10 



has no affect on indol production by P. vulgar is, inhibits its produc- 

 tion in the case of B. coli as readily as does dextrose. This naturally 

 leads to the conclusion that it is the acid and not the sugar which in- 

 hibits the production of indol. 



An attempt was made to settle this point more definitely ly inocu- 

 lating with P. vulgaris a sugar medium to which more than enough 

 powdered marble had been added to neutralize the acid formed. It was 

 found, however, that indol was not produced in any case where 1/ or 

 more of dextrose had been added to the medium. This seemed to dis- 

 prove the conclusion that the inhibition of indol is due to the acid 

 formed, but on titrating the solutions to which the marble had been 

 added, it was found that the acid was being produced much faster than 

 it was neutralized by the marble, so that even in this case the acid was 

 probably the chief factor inhibiting the proteolytic action of the bacteria. 

 As ordinary marble may contain some impurities, it was thought that 

 the impurities in this substance may have had something to do with 

 the results. To avoid this source of error, several flasks were prepared, 

 each containing 100 c. c. of a 1% solution of peptone to which were 

 added 1% of dextrose and more than enough chemically pure calcium 

 carbonate to neutralize all acid formed, together with sufficient of a 1% 

 azolitmin solution to give a blue color. The flasks were inoculated with 

 Proteus vulgaris and incubated at 37. In about 24 hours, the 

 litmus had turned red. A flask prepared in the same manner was in- 

 oculated with B. coli and this also turned red. Flasks prepared in the 

 same way without the addition of litmus and inoculated with B. coli or 

 P. vulgaris, failed to produce indol in four days. The calcium car- 

 bonate, while reducing the amount of acid produced, did not keep the 

 solution alkaline to litmus. Since acid sufficient to turn blue litmus 

 red is able to inhibit the action of the tryptic ferment, calcium carbonate 

 added to a medium is not sufficient to keep the degree of acidity low 

 enough for the proteolytic ferment of P. vulgaris to act on the peptone. 

 To obviate this difficulty a flask, prepared as in the other experiments, 

 was inoculated with P. vulgaris and the acid was neutralized from 

 time to time by the addition of a sterile solution of sodium carbonate 

 to the medium. In this flask, indol was formed as soon as all the 

 sugar had been used up. When lactic acid was added to the peptone 

 solution which had been inoculated with P. vulgaris so that the so- 

 lution was more than 0.5 % ac id, the production of indol was inhibited. 

 When less than 0.5 % was added, however, the production of indol was 

 not appreciably delayed. 



The above experiments seem to justify the conclusion that carbo- 

 hydrates are more available to bacteria than are the proteins. In a medium 

 containing sugar and protein substances, the sugar is usually the first 

 attacked. When this is used up if the acid produced does not retard 

 further action, the proteins are utilized. The inhibition of the proteolytic 

 action is due to the acid formed and not to the sugar itself, for if the 

 carbohydrate is not fermented the proteolytic action may not be retarded. 

 The ferment produced by P. vulgaris seems to be a tryptic ferment. 



Effect of carbohydrates on gelatin liquefaction. 



Since the addition of certain carbohydrates to peptone solution in- 

 hibited the production of indol by P. vulgaris, it was thought that 

 these sugars may also inhibit the power of these organisms to liquefy 



