New bacteriophages that infect the phytopathogen Ralstonia solanacearum

Four kinds of bacteriophage (phi RSL, phi RSA, phi RSM and phi RSS) were isolated from Ralstonia solanacearum, a soil-borne Gram-negative bacterium that is the causative agent of bacterial wilt in many important crops. The Myovirus-type phages phi RSL1 and phi RSA1 contained dsDNA genomes of 240 kbp and 39 kbp, respectively. These phages have relatively wide host ranges and gave large clear plaques with various host strains; especially phi RSA1 was able to infect all 15 R. solanacearum strains of different races or different biovars tested in this study. Three host strains contained phi RSA1 -related sequences in their genomic DNAs, suggesting a lysogenic cycle of phi RSA1. Two phages, phi RSM1 and phi RSS1, were characterized as Ff-type phages (Inovirus) based on their particle morphology, genomic ssDNA and infection cycle. However, despite their similar fibrous morphology, their genome size (9.0 kb for phi RSM1 and 6.6 kb for phi RSS1) and genome sequence were different. Strains of R. solanacearum that were sensitive to phi RSM1 were resistant to phi RSS1 and vice versa. Several R. solanacearum strains contained phi RSM1 -related sequences and at least one strain produced phi RSM1 particles, indicating the lysogenic state of this phage. These phages may be useful as a tool not only for molecular biological studies of R. solanacearum pathogenicity but also for specific and efficient detection (phi RSM1 and phi RSS1) and control of harmful pathogens (phi RSL and phi RSA) in cropping ecosystems as well as growing crops.