Ralstonia solanacearum is a Gram-negative bacterium and the causative agent of bacterial wilt in many important crops. We treated R. solanacearum with three lytic phages: RSA1, RSB1, and RSL1. Infection with RSA1 and RSB1, either alone or in combination with the other phages, resulted in a rapid decrease in the host bacterial cell density. Cells that were resistant to infection by these phages became evident approximately 30 h after phage addition to the culture. On the other hand, cells infected solely with RSL1 in a batch culture were maintained at a lower cell density (1/3 of control) over a long period. Pretreatment of tomato seedlings with RSL1 drastically limited penetration, growth, and movement of root-inoculated bacterial cells. All RSL1-treated tomato plants showed no symptoms of wilting during the experimental period, whereas all untreated plants had wilted by 18 days postinfection. RSL1 was shown to be relatively stable in soil, especially at higher temperatures (37 to 50°C). Active RSL1 particles were recovered from the roots of treated plants and from soil 4 months postinfection. Based on these observations, we propose an alternative biocontrol method using a unique phage, such as RSL1, instead of a phage cocktail with highly virulent phages. Using this method, RSL1 killed some but not all bacterial cells. The coexistence of bacterial cells and the phage resulted in effective prevention of wilting.

ϕRSL1 jumbo phage belongs to a new class of viruses within the Myoviridae family. Here, we report its three-dimensional structure determined by electron cryo microscopy. The icosahedral capsid, the tail helical portion, and the complete tail appendage were reconstructed separately to resolutions of 9 Å, 9 Å, and 28 Å, respectively. The head is rather complex and formed by at least five different proteins, whereas the major capsid proteins resemble those from HK97, despite low sequence conservation. The helical tail structure demonstrates its close relationship to T4 sheath proteins and provides evidence for an evolutionary link of the inner tail tube to the bacterial type VI secretion apparatus. Long fibers extend from the collar region, and their length is consistent with reaching the host cell surface upon tail contraction. Our structural analyses indicate that ϕRSL1 is an unusual member of the Myoviridae that employs conserved protein machines related to different phages and bacteria.

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