Massilia varians has potential as a probiotic to inhibit bacterial pathogens in aquaculture, but very limited information is available regarding its use in Chinese mitten crab Eriocheir sinensis for enhancing host immunity, antioxidant ability and intestinal microbiota homeostasis. In this study, a 40-day feeding trial was conducted to evaluate the protective effects of dietary supplementation with M. varians P2-4 on nonspecific immune response, antioxidant status, intestinal microbiota and resistance against Pseudomonas aeruginosa infection in E. sinensis. Results demonstrated that dietary supplementation with M. varians P2-4 at 6.0 × 10⁶ to 6.0 × 10⁸ CFU/g diet significantly boosted nonspecific immunity and improved antioxidant capability of E. sinensis, mainly as evidenced by markedly increased activities of plasma lysozyme, plasma superoxide dismutase, hepatopancreatic superoxide dismutase and catalase. Furthermore, crabs fed M. varians P2-4-supplemented diets exhibited markedly improvements in intestinal microbiota composition and diversity, and showed substantially enhanced survival following P. aeruginosa challenge, with 7-day relative percentage survival ranging from 76.9% to 100.0%. To the best of our knowledge, this is the first study to reveal that M. varians P2-4 supplementation functions as a new biocontrol strategy in E. sinensis by effectively improving the non-specific immunity, antioxidant status and intestinal microbiota to mitigate P. aeruginosa infection.

Nowadays, members of the genus Enterobacter have been documented as human and aquaculture pathogens. To date, no reports have described Enterobacter bugandensis infecting Hippocampus abdominalis. In the present study, an isolate of E. bugandensis, designated H4, was identified as a causative pathogen in cultured H. abdominalis following Koch’s postulate, and its virulence properties were further described. The isolate’s genome consisted of a single circular chromosome and harbored several virulence and resistance genes, including, but not limited to, csgG, acrB, hcp, gndA, galF, rpoS, fur, rcsB, and phoP involved in adherence, antimicrobial activity, effector delivery systems, immune modulation, and regulation, as well as baeR, blaACT-49, ramA, hns, ftsI, acrA, gyrA, fabI, crp, oqxB, parE, gyrB, phoP, rpoB, tuf, ptsI, and fosA2 functioning against aminoglycoside, cephamycin, disinfecting agent and antiseptic, fluoroquinolone, macrolide, peptide, and other antimicrobials. Additionally, the isolate exhibited multiple resistance to cephalosporins, penicillins, and tetracyclines and demonstrated a median lethal dose (LD₅₀) of 4.47 × 10⁵ CFU/mL in H. abdominalis. To our knowledge, this is the first study to describe E. bugandensis infecting H. abdominalis. These findings highlight the zoonotic potential of E. bugandensis and underscore the need for targeted health management in seahorse farming.