Therapeutic and immunomodulatory effects of phage therapy in red hybrid tilapia challenged with Klebsiella pneumoniae.

The increasing prevalence of antibiotic-resistant Klebsiella pneumoniae poses a serious threat to aquaculture sustainability and fish health. This study evaluated the therapeutic efficacy and immunomodulatory effects of Klebsiella pneumoniae Phage 5 (KPP5) in red hybrid tilapia experimentally challenged with K. pneumoniae strain 001 (KP001). In vitro characterisation demonstrated that the phage exhibited strong lytic activity, rapid replication, and a high burst size against the target bacterium. An in vivo challenge model was established to assess disease progression, survival, bacterial clearance, phage persistence, and host immune responses following intraperitoneal administration. Phage-treated fish exhibited a 41.67% relative survival, a complete hepatic bacterial clearance, and a sustained phage persistence. Phage titres persisted longer in infected fish, indicating active replication in the presence of the bacterial host. Immunological analyses revealed that phage therapy moderated excessive inflammatory responses while enhancing protective immunity, as evidenced by stabilised protein levels, regulated complement activity, partial restoration of lysozyme activity, and significantly elevated immunoglobulin M (IgM) concentrations in both serum and mucus. These findings demonstrate that KPP5 provides dual benefits through effective bacterial control and modulation of innate and adaptive immune responses. Overall, this study supports the potential of bacteriophage therapy as a safe and promising alternative strategy for managing antibiotic-resistant bacterial infections in aquaculture. This study was registered with the Institutional Animal Care and Use Committee (IACUC) of Universiti Malaya (Ref: G8/10032025/07012025-02/R).