Bacteriology. The minute organisms which are commonly called "bacteria"[1] are also known popularly under other designations, e.g. "microbes," "micro-organisms," "microphytes," "bacilli," "micrococci." All these terms, including the usual one of bacteria, are unsatisfactory; for "bacterium," "bacillus" and "micrococcus" have narrow technical meanings, and the other terms are too vague to be scientific. The most satisfactory designation is that proposed by Nägeli in 1857, namely "schizomycetes," and it is by this term that they are usually known among botanists; the less exact term, however, is also used and is retained in this article since the science is commonly known as "bacteriology." The first part of this article deals with the general scientific aspects of the subject, while a second part is concerned with the medical aspects.

I. The Study of Bacteria

The general advances which have been made of late years in the study of bacteria are clearly brought to mind when we reflect that in the middle of the 19th century these organisms were only known to a few experts and in a few forms as curiosities of the microscope, chiefly interesting for their minuteness and motility. They were then known under the name of "animalculae," and were confounded with all kinds of other small organisms. At that time nothing was known of their life-history, and no one dreamed of their being of importance to man and other living beings, or of their capacity to produce the profound chemical changes with which we are now so familiar. At the present day, however, not only have hundreds of forms or species been described, but our knowledge of their biology has so extended that we have entire laboratories equipped for their study, and large libraries devoted solely to this subject. Furthermore, this branch of science has become so complex that the bacteriological departments of medicine, of agriculture, of sewage, etc., have become more or less separate studies.

The schizomycetes or bacteria are minute vegetable organisms Definition. devoid of chlorophyll and multiplying by repeated bipartitions. They consist of single cells, which may be spherical, oblong or cylindrical in shape, or of filamentous or other aggregates of cells. They are characterized by the absence of ordinary sexual reproduction and by the absence of an ordinary nucleus. In the two last-mentioned characters and in their manner of division the bacteria resemble Schizophyceae (Cyanophyceae or blue-green algae), and the two groups of Schizophyceae and Schizomycetes are usually united in the class Schizophyta, to indicate the generally received view that most of the typical bacteria have been derived from the Cyanophyceae. Some forms, however, such as "Sarcina," have their algal analogues in Palmellaceae among the green algae, while Thaxter's group of Myxobacteriaceae suggests a relationship with the Myxomycetes. The existence of ciliated micrococci together with the formation of endospores - structures not known in the Cyanophyceae - reminds us of the flagellate Protozoa, e.g. Monas, Chromulina. Resemblances also exist between the endospores and the spore-formations in the Saccharomycetes, and if Bacillus inflatus, B. ventriculus, etc., really form more than one spore in the cell, these analogies are strengthened.

Schizomycetes such as Clostridium, Plectridium, etc., where the sporiferous cells enlarge, bear out the same argument, and we must not forget that there are extremely minute "yeasts," easily mistaken for Micrococci, and that yeasts occasionally form only one spore in the cell.

Nor must we overlook the possibility that the endospore-formation in non-motile bacteria more than merely resembles the development of azygospores in the Conjugatae, and some Ulothricaceae, if reduced in size, would resemble them. Meyer regards them as chlamydospores, and Klebs as "carpospores" or possibly chlamydospores similar to the endospores of yeast. The former also looks on the ordinary disjointing bacterial cell as an oidium, and it must be admitted that since Brefeld's discovery of the frequency of minute oidia and chlamydospores among the fungi, the probability that some so-called bacteria - and this applies especially to the branching forms accepted by some bacteriologists - are merely reduced fungi is increased. Even the curious one-sided growth of certain species which form sheaths and stalks - e.g. Bacterium vermiforme, B. pediculatum - can be matched by Algae such as Oocardium, Hydrurus, and some Diatoms. It is clear then that the bacteria are very possibly a heterogeneous group, and in the present state of our knowledge their phylogeny must be considered as very doubtful.

Nearly all bacteria, owing to the absence of chlorophyll, are saprophytic or parasitic forms. Most of them are colourless, but a few secrete colouring matters other than chlorophyll. In size their cells are commonly about 0.001 mm. (1 micromillimetre or 1 µ) in diameter, and from two to five times that length, but smaller ones and a few larger ones are known. Some of the shapes assumed by the cells are shown in fig. 1.

Fig. 1. Various forms of bacteria.

Fig. 1. - Preparations showing various forms of bacteria and the various types of cilia and their arrangement.

A. Bacillus subtilis, Cohn, and Spirillum undula, Ehrenb.
B. Planococcus citreus (Menge) Migula.
C. Pseudomonas pyocyanea (Gessard), Migula.
D. P. macroselmis, Migula.
E. P. syncyanea (Ehrenb.), Migula.
F. Bacillus typhi, Gaffky.
G. B. vulgaris (Hauser), Migula.
H. Microspira Comma (Koch), Schroeter.
J, K. Spirillum rubrum, Esmarsch.
L, M. S. undula (Müller), Ehrenb. (All after Migula.)

That bacteria have existed from very early periods is clear from Distribution in Time. their presence in fossils; and although we cannot accept all the conclusions drawn from the imperfect records of the rocks, and may dismiss as absurd the statements that geologically immured forms have been found still living, the researches of Renault and van Tieghem have shown pretty clearly that large numbers of bacteria existed in Carboniferous and Devonian times, and probably earlier.