BACTERIA. Literally the word bacte rium, bacteria being its plural, means a tiny rod or stick. As understood, however, by biologists, bacteria constitute a genius of lowly organized microscopic plants having forms other than that indicated by the literal meaning of the word. Briefly defined, bacteria are unicellular vegeta bles that multiply by the simple process of transverse division — they are, therefore, schiz otnycetes. In size they are all of microscopic dimensions requiring in most cases to be magni fied from 600 to 1,000 diameters before becom ing visible and even then they appear in many instances as scarcely more than tiny points. As encountered in nature they assume a variety of forms which may be conveniently arranged into three principal groups, namely: the spheri cal, the rod-like and the spiral. To the spheri cal forms the name cocci or micrococci (coccus, singular) is given, and, according to the man ner in which these tiny spheres develop and their progeny adhere to one another they are further severally designated as staphylococci, that is, cocci clustering irregularly together like grapes in a bunch; streptococci, that is, cocci adhering together like beads or pearls in a strand; diplo-cocci, that is, cocci occurring in pairs; tetracocci, that is, cocci clustered in fours, etc.
To the rod-like group — that is, those which are straight, having one diameter longer than another — the designation bacilli (bacillus, sin gular) is given. While the structure and mode of multiplication of many of the bacilli is as simple as is that of the micrococci — that is, one cell divides into two, two into four, and so on ad infinitum, without variation it is neverthe less in the group of bacilli that we encounter a number of species provided by nature with a more highly organized and complicated means for propagation and perpetuation. It is here that we encounter species in the course of whose life cycle there develops within each rod a single tiny, oval, highly resistant body, a • spore as it is called, which may be fairly com pared to the seeds of higher plants and which, like the seed, may be gathered and kept for almost indefinite periods, without losing their power of germination. Since such spores of bacteria are markedly tenacious of life even under the most unfavorable of circumstances it is obvious that the power to form spores is an important provision for the preservation of the species. It is of passing interest to know that the ability to form spores is possessed by some, but not many of the disease-producing bacteria, a fact that serves to explain in part the difficulties experienced by the sanitarian in eliminating certain types of infection; for it must be remembered that the infective species capable of entering the spore stage are by vir tue of that property much less vulnerable to the action of disinfectants and disinfecting proc esses than are the species not so endowed.
The spiral forms, spirilla, as they are called, comprise those bacteria having one or more curves in their long axis, that is, those that are twisted like a corkscrew. They are sometimes seen as homogeneous, long spiral threads with out segmentations, while again they may con sist of short curved segments adhering end to end. Spore formation is not a characteristic of the spiral bacteria.
In structure bacteria are non-nucleated masses of protoplasm surrounded by an en veloping zone appearing in some instances to be but a condensation of the central protoplasm, while in others it partakes somewhat of the nature of mucin. Many of the bacteria exhibit no evidence of independent mobility, while others, by virtue of special locomotive appara tus (flagella) move themselves about in fluids in a most energetic manner. As their structure is exceedingly simple, in so far as formed ele ments are concerned, their mode of nutrition is, physically speaking, correspondingly simple — that is, the nourishment is absorbed and their waste products discharged • directly through their enveloping membranes by the process of osmosis. This being the case it is obvious that bacteria can multiply and perform their physi ological functions only under conditions of moisture. Unlike the more highly organized plants bacteria are apparently without special provisions for gaseous exchange, that is, they are devoid of chlorophyl. They obtain their oxygen as such from the free air or from easily oxygen compounds. In the course of his early investigations in this field Pasteur discovered a group of bacteria that have ever proven to be of the greatest interest —a group that, paradoxical as it may seem, not only does not require free oxygen for its life processes but to the growth of which free oxygen is actually prohibitive. To these species he gave the designation microbic to distinguish them from the majority, the trrobic varieties, to which free oxygen is essential. In their rela tions to higher life bacteria may be regarded as allies or as enemies, according to the nature of species under consideration. Contrary to notions that have been more or less prevalent the majority of bacteria have nothing to do with disease production. Their natural role is that of scavengers. They are concerned in nature's great laboratory, the soil, in working over dead organic matters into forms appro priate to the nourishment of growing vegeta tion. Since in the course of this conversion dead bodies that would otherwise encumber the earth are caused to disappear they must from both the aesthetic and economic standpoints be regarded as, in the main, benefactors. In this group of saprophytic bacteria, as they are called, that is, those that live on dead matters, we encounter species of the greatest interest and importance. It is here that we perceive the omnipresent forms concerned in the reduction of dead animal and vegetable tissues into such simple forms as carbon dioxide, ammonia and water to be used by higher plants. It is in this group that we find the ever-present nitrifying, denitrifying and nitrogen fixing species— that is, those peculiar ferments that assist the legu minous plants in assimilating free atmospheric nitrogen; those 'that oxidize the ammonia of decomposition to the nitrous and nitric acids so essential to plant life, and those that, by their reducing function, reverse this phenomenon; those that convert the objectionable organic matter of sewage and polluted waters into an inert inorganic form and those that, through their specific activities, supply, where circum stances are favorable, the entire commercial world with its supply of saltpetre.