The _bacteria_ are also one-celled microörganisms, smaller than the yeast. Figure 31 shows the four types of bacterial cells. Their size is measured by the unit used in the microscope, called the micron, which is about 1/25000 of one inch. Bacteria may measure from one to three or four of these microns in length. Some bacteria are reproduced by means of spores which form within the cell. Bacteria, as they develop in some material, produce substances from the material that may or may not be injurious to us. One important truth about the bacteria is this: that many of them are harmless, and may even be made useful, as in the manufacture of fruit vinegar. The pleasant acid of buttermilk and of sour milk is due also to bacteria which are not harmful to us. However, there may be disease producing bacteria present in milk that is not clean, and their presence must not be tolerated. Other bacteria, developing in meat and fish, produce substances known as _ptomaines_, which are dangerous poisons; or, more often, the kinds of bacteria which thrive in meats and fish may themselves be directly injurious to man. It is evident, therefore, that the problem before us is the control of these lower organisms, that we may increase or destroy them as we will. =The control of microörganisms.=——With warmth, water, and food all living things flourish and grow; most organisms require air, but some of the microörganisms do not. Where these conditions are best met, the organism is most active and multiplies most rapidly. To retard growth or to destroy life, the conditions must be the reverse of favorable. While warmth, say a temperature from 70° to 90° F., promotes the life of most microörganisms, intense heat destroys it. The boiling temperature, 212° F., will kill these lower organisms, although this heat has to be continued for some length of time, particularly in the case of spores. The spores of certain bacteria are quite resistant. A temperature of 32° F. and lower retards growth, but it requires extreme cold to destroy bacteria. Since moisture is necessary to all the lower organisms, they do not develop in a dry material or dry place. We cannot destroy these lower forms of life by removing food from them, since they are ever present, but we can make the food unavailable to them through the introduction in the material of certain substances called preservatives which prevent their growth. The preservatives long familiar are salt, sugar, wood-smoke, spices, vinegar, and alcohol. While a small amount of sugar is necessary in the fermentation process, a large amount acts as a preservative, as in candied fruit. It is an interesting fact that alcohol and vinegar, products of fermentation processes, tend (when sufficiently concentrated) to stop the growth of the fermentation organisms.