72 APPLIED BACTERIOLOGY 



colonies develop only on the surface of the media, others in 

 the depth. The different colours of the colonies also afford 

 distinguishing traits. They are only rarely colourless and 

 transparent ; as a rule, they are more or less coloured. 

 The predominating colours are yellow and white ; these 

 occur in every possible tone. Not infrequently the colony 

 remains colourless, while the surrounding gelatine may 

 become coloured. In addition to the bacterial colonies 

 proper, colonies of various coloured moulds very frequently 

 appear, but these are never mistaken for colonies of bacteria, 

 as they are always characteristic on account of their raised 

 and feather-like hyphae. 



Having thus described the methods of isolating micro- 

 organisms from a mixture by means of plate cultures, it 

 becomes necessary to further separate and study each indi- 

 vidual colony. It should not be overlooked that isolated 

 colonies do not necessarily contain only one species, as 

 they may not have developed from a single cell. 



To further study the organisms thus isolated by means 

 of plate cultures, it is necessary to inoculate from the 

 colonies into tubes of various nutrient media to determine 

 the morphological and biological characters of the micro- 

 organisms under examination, and thus gain a knowledge 

 of the class to which the organism belongs. This proce- 

 dure is carried out as follows : A portion of a colony is 

 removed on the point of a sterile platinum wire and trans- 

 ferred to gelatine and agar in the form of ' streak ' and 

 ' stab ' cultures, and in some cases to other media. There 

 are several mechanical contrivances sold for the purpose of 

 accurately picking out the particular colony it is desired to 

 examine, but with a little skill the use of a simple inocu- 

 lating wire is perfectly satisfactory in practice. 



' Streak ' Cultures. — For the streak cultures we use tubes 

 in which the nutrient gelatine, or agar, has been solidified 



