126 BACTERIOLOGICAL LABORATORY TECHNIC 



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 ways, before it is certain 

 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 usually used since they have this advantage 

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

 used it must be melted at iooC. and cooled below 48C. and 

 above 43C. Above 48C. bacteria may be killed. Below 43C. 

 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 hardened, they must be invented 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 at 2o-25C. It is often of 

 advantage to cool the plates by means of ice, before they are 

 filled. 



The so-called "Stroke plates" are extremely useful for hospital 

 bacteriology. The agar is softened, poured into plates, allowed to 

 harden and the material to be examined is smeared upon the firm 

 surface by a flattened platinum rod, a "Spatula." The separate 

 colonies develop along the lines of spread and can be isolated to 

 individual tubes as given above. 



Roll Citllure. Instead of pouring out the contents of the in- 

 oculated tubes the gelatine may be made to harden on the walls of 



