SELECTION AND PREPARATION OF ORGANISMS 71 



should have at least one good laboratory course in bacteriology or micro- 

 biology. Only a brief, very general description can be provided here. 

 Every organism must be supplied with certain chemical materials in 

 order to grou^ and multiply. Microorganisms commonly are grown on 

 artificially prepared media which will provide a source of carbon, water, 

 nitrogen, several mineral salts, oxygen or other gases, and vitamins or 

 any other required compounds. Sometimes this medium is a liquid; other 

 times it is semi-solid because of the addition of gelatin or agar. If the 

 microorganism is autotrophic and can make its own carbon compounds 

 from carbon dioxide, it must be provided with light or another energy 

 source. The medium is sterilized in an autoclave by heating under pres- 

 sure to a temperature well above the boiling point at atmospheric pres- 

 sure. Some unwanted bacteria are able to withstand any less drastic 

 sterilization procedure. After the solution has cooled in its carefully 

 closed container, a small amount of the desired microorganisms is intro- 

 duced or "inoculated." With reasonable skill and some luck, only 

 cells of the desired type enter the nutrient medium and in a few days 

 will have multiplied to produce enough cells for experiment. The sterile 

 handling and aseptic techniques require constant practice. More than 

 one experiment has been ruined or delayed by the presence of foreign 

 organisms in a supposedly pure culture. This type of disaster becomes 

 even more dangerous if the investigator is not aware of the invasion by 

 the other species. Usually a routine microscopic examination is worth 

 the few minutes that it takes. 



Occasionally experiments are performed with "enrichment cultures" 

 of microorganisms, or, as they are sometimes called, "almost pure cul- 

 tures." Any such experiment is subject to some question— especially if 

 the proportion of the various species is changed by the growth during 

 several transfers to new media— because the measured responses might 

 be produced by the other organisms present. 



I cannot urge strongly enough that every biologist learn the sterile 

 techniques required in bacteriolog)'. Of course they apply directly to 

 microorganisms, but the general principles apply to the handling of the 

 larger plants and animals as well. 



Tissue Culture: Some kinds of cells or tissues can, if properly handled, 

 continue to grow after being removed from the higher plant or animal. A 

 solution or other growth medium is prepared which will contain all the 

 chemical compounds the cells jiormally require. Most of these solutions 

 are very complex mixtures, often containing extracts of poorly knowTi 

 composition from living or dead cells. A representative solution for the 



