78 BACTERIAL METABOLISM 
nently represented among the decomposition products of carbohydrates 
by the typhoid baciUus, while formic acid is either not present or 
j)resent in small amounts in corresponding cultures of colon bacilli. ^ 
Shaw^ has reported that several members of the dysentery-typhoid- 
coli group produce moderate amounts of formaldehyde, especially when 
glucose is present in the cultural medium. 
The qualitative changes produced in fats and lipoidal substances 
by bacteria are not well known. 
Bacterial S3mergism.— It has long been known^ that two bacteria, 
working together, may bring about chemical reactions that exceed 
the sum of their separate activities. Thus, a staphylococcus produces 
formic, as well as lactic and acetic acids from the fermentation of 
lactose. B. typhosus alpha (or beta) has no action whatsoever upon 
lactose. The two organisms, growing simultaneously in a lactose 
medium produce not only acid, but gas as well. This is a coupled 
reaction, in which the formic acid produced by the staphylococcus is 
transformed by the enzyme formiase (see p. 77), elaborated by the 
paratyphoid bacillus, into CO2 and H2. Many instances of this type 
of reaction are known,'* and in at least some of these coupled reactions 
the same mechanism is involved. 
Another type of coupled reaction is that one in which B. coli (which 
does not characteristically produce gas in milk) and a strongly proteo- 
lytic organism, as B. mesentericus (which has no action whatsoever 
upon lactose) together form considerable amounts of gas in this 
medium.^ The explanation seems to be that the proteolyte cleaves 
the proteins of milk into peptones and polypeptids, and these in turn 
cause the colon bacillus to form gas from the lactose as well as to 
produce acid. The same effect may be obtained by adding peptone 
to the milk; when this is done, the colon bacillus alone will produce both 
acid and gas from the milk. 
These coupled reactions have been but little studied, but they are of 
great importance, particularly in the intestinal tract, and in those 
naturally occurring processes where several types of bacterial activity, 
occurring serially, bring about important chemical transformations. 
VI. QUALITATIVE INFLUENCE OF UTILIZABLE CARBOHYDRATES 
UPON THE ELABORATION OF PROTEOLYTIC ENZYMES. 
Certain bacteria, as for example B. proteus, characteristically pro- 
duce proteolytic enzymes which rapidly dissolve gelatin by hydrolytic 
cleavage. These enzymes are exo-enzymes; that is, they may be 
obtained sterile and free from bacteria simply by passing gelatin 
'Clark: Science, November 7, 191.3. 
2 British Med. Jour., 1924, i, 461. 
3 Castellani and Taylor: British Med. Jour., 1917, ii, 855; 1919, i, 183. 
" See Holman: Bacterial Synergism, Jour. Infec. Dis., 1921, 39, 145, for other 
examples of bacterial synergism. 
' Kendall: Boston Med. and Surg. Jour., 1910, 163, 322, 
