Pathogenicity of Aeromonas salmonicida and protection effect of Bacillus velezensis on Macrobrachium nipponense against A. salmonicida

Abstract

Aeromonas salmonicida is a conditional pathogen and abundant in water environment, which causes infection in various aquatic animals. However, there are few reports on A. salmonicida infections in prawns. In this study, the pathogenic A. salmonicida CS3 was first isolated from diseased Macrobrachium nipponense. Obvious severe lesions in diseased M. nipponense were observed by histopathological analysis. Gill filament cells were obviously enlarged, the tissue boundaries were incomplete, hepatopancreas vacuoles were ruptured, and basal lamina was damaged. The infection test confirmed that A. salmonicida CS3 had high virulence, and the LD50 was 4.43 × 106 CFU/mL at 7 d post infection. Furthermore, detection of virulence-related genes showed that A. salmonicida CS3 was positive for alt, dna, sp, aer, lip, act, and hly. The antibacterial tests demonstrated that B. velezensis CPA1–1 exhibited antibacterial activity against A. salmonicida. Following the challenge with A. salmonicida, the survival rate of M. nipponense was significantly higher in B. velezensis treated group compared to the control group, and the mRNA expression of CDSP, gC1qR, relish, MyD88, hemocyanin, and α2M were increased to varying degrees. This study indicated that A. salmonicida was a pathogen causing the mass mortalities in M. nipponense, and B. velezensis CPA1–1 holds promise as a potential probiotic for preventing A. salmonicida infections in M. nipponense.