Three-dimensional structures of bacteriophage neck subunits are shared inPodoviridae,SiphoviridaeandMyoviridae

Integrative structural analysis of Pseudomonas phage DEV reveals a genome ejection motor

DEV is an obligatory lytic Pseudomonas phage of the N4-like genus, recently reclassified as Schitoviridae. The DEV genome encodes 91 ORFs, including a 3,398 amino acid virion-associated RNA polymerase. Here, we describe the complete architecture of DEV, determined using a combination of cryo-electron microscopy localized reconstruction, biochemical methods, and genetic knockouts. We built de novo structures of all capsid factors and tail components involved in host attachment. We demonstrate that DEV long tail fibers are essential for infection of Pseudomonas aeruginosa and dispensable for infecting mutants with a truncated lipopolysaccharide devoid of the O-antigen. We identified DEV ejection proteins and, unexpectedly, found that the giant DEV RNA polymerase, the hallmark of the Schitoviridae family, is an ejection protein. We propose that DEV ejection proteins form a genome ejection motor across the host cell envelope and that these structural principles are conserved in all Schitoviridae.

Neck and capsid architecture of the robust Agrobacterium phage Milano

Large gaps exist in our understanding of how bacteriophages, the most abundant biological entities on Earth, assemble and function. The structure of the "neck" region, where the DNA-filled capsid is connected to the host-recognizing tail remains poorly understood. We describe cryo-EM structures of the neck, the neck-capsid and neck-tail junctions, and capsid of the Agrobacterium phage Milano. The Milano neck 1 protein connects the 12-fold symmetrical neck to a 5-fold vertex of the icosahedral capsid. Comparison of Milano neck 1 homologs leads to four proposed classes, likely evolved from the simplest one in siphophages to more complex ones in myo- and podophages. Milano neck is surrounded by the atypical collar, which covalently crosslinks the tail sheath to neck 1. The Milano capsid is decorated with three types of proteins, a minor capsid protein (mCP) and two linking proteins crosslinking the mCP to the major capsid protein. The extensive network of disulfide bonds within and between neck, collar, capsid and tail provides an exceptional structural stability to Milano.

Psychrobacter Phage Encoding an Antibiotics Resistance Gene Represents a Novel Caudoviral Family

Psychrobacter is an important bacterial genus that is widespread in Antarctic and marine environments. However, to date, only two complete Psychrobacter phage sequences have been deposited in the NCBI database. Here, the novel Psychrobacter phage vB_PmaS_Y8A, infecting Psychrobacter HM08A, was isolated from sewage in the Qingdao area, China. The morphology of vB_PmaS_Y8A was characterized by transmission electron microscopy, revealing an icosahedral head and long tail. The genomic sequence of vB_PmaS_Y8A is linear, double-stranded DNA with a length of 40,226 bp and 44.1% G+C content, and encodes 69 putative open reading frames. Two auxiliary metabolic genes (AMGs) were identified, encoding phosphoadenosine phosphosulfate reductase and MarR protein. The first AMG uses thioredoxin as an electron donor for the reduction of phosphoadenosine phosphosulfate to phosphoadenosine phosphate. MarR regulates multiple antibiotic resistance mechanisms in Escherichia coli and is rarely found in viruses. No tRNA genes were identified and no lysogeny-related feature genes were detected. However, many similar open reading frames (ORFs) were found in the host genome, which may indicate that Y8A also has a lysogenic stage. Phylogenetic analysis based on the amino acid sequences of whole genomes and comparative genomic analysis indicate that vB_PmaS_Y8A contains a novel genomic architecture similar only to that of Psychrobacter phage pOW20-A, although at a low similarity. vB_PmaS_Y8A represents a new family-level virus cluster with 22 metagenomic assembled viral genomes, here named Minviridae. IMPORTANCE Although Psychrobacter is a well-known and important bacterial genus that is widespread in Antarctic and marine environments, genetic characterization of its phages is still rare. This study describes a novel Psychrobacter phage containing an uncharacterized antibiotic resistance gene and representing a new virus family, Minviridae. The characterization provided here will bolster current understanding of genomes, diversity, evolution, and phage-host interactions in Psychrobacter populations.

Structure of Vibrio Phage XM1, a Simple Contractile DNA Injection Machine

Antibiotic resistance poses a growing risk to public health, requiring new tools to combat pathogenic bacteria. Contractile injection systems, including bacteriophage tails, pyocins, and bacterial type VI secretion systems, can efficiently penetrate cell envelopes and become potential antibacterial agents. Bacteriophage XM1 is a dsDNA virus belonging to the Myoviridae family and infecting Vibrio bacteria. The XM1 virion, made of 18 different proteins, consists of an icosahedral head and a contractile tail, terminated with a baseplate. Here, we report cryo-EM reconstructions of all components of the XM1 virion and describe the atomic structures of 14 XM1 proteins. The XM1 baseplate is composed of a central hub surrounded by six wedge modules to which twelve spikes are attached. The XM1 tail contains a fewer number of smaller proteins compared to other reported phage baseplates, depicting the minimum requirements for building an effective cell-envelope-penetrating machine. We describe the tail sheath structure in the pre-infection and post-infection states and its conformational changes during infection. In addition, we report, for the first time, the in situ structure of the phage neck region to near-atomic resolution. Based on these structures, we propose mechanisms of virus assembly and infection.

