200 



Cultures, and Their Study 



is several days old, and indeed sometimes not until much 

 later. Again, many colonies make their first appearance as 

 minute, sharply circumscribed points, and later spread upon 

 the surface of the culture medium, either in the form of a 

 thin, homogeneous layer or a filamentous cluster. It is 

 particularly important that in describing new species of 

 bacteria an account of the appearance of the colonies from 

 day to day, comparing all of their variations for at least 

 two weeks, should be included. 



Pure Cultures. Single colonies also subserve a second 

 very important purpose, that of enabling us to secure pure 

 cultures of bacteria from a mixture. The usual method of 

 doing this is by the use of plates, Petri dishes, or Esmarch 

 tubes according to the methods already described. For 



Fig. 47. Microscopic structure of colonies: 1, Areolate; 2, grumose; 

 3, moruloid; 4, clouded; 5, gyrose; 6, marmorated; 7, reticulate, 

 8, repand; 9, lobate; 10, erose; 11, auriculate; 12, lacerate; 13, firn- 

 bricate; 14, ciliate (Frost). 



this purpose an isolated colony is selected and carefully 

 examined to see that it is single and not a mixture of two 

 closely approximated colonies of different kinds, and then 

 transplanted to a tube of an appropriate culture medium. 

 If the colonies are few and of good size, each is transplanted 

 to the medium directly and without instrumental assistance. 

 If, however, the colonies are numerous, of small size, and 

 close together, it may be necessary to do it under a dis- 

 secting microscope or even a low power of the ordinary 

 bacteriologic microscope. This operation of transplantation 

 is familiarly known as fishing. 



Fishing. It requires considerable practice and skill to 

 fish successfully, and the student should early begin to 

 practise it. The colony to be transplanted, selected because 

 of its isolation, its typical appearance, and convenient posi- 



