ROLL CULTURE III 



wiped with the cotton plug and then held in the flame to destroy all 

 bacteria clinging to it. The lid of a petri dish is carefully and par- 

 tially lifted and the contents of the tube rapidly and evenly poured 

 over the bottom of the plate, and the lid quickly replaced. 



This procedure is followed with the other tubes, and then the 

 plates or dishes are put in a cool dark place, and the tubes are put 

 into a solution of bichloride of mercury, or into boiling water. 



The plates should be examined from time to time. After several 

 days a perfect cloud of round colonies are seen in number one; a 

 large number in No. 2 and a much fewer number, say fifty, in No. 3. 

 It is an easy matter then to pick out a colony that is surrounded by 

 a bluish green halo and transfer it to a tube of agar or bouillon. 

 In the case of pus it is more than probable that the colony is that of 

 the pyocyaneus bacillus, and that it contains nothing but these 

 bacilli. It must be studied in a dozen other ways, before it is cer- 

 tain that it is this bacillus, but the preceding method is a necessary 

 primary step to secure this organism in pure culture and may be 

 taken as a pattern 'for all plate methods. 



Agar plates are often used since they have this advantage they 

 do not melt at 37 C. incubator temperature. When agar is used it 

 must be melted at 100 C. and cooled below 45 C. and above 39 C. 

 Above 45 C. bacteria may be killed. Below 39 C. the agar begins 

 to harden, so this method must be performed quickly; the plates 

 should be slightly warmed, the culture poured on and the agar hard- 

 ened, they then must be inverted in the incubator, since the water 

 of condensation forming in the lids of the plates often falls and 

 washes one colony into another. 



When gelatine plates are made, they must be kept in a cool place. 

 It is often of advantage to cool the plates by means of ice, before 

 they are filled. 



Roll Culture. 



Instead of pouring out the contents of the inoculated tubes the 

 gelatine may be made to harden on the walls of the tubes by quickly 



