Physical Characteristics of Bacteria

Bacteria are distinguished from other living things because of their cell structure: they have no distinct nucleus—that is, their DNA is not enclosed by a membrane—and they lack most of the internal structures found in the cells of higher organisms. This classifies bacteria as prokaryotes and places them in the kingdom Monera. (See also cell; living things.)

Scientists divide the bacteria into groups based on shape: spherical cells, which are labeled as cocci (sing., coccus); rod-shaped cells, called bacilli (bacillus); curved rods, known as vibrios; and spiral-shaped bacteria. The latter group is further divided into spirilla (spirillum), which are rigid, thick spirals; and spirochetes, which are thin and flexible.

With one exception, all bacteria have a cell wall surrounding a cell membrane, inside of which lies cytoplasm, a jellylike substance composed of water, proteins, and other molecules. Only the mollicutes lack a cell wall. A single circular strand of DNA floats freely in the cytoplasm, as do ribosomes—structures that help the bacteria make proteins (see cell). Many species have plasmids, independent pieces of DNA that can be transferred to other bacteria during reproduction. In some species, a gelatinous capsule encloses the cell wall.

Where Bacteria Live

Bacteria are found in more habitat types than any other living thing. One teaspoon (5 milliliters) of fertile soil may contain as many as 500 million individual bacteria. Some species live on and within other living things, such as animals or fungi (see symbiosis). Bacteria inhabit oceans, deserts, hot springs, and even snow. They have been found high in the atmosphere, at the bottom of rivers and lakes, and in the deepest mines. Some species can survive at temperatures high enough to kill other forms of life; polar-dwelling species are well adapted to subzero temperatures.

The Size of Bacteria

Bacteria are smaller than other cells, but larger than viruses. Bacteria are measured in units of length called micrometers, or microns. One millimeter is equal to 1,000 microns; it takes about 25,000 microns to make up one inch (2.5 centimeters). Some bacteria measure only one half a micron. Thiomargarita namibiensus, a bacterium that lives buried in ocean sediment, may measure as much as 750 microns wide (about 0.03 inch, or 0.75 millimeter), and can be seen without a microscope. Epulopiscium fishelsoni, which lives in the intestines of some fish species, averages around 500 microns (about 0.02 inch, or 0.5 millimeter) in length.

Most bacteria range from 1 to 5 microns in size. The light microscope can magnify an image approximately 1,000 times, and is used for basic studies of bacteria. Specialized microscopes, such as electron or phase-contrast microscopes, are used to examine special details of bacterial structure. (See also microscope.)

How Bacteria Live

Significance in Nature

The decomposition of organic (substances that contain carbon) material in nature is brought about chiefly by vast numbers of saprophytic bacteria, though certain fungi contribute to the process. If there were no decay, the remains of dead organisms and the waste of cities would accumulate so fast that they would soon interfere with everyday life. As saprophytes break down organic matter, such as rotting leaves and dead insects, they enrich the soil by returning minerals and nutrients to it. Carbon dioxide, a by-product of decomposition, is released into the air for plants to use in photosynthesis. (See also photosynthesis.)

Although plants need nitrogen to grow, and nitrogen is abundant in the air, green plants cannot use it in its gaseous form. Several kinds of bacteria can take nitrogen from the atmosphere and convert it into a form that is usable by plants, a process called nitrogen fixation. Some nitrogen-fixing bacteria, such as Rhizobium, live in a mutualistic relationship with plants such as legumes (peas and beans) by forming nodules on the plant's roots (see symbiosis). Free-living nitrogen-fixing soil bacteria, such as Azotobacter, release fixed nitrogen into the soil, where it is taken up by the plant's roots. (See also nitrogen; soil.)

How Bacteria Cause Disease

Pathogenic bacteria may enter the body in many ways, such as through the mouth or through cuts in the skin, where they may multiply sufficiently to cause an infection. The infection may be caused by the microbes themselves, or by poisons called toxins that they produce. Some toxins, such as those produced by Staphylococcus aureus, are more dangerous than the bacteria. Plants, too, are vulnerable to bacterial infections, though they are plagued by different species than are animals. (See also disease, human; immune system; plants, diseases of.)

