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 formation of gas) are due to these organisms. Other bacteria oxidize alcohol to acetic acid, and produce lactic acid in milk and butyric acid in butter. Bacteria live in the mouth, stomach, intestines, and on the surface of the skin of animals. Some secrete gelatinous sheaths around themselves; others secrete sulfur or iron, giving the substratum a vivid color.

Were it not for bacteria, man could not live on the earth, for not only are they agents in the process of decay, but they are concerned in certain healthful processes of plants and animals. We have learned in Chap. VIII how bacteria are related to nitrogen-gathering.

Bacteria are of economic importance not alone because of their effect on materials used by man, but also because of the disease-producing power of certain species. Pus is caused by a spherical form, tetanus or lock-jaw by a rod-shaped form, diphtheria by short oblong chains, tuberculosis or "consumption" by more slender oblong chains, and typhoid fever, cholera, and other diseases by other forms. Many diseases of animals and plants are caused by bacteria. Disease-producing bacteria are said to be pathogenic.

The ability to grow in other nutrient substances than the natural one has greatly facilitated the study of these minute forms of life. By the use of suitable culture media and proper precautions, pure cultures of a particular disease-producing bacterium may be obtained with which further experiments may be conducted.

Milk provides an excellent collecting place for bacteria coming from the air, from the coat of the cow and from the milker. Disease germs are sometimes carried in milk. If a drop of milk is spread on a culture medium (as agar), and provided with proper temperature, the bacteria will multiply, each one forming a colony visible to the naked eye. In this way, the number of bacteria originally contained in the milk may be counted.

Bacteria are disseminated in water, as the germ of typhoid fever and cholera; in milk and other fluids; in the air; and on the bodies of flies, feet of birds, and otherwise.

Bacteria are thought by many to have descended from algæ by the loss of chlorophyll and decrease in size due to the more specialized acquired saprophytic and parasitic habit.

The algæ comprise most of the green floating "scum" which covers the surfaces of ponds and other quiet waters. The masses of plants are often called "frog spittle." Others are attached to stones, pieces of wood, and other objects submerged in streams