Bacteria are every day becoming more and more important on account of the relation in which they are found to stand to various diseases. Anthrax, diphtheria, phthisis, and typhoid fever, are probably all due to various species of bacteria introduced into the body, and affecting various organs in it. It is, therefore, of the greatest possible importance that their life-history should be learned, and that we should know what the conditions are under which they thrive best, and what the conditions are which will destroy their life and prevent their development.

They appear to increase in two ways : first, by simple multiplication of their parts, and secondly, by forming spores.

Bacteria require water, organic matter, and salts, for their life. Some of them also require the presence of free oxygen; others do not; hence they have been divided by Pasteur into two classes : aerobious and anaerobious. To the anaerobious bacteria oxygen is not merely unnecessary but hurtful, and even the aerobious bacteria, although they require oxygen in a certain quantity, are injured or destroyed by it when it is in excess.

Fig. 15. Blastomycetes, or Yeasts

Fig. 15. Blastomycetes, or Yeasts

Torula, or Saccharomyces (Fig. 15) or Mycoderma.

Fig. 16. Hyphomycetes, or Moulds

Fig. 16. Hyphomycetes, or Moulds.

Mucor.

Penicillium.

Oidium.

Achorion.

Trichophyton.

Microsporon.

Bacteria Or Schizomycetes 44

Fig. 17.

Schizomycetes, or Bacteria.

Sphaerobacteria (globular cells) .

Micrococcus (1 (a & b) & 2, Fig. 16).

Sarcina (3).

Microbacteria, or Bacteria proper (small, rod-like cells)

Bacterium .

Bacterium termo (4).

B. lineola (5).

Desmobacteria, or

Filobacteria (larger rod-like or thread- like cells)

Bacillus (straight) .

B. subtilis (6).

B. anthracis (7).

J B. septicaemia.

1 B. malarias (8).

B. tuberculosis (12).

V B. leprae.

Vibrio (wavy) .

Vibrio serpens (9).

Spirobacteria (twisted or spiral -cells)

Spirochseta (long, flexible, close-wound spirals) .

Spiroehasta. Ober-meyeri (10).

Spirillum (short, stiff, open spirals) .

S. volutans (11).

The soil which is most favourable to different classes of bacteria varies with each class. A struggle for existence goes on between bacteria and other organised ferments, and between different kinds of bacteria themselves, in the same way as amongst higher plants. Just as an abundant crop of one kind of higher plants will occupy a whole field and choke other plants, so that kind of bacterium which grows most readily in a particular soil will choke others and prevent them growing at the same time with itself. During their growth they alter the soil or substance in which they grow, either by exhausting the nutriment it affords, or by forming in it new substances which are injurious to themselves, and thus they gradually die out.

But the soil which is no longer suitable for one kind of bacterium then becomes suitable for another, and their spores, which may have lain without germinating during the time the first kind was growing, now begin to grow actively.

Thus, if a number of germs of different classes of fungi be added at the same time to a saccharine solution, the bacteria only will grow and set up lactic fermentation. If a small quantity of tartaric acid be now added (1/2 per cent.) the yeast alone will grow and alcoholic fermentation begins. If more tartaric acid be added (4-5 per cent.) the alcoholic fermentation stops, and mould begins to grow. In this process neither the bacteria nor the yeast are killed by the addition of tartaric acid, which, in different proportions, merely renders the liquid more favourable for the growth of the yeast and mould respectively, and enables them to flourish best, although the others are still present.

In fresh grape-juice many germs are present, but the composition of the liquid being more favourable to the growth of the yeast-plant than to other fungi, it alone grows. When it has converted the sugar into alcohol its growth stops, and bacteria may then multiply and convert the alcohol into acetic acid. This in turn checks the growth of the bacteria, and mould-fungi then find the soil favourable. In their growth they consume the lactic acid, and the liquid once more affords a favourable soil for bacteria, which may then grow and cause putrefaction.

The same struggle for existence occurs between the different species of bacteria themselves. Thus micrococci may be prevented from growing by micro-bacteria, and bacilli may be killed by bacterium termo when the supply of oxygen is insufficient for both.1

It is to be noted, however, that in the struggle for existence the formation of poisonous products by bacteria may be, and probably is, beneficial to them. No doubt these poisonous products check their own growth and finally destroy them; but in the struggle for existence between bacteria and living tissues these poisons may be beneficial to the bacteria by killing the tissues, and thus giving the bacteria a more ample supply of nutriment.

1 Ziegler's Pathological Anatomy, translated and edited by MacAlister, p. 272. This work contains a very lucid and complete account of disease-germs.

In investigating any problem it is always best to take the simplest case, and if we look at the struggle for existence between bacilli and an amoeba, or white blood-corpuscle, we shall see that the formation of poisonous products by the bacteria may enable them to destroy the amoeba or leucocyte instead of their being destroyed by it (Fig. 25, p. 87).

These poisonous products in fact may prepare the soil for bacteria, and this supposition is confirmed by the observations of Rossbach and Rosenberger. Rossbach found that when papain was injected into the vessels, micrococci developed in the blood with extraordinary rapidity, the ferment seeming to have altered the blood to such an extent that it became an exceptionally favourable soil for the micrococci. A similar result was observed by Rosenberger from the injection of sterilised septic blood. In this blood the bacteria themselves were destroyed, but the poisonous substances which they had formed were present, and these seemed to have a similar action to the papain.