_cilia_ (sing. _cilium_) that are developed on the
outer surface of the cell. By means of the rapid vibration of these
organs, the cell is propelled through the medium. Nearly all cocci are
immotile, while the bacilli may or may not be. These cilia are so
delicate that it requires special treatment to demonstrate their
presence.

~Classification.~ In classifying or arranging the different members of any
group of living objects, certain similarities and dissimilarities must
be considered. These are usually those that pertain to the structure and
form, as such are regarded as most constant. With the bacteria these
differences are so slight that they alone do not suffice to distinguish
distinctly one species from another. As far as these characters can be
used, they are taken, but in addition, many characteristics of a
physiological nature are added. The way that the organism grows in
different kinds of cultures, the by-products produced in different
media, and effect on the animal body when injected into the same are
also used as data in distinguishing one species from another.

~Conditions favoring bacterial growth.~ The bacteria, in common with all
other living organisms are affected by external conditions, either
favorably or unfavorably. Certain conditions must prevail before
development can occur. Thus, the organism must be supplied with an
adequate and suitable food supply and with moisture. The temperature
must also range between certain limits, and finally, the oxygen
requirements of the organism must be considered.

~Food supply.~ Most bacteria are capable of living on dead, inert, organic
matter, such as meats, milk and vegetable material, in which case, they
are known as _saprophytes_. In contradistinction to this class is a
smaller group known as _parasites_, which derive their nourishment from
the living tissues of animals or plants. The first group comprise by far
the larger number of known organisms which are concerned for the most
part in the decomposition of organic matter. The parasitic group
includes those which are the cause of various communicable diseases.
Between these two groups there is no sharp line of division, and in some
cases, certain species possess the faculty of growing either as
parasites or saprophytes, in which case they are known as _facultative_
parasites or saprophytes.

The great majority of bacteria of interest in dairying belong to the
saprophytic class; only those species cap