Resistance of Dickeya solani strain IPO 2222 to lytic bacteriophage vB_Dsol_D5 ({Phi}D5) results in fitness tradeoffs for the bacterium during infection

Abstract

Resistance to bacteriophage infections protects bacteria in phage-full environments, allowing them to survive and multiply in the presence of their viral predators. However, such resistance may cause direct costs for strains linked with the ecological fitness expressed as reduced competitiveness for resources or reduced virulence or both. Unfortunately, limited knowledge exists about such costs paid by phage-resistant plant pathogenic bacteria in their natural environments. This study analyzed the costs of phage resistance paid by broad host phytopathogenic pectinolytic bacterium Dickeya solani both in vitro and in potato (Solanum tuberosum L.) plants. Thirteen D. solani IPO 2222 Tn5 mutants were identified that exhibited resistance to infection caused by lytic bacteriophage vB_Dsol_D5 ({Phi}D5). The genes disrupted in these 13 mutants encoded proteins involved in the synthesis of the bacterial envelope components (viz. LPS, EPS and capsule). The ability of {Phi}D5-resistant D. solani mutants to colonize and cause symptoms on potato plants as well as other phenotypes that are known to contribute to the ecological fitness of D. solani in-plant environment, including growth rate, production of effectors, swimming and swarming motility, use of various carbon and nitrogen sources and biofilm formation were assessed. Although phage resistance did not affect most of the phenotypes of {Phi}D5-resistant D. solani evaluated in vitro, all phage resistant mutants were significantly compromised in their ability to survive on and colonize and cause disease symptoms in potato plants. This study is, to our knowledge, one of few to show the direct link between phage resistance and the fitness of plant pathogenic bacteria and the first one to assess phage-host associations for D. solani.