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remaining associated in pairs; both of the foregoing groups exhibit this form, the term being applied to a given species when it occurs most commonly in the diplococcal form (Diplococcus pneumonia).

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FIG. 1.-MORPHOLOGICAL FORMS OF BACTERIA.

a, Cocci; b, diplococci, diplococcus with capsule; c, staphylococci; d, streptococci; e, tetracoccus and sarcina; f, various types of bacilli; g, streptobacilli; h, spirillum, comma forms and spirochete; i, streptothrix branched threads.

(4) MERISMOPEDIA.—Division in two directions in the same plane, the cocci remaining attached in groups of four, or "tetrads," as Micrococcus tetragenous.

(5) SARCINE.-Division in three planes, one at right angles to the other two, the cocci remaining attached in cubical packets of eight, which are often associated in masses. These cocci are generally of larger size than the other groups.

The term micrococcus is applied to all the above species, and is also used to designate cocci occurring separately, as mono-cocci; such mono-coccal form is common among the staphylococci.

(b) BACILLI.-Rod-shaped organisms, the greatest diameter being more than twice the lesser; the cells may be long or short, but, as a rule, are not wider than 1·0 μ. Division generally takes place at right angles to the long axis. The cells of some species have flagella and are motile. Two bacilli may remain united by their capsule, forming a diplobacillus, or the individual elements remaining united in chains a streptobacillus is formed. These two morphological forms are not so constant as the corresponding cocci, and are of little use in classification. Bacilli frequently grow out into long threads, particularly in fluids, to which form the term "leptothrix ' is often wrongly applied. Endogenous spore formation occurs in many species. The spores may be central or terminal in position; round, oval or fusiform in shape. In the latter case the organism containing the spore becomes swollen and spindle-shaped and is termed a Clostridium.

(c) SPIRILLA.-Curved or comma-shaped rods and spiral filaments. Reproduction by binary fission or simple division. Generally motile, movement in direction of long axis and rotation upon same axis. In the long spiral threads, also termed spirochate, the motility does not always appear to be due to the presence of flagella, the organisms possessing contractility. The comma-shaped cells generally possess flagella, which may be single or multiple and situated at one or both poles.

The term "vibrio " is often applied as a generic term to this group; some authors however use "vibrio" in a special sense, thus Hueppe calls forms without endospores "spirochete," ," "vibrio " those having endospores. Migula applies the term vibrio to those organisms with only one or two polar flagella, spirilla those with bunches of flagella, and spirochete those without flagella. Flügge employs "vibrio" to denote forms with but slightly marked undulations.

The Higher Bacteria.―These organisms show a distinct advance on the lower group, but at present our knowledge of them is fragmentary and includes only isolated species. Some of the species show differentiation of the two extremities; one end

may be specialised for attachment, the other for reproduction-a method approaching the sporulation of the Moulds, to which the High Bacteria are somewhat closely related. The filaments of these bacteria are generally segmented, but special methods are required to bring out the divisions. There is often a capsule common to the whole thread.

Spirulina (Hueppe). The cells are sometimes rod-shaped, sometimes spiral, and in some media may grow out into long spiral, undulatory or straight filaments. The threads break up into coccilike reproductive bodies-" arthrospores." Under this head the Proteus group was first described; they are now generally placed with the bacilli.

Leptothrix (Zoph).-Rod-shaped, spherical and filamentous forms, the last showing a difference between base and apex. Filaments straight or spiral. Spore formation unknown.

This definition of leptothrix is the one I have adopted in the following pages, bacilli forming filaments or threads are not included in it. (See later.)

Cladothrix. Spherical, rod-shaped, and filamentous forms, the latter show pseudo branching. Reproduction by arthrospores.

Streptothrix.-Felted mycelium-like filaments, showing true dichotomous branching. Club-shaped thickenings appear at the ends of some of the threads. Various forms are produced by breaking up of threads simulating cocci, bacilli and spirilla; from these new individuals may be formed.

Bacteria are also classified according to the environment necessary to their development, as Saprophytes and Parasites.

The saprophytes are those whose existence is possible apart from a living host, and which obtain the nutriment necessary for their growth from dead organic matter, or from simple organic salts and water.

