72 CULTURES AND THEIR STUDY. 



frequent handling and exposure of the medium are not 

 required. Tubes used for this purpose should contain not 

 more than 5 c.c. of culture-medium. The medium is lique- 

 fied and inoculated as in the preceding method. The cotton 

 plug is burnt, pushed down even with the mouth of the tube, 

 and covered with a rubber cap. The tube is then held under 

 cold running water or laid on a block of ice and twirled 

 rapidly between the fingers, so that the medium will solidify 

 on the wall of the tube in a thin film. The colonies form 

 on the sides of the tube, and are studied in the same way as 

 those in the Petri dish. When the tube is twirled, care 

 should be taken not to wet the end of the cotton plug, so 

 that it will not adhere to the tube when the gelatin or agar 

 solidifies. 



Tube cultures : After the colonies have fully developed, 

 tubes are inoculated from them. Any kind of medium may 

 be used, but it is customary to inoculate a set consisting of a 

 tube each of beef-tea, gelatin, agar, potato, blood-serum, and 

 glycerin-agar. If the suspected material is believed to con- 

 tain bacteria which will grow only on special media, such 

 media must be used ; as, for instance, in the case of the 

 tubercle bacillus, typhoid bacillus, diphtheria bacillus, etc. 



The tubes and needle are held as before, the platinum wire 

 being perfectly straight and without a loop at the end. The 

 cover of the Petri dish is raised slightly, and with the sterile 

 needle a very small amount of the colony is removed and 

 transferred to the test-tube. The wire must not come in con- 

 tact with anything except the colony and the medium to be 

 inoculated. 



For bouillon inoculation, or any other liquid medium, the 

 needle is passed into the medium and the adhering culture 

 shaken off. 



In gelatin the so-called stab culture (Fig. 27) is made. The 

 wire is passed vertically into the centre of the medium for 

 about half its length and is then slowly withdrawn. 



Stab cultures are made principally for the purpose of 

 studying liquefaction. The growth in or on gelatin is varied, 

 and often quite characteristic. Thus, it may be limited to the 

 surface of the medium; or to the stab; or it may appear both 



