GENERAL PRINCIPLES 159 



Colour Reduction is measured by the addition of some 

 easily discoloured substance to the medium. Litmus, 

 methylene-blue, and sodium sulphindigotate are used. As 

 the bacteria grow, the colour is discharged in the anaerobic 

 parts of the culture. In a fluid medium, shaking restores 

 the colour. 



Proteinochrome formation is observed in 5 per cent 

 peptone broth or 3 per cent peptone water. Add a few 

 drops of acetic acid and then fresh chlorine water, when 

 a red-violet colour indicates proteinochrome formation. 



Test for Indol Formation. To a pure culture in 

 1 broth or peptone water add i c.c. of o-oi per cent sodium 

 nitrite solution and i c.c. of purest sulphuric (25 per cent), 

 or HC1. A red coloration within five minutes indicates 

 that indol is present. A second test which gives a positive 

 result with B. coli within forty-eight hours at 37 C. is to 

 add i c.c. of an acid solution of paradimethylamido- 

 benzaldehyde, when a rose or cherry-red colour develops 

 in 2 or 3 minutes if indol is present. (Solution A. : para. 

 8 grm., HC1 160 c.c., absolute alcohol 760 c.c. ; Solution 

 B. : cold saturated solution of potassium sulphate. Use 

 i c.c. of A, and shake, and add i c.c. of B. and allow to 

 stand.) If there is any suspicion of the micro-organism 

 having the power of reducing nitrates to nitrites, add the 

 sulphuric acid first and wait ; if no coloration develops, 

 then add the nitrite solution. 



Spirillum cholerae has this power, and hence the addition 

 of the sulphuric acid is alone required. This is called 

 the nitroso-indol reaction. 



LIQUEFACTION OF GELATIN. 



This is due to the development of enzymes from the 

 growth of bacteria in proteid media. These proteolytic 

 enzymes are not always secretions of the bacterial cell, 

 but are in some cases closely bound to the cell-body, and 

 are separable from it only after its death. When they are 

 true secretory products, they can be separated from the 

 micro-organisms by filtration through a Berkefeld filter 

 candle, and from such filtrates they can be obtained in 



