1 9 



j.^ 



blue, but in all cases where gelatin was not liquified, turned decidedly 

 red. The acid formed by the fermentation of the sugar is probably 

 therefore, the chief factor at work in preventing gelatin liquefaction by 

 P. vulgaris. 



In view of the experiments of Ogata (29), Labor de (30), Gold- 

 thwait (31) and Lindsay (32), in which these authors seem to have 

 demonstrated that the addition of carbohydrates to artificial digestion 

 solutions retards the digestion of protein substances, it would seem that 

 sugars themselves should retard the proteolytic action of bacterial fer- 

 ments as well. Bearing in mind that these authors, experiments were 

 conducted with peptic ferments, the idea might be entertained that 

 sugars likewise exert some retarding effect in the presence of tryptic 

 ferments such as are produced by P. vulgaris. That the mere presence 

 of sugar is not effective, however, is conclusively demonstrated by the 

 following experiments. 



A flask containing 50 c. c. of nutrient gelatin with a reaction neutral 

 to phenolphthalein was inoculated with P. vulgaris I and allowed to 

 stand until completely liquefied. A portion of the liquefied gelatin was 

 passed through a hard filter to remove the bacteria. Two series of 

 flasks were prepared as indicated in the following table. To each of the 

 first series of flasks one c. c. of the unfiltered gelatin was added, while 

 to each of the second series of flasks 1 c. c. of the filtered gelatin was 

 added. Both series were allowed to stand under the same conditions. 

 The results for the sake of convenience are given in tabulated form. 



1. 50 c. c. nutrient gelatin + 2 / dextrose +1 c. c. filtered gelatin = no liquefaction in 



20 days. 



2. 50 c. c. + 2 % TJ +1 c - c - filtered gelatin = liquefaction in 4 



days. 



3. 50 c. c. + 2 % + 1 c. c. unfiltered gelatin + 1 c. c. filtered 



gelatin = no liquefaction in 8 days. 



4. 50 c. c. +2 % lactose +1 c. c. unfiltered gelatin liquefaction in 



4 days. 



5. 50 c. c. 4- 2 / n +1 c. c. filtered gelatin = liquefaction in 4 



6. 50 c. c. +1 c. c. unfiltered gelatin = liquefaction in 



4 days. 



7. 50 c. c. + 1 c. c. filtered gelatin = liquefaction in 4 



days. 



8. 50 c. c. +2 % HC1 +1 c. c. filtered gelatin = no liquefaction in 



10 days. 



All the flasks were tested at the completion of the experiments and 

 pure cultures of P. vulgaris were found in all of the flasks to which 

 the unfiltered gelatin had been added while those flasks which had re- 

 ceived the filtered gelatin proved sterile. Flasks one and three deve- 

 loped an acid reaction while the reaction of the other flasks remained 

 unchanged. 



The results are striking and conclusive. In those flasks to which 

 acid had been added or which developed an acid reaction no liquefaction 

 resulted, while in the flasks in which no acid was produced liquefaction 

 took place at a fairly uniform rate. There can be but one interpretation 

 of these results: the liquefaction of the gelatin was inhibited by the 

 acid. We know from the above experiments that dextrose has no effect 

 upon the proteolytic ferment when formed. If to the dextrose gelatin 

 we add in addition to the filtered gelatin a suspension of Proteus vul- 

 garis we get no liquefaction. In this case, however, the bacteria attacked 



