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Bacteria are single-celled microorganisms that serve many functions.
Beneficial bacteria. Most bacteria are very useful. They live in a variety of places and grow whenever conditions are suitable. A few beneficial functions of bacteria are the production of food products including dairy products, such as yogurt, sauerkraut, fermented meat products such as summer sausage, and vinegar. Bacteria also help fix nitrogen in the soil and are responsible for decomposing organic materials, which returns important nutrients back to the soil. The beneficial aspects of microorganisms far outweigh their harmful effects.
Pathogenic bacteria. These bacteria produce diseases in humans, animals and plants and are called pathogenic bacteria. They are a relatively few in number and produce disease by growing on or in certain tissues and producing harmful poisons or toxins, which people and animals consume.
Spoilage bacteria. As bacteria live and grow they produce changes in food products that damage flavor, texture and composition. Specific bacteria can cause milk to sour or develop off flavors; meat to spoil; and wine to turn to vinegar
The temperature of food must be frequently monitored to control bacterial growth.
Spore-forming bacteria. When certain bacteria grow, they have the ability to develop resistance to extreme heat, dryness and chemicals. These bacteria are called spore formers because they develop a "shell" which is capable of protecting the cell under adverse conditions. The spore is the "resting stage" of the live bacteria and it can begin to grow into an active cell when proper growth conditions are provided. Because spores are resistant to heat, higher temperatures and pressure are used in food canning to destroy them.
Toxin-forming bacteria. Some bacteria form toxins (or poisons) that can damage host cells and tissues causing foodborne illness. There are three types of toxins -- exotoxins (common to most toxin-forming bacteria that cause foodborne illness), endotoxins, and enterotoxins (common to E. coli). The differences between exotoxins and endotoxins is significant and are described below. Enterotoxins exhibit characteristics of both.