Throughout history, warring nations have used or planned to use pathogenic bacteria to spread sickness among the enemy and to destroy livestock and farm crops. Although biological weapons have not been used since World War II, the relative ease of developing such weapons has increased the risk of their use by terrorist organizations. (See also chemical and biological terrorism; chemical and biological warfare.)

How Bacteria Fight Disease

Scientists use pathogenic bacteria and their toxins to create vaccines—preparations that can protect humans and animals from infections. Vaccines are prepared with killed bacteria dissolved in a solution that is injected into the body. The presence of the killed bacteria stimulates the individual's immune system to form antibodies against the microbe, thus protecting the individual from becoming ill if infected in the future (see immune system).

Antitoxin injections contain preformed antibodies. They are prepared by injecting bacterial toxins into live animals. The blood is later collected and the serum is separated from it by chemical means. Antitoxins act more quickly than vaccines, but the protection usually lasts no more than a few weeks or months. Vaccines, however, protect for a year or longer. (See also antitoxin; vaccine.)

Some kinds of microorganisms secrete substances called antibiotics that can destroy other microbes (see antibiotic). Penicillin, the best-known antibiotic, is the product of a mold. Bacitracin and polymyxin are made by bacteria. Many widely used antibiotics, such as streptomycin, neomycin, erythromycin, and tetracycline, come from members of the genus Streptomyces, a group of soil bacteria with mold-like characteristics.

Bacteria in Foods

The bacteria that bring about decay are the chief cause of food spoilage. For fresh foods, the decay process may be slowed down by refrigeration or checked by freezing. When frozen food is thawed, however, the bacteria become active again.

Many kinds of bacteria can be killed by heat. Because few species grow in foods that are acidic, dried, salted, or very sweet, it is easier to process foods with these qualities. For example, canning acidic foods such as tomatoes is less complicated than processing green beans. The amount of heat required to can nonacidic foods successfully is very high because of the need to destroy thermophilic (heat-loving) bacteria and their spores. (See also food processing.)

As some bacteria and other microbes “digest” nutrients, they produce certain chemical products through a process called fermentation. Dairy products are fermented in many ways to produce products such as sour cream, buttermilk, cheese, and yogurt. Cheeses owe their various flavors almost entirely to different kinds of combinations of bacteria and mold. (See also dairy industry; fermentation; yogurt.)

Bacteria in the Chemical Industry

Fermentation is also important to the chemical industry. Because they can be grown quickly and in vast quantities, bacterial cells are used as miniature chemical factories, producing a wide range of substances—some of which can only be produced by bacteria. The solvents butyl alcohol (used in lacquers) and acetone are formed by Clostridium bacteria. A Lactobacillus species makes lactic acid from corn starch or whey. Acetobacter uses alcohol to synthesize acetic acid, the ingredient that gives vinegar its characteristic flavor. Dextran, which is used to expand blood volume in cases of hemorrhage or dehydration, is made by a Leuconostoc bacterium from sucrose, common table sugar.

Bacteria in the Laboratory

In the laboratory, bacteria are grown in a substance called a culture medium. Liquid media in test tubes is used for many types of research. In other studies, a solidifying agent such as agar (obtained from seaweeds) is added to the mixture, which is poured into a shallow plate called a petri dish and allowed to congeal. Bacteria are applied to the surface of the jellylike medium, where they grow and form patches called colonies, each containing millions of individual bacterial cells.

The nutrients added to the cultures depend on the species' requirements. Autotrophic bacteria can grow on powdered sulfur and other inorganic substances and get carbon from carbon dioxide in the air. Sugars, amino acids, and vitamins are used for some heterotrophs. Blood may be added to cultures used to grow some pathogenic species.

The identification of bacteria can be difficult and requires application of numerous criteria. Among the most basic are microscopic observations of the size, shape, and appearance of the bacteria after they have been stained, and the colors and shapes displayed by colonies growing on agar plates.

In the late 20th century scientists discovered a way to transfer genes from plants and animals into certain bacterial cells. As the latter multiplied, they produced the product coded for by the transferred gene. Today the technology is used to produce human insulin for diabetics, and has many applications in fields ranging from medicine to agriculture. (See also disease, human; DNA; genetic engineering.)