Accelerated degradation of cellulose in silkworm excrement by the interaction of housefly larvae and cellulose-degrading bacteria

Abstract

The environmental pollution caused by silkworm (Bombyx mori) excrement is prominent, and rich in refractory cellulose is the bottleneck restricting the efficient recycling of silkworm excrement. This study was performed to investigate the effects of housefly larvae vermicomposting on the biodegradation of cellulose in silkworm excrement. After six days, a 58.90% reduction of cellulose content in treatment groups was observed, which was significantly higher than 11.5% of the control groups without housefly larvae. Three cellulose-degrading bacterial strains were isolated from silkworm excrement, which were identified as Bacillus licheniformis, Bacillus amyloliquefaciens, and Bacillus subtilis based on 16S rRNA gene sequence analysis. These three bacterial stains had a high cellulose degradation index (HC value ranged to between 1.86 and 5.97 and FPase ranged from 5.07 U/mL to 7.31 U/mL). It was found that housefly larvae increased the abundance of cellulose-degrading bacterial genus (Bacillus and Pseudomonas) by regulating the external environmental conditions (temperature and pH). Carbohydrate metabolism was the bacterial communities' primary function during vermicomposting based on the PICRUSt. The results of Tax4Fun indicated that the abundance of endo-β-1,4-glucanase and exo-β-1,4-glucanase increased rapidly and maintained at a higher level in silkworm excrement due to the addition of housefly larvae, which contributed to the accelerated degradation of cellulose in silkworm excrement. The finding of this investigation showed that housefly larvae can significantly accelerate the degradation of cellulose in silkworm excrement by increasing the abundance of cellulose-degrading bacterial genera and cellulase.

Keywords: Cellulose; Cellulose-degrading bacteria; Housefly larvae; Silkworm excrement; Vermicomposting.