PRODUCTS FORMED IN CULTURES 77 



separated milk in the middle. The cream and milk may then be 

 carefully decanted, and the sediment will be available for examina- 

 tion. 



For methods of examination for special organisms see page 67. 



Products formed in Cultures 

 (rt) Gas- 

 To test an organism for gas production, inoculate a fermenta- 

 tion tube {see Fig. 15) containing sterile glucose or lactose bouillon 

 (or dextrose - free bouillon to which a defi- 

 nite amount of sugar has been added), and 

 incubate at 37° C. Care must be taken that 

 the culture liquid, prior to inoculation, com- 

 pletely fills the upright branch. The gas 

 generated gathers at the top of this branch, 

 and when the evolution has ceased the length 

 of the column of gas can, if desired, be 

 measured with a scale and expressed as a 

 percentage of the total length of the column 

 of liquid prior to inoculation. 



{b) Indol— 



To test for the presence of indol, inocu- 

 late test-tubes of peptone water with the 

 organism to be tested and incubate at 



V' c. 



•,,-, ^t 1 i 1 1 Y\Q. 15. — Fermentation 



When growth has taken place remove tube, 



the cultures from the incubator and, after 



allowing them to cool down to room temperature, run in by 

 means of a sterile pipette i c.c. of pure sulphuric acid. 



Allow the tubes to stand upright in a rack for ten minutes. 

 If a red or rose-pink colour is developed within this time it 

 will indicate the presence of both indol and a nitrite. 



If no colour change takes place, add i c.c. of 02 per cent. 

 solution of potassium nitrite and place aside for a further period 

 of ten minutes. The production of a red coloration will indicate 

 the presence of indol. Should the coloration prove feeble, a small 

 quantity of amylic alcohol may be shaken up with the culture, 

 when the rose colour will show very distinctly after standing. 



