Abstract
Indigo fermentation, which depends on the indigo-reducing action of microorganisms, has traditionally been performed to dye textiles blue in Asia as well as in Europe. This fermentation process is carried out by naturally occurring microbial communities and occurs under alkaline, anaerobic conditions. Therefore, there is uncertainty regarding the fermentation process, and many unknown microorganisms thrive in this unique fermentation environment. Until recently, there was limited information available on bacteria associated with this fermentation process. Indigo reduction normally occurs from 4 days to 2 weeks after initiation of fermentation. However, the changes in the microbiota that occur during the transition to an indigo-reducing state have not been elucidated. Here, the structural changes in the bacterial community were estimated by PCR-based methods. On the second day of fermentation, a large change in the redox potential occurred. On the fourth day, distinct substitution of the genus Halomonas with the aerotolerant genus Amphibacillus was observed, corresponding to marked changes in indigo reduction. Under open-air conditions, indigo reduction during the fermentation process continued for 6 months on average. The microbiota, including indigo-reducing bacteria, was continuously replaced with other microbial communities that consisted of other types of indigo-reducing bacteria. A stable state consisting mainly of the genus Anaerobacillus was also observed in a long-term fermentation sample. The stability of the microbiota, proportion of indigo-reducing microorganisms, and appropriate diversity and microbiota within the fluid may play key factors in the maintenance of a reducing state during long-term indigo fermentation. Although more than 10 species of indigo-reducing bacteria were identified, the reduction mechanism of indigo particle is riddle. It can be predicted that the mechanism involves electrons, as byproducts of metabolism, being discarded by analogs mechanisms reported in bacterial extracellular solid Fe3+ reduction under alkaline anaerobic condition.
Highlights
Ancient human beings developed a number of environmentally friendly and renewable bioprocesses to benefit their communities
During a trial aimed at the isolation of indigo-reducing bacteria concomitant with the above analysis of the microbiota from fermentation fluid that we prepared in our laboratory and aged (10-month-old) fermentation fluid obtained from the craft center in Date City, Hokkaido, Japan, using a medium containing 0.2% indigo carmine, indigo-reducing Oceanobacillus indicireducens and Amphibacillus spp. were isolated (Aino et al, 2011)
To determine the microbiological basis of the maintenance of indigo fermentation, we examined the microbiota in fermentation fluids maintained for more than 6 months (Okamoto et al, 2017)
Summary
Ancient human beings developed a number of environmentally friendly and renewable bioprocesses to benefit their communities. Indigo-containing plants are composted by microorganisms, and indican is transformed to indigo via the formation of indoxyl during this process (Figure 1). This procedure is performed in Japan, Europe, northeastern India, and West Africa. The development of conventional procedures, including management systems that do not involve the use of chemical reagents, is indispensable for the reemerging of the technique of indigo reduction by microorganisms To achieve this goal, it may be helpful to identify the microorganisms responsible for indigo reduction and the mechanisms of indigo reduction during both the initial transitional changes and the stable state in the fermentation process. Analysis of many kinds of fermentation fluids in various fermentation stages is necessary for elucidation of the core mechanisms associated with the transitional changes in the microbiota during indigo fermentation
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