New bacteriophages that infect the phytopathogen Ralstonia solanacearum

Abstract

Four kinds of bacteriophage (phiRSL, phiRSA, phiRSM and phiRSS) 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 phiRSL1 and phiRSA1 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 phiRSA1 was able to infect all 15 R. solanacearum strains of different races or different biovars tested in this study. Three host strains contained phiRSA1-related sequences in their genomic DNAs, suggesting a lysogenic cycle of phiRSA1. Two phages, phiRSM1 and phiRSS1, 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 phiRSM1 and 6.6 kb for phiRSS1) and genome sequence were different. Strains of R. solanacearum that were sensitive to phiRSM1 were resistant to phiRSS1 and vice versa. Several R. solanacearum strains contained phiRSM1-related sequences and at least one strain produced phiRSM1 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 (phiRSM1 and phiRSS1) and control of harmful pathogens (phiRSL and phiRSA) in cropping ecosystems as well as growing crops.