Movement. Many bacteria are unable to move from place to place. They have, however, a vibrating movement known as the Brownian motion that is purely physical. Many other kinds are endowed with powers of locomotion. Motion is produced by means of fine thread-like processes of protoplasm known as cilia (sing. cilium) that are developed on the outer surface of the cell. By means of the rapid vibration of these organs, the cell is propelled through the medium. Nearly all cocci are immotile, while the bacilli may or may not be. These cilia are so delicate that it requires special treatment to demonstrate their presence.
Classification. In classifying or arranging the different members of any group of living objects, certain similarities and dissimilarities must be considered. These are usually those that pertain to the structure and form, as such are regarded as most constant. With the bacteria these differences are so slight that they alone do not suffice to distinguish distinctly one species from another. As far as these characters can be used, they are taken, but in addition, many characteristics of a physiological nature are added. The way that the organism grows in different kinds of cultures, the by-products produced in different media, and effect on the animal body when injected into the same are also used as data in distinguishing one species from another.
Conditions favoring bacterial growth. The bacteria, in common with all other living organisms are affected by external conditions, either favorably or unfavorably. Certain conditions must prevail before development can occur. Thus, the organism must be supplied with an adequate and suitable food supply and with moisture. The temperature must also range between certain limits, and finally, the oxygen requirements of the organism must be considered.
Food supply. Most bacteria are capable of living on dead, inert, organic matter, such as meats, milk and vegetable material, in which case, they are known as saprophytes. In contradistinction to this class is a smaller group known as parasites, which derive their nourishment from the living tissues of animals or plants. The first group comprise by far the larger number of known organisms which are concerned for the most part in the decomposition of organic matter. The parasitic group includes those which are the cause of various communicable diseases. Between these two groups there is no sharp line of division, and in some cases, certain species possess the faculty of growing either as parasites or saprophytes, in which case they are known as facultative parasites or saprophytes.
The great majority of bacteria of interest in dairying belong to the saprophytic class; only those species capable of infecting milk through the development of disease in the animal are parasites in the strict sense of the term. Most disease-producing species, as diphtheria or typhoid fever, while parasitic in man lead a saprophytic method of life so far as their relation to milk is concerned.
Bacteria require for their growth, nitrogen, hydrogen, carbon, oxygen, together with a limited amount of mineral matter. The nitrogen and carbon are most available in the form of organic compounds, such as albuminous material. Carbon in the form of carbohydrates, as sugar or starch, is most readily attacked by bacteria.
Inasmuch as the bacteria are plant-cells, they must imbibe their food from material in solution. They are capable of living on solid substances, but in such cases, the food elements must be rendered soluble, before they can be appropriated. If nutritive liquids are too highly concentrated, as in the case of syrups and condensed milk, bacteria cannot grow therein, although all the necessary ingredients may be present. Generally, bacteria prefer a neutral or slightly alkaline medium, rather than one of acid reaction; but there are numerous exceptions to this general rule, especially among the bacteria found in milk.
Temperature. Growth of bacteria can only occur within certain temperature limits, the extremes of which are designated as the minimum and maximum. Below and above these respective limits, life may be retained in the cell for a time, but actual cell-multiplication is stopped. Somewhere between these two cardinal temperature points, and generally nearer the maximum limit is the most favorable temperature for growth, known as the optimum. The temperature zone of most dairy bacteria in which growth occurs ranges from 40°-45° F. to somewhat above blood-heat, 105°-110° F., the optimum being from 80°-95° F. Many parasitic species, because of their adaptation to the bodies of warm-blooded animals, generally have a narrower range, and a higher optimum, usually approximating the blood heat (98°-99° F). The broader growth limits of bacteria in comparison with other kinds of life explain why these organisms are so widely distributed in nature.
Air supply. Most bacteria require as do the green plants and animal life, the free oxygen of the air for their respiration. These are called aerobic. Some species, however, and some yeasts as well possess the peculiar property of taking the oxygen which they need from organic compounds such as sugar, etc., and are therefore able to live and grow under conditions where the atmospheric air is excluded. These are known as anaerobic. While some species grow strictly under one condition or the other, and hence are obligate aerobes or anaerobes, others possess the ability of growing under either condition and are known as facultative or optional forms. The great majority of milk bacteria are either obligate or facultative aerobes.
Rate of growth. The rate of bacterial development is naturally very much affected by external conditions, food supply and temperature exerting the most influence. In the neighborhood of the freezing point but little growth occurs. The rate increases with a rise in temperature until at the optimum point, which is generally near the blood heat or slightly below (90°-98° F.), a single cell will form two cells in 20 to 30 minutes. If temperature rises much above blood heat rate of growth is lessened and finally ceases. Under ideal conditions, rapidity of growth is astounding, but this initially rapid rate of development cannot be maintained indefinitely, for growth is soon limited by the accumulation of by-products of cell activity. Thus, milk sours rapidly at ordinary temperatures until the accumulation of acid checks its development.