These properties explain the regular distribution of bacteria in liquids exposed to the air.  Thus, in water in which plants have been macerated the surface of the liquid is occupied by Bacillus subtilis. which has need of free oxygen in order to live, while in the bulk of the liquid, in the vegetable tissues, we find other bacteria, notably B. amylobacter, which lives very well by consuming oxygen in a state of combination.  Bacteria, then, can only live in organic matters, now in the presence and now in the absence of air.

What we have just said allows us to understand the process of cultivating these organisms.  When it is desired to obtain these algae, we must take organic matters or infusions of such.  These liquids or substances are heated to at least 120 deg. in order to kill the germs that they may contain, and this is called “sterilizing.”  In this sterilized liquid are then sown the bacteria that it is desired to study, and by this means they can be obtained in a state of very great purity.

The Bacteriaceae are very numerous.  Among them we must distinguish those that live in inert organic matters, alimentary substances, or debris of living beings, and which cause chemical decompositions called fermentations.  Such are Mycoderma aceti, which converts the alcohol of fermented beverages into vinegar; Micrococcus ureae, which converts the urea of urine into carbonate of ammonia, and Micrococcus nitrificans, which converts nitrogenized matters into intrates, etc.  Some, that live upon food products, produce therein special coloring matters; such are the bacterium of blue milk, and Micrococcus prodigiosus (Fig. 2, I.), a red alga that lives upon bread and forms those bloody spots that were formerly considered by the superstitious as the precursors of great calamities.

[Illustration:  Fig. 2.—­VARIOUS MICROBES. (Highly magnified.)]

Another group of bacteria has assumed considerable importance in pathology, and that is the one whose species inhabit the tissues of living animals, and cause more or less serious alterations therein, and often death.  Most contagious diseases and epidemics are due to algae of this latter group.  To cite only those whose origin is well known, we may mention the bacterium that causes charbon, the micrococcus of chicken cholera, and that of hog measles.

It will be seen from this sketch how important the study of these organisms is to man, since be must defend his body against their invasions or utilize them for bringing about useful chemical modifications in organic matters.