Synergistic effects of gentamicin, levofloxacin, and cefotaxime against Aeromonas salmonicida: In vitro and In vivo evaluation.

The increasing prevalence of multidrug-resistant (MDR) Aeromonas salmonicida (AS) threatens both animal and human health by limiting the effectiveness of conventional antibiotics. To identify effective therapeutic strategies, we evaluated the activity of gentamicin (GENTA), levofloxacin (LEV), and cefotaxime sodium (CFT) as individual treatments and in combination using zebrafish and rainbow trout infection models. In vitro checkerboard assays showed consistent synergistic effects in the GENTA-CFT combination, as confirmed by Combenefit analysis. In vivo experiments revealed that AS infection caused high mortality, inflammation, and impaired swimming performance in rainbow trout. Monotherapies with GENTA and CFT provided partial protection, whereas combination treatment significantly improved survival and reduced behavioral abnormalities and tissue pathology. Gene-expression profiling further showed that combination therapy markedly suppressed key inflammatory and immune-stress markers compared with infected controls. These findings highlight the superior protective efficacy of antibiotic combinations, particularly GENTA with CFT, in improving survival, alleviating tissue damage, and modulating host immune responses during AS infection in rainbow trout. The study underscores the potential of rational combination therapy as a strategy to overcome antimicrobial resistance, enhance treatment outcomes, and reduce the impact of pathogens on host health.