingdom Monera, which also includes blue-green algae (see Taxonomy), bacteria generally are classified into three groups based on their shape: spherical (coccus), rodlike (bacillus), or spiralor corkscrew-shaped (spirochete). Some bacteria also have a shape like that of a comma and are known as vibrio. Spirochetes, which are linked to such diseases as syphilis, sometimes are considered a separate type of creature; hence, Monera occasionally is defined as consisting of blue-green algae, bacteria, and spirochetes.
The cytoplasm (material in the cell interior) of all bacteria is enclosed within a cell membrane that itself is surrounded by a rigid cell wall. Bacteria produce a thick, jellylike material on the surface of the cell wall, and when that material forms a distinct outer layer, it is known as a capsule. Many rod, spiral, and comma-shaped bacteria have whiplike limbs, known as flagella, attached to the outside of their cells. They use these flagella for movement by waving them back and forth. Other bacteria move simply by wiggling the whole cell back and forth, whereas still others are unable to move at all.
Bacteria most commonly reproduce by fission, the process by which a single cell divides to produce two new cells. The process of fission may take anywhere from 15 minutes to 16 hours, depending on the type of bacterium. Several factors influence the rate at which bacterial growth occurs, the most important being moisture, temperature, and pH, or the relative acidity or alkalinity of the substance in which they are placed.
Bacterial preferences in all of these areas vary: for example, there are bacteria that live in hydrothermal vents, or cracks in the ocean floor, where the temperature is about 660°F (350°C), and some species survive at a pH more severe than that of battery acid. Most bacteria, however, favor temperatures close to that of the human body—98.6°F (37°C)—and pH levels only slightly more or less acidic than water. Since they are composed primarily of water, they thrive in a moist environment.
Not all bacteria are harmful; in fact, some even are involved in the production of foods consumed by humans. For example, bacteria that cause milk to become sour are used in making cottage cheese, buttermilk, and yogurt. Vinegar and sauerkraut also are produced by the action of bacteria on ethyl alcohol and cabbage, respectively. Other bacteria, most notably Escherichia coli (E. coli) in the human intestines, make it possible for animals to digest foods and even form vitamins in the course of their work. (See Digestion for more on these subjects.) Others function as decomposers (see Food Webs), aiding in the chemical breakdown of organic materials, while still others help keep the world a cleaner place by consuming waste materials, such as feces.
Despite its helpful role in the body, certain strains of E. coli are dangerous pathogens that can cause diarrhea, bloody stools, and severe abdominal cramping and pain. The affliction is rarely fatal, though in late 1992 and 1993 four people died during the course of an E. coli outbreak in Washington, Idaho, California, and Nevada. More often the outcome is severe illness that may bring on other conditions; for example, two teenagers among a group of 11 who became sick while attending a Texas cheerleading camp had to receive emergency appendectomies. The pathogen is usually transmitted through under-cooked foods, and sometimes through other means; for example, a small outbreak in the Atlanta area in the late 1990s occurred in a recreational water park.
Many bacteria attack the skin, eyes, ears, and various systems in the body, including the nervous, cardiovascular (heart, lung), respiratory, digestive, and genitourinary (i.e., reproductive and urinary) systems. The skin is the body's first line of defense against infection by bacteria and other microorganisms, although it supports enormous numbers of bacteria itself. Bacteria play a major role in a skin condition that is the bane of many a young man's (and, less frequently, a young woman's) existence: acne. Pimples or “zits,” known scientifically as Acne vulgaris, constitute one of about 50 varieties of acne, or skin inflammation, which are caused by a combination of heredity, hormones, and bacteria—particularly a species known as Propionibacterium acnes. When a hair follicle becomes plugged by sebum, a fatty substance secreted by the sebaceous, or oil, glands, this forms what we know as a blackhead; a pimple, on the other hand, results when a bacterial infection, brought about by P. acnes, inflames the blackhead and turns it red. For this reason, antibiotics may sometimes cure acne or at least alleviate the worst symptoms.
Acne may seem like a life-and-death issue to a teenager, but it goes away eventually. On the other hand, toxic shock syndrome (TSS), caused by other bacteria at the surface of the skin—species of Staphylococcus (Staphylococci) and Streptococcus — can be extremely dangerous. The early stages of TSS are characterized by flulike symptoms, such as sudden fever, fatigue, diarrhea, and dizziness, but in a matter of a few hours or days the blood pressure drops dangerously, and a sunburn-like rash forms on the body. Circulatory problems arise as a result of low blood pressure, and some extremities, such as the fingers and toes, are deprived of blood as the body tries to shunt blood to vital organs. If the syndrome is severe enough, gangrene may develop in the fingers and toes.
In 1980, several women in the United States died from TSS, and several others were diagnosed with the condition. As researchers discovered, all of them had been menstruating and using high-absorbency tampons. It appears that such tampons provide an environment in which TSS-causing bacteria can grow, and this led to recommendations that women use lower-absorbency tampons if possible, and change them every two to four hours. Since these guidelines were instituted, the incidence of toxic shock has dropped significantly, to between 1 and 17 cases per 100,000 menstruating women.
Many bacteria produce toxins, poisonous substances that have effects in specific areas of the body. An example is Clostridium tetani, responsible for the disease known as tetanus, in which one's muscles become paralyzed. A related bacterium, C. botulinum, releases a toxin that causes the most severe form of food poisoning, botulism. Salmonella poisoning comes from another genus, Salmonella, which includes S. typhi, the cause of typhoid fever.