Diversity and activity of cellulolytic bacteria, isolated from the gut contents of grass carp (Ctenopharyngodon idellus) (Valenciennes) fed on Sudan grass (Sorghum sudanense) or artificial feedstuffs

Herbivorous grass carp (Ctenopharyngodon idellus) has a powerful capability to digest cellulose from aquatic plants, depending on the cellulase complex produced by the cellulolytic bacterial community in the gastrointestinal (GI) tract. However, it remains uncertain which bacteria taxa may actively participate in the digestion of food fibre. In this study, a total of 499 cellulolytic bacteria from the gut content of grass carp fed on Sudan grass (242 strains) and artificial feedstuffs (257 strains) were randomly isolated and characterized using carboxymethyl-cellulose, microcrystalline cellulose and cellobiose agar media. The results showed that more than half of the isolates were capable of degrading carboxymethyl-cellulose and cellobiose, while the remaining isolates were restricted to microcrystalline cellulose decomposition, exclusively. The cellulolytic bacterial community was dominated by Aeromonas, followed by Enterobacter, Enterococcus, Citrobacter, Bacillus, Raoultella, Klebsiella, Hydrotalea, Pseudomonas, Brevibacillus and some unclassified bacteria, as revealed by 16S rDNA sequence analysis. Notably, grass carp fed on grass with high-fibre content harboured a higher diversity of cellulolytic bacteria than the ones fed on low-fibre feedstuffs. Our results provided evidence for a positive correlation between the content of food fibre and the diversity of cellulolytic bacteria in grass carp intestines. Thus, improving growth conditions and cellulase activities for GI cellulolytic microorganisms in grass carp intestines are critical for effective utilization of feedstuffs containing high fibre levels.