are chiefly
exerted towards this particular bacterium. Such a serum may accordingly
within certain limits be used for differentiating this organism from others
closely allied to it (_vide infra_).

The modes of cultivation described apply only to organisms which grow in
presence of oxygen. Some, however--the strictly _anaerobic_ bacteria--grow
only in the absence of oxygen; hence means must be adopted for excluding
this gas. It is found that if the inoculation be made deep down in a solid
medium, growth of an anaerobic organism will take place, especially if the
medium contains some reducing agent such as glucose. Such cultures are
called "deep cultures." To obtain growth of an anaerobic organism on the
surface of a medium, in using the plate method, and also for cultures in
fluids, the air is displaced by an indifferent gas, usually hydrogen.

[Sidenote: Inoculation.]

In testing the effects of bacteria by inoculation the smaller rodents,
rabbits, guinea-pigs, and mice, are usually employed. One great drawback in
certain cases is that such animals are not susceptible to a given
bacterium, or that the disease is different in character from that in the
human subject. In some cases, _e.g._ Malta fever and relapsing fever,
monkeys have been used with success, but in others, _e.g._ leprosy, none of
the lower animals has been found to be susceptible. Discretion must
therefore be exercised in interpreting negative results in the lower
animals. For purposes of inoculation young vigorous cultures must be used.
The bacteria are mixed with some indifferent fluid, or a fluid culture is
employed. The injections are made by means of a hypodermic syringe into the
subcutaneous tissue, into a vein, into one of the serous sacs, or more
rarely into some special part of the body. The animal, after injection,
must be kept in favourable surroundings, and any resulting symptoms noted.
It may die, or may be killed at any time desired, and then a post-mortem
examination is made, the conditions of the organs, &c., being observed and
noted. The various tissues affected are examined microscopically and
cultures made from them; in this way the structural changes and the
relation of bacteria to them can be determined.

[Sidenote: Separation of toxins.]

Though the causal relationship of a bacterium to a disease may be
completely established by the methods given, another very important part of
bacteriology is concerned with the poisons or toxins