96 A MANUAL OF BACTERIOLOGY 



p. 264). The fluid is then diluted by the addition of some 

 sterile fluid so that a given volume of the dilution contains 

 a single organism only, assuming the organisms to be 

 evenly distributed throughout the fluid. By transferring 

 this volume of the dilution to tubes of sterile media pure 

 cultivations can in some cases be obtained, a single 

 organism having been sown in a tube. This method is at 

 best an uncertain one. 



The plate-culture method is now commonly employed 

 and is, as a rule, far more reliable. It depends upon the 

 following principles : Gelatin and agar media, when 

 melted, remain fluid down to 25 and 45 C. respectively, 

 temperatures which will not affect the vitality even of 

 delicate organisms. By inoculating the fluid gelatin or 

 agar, thoroughly mixing, and then pouring on to a level 

 sterilised surface, so that the medium solidifies in a thin 

 film ("plating"), the organisms, wherever they may be 

 situated, are fixed and are unable to wander, and, being 

 in a good nutrient soil, grow and multiply and ultimately 

 form visible growths or colonies. Many of these colonies 

 will have arisen from a single organism ; the colony, 

 therefore, is a " pure growth," i.e. consists of a single 

 species, and pure cultures can be obtained by inoculating 

 tubes of sterile media from them. 



When suitable, sterile nutrient gelatin is usually 

 employed for the preparation of plate cultivations, as 

 it is more easily manipulated than agar. Three tubes 

 of sterile nutrient gelatin are melted at a low temperature 

 in a beaker of water (gelatin melts at 24 C. ; the tempera- 

 ture should not exceed about 45 C.). The tubes may 

 be termed respectively 1, 2, and 3. Tube No. 1 is inocu- 

 lated, by means of a platinum needle, with a trace of the 

 growth from which pure cultivations are desired. The 

 trace of growth is thoroughly mixed up and distributed 

 throughout the melted gelatin. If this mixture be 



