Isolation, identification, and pathogenic potential of Bacillus cereus from diseased big‐belly seahorse Hippocampus abdominalis

Abstract Members of Bacillus cereus have been documented as important bacterial pathogens in aquaculture. However, scarce information is available on B. cereus isolates as causal pathogens of big‐belly seahorses Hippocampus abdominalis . In the present study, a B. cereus isolate (SH1), recovered from a disease outbreak on a seahorse farm, was identified as a bacterial pathogen of big‐belly seahorses, and its pathogenic potential in terms of pathogenicity‐related factors, antimicrobial resistance, and virulence were analyzed. The isolate possessed β‐hemolytic and proteolytic activities, and harbored 31 pathogenicity‐related genes, including 20 virulence genes inhA , alo , cytK , hblA , hblC , hblD , nheA , nheB , asbA , asbB , asbC , asbD , asbE , asbF , dhbA , dhbB , dhbC , dhbE , dhbF , and hal encoding exoenzymes, exotoxins, and nutritional/metabolic factors, as well as 11 resistance genes BcI , BcII , BcIII , EF‐Tu , vanR , vanS , mphM , satA , rpoB , fosB , tet (45) conferring resistance to β‐lactams, elfamycins, glycopeptides, macrolides, nucleosides, rifamycins, phosphonic acids, and tetracyclines. Furthermore, the isolate correspondingly displayed phenotypic resistance to chephalosporins and penicillins and showed in vivo virulence to big‐belly seahorses with the seven‐day median lethal dose (LD 50 ) value of 1.51 × 10 6 CFU/mL. To our knowledge, this is the first study to describe B. cereus as a pathogen of big‐belly seahorses. The findings of this study underscore the need for targeted management of B. cereus infection in seahorses.