346 



drew attention to this fact was Alvarez, but he went too far by admitting the 

 existence of specific bacteria for indigo-fermentation*). By bringing a drop of the 

 first crude fermentation into a second quantity of a decoction and so on, an accumu- 

 lation, sometimes a pure culture oi Aërobacter is obtained"). 



By sowing out an Aërobacter-iermtntAtion on indican-gelatine, not only the 

 Aërobacter-colonies, but also these of various ether bacteria celour deeply blue by 

 indigo. Commonly, however, the Aërobacter-species are recognised by their number. 

 But the chief characteristic of Aérohacter is its fermenting power and its teniperary 

 anaerobiesis, 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 Aërobacter in liquid cultures and the prevailing part 

 which these bacteria have in the splitting of the indican in the spentaneous indigo- 

 fermentations. In pure cultures this splitting can of course be as well effected by 

 \'aricius comnion aerobics, albeit more slowly. 



The deeomposition of indican by Aërobacter is operated katabolically, as in all 

 ether examined bacteria also, so that killed bacteria are inactive and indigo-enzyme 

 cannot be separated out. The optimum temperature for the deeomposition agrees 

 with that of the grnwth and is, for iiistance, 28" C. for a variety of A. acrogenes 

 isolated from milk. 



The number of Aërohacter-iorms obtained by sowing out from the decoctions is 

 verv great but mav be reduced te three chief species, described by me elsewhere (1. c. 

 pag. 200). They are Aërobacter aërogenes A. col: and A. Uquefaciens, all represented 

 by many varieties and allied by intermediate ferms. Not all varieties are equally 

 active. Se, among the forms of A. coli. whïch for the greater part decompose most 

 vigorously, the variety A. coli var. commune, isolated from the intestines er from 

 faeces, is but feebly active or not active at all and recognisable by this feature. 



The products of the deeomposition of indican by Aërobacter (and by bacteria in 

 general) are the same as by enzyme action, i. e. indoxyl and glucose. If a nutriënt 

 liquid centaining indican, e. g. decoct of indigo-plants, broth, or yeast-water, is passed 

 into a fermentation tube and infected with Aërobacter. indigo-blue is formed in the 

 open end, while in the clesed one carbenic acid and hydrogen eriginate from the 

 glucose of the indican ^). and indoxyl which remains a long time unchanged. 



Tn preportion 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 indoxyl can proceed 

 with much rapidity by the action of enzymes and still more rapidly by acids in pre- 

 sence of ferrichloride, while it is impossible to make the process ge on as quickly by 



') Comptes rendvis. T. 105, paR. 287, 1887. 



') In scveral other plant-infusions. not from indigo-plants, qiiite tho s;inic is ob- 

 servcd. The strongest /4è>obac(er-fcrmentations are obtained by niixing rye-flour with 

 water to a thick pap and placing it at 280 c. .A.ftcr a few hours the development sets 

 in of carbonic acid and hydrogen, caused by the Acrobaclcr-^\iCc\QS. never wanting in 

 flour. which in the bcginning supplant all other bacteria. 



•) That the development of gas is duc to the sugar of the indican, and not to the 

 free sugar already present in the decoctions or the indican prcparations, is proved by 

 the fact that the gas-developmcnt is the same whcn beforehand all frcc sugar has 

 been removed from the H<niid by nicans of pure beer-yeast, wliich aots not on indican. 



