Nettle manure: an unsuspected source of bacteriophages active against various phytopathogenic bacteria

Screening of 10 environmental samples (mainly of rhizospheric origin) for lytic activity against two bacterial phytopathogens, Pseudomonas syringae pv. tomato DC3000 (CFBP2212) and Xanthomonas hortorum pv. vitians (CFBP3979), revealed that four samples harboured phages that were active against one strain. Only one sample, composed of an artisanal nettle liquid manure, contained phages able to lyse both strains. Electron microscopy revealed the presence of tailed bacteriophages, with all phages isolated on the Xanthomonas strain displaying a contractile tail typical of members of the family Myoviridae, whereas phages isolated on the Pseudomonas strain were related to members of the family Siphoviridae and short-tailed members of the family Podoviridae. Sequence analysis of the two Podoviridae-like bacteriophages isolated on Pseudomonas syringae pv. tomato, Pst_GM1 isolated from nettle manure and Pst_GIL1 isolated from infected lettuce leaves, revealed (i) strong homology between the two isolated phages, (ii) a high degree of sequence similarity to various phages isolated from various environments and from different geographical locations, and (iii) similarity of these phages to members of the family Autographiviridae, and more precisely, the genus Ghunavirus. Further investigation of the potential of nettle manure to host phages that could be active against a wider range of strains revealed that it contained phages active against 10 phytopathogens (out of 16 tested). Thus, nettle manure (and likely other plant manures) could represent a valuable source of phages, especially those targeting bacterial phytopathogens, in the same way that anthropized environments such as sewage are widely used as sources of phages active against opportunistic or acute pathogens of humans.