Domain Bacteria

Characteristics of bacteria

Domain Bacteria includes the prokaryotes people encounter on an everyday basis. 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, meaning they synthesize their own foods. Such bacteria engage in the process of photosynthesis. They use pigments dissolved in their cytoplasm for the photosynthetic reactions (see Chapter 5). Two groups of photosynthetic bacteria are the green sulfur bacteria and the purple bacteria. The pigments in these bacteria resemble plant pigments. Some autotrophic bacteria are chemosynthetic. 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, those living at human body temperatures are mesophilic, and those 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 oxygen; they are described as facultative. Most bacterial species live in a neutral pH environment (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 other debris in the environment. These bacteria recycle the essential elements in the organic matter.

In the food industry, bacteria are used to prepare 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 (see Chapter 11).