SIGNIFICANCE OF BACTERIAL METABOLISM 81 
breakdown. Thus B. alcaligenes does not utilize any carbohydrate; 
as is well known, it is carnivorous. The addition of insulin to cultures 
of this organism does not cause it to utilize glucose.^ B. dysenteriae 
can utilize olucose, and consequently it produces acid in a medium 
containing both protein derivatives and this sugar; similarly, B. 
proteus and B. coli ferment glucose 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 fermentation of 
the milk sugar (lactose) precipitates 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. 
Also, the process may be reversed, and bacteria may be utilized in a 
very practical manner to identify carbohydrates, both singly and in 
mixture. As little as 0.001 per cent of carbohydrate may thus be 
detected under favorable conditions.- Carbohydrate derivatives, as 
the hexose acids, mono and di, and the hexose alcohols may be simi- 
larly identified.^ If bacteria did not habitually utilize carbohydrate in 
preference to protein for their fuel needs, these fermentation reactions 
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 acid in 
reaction due to the liberation of peptids and amino-acids.* If the 
organisms are grown in glucose gelatin no liquefaction takes place. The 
bacilli produce CO2 and H2 as well as acid in glucose 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 glucose 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 
instances cited) from bacterial attack, and inasmuch as proteus bacilli 
prepare gelatin for assimilation through the action of a proteolytic 
ferment, the ferment is not elaborated by them under these condi- 
tions.^ A precisely similar restriction of the development of gelatin- 
liquefying enzymes by utilizable sugars occurs in cultures of cholera 
• Kendall: Jour. Infec. Dis., 1925, 37, ,329. 
2 Kendall and Yoshida: Jour. Infec. Dis., 192.3, 32, 355, 362, 369. 
3 Kendall, Bly and Haner: Jour. Infec. Dis., 1923, 32, 377. 
* Jordan: Biological Studies by the Pupils of William Thompson Sedgwick, 1906, 
p. 124. 
'' Kendall, Cheetham and Hamilton: Jour. Infec. Dis., 1922, 30, 251. Kendall and 
Keith: Ibid., 1926, 37, 193. 
