672 G ASTRO-INTESTINAL BACTERIOLOGY 
observations are in harmony, in essential features at least, with those 
made under like conditions in man. A monotonous diet in which lac- 
tose and protein are fed in proportions and amounts similar to breast 
milk leads to the gradual development of an intestinal flora in experi- 
mental animals closely simulating that of nurslings. A preponderance 
of protein, on the other hand, encourages the development of bacteria 
which are more proteolytic in nature. 
It is a striking fact that the above alternation in intestinal bacteria 
following changes along definite lines in the diet is elicited only when 
the feeding is maintained for several days; rapid alternations between 
a purely protein diet and a diet rich in sugar (as cow's milk diluted 
with an equal volume of 4 per cent lactose solution) do not ordinarily 
lead to such noteworthy changes in the types of bacteria excreted in 
the feces. ^ The general trend of such rapid alternations between a 
protein regimen and one in which sugars predominate (starches do 
not necessarily react in this manner) is to establish a flora which is 
relatively heterogeneous, in which there is neither a decided predomi- 
nance of obligately carbohydrophilic bacteria, as B. bifidus or acido- 
philus, nor of obligately proteolytic bacteria. 
A most striking and important influence of diet upon bacterial 
activity in the intestinal tract does not manifest itself in a study con- 
fined exclusively to the changes in bacterial types of the intestinal 
flora. The monotony of the typical nursling flora depends in a large 
measure on the continual presence of lactose (a sugar not fermented 
by a majority of bacteria) throughout the intestinal tract. A sub- 
stitution of other sugars— as glucose, saccharose or maltose— leads 
to a replacement of B. bifidus by other more or less obligately fer- 
mentative organisms, provided an excess of the respective carbohydrate 
be maintained, but the same monotony of types is observed. 
The proportion of carbohydrate to protein in the diet of normal 
adults is far less than in nurslings and, furthermore, a considerable 
proportion of the carbohydrate is in the form of starches which, as 
such, are not readily fermented by most bacteria. Again, sugars, if 
they are present, are largely absorbed from the higher levels of the 
small intestine, leaving residual unhydrolyzed starches and protein 
in relatively great concentration in the lower levels of the large intes- 
tine. It is not surprising, under these conditions, to find that the 
more obligate fermentative bacteria— the Cocci— are prominent at 
the higher levels, as is the case normally in infants; that facultative 
bacteria, as B. coli, are common in a transitional zone between a 
medium containing moderate amounts of utilizable carbohydrate and 
one in which the utilizable carbohydrate is frequently absent ,2 and 
' This probably explains some of the irregularities experienced during brief feeding 
experiments. 
'^ Bacillus coli and various closely related bacilli are among the most labile of intes- 
tinal bacteria in adapting their metabolism to the composition of the intestinal contents. 
In a medium containing both utilizable carbohydrate and utilizable protein these 
organisms act principally upon the carbohydrate, forming lactic and smaller amounts 
of other acids. In a protein medium the products of metabolism are indol, phenols 
and other products of proteolysis. 
