In their growth bacteria are engaged for the most part in splitting up organic compounds, and in building up others. In this process they frequently eliminate chemical principles of greater or lesser complexity. In the souring of milk, for instance, the bacillus acidi lactici changes milk sugar into lactic acid. Then in ordinary putrefactive processes the albuminous principles are decomposed, while various principles are produced, some of them alkaloids, and some gases, which present peculiar disagreeable odours. There are some which produce pigments; the bacillus prodigiosus, for example, is characterized by the brilliant red appearance which it presents when growing in masses. Other bacteria produce blue or yellow pigments. These pigments are, as it were, secretions of the bacteria, and in many cases, at least, the pigment is not in the substance of the bacteria, but lies between them. It is now generally acknowledged that decompositions of organic matter and fermentations are the result of the action of bacteria, and it is recognized that each form of bacterium, when under similar conditions, produces the same chemical results.
In relation to the causation of disease, it is of importance to note the nature of some of the chemical products of the bacteria. Certain basic substances or alkaloids have been separated from decomposing matters and from cultures of bacteria, and some of them have been obtained in the crystalline form. To these alkaloids the name Ptomaines ( = a dead body) has been given. Like the alkaloids i-volved by other members of the vegetable kingdom, some of these are poisonous and some are not. The poisonous varieties are called Toxines (Brieger). But the bacteria evolve poisonous products which are not alkaloids, and it seems desirable to extend the term Toxine (which means simply a poison) to these. It may be used therefore as equivalent to a poison evolved by bacteria in their growth, whether in artificial cultures or in their natural habitats.
Some of the toxines are albuminoid substances, and are properly called Toxalbumins, but some of them (as, for example, Tuberculin) are not coagulated, as albumins are, by boiling, and hence are not toxalbumins. To these the term Proteins may be applied. In this way, although the nomenclature is not fully established and there are differences in the uses of the terms, the toxines may be divided into the three forms - Ptomaines, Toxalbumins, and Proteins. The toxines and their effects are more fully considered further on.
The Ptomaines include many different alkaloids, some of which have been at least partially isolated. Panum isolated a virulent poison which was soluble in water but not in alcohol, which he called the Putrid poison. Bergmann and Schmiedeberg isolated a crystalline substance which they called Sepsin, and several other crystalline alkaloids have been obtained, especially by Brieger. Amongst the latter may be mentioned Neurine, an alkaloid obtained from decomposing muscle. It possesses properties similar to those of muscarine, a poison obtained from some fungi, and peculiarly fatal to flies. Poisonous products evolved by the decomposition of animal foods sometimes lead to serious and even fatal results. There are not a few cases of poisoning by sausages, in some of which the symptoms have resembled those of poisoning by atropine (Brunton). There have been also cases where fish of various kinds, but especially crabs, have had this effect, and there are also cases in which tinned meats have apparently had poisonous properties. It appears as if the poisons might be evolved without the ordinary signs of putrefactive decomposition. In that case the bacteria concerned would not be associated with the ordinary septic kinds.
Somewhat more problematical are the so-called Leucomaines. It is asserted that in the physiological processes, such as muscular action, alkaloids are formed, which, like those arising from bacteria, are poisonous. To these physiological alkaloids the name Leucomaines is given.
Bacteria are present almost everywhere in nature, but hey are specially abundant where water and decomposable organic matter concur. They are present in water, - even the purest waters contain them in considerable abundance. They exist in the air, provided it contains watery vapour. If the air is dry then spores will be present rather than the bacteria themselves. In the earth they are present, but, as they are here liable to drying, it is often chiefly spores which are found. There are not many below the level of three feet from the surface. They are present abundantly in the human body; on the skin wherever there is dirt, and on the mucous membranes.