METHODS USED IN CULTIVATION OF BACTERIA 177 



of bacteria which are being studied. Often in the second dilution, 

 more frequently in the third, the colonies will be found well apart 

 and can then be "fished." The process of "colony-fishing" is one 

 which requires practice and should always be done with care, for 

 upon its success depends the purity of the sub-culture obtained. 

 Colonies should never be fished under the naked eye, no matter how 

 far apart and discrete they may appear, since not infrequently close 

 to the edge of or just beneath a larger colony there may be a 

 minute colony of another species which may be too small to be 

 visible to the naked eye, but which, nevertheless, if touched by 

 accident will contaminate the sub-culture. 



For proper "fishing," the Petri plate with cover removed, should 

 be placed upon the stage of the microscope and examined with a 

 low power objective, such as Leitz No. 2 or Zeiss AA. The sterilized 

 platinum needle, held in the right hand, is then carefully directed 

 into the line of focus of the lens, while the small finger of the 

 hand is steadied upon the edge of the microscope stage. When the 

 point of the needle is clearly visible through the microscope, it is 

 gently depressed until it is seen to touch the colony and to carry 

 away a portion of it, The needle is then withdrawn without again 

 touching the nutrient medium or the edges of the glass or the lens, 

 and transferred to a tube of whatever medium is desired. In this 

 way, individuals of one colony, descendants of a single bacterium 

 of the original mixture are carried over to the fresh medium. 



Esmarch Roll Tubes. 1 A simple method of obtaining separate colonies 

 is that devised by von Esmarch and known as "roll-tube ' ' cultivation. Tubes 

 of melted gelatin are inoculated with various dilutions of the bacterial 

 mixture and, while still liquid, are laid in an almost horizontal position 

 upon a block of ice, which has been grooved slightly by means of a test 

 tube filled with hot water. The test tube containing the gelatin, after being 

 placed in this groove, is rapidly revolved by passing the fingers of the right 

 hand across it, while its base is carefully steadied with the left hand. If 

 the revolving is carried out with sufficient speed, the gelatin will harden 

 in a thin layer on the inner surface of the tube. The colonies will develop 

 in this layer and may be "fished" by means of a platinum wire with bent 

 point introduced into the tube. This method is useful for certain purposes, 

 but is too inconvenient for routine work. It is now rarely used. 



1 Esmarch, Zeit. f. Hyg., i, 1886. 



