VITAL ACTIVITIES OF BACTERIA 151 



those of most value for identifying species are indol, skatol,. 

 phenol, and tyrosin. The results of careful investigations 

 have only been recorded relative to the occurrence of indol 1 

 and phenol. 



Decomposition of fats. Pure fat is not a suitable culture 

 medium for bacteria. The rancidity of butter is brought 

 about (1) as the result of a purely chemical decomposition of 

 the butter by the oxygen of the air under the influence of 

 sunlight, and (2) through the formation of lactic acid from 

 the milk-sugar left in the butter. Fats are, however, attacked 

 by bacteria when mixed with gelatin and used as culture 

 media, with the production of acid. 



Putrefaction. By putrefaction is commonly understood 

 the decomposition due to bacteria which results in the produc- 

 tion of malodorous substances. Technically putrefaction de- 

 pends upon the decomposition of complex organic compounds,, 

 albuminous substances due largely if not entirely to anaerobic; 

 bacteria. 



1 Demonstration of indol and phenol according to Lehmann and 

 Neuman is as follows: Indol. To a bouillon culture, which should, 

 if possible, be not under eight days old and tree from sugar, is added 

 half its volume of 10 per cent sulphuric acid. If in heating to about 

 80 C. a pink or bluish-pink coloration is immediately produced it 

 indicates the presence of both indol and the nitrites, the above- 

 described nitroso-indol reaction requiring the presence of both these 

 substances for its successful operation. This is the so-called: 

 "cholera-red reaction" of the cholera spirillum and once thought to- 

 be diagnostic of it. As a rule, however, the addition of sulphuric 

 acid alone is not sufficient, and a little nitrite must be added; this 

 may be done later, the culture being first warmed without nitrite, 

 when, if there is no reaction or a doubtful one, 1 to 2 cc. of 0.005 per 

 cent solution of sodium nitrite is added until the maximum reaction 

 is obtained. The addition of strong solutions of nitrite colors the 

 acid liquid brownish-yellow and ruins the test. Phenol. The culture 

 in sugar-free bouillon receives the addition of about one-fifth its 

 volume of hydrochloric acid and is then distilled. The distillate 

 gives a fiocculent precipitate when treated with bromin water. If 

 carefully neutralized with calcium carbonate, the addition of neutral 

 very dilute chlorid of iron gives a violet color. 



