THE SIGNIFICANCE OF BACTERIAL METABOLISM 81 



2. The Recognition of Bacteria. The recognition of many kinds 

 of bacteria, as for example members of the intestinal group, depends 

 upon the reactions these organisms induce in various sugars. Thus, 

 B. alcaligenes ferments no sugars; B. dysenterise ferments dextrose 

 with the production of acid; B. proteus ferments dextrose and sac- 

 charose with the evolution of gas and the production of acid; B. 

 coli ferments dextrose and lactose with the evolution of gas and the 

 production of acid; B. coli coagulates milk, while B. proteus charac- 

 teristically peptonizes it. All of these reactions are explained per- 

 fectly upon the theory that utilizable carbohydrate protects protein 

 from bacterial breakdown. Thus, B. alcaligenes does not utilize 

 any carbohydrate; as is well known, it is carnivorous. B. dysenterise 

 can utilize dextrose, and consequently it produces acid in a medium 

 containing both protein derivatives and this sugar: similarly, B. 

 proteus and B. coli ferment dextrose and in addition a specific biose. 

 B. proteus, however, does not ferment lactose, hence it attacks the 

 protein of milk; while B. coli, which does ferment lactose, produces 

 an acid coagulation in milk: the acid resulting from the fermenta- 

 tion of the milk sugar (lactose) protects the proteins of the milk. 

 In each instance the organisms attack the utilizable carbohydrate 

 whenever it is present, in preference to the protein for their energy 

 requirements. If bacteria did not habitually utilize carbohydrate 

 in preference to protein for their fuel needs, these fermentation reac- 

 tions would be of no value whatsoever -as diagnostic tests for these 

 various microorganisms. 



3. Certain bacteria, notably B. proteus, produce active, soluble 

 (extracellular) enzymes when grown in sugar-free gelatin, that 

 bring about an energetic liquefaction of this medium, which becomes 

 alkaline in reaction. If the organisms are grown in dextrose gelatin 

 no liquefaction takes place; the bacilli produce CO 2 and H 2 as well 

 as acid in dextrose gelatin, using the sugar in preference to the protein 

 for their energy needs. The liquefied gelatin containing the soluble 

 gelatinase may be sterilized by passage through a Berkefeld filter, 

 thus removing all bacteria. The filtrate will liquefy sterile plain or 

 sterile dextrose gelatin, thus proving that the soluble enzyme, which 

 prepares gelatin for assimilation by proteus bacilli (and which is 

 only produced in a carbohydrate-free medium), acts specifically on 

 the protein irrespective of other substances which may be present. 

 In this instance the presence of utilizable sugar in cultures of living 

 proteus bacilli protects the protein (gelatin in the instance cited) 



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