Cellulolytic activity of gut bacteria isolated from the eri silkworm larvae, Samia ricini, (Lepidoptera: Saturniidae)

The eri silkworm, Samia ricini, is associated with symbiotic bacteria believed to be playing several physiological functions to the host. The larvae of S. ricini were subjected to isolation of gut bacteria using culture-dependent 16S rRNA gene amplification and sequencing; and qualitative and quantitative enzymatic assays. The results of qualitative cellulolytic assays indicated that 58% of the screened gut bacterial isolates had cellulolytic activity. The cellulolytic bacterial isolates belonged to Bacillus sp. (88%), Pseudomonas sp. (8%) and Enterococcus sp. (4%). Quantitative assays indicated that the highest beta-glucosidase (13.42 U/mg), endoglucanase (3.60 U/mg) and exoglucanase (2.60 U/mg) activity were in isolates ERI011, ERI009 and ERI097, respectively. Our results further indicate that significant activities of endogenous cellulases are present in the gut fluids of S. ricini. However, a comparative analysis of the cellulolytic activity showed that the endoglucanase activity of bacterial origin was 13 and 18-fold higher in the foregut and midgut, respectively, than that in gut fluids. Similarly, the beta-glucosidase activity was 20 and ninefold higher compared with gut fluids from the same compartments. The higher cellulolytic activity of bacterial origin compared with that of insect origin suggests that more cellulose digestion may be aided by cellulases of bacterial origin. These cellulolytic gut bacterial isolates may be good sources for profiling novel genes and biomolecules for biotechnological application.