used to denote a power of counteracting bacteria and their products.

#General Characters of Bacteria.#--A _bacterium_ consists of a finely
granular mass of protoplasm, enclosed in a thin gelatinous envelope.
Many forms are motile--some in virtue of fine thread-like flagella, and
others through contractility of the protoplasm. The great majority
multiply by simple fission, each parent cell giving rise to two daughter
cells, and this process goes on with extraordinary rapidity. Other
varieties, particularly bacilli, are propagated by the formation of
_spores_. A spore is a minute mass of protoplasm surrounded by a dense,
tough membrane, developed in the interior of the parent cell. Spores are
remarkable for their tenacity of life, and for the resistance they offer
to the action of heat and chemical germicides.

Bacteria are most conveniently classified according to their shape. Thus
we recognise (1) those that are globular--_cocci_; (2) those that
resemble a rod--_bacilli_; (3) the spiral or wavy forms--_spirilla_.

_Cocci_ or _micrococci_ are minute round bodies, averaging about 1 µ in
diameter. The great majority are non-motile. They multiply by fission;
and when they divide in such a way that the resulting cells remain in
pairs, are called _diplococci_, of which the bacteria of gonorrhœa and
pneumonia are examples (Fig. 5). When they divide irregularly, and form
grape-like bunches, they are known as _staphylococci_, and to this
variety the commonest pyogenic or pus-forming organisms belong (Fig. 2).
When division takes place only in one axis, so that long chains are
formed, the term _streptococcus_ is applied (Fig. 3). Streptococci are
met with in erysipelas and various other inflammatory and suppurative
processes of a spreading character.