QUALITATIVE EXAMINATION OF MILK 65 



Procedure— 



1. Introduce into the tube marked i, by means of one of the 

 pipettes or the platinum needle, a single drop of the fluid under 

 examination. Replace the cotton-wool plug and roll the tube in 

 the hand for at least two minutes in order to ensure an even dis- 

 tribution of organisms throughout the medium. 



2. W'^ith a second pipette, lightly passed through the flame, or 

 with the platinum needle freshly flamed, remove three drops of the 

 liquid from tube i, into the second tube. Roll in the hand for one 

 minute as above. 



3. Transfer three drops from tube 2 to tube 3 in the same 

 fashion. Roll the tube as above and set aside for a few moments 

 with the others. 



4. Take the tube marked i, and the first Petri dish, and, after 

 removing the cotton-wool plug from the former and flaming the 

 tube orifice, raise the cover of the dish sufficiently to allow the 

 liquefied medium to be poured into it with an even flow. Replace 

 the cover as soon as the last drop of gelatine has fallen, and, keep- 

 ing it as level as possible, oscillate the dish gently in order to 

 obtain an even surface of the medium. Set aside on a level 

 surface, and when complete solidification has taken place incubate 

 at 20° C. or lower temperature. Treat tubes 2 and 3 in a similar 

 manner. 



Examine the dishes carefully each day. Note the characters of 

 the developing colonies, and inoculate a tube of broth from each 

 distinctive colony ; from this tube subsequent cultures can be made 

 on any media desired. The tubes should be clearly marked to 

 correspond with the colonies from which the growth was taken and 

 the colonies themselves may be marked {a, b, c, etc) in ink on the 

 under side of the bottom dish. 



2. On nutrient ag'ar — 



Proceed as in the case of gelatine. It is necessary to act 

 quickly, as the medium, of which the melting point is 100° C, 

 resolidifies on the fall of the temperature to 40° C. The tubes of 

 agar are first melted by boiling in a beaker of water, then allowed 

 to cool until th. reach a temperature just above the solidifying 

 point. They should then be placed, and kept during the succeeding 

 manipulations, in a beaker of water maintained at from 40"^ to 

 45^ C. The manipulations should be carried out as quickly as 

 possible, and to prevent an uneven surface due to the too rapid 

 solidification of the medium, the Petri dishes should be previously 



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