The significance of sukumo (composted indigo leaves) as a microbial source for traditional Japanese indigo dyeing

A critical process in traditional Japanese indigo dyeing is the microbial reduction of indigo within the dye suspension, which consists solely of sukumo (fermented indigo leaves), wood ash lye, and microbial nutrients such as wheat bran. Although sukumo has long been recognized as a potential microbial source, the microbial community dynamics during its production is still largely unexplored, and its contribution to dyeing performance via microbial supply remains poorly characterized. In this study, we investigated the significance of sukumo as a microbial source, in addition to its established role as a pigment source. We conducted a time-series analysis of microbial communities throughout the four-month fermentation process of sukumo, using weekly samples collected from two geographically distinct indigo dyeing studios. Microbial profiling revealed similar successional patterns between the two sites. Notably, in the later stages of fermentation, known indigo-reducing bacteria, such as the genus Oceanobacillus and the family Tissierellaceae, emerged at both locations. Laboratory-scale dyeing experiments using immature sukumo demonstrated that supplementation with a small amount of mature sukumo restored dyeing activity and increased the abundance of Oceanobacillus and Tissierellaceae. Furthermore, the addition of the indigo-reducing isolate Tissierellaceae strain TU-1 to the immature sukumo-based dye suspension led to a marked enhancement in dyeing performance. These findings highlight the critical role of sukumo as a microbial source in traditional indigo dyeing and suggest that prolonged fermentation is essential for nurturing functional indigo-reducing bacteria. This insight provides a foundation for improving dye suspension performance through targeted microbial community management.