The saprophytic group also comprehends many species used in the arts and industries such, for instance, as those concerned in the production of certain organic acids; those em ployed in the manufacture of indigo by the fermentation process and in the preparation of hemp; and those utilized in the manufacture of cheese and butter. In the study of this large group one constantly encounters species presenting most engaging characteristics some of these, the chromogenic varieties, have the property of producing during the course of their growth pigments of great beauty — bril liant reds, delicate pinks, rich purples, yellows ranging from the palest lemon to the deepest orange, are those most often encountered. In another group, the photogenic, we meet with species having the emission of light as their most singular peculiarity. When growing these forms glow with a peculiar phosphores cence, and it is significant to note that these luminous varieties have been most frequently encountered in the sea and upon articles from the sea. The evil odors of putrefaction are the results of saprophytic bacterial development In the parasitic group of bacteria we encounter those species that exist always at the expense of a living host, either animal or vegetable, and in doing so not only appropriate materials necessary to life, but give off in return waste products that may act as direct poison to the host. Fortunately this is a much smaller group than is the saprophytic mentioned above. In no particulars, save for their ability to exist at the expense of a living host and cause disease, are the disease-producing bacteria distinguishable from the innocent varieties. The essential dif ference between the disease-producing and the innocent bacteria species is that the former possess as their most striking physiological peculiarity the power of elaborating poisons, toxins, technically speaking, that have a direct destructive action upon the tissues of their host In some cases the poisons may properly be re garded as secretions of the bacteria,. and, under
artificial conditions of cultivation, may easily be separated from the living bacteria elaborat ing them. This is especially true of the poisons of diphtheria and tetanus or lock-jaw. When thus separated such poisons, entirely independ ent of the living bacteria, retain the specific property of causing the symptoms and many of the pathological changes that characterize the growth of the living bacteria in the tissues. In other cases the poisons cannot be so readily separated; they appear to be an integral con stituent of the protoplasm of which the bacteria are composed. This is especially the case with the toxins of bacillus typhosus, bacillus dysen and spirillum cholera Asiatic& — the or ganisms concerned in the causation of typhoid fever, epidemic dysentery and Asiatic cholera, respectively. In the case of still other patho genic species there is little doubt that specific intoxicants are in one way or another elabo rated during infection, but as yet they have not been satisfactorily demonstrated. Nevertheless, it may be said that, in general, infection by bacteria is to-day regarded as essentially a chemical phenomenon — that is, as a reaction between the poisons elaborated by the bacteria and the tissues with which they come in con tact ; the result of the reaction being the partial or complete death of the host in which the phenomenon is in operation.
Bibliography.— Newman, ; Lip man, 'Bacteria in Relation to Country Life' ; Slater and Spitta, 'An Atlas of Bacteriology' ; G. Sims Woodhead, M.D., (Bacteria and Their Products.) A. C. ABBOTr, Bacteriologist, University of Pennsylvania.