Cellulolytic bacteria isolated from agar waste as candidate seaweed fermentation agents in rabbitfish (Siganus guttatus) feed bioprocessing

This study aimed to isolate, characterize and identify cellulolytic bacteria from solid agar waste as seaweed fermentation agents for rabbitfish (Siganus guttatus) feed bioprocessing. Bacteria were isolated using a spread plate method on a carboxymethyl cellulose (CMC) medium. Characterization included morphology, Gram staining, cellulolytic index, CMCase activity and bacterial pathogenicity. Bacteria with a high cellulolytic index and CMCase activity were selected as fermentation agent candidates. The pathogenicity test used an intramuscular injection method. Selected bacterial isolates (density 106 CFU mL-1 ) were injected into healthy juvenile rabbitfish (10 fish container-1 ) with a saline solution as control. The juvenile fish were monitored for 10 days; parameters observed were survival rate, disease symptoms, fish behaviour and leucocyte counts. Bacterial identification was carried out by BLAST analysis of bacteria DNA sequences. Data on bacteria characteristic and identity were analysed descriptively, while data on survival rate and leucocyte counts were analysed using analysis of variance (ANOVA). Nineteen bacteria were isolated of which 9 were selected as candidate fermentation agents including 9 Gram-negative bacteria. SIM91 had the highest cellulolytic index (3.03) and CMCase activity (1.2 U mL-1 ). Eight isolates were not pathogenic and 1 isolate (SIM103) was pathogenic with symptoms including swollen eyes and pale gills. Survival rate and leucocyte counts of juvenile rabbitfish were not significantly different between injected fish and the control (p > 0.05). BLAST analysis of bacteria DNA identified four species of bacteria (Pseudomonas stutzeri, P. chengduensis, P. aeruginosa, and P. songnenensis) with 97-99% similarity. This study resulted in the identification of bacteria with potential as fermentation agents in rabbitfish (S. guttatus) feed bioprocessing.