( 506 ) 
these enzymes, that of S. sphaericus, which acts the most strongly of 
all, will be treated in § 6. 
Here I wish to remark that indigo-enzymes originate in the 
yeast-cells only then, when cultured on a solid medium e.g. on 
wort-gelatine, with abundant access of air. When cultured in nutrient 
liquids, even with a current of air passing through, they produce no 
or only very little enzyme. 
The indigo-blue, formed by most moulds and yeast-species in 
the decomposition of indican, is for the greater part confined within 
the protoplasm, as was already described and figured by Mr. Motiscu 
(le); but in those cases when decomposition is very strong, as 
with many bacteria, the indoxyl streams out and also precipitates 
outside of the cell in granules of indigo-blue. 
5. Indigo-fermentation by Aërobacter. 
When a decoction of Indigofera or Polygonum is infected with 
garden-soil, canal-water or mud, and placed at 28°C., there origin- 
ates, during a copious formation of indigo, a rich bacteria-flora in 
which the common gas-producing ferments, which I recently united ') 
in the genus Aérobacter, perform the chief part. The first who drew 
attention to this fact was ALVAREZ, but he went too far by admitting 
the existence of specific bacteria for indigo-fermentation *). By bring- 
ing a drop of the first crude fermentation into a second quantity 
of a decoction and so on, an accumulation, sometimes a pure culture 
of Aérobacter is obtained *). 
By sowing out an Aérobacter-fermentation on indican-gelatine, not 
only the Aérobacter-colonies, but also those of various other bacteria 
colour deeply blue by indigo. Commonly, however, the Aérobacter-species 
are recognised by their number. But the chief characteristic of Aéro- 
bacter is its fermenting power and its temporary anaerobiosis, by which 
the splitting of indican goes on even at temporary exclusion of air, 
which is not the case with the aerobics. On this characteristic is 
based the supplanting of the aerobics by Aérobacter in liquid cultures 
and the prevailing part which these bacteria have in the splitting 
’) Centralblatt für Bacteriologie. 2e Abt. Bd. 6 N° 7, 1900. 
2) Comptes rendus, T. 105, pag. 287, 1887. 
3) In several other plant-infusions, not from indigo-plants, quite the same is ob- 
served. The strongest érobacter-fermentations are obtained by mixing rye-flour with 
water to a thick pap and placing it at 28° C. After a few hours the development 
sets in of carbonic acid and hydrogen, caused by the -dérobacter-species, never 
wanting in flour, which in the beginning supplant all other bacteria. 
