242 UTILIZATION OF CARBOHYDRATES 



It is by no means definitely proved that the mechanism of the reaction in every 

 instance involves the formation of formic acid by the fermenting member, but this 

 certainly happens in at least some of the synergetic couples tried thus far: in fact, 

 formic acid has been separated and measured with considerable precision from several 

 of these synergetic reactions. 



This reaction has significance in several fields of bacteriology: It is of prime im- 

 portance, as Sears and Putnam pointed out' in the "presumptive test" for B. coli 

 in water. It has an important bearing upon the gas formation by intestinal bacteria, 

 and it occasionally leads to false conclusions with respect to gas formation in contam- 

 inated and in mixed cultures. 



Another type of coupled reaction is one described by Kendall- in relation to gas 

 formation in milk by B. coli and an associated, actively proteolytic organism. Under 

 normal conditions B. coli does not produce gas in milk, although this medium is rich 

 in lactose, but does ferment lactose energetically in nutrient fermentation media with 

 a considerable evolution of gas. If, however, B. coli be inoculated into milk together 

 with an actively growing, strongly proteolytic strain of microbe which does not fer- 

 ment lactose, as, for example, B. mesentericus, gas is formed in considerable amounts. 



Here the explanation seems to depend upon the cleavage of the casein by the 

 proteolyte with the liberation of amino acid complexes. These in turn, in some man- 

 ner not well understood, cause the colon bacillus to liberate gas in addition to the 

 acid which it normally produces in milk. Similar results may be obtained by digesting 

 the milk prior to inoculation, with trypsin or by repeated heating of the milk, which 

 of course tends to break down the protein, and thus accomplish the same end as the 

 cleavage by a proteolyte. 



Coupled reactions are as yet but little studied, but there is little doubt that some 

 very important results await discovery in this field of bacterial synergism. 



' Sears, H. J., and Putnam, J. J.: /. Infect. Dis., 32, 270. 1923. 

 * Kendall, A. I.: Boston M. b' S. J., 163, 322. 1910. 



