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of the indican in the spontaneous indigo-fermentations. In pure 
cultures this splitting can of course be as well effected by various 
common aerobics, albeit more slowly. 
The decomposition of indican by Aërobacter is operated katabol- 
ically, as in all other examined bacteria also, so that killed 
bacteria are inactive and indigo-enzyme cannot be separated out. 
The optimum temperature for the decomposition agrees with that 
of the growth and is, for instance, 28° C. for a variety of A. 
aérogones isolated from milk. 
The number of Aërobacter-forms obtained by sowing out from 
the decoctions is very great but may be reduced to three chief 
species, described by me elsewhere (le. pag. 200). They are Aéro- 
bacter aörogenes, A. coli and A. liquefaciens, all represented by many 
varieties and allied by intermediate forms. Not all varieties are 
equally active. So, among the forms of A. coli, which for the greater 
part decompose most vigorously, the variety A. coli var. commune, 
isolated from the intestines or from faeces, is but feebly active or 
not active at all and recognisable by this feature. 
The products of the decomposition of indican by Aérobacter (and 
by bacteria in general) are the same as by enzyme action, i. e. indoxyl 
and glucose. If a nutrient liquid containing indican, e. 2. decoct 
of indigo-plants, broth, or yeast-water, is passed into a fermentation tube 
and infected with Aérobacter, indigo-blue is formed in the open 
end, while in the closed one carbonic acid and hydrogen originate 
from the glucose of the indican!), and indoxyl which remains a 
long time unchanged. 
In proportion as the oxidation of the indoxyl proceeds more slowly, 
more indigo-red is produced, similarly to the splitting of indican by 
enzymes and acids. Now the splitting of the indican, and consequently 
the oxidation of the indoxy! can proceed with much rapidity by 
the action of enzymes and still more rapidly by acids in presence of 
ferrichloride, while it is impossible to make the process go on as 
quickly by bacteria. So it is inevitable that the formation of indigo- 
red is very great in the case of the bacterial fermentation of the 
indican, while it is possible to reduce its amount practically to zero 
in the case of chemical decomposition. As it is besides hardly 
possible to separate the indigo-blue from the substance of the bacteria, 
') That the development of gas is due to the sugar of the indican, and not to the 
free sugar already present in the decoctions or the indican preparations, is proved by 
the fact that the gas-development is the same when beforehand all free sugar has 
been removed from the liquid by means of pure beer-yeast, which acts not on indican, 
37 
