Characterization of antibacterial substances produced by phenotypic conversion mutants of Ralstonia solanacearum species complex strains of phylotype I and IV

Strains of Ralstonia species, previously belonging to the R. solanacearum species complex (RSSC), spontaneously undergo phenotypic conversion (PC). The effect of mutations on antibacterial production and resistance to antibacterial substances, and characterization of antibacterial substances of these PC mutants has not previously been investigated in detail. In this study, we evaluated the ability of 25 PC mutants of R. pseudosolanacearum (phylotype I) and R. syzygii (phylotype IV) to produce antibacterial substances and siderophores, cause brown discolouration of media, and suppress tomato bacterial wilt disease following inoculation. Of the 25 PC mutants, 12 strains showed antibacterial activity on the agar medium, and four strains (8105PC, 82-1PC, 101PC, and Suzaki-5PC) produced antibacterial substances on agar and in liquid media. However, the production of antibacterial substances by the PC mutants was not directly related to siderophore production, brown discolouration of media, and the suppression of tomato bacterial wilt disease. The susceptibility to antibacterial substances of the PC mutants differed among the 25 wild-type strains. Evaluation of the antibacterial characteristics of these PC mutants revealed that the antibacterial substances produced by 8105PC, 82-1PC, and 101PC strains were heat labile, proteinase K-sensitive high-molecular proteinaceous substances (> 100 kDa). Suzaki-5PC produced highly proteinase K‒resistant antibacterial substances. Cross-response tests revealed that the efficacy of antibacterial substances and resistance to antibacterial substances differed between wild-type and PC-mutant strains. Our study indicated that productivities, resistances and properties of antibacterial substance in R. pseudosolanacearum and R. syzygii subsp. indonesiensis strains changed by strains and phenotypic mutation.