Bacillus lepræ produces leprosy; Bacillus mallei produces glanders; Bacillus tetani, tetanus (the tetanus bacillus is very common in soil; anaerobic); Bacillus diphtheriæ, diphtheria; Bacillus typhosus, typhoid fever, etc.
Pathogenic Spiral Bacteria. Spirochæte obermeieri (Fig. [24]) produces intermittent fever (febris recurrens); it makes its appearance in the blood during the attacks of fever, but it is not to be found during intervals when there is no fever. Obligate parasite.
Spirillum choleræ asiaticæ (Microspira comma) without doubt produces Asiatic cholera; an exceedingly motile spirillum, which is also found in short, bent rods (known as the “Comma-bacillus”), it lives in the intestines of those attacked by the disease, and gives off a strong poison which enters the body. It is easily cultivated as a saprophyte.
A great many circumstances seem to show that a number of other infectious diseases (syphilis, small-pox, scarlet-fever, measles, yellow-fever, etc.) owe their origin to parasitic Bacteria, but this has not been proved with certainty in all cases.
It has been possible by means of special cultivations (ample supply of oxygen, high temperature, antiseptic materials) to produce from the parasitic Bacteria described above (e.g. the fowl-cholera and the anthrax Bacteria) physiological varieties which are distinct from those appearing in nature and possess a less degree of “virulence,” i.e. produce fever and less dangerous symptoms in those animals which are inoculated with them. The production of such physiological varieties has come to be of great practical importance from the fact that they are used as vaccines, i.e. these harmless species produce in the animals inoculated with them immunity from the malignant infectious Bacteria from which they were derived. This immunity is effected by the change of the products of one or more of the Bacteria, but we do not yet know anything about the way in which they act on the animal organism. The white blood corpuscles, according to the Metschnikoff, play the part of “Phagocytes” by absorbing and destroying the less virulent Bacteria which have entered the blood, and by so doing they are gradually enabled to overcome those of a more virulent nature.
Fig. 34.—a and b The same blood-cell of a Frog: a in the act of engulfing an anthrax-bacillus; b after an interval of a few minutes when the bacillus has been absorbed.
Class 5. Conjugatæ.
The Algæ belonging to this class have chlorophyll, and pyrenoids round which starch is formed. The cells divide only in one direction, they live solitarily, or united to form filaments which generally float freely (seldom attached). Swarm-cells are wanting. The fertilisation is isogamous (conjugation) and takes place by means of aplanogametes. The zygote, after a period of rest, produces, immediately on germination, one or more new vegetative individuals; sometimes akinetes or aplanospores are formed in addition. They only occur in fresh or slightly brackish water.
Order 1. Desmidiaceæ. The cells generally present markings on the outer wall, and are mostly divided into two symmetrical halves by a constriction in the middle, or there is at least a symmetrical division of the protoplasmic cell-contents. The cell-wall consists nearly always of two layers, the one overlapping the other (Fig. [35] C). The cells either live solitarily or are united into unbranched filaments. The mass of protoplasm formed by the fusion of the two conjugating cells becomes the zygote, which on germination produces one (or after division 2, 4 or 8) new vegetative individual. The chromatophores are either star-, plate-, or band-shaped, and regularly arranged round the long axis of the cell.