Most importance bacteria are harmless to humans, and many bacteria are essential or useful to the existence of plant and animal life. Only a small fractions of bacteria causes disease; most bacteria attack organic matter only after it is dead. Were it not for bacteria that decomposed animals and plants. These material would accumulate almost indefinitely.
Bacteria also enrich the soil in various ways. The so-called nitrogen fixing bacteria take nitrogen gas fro the atmosphere and convert it to a form (nitrate) that green plants use for growth. Bacteria also create fertilizer by breaking down compost heaps made of soil and dead plat matter.
Bacteria are important industrially in the production of cheese, yogurt, buttermilk, vinegar, and sauerkraut; in the preparations of antibiotic such as streptomycin, which is extracted from soil bacteria; in the tanning of leather and hides and the curing of tobacco; and in the sewage disposal plants to render organic wastes harmless. Cattle, sheep, and goats live on the grass; yet without bacteria they would not be able to digest the tough fibers of plant cellulose.
Improperly processed foods are subject to spoilage by bacteria. Poisonous toxins are sometimes produced by such spoiling bacteria as staphylococci, Streptococci, and Salmonella. They cause severe illness human eating affected food. Clostridium botolinum, growing in canned or smoked foods that have been improperly processed, produces a toxin that causes Botulism, frequenly fatal disease.