254 MICROSCOPIC AND CULTURAL STUDY OF BACTERIA 
An explanation for the phenomenon set forth in the table does 
not readily suggest itself. Somewhat similar selective action upon 
specific amino-acids by other bacteria is known, qualitatively at 
least. Thus, members of the hemorrhagic septicemia group, particu- 
larly those derived from animal sources, produce indol in plain broth 
media, although their action upon nitrogenous substance is other- 
wise very limited. Such organisms appear to be tryptophanophilic. 
Typhoid bacilli, diphtheria bacilli and many other pathogenic 
organisms usually fail to produce indol in ordinary media under 
similar conditions. It is possible that this noteworthy action of 
members of the hemorrhagic septicemia group upon tryptophan 
may be related to the fact that this amino-acid is an important con- 
stituent of the hemoglobin, the coloring matter of the blood; the 
hemorrhagic septicemia bacteria are particularly likely to grow in 
the blood stream of infected animals. 
Fermentation reactions of bacteria in varied carbohydrate media 
are of importance in their cultural identification. The table on page 
353 illustrates the separation of members of the intestinal group of 
bacteria by their fermentation reaction in various carbohydrates. 
il/?7A-.— Milk is an important natural medium for bacterial growth. 
It contains protein, carbohydrate and fat, together with inorganic 
salts. A variety of reactions and changes in milk are produced by 
bacterial development. 
(a) Change in Reaction.— Mi\k contains, in addition to protein, 
two carbohydrates, which play a prominent part in determining the 
reaction of the medium. The principal carbohydrate is lactose, but 
fresh milk contains in addition, a small amount— about 0.08 per cent — 
of a sugar reacting like glucose.^ Changes in the reaction of milk 
caused by bacterial activity, therefore, may be of several types. An 
initial acidity followed by an alkaline reaction, as exliibited by the 
dysentery bacilli and other organisms, is probably due to the initial 
fermentation of the small amount of glucose, which results in the 
formation of acid— and then the production of alkaline products from 
the decomposition of protein when the glucose is exliausted. These 
organisms do not ferment lactose. 
A permanent acid reaction is induced either by bacteria which 
ferment lactose, or less commonly, by the decomposition of fat with 
the liberation of fatty acids with or without the fermentation of glycerin. 
B. typhosus and B. paratyphosus alpha produce a permanently acid 
reaction in milk, but do not ferment lactose. The exact chemistry of 
their activity in this medium is not known. An alkaline reaction in 
milk is usually an indication of proteolytic action with the formation of 
basic products of protein decomposition. 
The accumulation of acid incidental to the fermentation of lactose, 
as, for example, by B. coli, may be sufficient in amount to cause an 
1 Theobald Smith: Jour. Boston Soc Med. Sci., 1897, 2, 236. Jones: Jour. Infec. 
Dis., 1914, 15, 357. 
