Pathogenomic profiling: Decoding the pathogenic potential of virulent Bacillus cereus associated with mortality of farmed Labeo rohita (Hamilton, 1822), in India

Abstract

Bacillus cereus is widely used as probiotics in aquaculture. However, present research work identified B. cereus from diseased fish (Labeo rohita) associated with mass mortality in Hooghly, West Bengal, India. The bacterium was identified based on morphological, biochemical characterization and molecular identification, which comprise 16S rRNA gene and housekeeping gene sequencing analysis. Furthermore, the pathogenic potential of the isolates was confirmed by the presence of different toxin-encoding genes viz. hemolytic enterotoxin (hblA, hblC, hblD), non-hemolytic enterotoxin (nheA, nheB, nheC), cytotoxin K (cytK) and enterotoxin FM (entFM), Diarrheal enterotoxin (bceT) and Cereolysin AB. All these toxin-encoding genes were regulated by a master virulence regulating transcription factor plcr, which is also present in this isolate. Additionally, this bacterium also secretes caseinase and lecithinase, which helps the bacteria to invade the host tissue. The intraperitoneal injection of B. cereus at a dose of 3.6× 106 CFU ml−1 resulted in 100 % mortality of L. rohita. The histopathological changes revealed that the bacterium induces severe damage and necrosis in the liver and kidney of the infected fish. To understand the innate immune response once bacteria invade the host, a quantitative gene expression study was carried out using different immune-related genes and stress-related genes viz. Complement factor 3a (C3a), Toll-like receptor (TLR), Myeloid differentiation primary response 88 (Mydd88), Interleukin 6 (IL6), Nucleotide oligomerization domain (NOD)-like receptor, Catalase (CAT), Glutathione peroxidase (GPx) and Superoxide dismutase (SOD) in the hepatic and renal tissue. The over-expression of TLR, NOD and Mydd88 shared a common downstream inflammatory signaling pathway and the upregulation of antioxidant enzymes SOD, CAT and GPx was evident for combating strategies as well as safeguarding the host's cells from oxidative damage. Overall, this study showed that an isolated strain of B. cereus exhibits high pathogenicity, which may be attributed to climate change, urbanization or horizontal gene transfer. This presents a potential threat to the aquaculture industry and raises concerns about the possibility of cross-contamination within the human population.