cilia_ (singular cilium) which are variously
distributed on the surface of the cell. By the movement of these
relatively long, thread-like appendages the individual cell is able
to move in liquids. It must be remembered, when these moving cells
are observed under the microscope, that their apparent rate of
movement is magnified relatively as much as their size.

=Conditions for growth.= All kinds of living things need certain
conditions for growth such as food, moisture, air and a favorable
temperature. The bacteria prefer as food such organic matter as
milk, meat, and vegetable infusions. Those living on dead organic
matter are known as _saprophytes_, while those which are capable of
thriving in the tissues of the living plant or animal are known as
_parasites_. Certain of the parasitic forms are capable of causing
disease in plants and animals. In the first group are embraced most
of the bacteria that are able to develop in milk or its products,
such as those forms concerned in the spoiling of milk or its
fermentation. It is true that milk may contain disease-producing
bacteria coming either from a diseased animal or from a diseased
human being. It is also true that some of such harmful forms are
able to grow in milk, such as the organisms causing typhoid fever
and diphtheria.

=Food.= The bacteria like all other plants must have their food in
solution. Where they apparently live on solids, such as meats,
fruits, etc., they dissolve the food substances before utilizing the
same. If the solutions are highly concentrated, as in the case of
syrups, preserves and condensed milk, the bacteria cannot readily
grow, although all of the necessary food ingredients are present.
When such concentrated solutions are diluted, bacterial growth will
take place and the solutions will spoil.

[Illustration: Fig. 3.--Photomicrograph of Lactic Acid Bacteria.

Each cell is an individual organism, magnified 1250 diameters.]

Generally speaking the bacteria grow best in a neutral or slightly
alkaline solution rather than in acid liquids.

=Temperature.= One of the most important conditions influencing the
rate of growth of bacteria is the temperature. Each form has a
_minimum_ temperature below which growth can not take place; also a
_maximum_ above which growth is again impossible. For the majority
of species the minimum temperature ranges from 40 to 45 deg. F. the
maximum from 105 to 110 deg. F. Growth takes place most rapidly at t