Cultivation of a Lytic Double-Stranded RNA Bacteriophage Infecting Microvirgula aerodenitrificans Reveals a Mutualistic Parasitic Lifestyle

Identification of receptor-binding protein and host receptor of non-lytic dsRNA phage phiNY

To date, complete genome sequences of 14 double-stranded RNA (dsRNA) phages are available, and studies have shown that the host range of dsRNA phages is limited. The hosts of most dsRNA phages belong to the genus Pseudomonas. However, the dsRNA phage phiNY, which has a non-lytic life cycle, was isolated from Microvirgula aerodenitrificans. Currently, the interaction between dsRNA phage phiNY and its host bacteria is unclear, which is not beneficial to a comprehensive understanding of dsRNA phage biology and the exploitation of dsRNA phage with non-lytic life cycle for biomedical applications and others. Phage adsorption is a crucial step through the interactions between receptor-binding protein (RBP) of the phage and its receptors to initiate the infection process, which dictates host range specificity. Thus, we identified the RBP and host receptor of phiNY. Through homology alignment, amino acid sequence similarity analysis, and the phylogenetic tree analysis, orf11, located in the M-segment of dsRNA phage phiNY, encodes a putative RBP. We further performed the whole-cell enzyme-linked immunosorbent assay (ELISA), western blotting assay, and indirect immunofluorescence assay and demonstrated that this orf11 is an RBP. Finally, using affinity chromatography, ELISA, and dynamic light scattering, we identified lipopolysaccharides (LPSs) on the surface of the host M. aerodenitrificans strain LH9 as host receptors involved in the adsorption of the dsRNA bacteriophage phiNY and observed the state of phiNY RBP after combining with LPS by atomic force microscopy. These results will guide future studies on phage-host interaction in a dsRNA phage with a non-lytic life cycle.IMPORTANCEThe interactions between the lytic dsRNA phages and their host receptors have been clarified in previous studies. However, the interaction between the dsRNA phage phiNY (which has a non-lytic life cycle) and its host receptors during the dsRNA phage adsorption process was unknown. Here, we found that phiNY uses the orf11 protein as a receptor-binding protein (RBP). In addition, we found that this orf11 recognizes lipopolysaccharide from the host bacterium Microvirgula aerodenitrificans strain LH9 as a specific receptor. These results suggest that phiNY, like lytic dsRNA phages, uses an RBP to bind to a similar host receptor (i.e., lipopolysaccharide). Determining the interaction between the dsRNA phage phiNY and its host receptors will help to elucidate the mechanisms underlying the phiNY non-lytic life cycle and enhance our understanding of its infection mechanism.

Bacteriophage protein Dap1 regulates evasion of antiphage immunity and Pseudomonas aeruginosa virulence impacting phage therapy in mice

Bacteriophages have evolved diverse strategies to overcome host defence mechanisms and to redirect host metabolism to ensure successful propagation. Here we identify a phage protein named Dap1 from Pseudomonas aeruginosa phage PaoP5 that both modulates bacterial host behaviour and contributes to phage fitness. We show that expression of Dap1 in P. aeruginosa reduces bacterial motility and promotes biofilm formation through interference with DipA, a c-di-GMP phosphodiesterase, which causes an increase in c-di-GMP levels that trigger phenotypic changes. Results also show that deletion of dap1 in PaoP5 significantly reduces genome packaging. In this case, Dap1 directly binds to phage HNH endonuclease, prohibiting host Lon-mediated HNH degradation and promoting phage genome packaging. Moreover, PaoP5Δdap1 fails to rescue P. aeruginosa-infected mice, implying the significance of dap1 in phage therapy. Overall, these results highlight remarkable dual functionality in a phage protein, enabling the modulation of host behaviours and ensuring phage fitness.

Diversity and Current Classification of dsRNA Bacteriophages

Half a century has passed since the discovery of Pseudomonas phage phi6, the first enveloped dsRNA bacteriophage to be isolated. It remained the sole known dsRNA phage for a quarter of a century and the only recognised member of the Cystoviridae family until the year 2018. After the initial discovery of phi6, additional dsRNA phages have been isolated from globally distant locations and identified in metatranscriptomic datasets, suggesting that this virus type is more ubiquitous in nature than previously acknowledged. Most identified dsRNA phages infect Pseudomonas strains and utilise either pilus or lipopolysaccharide components of the host as the primary receptor. In addition to the receptor-mediated strictly lytic lifestyle, an alternative persistent infection strategy has been described for some dsRNA phages. To date, complete genome sequences of fourteen dsRNA phage isolates are available. Despite the high sequence diversity, similar sets of genes can typically be found in the genomes of dsRNA phages, suggesting shared evolutionary trajectories. This review provides a brief overview of the recognised members of the Cystoviridae virus family and related dsRNA phage isolates, outlines the current classification of dsRNA phages, and discusses their relationships with eukaryotic RNA viruses.

A novel mitovirus isolated from the filamentous fungus Hypoxylon fendleri

Members of the fungal genus Hypoxylon of the family Xylariaceae are known to produce secondary metabolites with significant chemical diversity. There are more than 200 species in the genus, including the filamentous fungus Hypoxylon fendleri. To the best of our knowledge, there have been no reports of mycoviruses in H. fendleri. In this study, a novel mycovirus, designated "Hypoxylon fendleri mitovirus 1" (HfMV1), was isolated from this fungus. The genome of HfMV1 is 2850 nt in length with a G + C content of 36% and contains a large open reading frame (ORF) encoding an RNA-dependent RNA polymerase (RdRp). BLASTp analysis revealed that the RdRp domain of HfMV1 had 28.30-50.90% sequence identity to those of members of the genus Duamitovirus and had the highest identity (50.90%) to Fusarium graminearum mitovirus 2-2 (FgMV2-2). Phylogenetic analysis further indicated that HfMV1 is a member of the genus Duamitovirus of the family Mitoviridae. This is the first report of a mycovirus in H. fendleri.

Characterization of Pseudomonas aeruginosa Bacteriophage L5 Which Requires Type IV Pili for Infection

Pseudomonas aeruginosa is a common opportunistic human pathogen. With the emergence of multidrug-resistant (MDR) clinical infection of P. aeruginosa, phage therapy has received renewed attention in treating P. aeruginosa infections. Moreover, a detailed understanding of the host receptor of lytic phage is crucial for selecting proper phages for therapy. Here, we describe the characterization of the P. aeruginosa bacteriophage L5 with a double-stranded DNA genome of 42,925 bp. The genomic characteristics indicate that L5 is a lytic bacteriophage belonging to the subfamily Autographivirinae. In addition, the phage receptors for L5 were also identified as type IV pili, because the mutation of pilZ, which is involved in pili synthesis, resists phage infection, while the complementation of pilZ restored its phage sensitivity. This research reveals that L5 is a potential phage therapy candidate for the treatment of P. aeruginosa infection.