PLATE CULTIVATIONS 77 



three purposes : (1) for obtaining pure cultivations, i.e. 

 cultures containing a single species, from a mixture of 

 organisms ; (2) for the enumeration of organisms ; and 

 (3) for ascertaining the characters of the colonies of 

 organisms as an aid in the identification of species. 



Before the introduction of plate cultivations pure 

 cultures of organisms could only be obtained by chance, 

 or by the dilution method, which was also by no means 

 certain. The dilution method consisted in estimating 

 approximately the number of organisms in a given 

 volume of fluid by means of an instrument on the same 

 principle as the hsematocytometer. 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 to 

 tubes of sterile media pure cultivations can in some cases 

 be obtained, a single organism having been sown in a tube. 



It is obvious, however, that this method is at best an 

 uncertain one, but the plate-culture method to a large 

 extent obviates this uncertainty. 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 growth, 

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

 pure cultures can be obtained by inoculating tubes of 

 sterile media from them. 



