658 MICROBIOLOGY OF SPECIAL INDUSTRIES 



INDIGO* 



Indigo is now for the most part made synthetically. This dye was 

 formerly made from certain species of Indigo/era, principally I. tinctoria. 

 This plant contains a glucoside, indican, which by fermentation and 

 oxidation yields indigo. 



The plants are placed in water at a temperature of 25 to 35 and 

 undergo a spontaneous alkaline fermentation which splits up the indican 

 into a sugar (indiglucin) and indigo white which remain in the solution. 

 This solution is then thoroughly aerated and the indigo white oxidized 

 into indigo blue which is insoluble and forms a sediment. This sedi- 

 ment is dried and constitutes the old indigo of commerce. 



Many bacteria are found in the fermenting liquid, but the cause 

 of transformation has been shown to be a specific form, Bacillus indigo- 

 genus, closely related to Friedlander's pneumonia bacillus. It is 

 strongly aerobic and surrounded by a gelatinous envelope. 



RETTING* 



The separation of the fibers of flax, hemp, ramie and similar plants 

 is brought about by a complex spontaneous fermentation. The plants 

 are either left on the surface of grassy meadows exposed to alternate 

 wetting and drying or immersed in water. In either case, the tissues 

 are gradually disintegrated by microbial action, more rapidly in the 

 wet process. 



The fermentation, principally bacterial, is due to many species. 

 Several have been described as being the principal agent in the process 

 but it is probable that the effects are due to the united action of several, 

 both aerobic and anaerobic. 



Among the forms to which the retting has been attributed are B. 

 amylobacter of van Tieghem, an anaerobic form which attacks the pec- 

 tic matters and to some extent the cellulose. Granulobacter pectino- 

 vorum of Beyerinck and van Delden, also anaerobic, transforms the 

 pectic matters into sugars which it decomposes, producing butyric 

 acid. Many other forms have been described and part of the work has 

 been ascribed to Mucor, PenicilUum, and various molds. 



Cultures of Granulobacter pectinowrum and other forms have been 

 successfully used to hasten the process. 



* Prepared by F. T. Bioletti. 



