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.