The strict or obligatory parasites are unable to exist apart from a living host, in whose tissues they multiply, often producing profound pathological changes; some few species may exist in the tissues of an animal without any harm arising.

Such bacteria as are capable of leading a saprophytic existence, but when gaining access to the tissues of the body will develop there, are termed facultative parasites.

Of the obligatory parasites the leprosy bacillus affords a good example; all attempts at its culture have failed, and it is unknown

apart from the disease with which its name is associated.

The cholera spirillum, typhoid bacillus, and most pathogenic bacteria are examples of the facultative parasite, which, besides the production of disease by development in living animal tissues, is enabled to exist outside the body as a saprophyte.

There are many transitional forms between the obligatory parasite and the saprophytic bacteria; many of these have only been obtained in pure culture within recent years, as, for instance, the influenza bacillus and the gonococcus. An organism living a parasitic existence as a general rule does not grow in artificial media as well as one which has for some time led a purely saprophytic existence. For this reason many bacteria require some little time before they develop their "laboratory habit." It seems not improbable that the various pathogenic bacteria which are to-day associated with disease were at one time simple saprophytes, and that some of the simple saprophytes, as we know them to-day, may yet attain pathogenic powers.

Not to admit such a development of pathogenic power entails the obscession that pathogenic bacteria were created by design to destroy human life, and, moreover, such a refusal places us at variance with the Monistic conception of the universe, and the orderly operation of the laws of evolution with which all observed phenomena accord.

MORPHOLOGY has been already referred to as the general basis of classification. Morphological form, however, is extremely variable, so that only the predominating or average form in any given species is taken as representing that species. Thus, for instance, the Pneumococcus or Diplococcus pneumoniæ occurs generally in the form of diplococci, but also constantly presents a monococcal and streptococcal form. The streptococcus of the mouth occurs in that cavity as a diplococcus almost without exception, whilst in the majority of culture media the streptococcal form predominates. Under certain circumstances the individual cocci may become so much elongated that a form allied to a strepto-bacillus is produced. It is for this reason, as well as for the fact that three chief morphological forms are common to a great number of families, that mycologists have adopted the methods of cultivation in the determination of species.

Chemistry. The determination of the chemistry of the bacterial cell was first undertaken by Nencki and Schäffer, who found that

the bodies of bacteria were very rich in a nitrogenous substance to which the term micro-protein was applied; with nitric acid this body does not give the xanthoproteic reaction. The percentage of nitrogen is generally about 14.75.

The bacterial bodies also contain some 3.5 per cent. of fat; Bullock has recently extracted a considerable quantity of fat from tubercle bacilli. Bacteria elaborate within their own protoplasm various ferments or enzymes and poisons or toxines; some of these are retained within the bacterial cell and may be obtained from the washed bodies of bacteria by appropriate methods. Thus Buchner obtained an active body, which fermented sugar with the production of alcohol, by expressing the contents of washed and dried yeast. The fermentation produced in no way differed from that brought about by living yeast, the fluid being demonstrably free from living organisms. Quite recently Macfadyen1 and Rowland prepared a principle from the washed bodies of typhoid bacilli by triturating them with sterile sand in a special apparatus. The glycerine extract obtained when injected into rabbits induced an agglutinative power in the serum similar to that developed during an attack of typhoid fever in the human subject. Some bacteria give a blue colouration with acidulated iodine, due to the presence of granulose; a number of these are met with in the mouth at various times. The butyric acid-bacilli give this blue reaction with iodine, and are grouped generically as Granulobacteria (Lefar).

Gram's method of staining (p. 46) also depends on some chemical difference in the composition of the given organism, some retaining the stain, others are quickly decolourised with the alcohol used.

Structure. The bacterial cell consists of a thin cell wall enclosing clear and, as far as is at present known, structureless contents.

The cell wall, allied but not identical with cellulose, is said to be of a radiate, honeycombed appearance, and that in this layer is situated the colouring matter of the chromogenic organisms, whilst the sulphur granules appearing in the Beggiatoa are contained in the internal plasma.

The plasm also contains at times highly refractile granules which are not spores; these granules are said to be homologous with the chromatin granules of higher plants from the avidity with

'Cent. für Bak., xxiv., 1902.

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