Presentation of Three Novel Tailed Phages Targeting Multiple Strains of Pseudomonas syringae

Background: Pseudomonas syringae are ubiquitous epiphytic plant pathogens infecting a wide range of important agricultural plant species. Bacteriophages has been proposed as biocontrol agents against plant pathogens, however, in order to utilize this approach, a deeper understanding of phage diversity and phage-host interactions is required. Materials and Methods: Phages targeting P. syringae GAW0113 were isolated from organic waste samples. Three distinct phage isolates were purified and subjected to whole-genome sequencing, comparative genomics, transmission electron microscopy and host-range assay using a wide selection of diverse P. syringae isolates. Results: The three phage isolates, Pseudomonas phage Bertil, Misse, and Strit, were shown to have podovirus morphology with a short tail stub and isometric head. They had double-stranded DNA ranging from 41,342 to 41,374 bp in size comprising 50-51 open reading frames. The three phage genomes were highly similar and genomic comparison analyses showed that they all belong to the Autographiviridae family of the order Caudovirales. All three phages were shown to have a narrow host-range. Conclusions: The three phages were shown to share morphological and genomic features with other phages in the Autographiviridae family, however, based on the limited nucleotide similarity we propose that the phages constitute a novel genus. All three phages were found to infect multiple strains of P. syringae covering several phylogroups.

Detection of preQ0 deazaguanine modifications in bacteriophage CAjan DNA using Nanopore sequencing reveals same hypermodification at two distinct DNA motifs

In the constant evolutionary battle against mobile genetic elements (MGEs), bacteria have developed several defense mechanisms, some of which target the incoming, foreign nucleic acids e.g. restriction-modification (R-M) or CRISPR-Cas systems. Some of these MGEs, including bacteriophages, have in turn evolved different strategies to evade these hurdles. It was recently shown that the siphophage CAjan and 180 other viruses use 7-deazaguanine modifications in their DNA to evade bacterial R-M systems. Among others, phage CAjan genome contains a gene coding for a DNA-modifying homolog of a tRNA-deazapurine modification enzyme, together with four 7-cyano-7-deazaguanine synthesis genes. Using the CRISPR-Cas9 genome editing tool combined with the Nanopore Sequencing (ONT) we showed that the 7-deazaguanine modification in the CAjan genome is dependent on phage-encoded genes. The modification is also site-specific and is found mainly in two separate DNA sequence contexts: GA and GGC. Homology modeling of the modifying enzyme DpdA provides insight into its probable DNA binding surface and general mode of DNA recognition.

Two novel bacteriophage genera from a groundwater reservoir highlight subsurface environments as underexplored biotopes in bacteriophage ecology

Although bacteriophages are central entities in bacterial ecology and population dynamics, there is currently no literature on the genomes of bacteriophages isolated from groundwater. Using a collection of bacterial isolates from an aquifer as hosts, this study isolated, sequenced and characterised two bacteriophages native to the groundwater reservoir. Host phylogenetic analyses revealed that the phages targeted B. mycoides and a novel Pseudomonas species. These results suggest that both bacteriophages represent new genera, highlighting that groundwater reservoirs, and probably other subsurface environments as well, are underexplored biotopes in terms of the presence and ecology of bacteriophages.

7-Deazaguanine modifications protect phage DNA from host restriction systems

Genome modifications are central components of the continuous arms race between viruses and their hosts. The archaeosine base (G+), which was thought to be found only in archaeal tRNAs, was recently detected in genomic DNA of Enterobacteria phage 9g and was proposed to protect phage DNA from a wide variety of restriction enzymes. In this study, we identify three additional 2'-deoxy-7-deazaguanine modifications, which are all intermediates of the same pathway, in viruses: 2'-deoxy-7-amido-7-deazaguanine (dADG), 2'-deoxy-7-cyano-7-deazaguanine (dPreQ0) and 2'-deoxy-7- aminomethyl-7-deazaguanine (dPreQ1). We identify 180 phages or archaeal viruses that encode at least one of the enzymes of this pathway with an overrepresentation (60%) of viruses potentially infecting pathogenic microbial hosts. Genetic studies with the Escherichia phage CAjan show that DpdA is essential to insert the 7-deazaguanine base in phage genomic DNA and that 2'-deoxy-7-deazaguanine modifications protect phage DNA from host restriction enzymes.

Unlocking the Potential of 46 New Bacteriophages for Biocontrol of Dickeya Solani

Modern agriculture is expected to face an increasing global demand for food while also needing to comply with higher sustainability standards. Therefore, control of crop pathogens requires new, green alternatives to current methods. Potatoes are susceptible to several bacterial diseases, with infections by soft rot Enterobacteriaceae (SRE) being a significant contributor to the major annual losses. As there are currently no efficient ways of combating SRE, we sought to develop an approach that could easily be incorporated into the potato production pipeline. To this end, 46 phages infecting the emerging potato pathogen Dickeya solani were isolated and thoroughly characterized. The 46 isolated phages were grouped into three different groups based on DNA similarity, representing two distinct clusters and a singleton. One cluster showed similarity to phages previously used to successfully treat soft rot in potatoes, whereas the remaining phages were novel and showed only very limited similarity to previously isolated phages. We selected six diverse phages in order to create the hereto most complex phage cocktail against SRE. The cocktail was applied in a proof-of-principle experiment to treat soft rot in potatoes under simulated storage conditions. We show that the phage cocktail was able to significantly reduce the incidence of soft rot as well as disease severity after 5 days of storage post-infection with Dickeya solani. This confirms results from previous studies that phages represent promising biocontrol agents against SRE infection in potato.