146 PURE CULTURES FROM PATHOLOGIC MATERIAL 



is obvious, of course, that the former pathogenic bacterium is respon- 

 sible for the suppuration, while the latter harmless saprophyte 

 represents an accidental contamination. 



When there are on one or more of the Petri dishes discrete (i.e.) not 

 confluent) colonies and the microscopic examination of stained cover- 

 glass preparations shows these to contain one microorganism only 

 which is considered the pathogenic causative bacterium and which 

 is desired in pure culture, another set of Petri dishes should be pre- 

 pared from this colony. This second set, provided it is inoculated 

 from an uncontaminated colony, should contain colonies of one type 

 only. From one of the colonies of the second set of plates a number 

 of culture tubes of various media should then be inoculated for the 

 further study and identification of the organism. 



"Fishing" for Colonies. It is sometimes not easy to find the young 

 small colonies which may have developed after twenty-four hours in 

 the Petri dishes, and it may not only be necessary to hunt for them 

 with the low power of the microscope, but also to remove some of 

 the material of such a colony with the platinum loop while looking 

 through the instrument. This procedure is called fishing for the 

 colony. It is not an easy one for the beginner, and in order to facili- 

 tate the work a special instrument with large stage and low-power 

 lenses having a large field of vision is used. 



Contamination with Moulds. Plates and Petri dishes are often, 

 in spite of all precautions, contaminated with moulds which have 

 fallen from the air upon the culture soil. It is, as a rule, quite easy 

 to distinguish the mould colonies from the bacterial colonies. The 

 former, however, may grow very rapidly, and if moulds are discovered, 

 subcultures, or transplants, should be made at once. 



Modifications. Sometimes it is very difficult to pour Petri dishes 

 on account of the surroundings, as, for instance, in a barn. In such 

 places a simple method of dilution is practised which often gives very 

 good results. The method is as follows : 



Have ready a number of slanting agar tubes. They may be 

 kept upright in the vest or coat pocket if there is no chance to place 

 them on a table. Sterilize the platinum loop, flame tube No. 1 over 

 an alcohol lamp held by an assistant, open tube and inoculate the 

 condensed water of the agar three times with the platinum loop. 

 Sterilize the latter, open tube No 1 again, enter with a sterile loop 

 and rub the condensed water well over the agar surface. Now inoculate 

 tube No. 2 from No. 1. Sterilize loop again and enter tube No. 2 and 

 rub over surface of agar and inoculate with the material obtained from 

 the condensed water of tube No. 3. In this manner a dilution of the 

 original material is obtained which will lead to the formation of a few 

 discrete individual colonies in tube No. 3 or No. 4. If the tubes are 

 dry and do not contain any condensed water, then the same method 

 may be practised by simply rubbing the loop each time over the 

 whole surface of the agar, sterilizing the platinum each time between 



