Members of the kingdom Monera are microscopic organisms that include the bacteria and cyanobacteria. Both the bacteria and cyanobacteria are prokaryotes. Prokaryotes lack a nucleus, and they have no organelles except ribosomes. The hereditary material exists as a single loop of double‐stranded DNA in a nuclear region, or nucleoid. Members of the kingdom multiply by an asexual process called binary fission. No evidence of mitosis is apparent in the reproductive process.

Bacteria live in virtually all the environments on earth, including the soil, water, and air. They have existed for approximately 3 billion years, and they have evolved into every conceivable ecological niche on, above, and below the surface of the earth.

Most bacteria can be divided into three groups according to their shapes. The spherical bacteria are referred to as cocci (the singular is coccus); the rod-shaped bacteria are bacilli (the singular is bacillus); and the spiral bacteria are spirochetes if they are rigid or spirilla (the singular is spirillum) if they are flexible.

Cocci may occur in different forms. Those cocci that appear as an irregular cluster are staphylococci (the singular is staphylococcus) and are the cause of “staph” infections. Cocci in beadlike chains are streptococci, and bacteria in pairs arediplococci. One streptococcus is the cause of “strep throat,” while another is a harmless organism used to make yogurt. A species of diplococcus is the agent of pneumonia, while a second causes gonorrhea, and a third is the agent of meningitis.

Characteristics of bacteria

Most bacterial species are heterotrophic, that is, they acquire their food from organic matter. The largest number of bacteria are saprobic, meaning that they feed on dead or decaying organic matter. A few bacterial species are parasitic. These bacteria live within host organisms and cause disease.

Certain bacteria are autotrophic, that is, they synthesize their own foods. Such bacteria often engage in the process of photosynthesis. They use pigments dissolved in their cytoplasm for the photosynthetic reactions. Two groups ofphotosynthetic bacteria are the green sulfur bacteria and the purple bacteria. The pigments in these bacteria resemble plant pigments. Some autotrophic bacteria arechemosynthetic. These bacteria use chemical reactions as a source of energy and synthesize their own foods using this energy.

Bacteria may live at a variety of temperatures. Bacteria living at very cold temperatures are psychrophilic, while those species living at human body temperatures are said to be mesophilic. Bacteria living at very high temperatures are thermophilic. Bacteria that require oxygen for their metabolism are referred to as aerobic, while species that thrive in an oxygen-free environment are said to be anaerobic. Some bacteria can live with or without air; they are described asfacultative. Most bacterial species live in a neutral pH environment (about pH 7), but some bacteria can live in acidic environments (such as in yogurt and sour cream) and others can live in alkaline environments. Certain bacteria are known to live at the pH of 2 found in the human stomach.

Activities of bacteria

Bacteria play many beneficial roles in the environment. For example, some species of bacteria live on the roots of pod-bearing plants (legumes) and “fix” nitrogen from the air into organic compounds that are then available to plants. The plants use the nitrogen compounds to make amino acids and proteins, providing them to the animals that consume them. Other bacteria are responsible for the decay that occurs in landfills and the other debris in the environment. These bacteria recycle the essential elements in the organic matter.

In the food industry, bacteria are used to prepare many products, such as cheeses, fermented dairy products, sauerkraut, and pickles. In other industries, bacteria are used to produce antibiotics, chemicals, dyes, numerous vitamins and enzymes, and a number of insecticides. Today, they are used in genetic engineering to synthesize certain pharmaceutical products that cannot be produced otherwise.

In the human intestine, bacteria synthesize several vitamins not widely obtained in food, especially vitamin K. Bacteria also often break down certain foods that otherwise escape digestion in the body.

Unfortunately, many bacteria are pathogenic; that is, they cause human disease. Such diseases as tuberculosis, gonorrhea, syphilis, scarlet fever, food poisoning, Lyme disease, plague, tetanus, typhoid fever, and most pneumonias are due to bacteria. In many cases, the bacteria produce powerful toxins that interfere with normal body functions and bring about disease. The botulism (food poisoning) and tetanus toxins are examples. In other cases, bacteria grow aggressively in the tissues (for example, tuberculosis and typhoid fever), destroying them and thereby causing disease.

Other bacteria

There are some exceptionally small forms of bacteria that can be seen only with the electron microscope. The rickettsiae are one group. These ultramicroscopic organisms are usually transmitted by arthropods such as fleas, ticks, and lice. They cause several human diseases including Rocky Mountain spotted fever and typhus.

Another type of ultramicroscopic bacteria is the chlamydiae. Like the rickettsiae, the chlamydiae can be seen only with the electron microscope. In humans, chlamydiae cause several diseases, including an eye infection called trachoma and a sexually transmitted disease called chlamydia.

Probably the smallest forms of bacteria are the mycoplasmas. Mycoplasmas occur in many shapes and have no cell walls. This latter characteristic distinguishes them from the other bacteria (where cell walls are prominent). Mycoplasmas can cause a type of pneumonia.

Another type of bacteria is the archaebacteria. Archaebacteria are ancient species of bacteria identified in recent years. They are separated from the other bacteria on the basis of their ribosomal structure and metabolic patterns. Archaebacteria are anaerobic species that use methane production as a key step in their energy metabolism. They are found in marshes and swamps. Some scientists believe there are two major subdivisions of bacteria: the Archaebacteria and all others, which they designate Eubacteria.