Ralstonia solanacearum Species Complex Resists Bacteriophage and/or Antibiotic by Reducing Virulence and Expressing Resistance Genes

The Ralstonia solanacearum species complex (RSSC) is a globally significant plant pathogenic bacterium. Given the lack of effective chemical controls, phage therapy has emerged as a promising biocontrol alternative. While combining phages with antibiotics can counteract phage resistance, RSSC may still evolve concurrent resistance to both agents. However, the fitness consequences and underlying mechanisms of such resistance remain unclear. In this study, a novel RSSC phage was isolated to experimentally investigate the trade-offs between resistance and virulence in evolved strains. Compared to the wild-type, phage-resistant, antibiotic-resistant, and dual-resistant mutants showed no significant differences in growth rate, exopolysaccharide and lipopolysaccharide production. However, their motility, soil survival, and biofilm formation were significantly impaired, with the most severe decline observed in the dual-resistant mutants. Furthermore, phage-resistant strains exhibited enhanced antibiotic resistance, while antibiotic-resistant strains displayed cross-resistance. The antibiotic resistance gene blaOXA-249 was upregulated only in antibiotic-resistant strains. In phage-resistant bacteria, the abortive infection system was activated. A reduction in bacterial cell numbers post-infection indicated that phage resistance limits phage propagation via a “suicidal” mechanism. These findings reveal that resistance evolution in RSSC carries substantial fitness costs and highlight phage steering as a novel strategy for designing phage agents.