combined as to be unavailable for the bacteria. Bacteria may further
be divided into those which are saprophytic or which find favorable
conditions for life outside of the body, and the parasitic. Many are
exclusively parasitic or saprophytic, and many are facultative, both
conditions of living being possible. It has been found possible by
varying in many ways the character of the culture medium and
temperature to grow under artificial conditions outside of the body
most, if not all, of the bacteria which cause disease. Thus, such
bacteria as tubercle bacilli and the influenza bacillus can be
cultivated, but they certainly would not find natural conditions which
would make saprophytic growth possible.

Bacteria may be very sensitive to the presence of certain substances
in the fluid in which they are growing. Growth may be inhibited by the
smallest trace of some of the metallic salts, as corrosive sublimate,
although the bacteria themselves are not destroyed. If small pieces of
gold foil be placed on the surface of prepared jelly on which bacteria
have been planted, no growth will take place in the vicinity of the
gold foil.

Variations can easily be produced in bacteria, but they do not tend to
become established. In certain of the bacterial species there are
strains which represent slight variations from the type but which are
not sufficient to constitute new species. If the environment in which
bacteria are living be unusual and to a greater or less degree
unfavorable, those individuals in the mass with the least power of
adaptibility will perish, those more resistant and with greater
adaptability will survive and propagate; and the peculiarity being
transmitted a new strain will arise characterized by this
adaptability. Bacteria with slight adaptability to the environment of