Race-specific genotypes of Pseudomonas syringae pv. tomato are defined by the presence of mobile DNA elements within the genome

Pseudomonas syringae pv. tomato is the causal agent of bacterial speck of tomato, an important disease that results in severe crop production losses worldwide. Currently, two races within phylogroup 01a (PG01a) are described for this pathogen. Race 0 strains have avirulence genes for the expression of type III system-associated effectors AvrPto1 and AvrPtoB, that are recognized and targeted by the effector-triggered immunity in tomato cultivars having the pto race-specific resistance gene. Race 1 strains instead lack the avrPto1 and avrPtoB genes and are therefore capable to aggressively attack all tomato cultivars. Here, we have performed the complete genome sequencing and the analysis of P. syringae pv. tomato strain DAPP-PG 215, which was described as a race 0 strain in 1996. Our analysis revealed that its genome comprises a 6.2 Mb circular chromosome and two plasmids (107 kb and 81 kb). The results indicate that the strain is phylogenetically closely related to strains Max13, K40, T1 and NYS-T1, all known race 1 strains. The chromosome of DAPP-PG 215 encodes race 1-associated genes like avrA and hopW1 and lacks race 0-associated genes like hopN1, giving it a race 1 genetic background. However, the genome harbors a complete ortholog of avrPto1, which allows the strain to display a race 0 phenotype. Comparative genomics with several PG01a genomes revealed that mobile DNA elements are rather involved in the evolution of the two different races.

Structural atlas of a human gut crassvirus

CrAssphage and related viruses of the order Crassvirales (hereafter referred to as crassviruses) were originally discovered by cross-assembly of metagenomic sequences. They are the most abundant viruses in the human gut, are found in the majority of individual gut viromes, and account for up to 95% of the viral sequences in some individuals1-4. Crassviruses are likely to have major roles in shaping the composition and functionality of the human microbiome, but the structures and roles of most of the virally encoded proteins are unknown, with only generic predictions resulting from bioinformatic analyses4,5. Here we present a cryo-electron microscopy reconstruction of Bacteroides intestinalis virus ΦcrAss0016, providing the structural basis for the functional assignment of most of its virion proteins. The muzzle protein forms an assembly about 1 MDa in size at the end of the tail and exhibits a previously unknown fold that we designate the 'crass fold', that is likely to serve as a gatekeeper that controls the ejection of cargos. In addition to packing the approximately 103 kb of virus DNA, the ΦcrAss001 virion has extensive storage space for virally encoded cargo proteins in the capsid and, unusually, within the tail. One of the cargo proteins is present in both the capsid and the tail, suggesting a general mechanism for protein ejection, which involves partial unfolding of proteins during their extrusion through the tail. These findings provide a structural basis for understanding the mechanisms of assembly and infection of these highly abundant crassviruses.

Insights into the global freshwater virome

Viruses are by far the most abundant life forms on this planet. Yet, the full viral diversity remains mostly unknown, especially in environments like freshwater. Therefore, we aimed to study freshwater viruses in a global context. To this end, we downloaded 380 publicly available viral metagenomes (>1 TB). More than 60% of these metagenomes were discarded based on their levels of cellular contamination assessed by ribosomal DNA content. For the remaining metagenomes, assembled contigs were decontaminated using two consecutive steps, eventually yielding 273,365 viral contigs longer than 1,000 bp. Long enough contigs (≥ 10 kb) were clustered to identify novel genomes/genome fragments. We could recover 549 complete circular and high-quality draft genomes, out of which 10 were recognized as being novel. Functional annotation of these genomes showed that most of the annotated coding sequences are DNA metabolic genes or phage structural genes. On the other hand, taxonomic analysis of viral contigs showed that most of the assigned contigs belonged to the order Caudovirales, particularly the families of Siphoviridae, Myoviridae, and Podoviridae. The recovered viral contigs contained several auxiliary metabolic genes belonging to several metabolic pathways, especially carbohydrate and amino acid metabolism in addition to photosynthesis as well as hydrocarbon degradation and antibiotic resistance. Overall, we present here a set of prudently chosen viral contigs, which should not only help better understanding of freshwater viruses but also be a valuable resource for future virome studies.

Evolutionary Dynamics between Phages and Bacteria as a Possible Approach for Designing Effective Phage Therapies against Antibiotic-Resistant Bacteria

With the increasing global threat of antibiotic resistance, there is an urgent need to develop new effective therapies to tackle antibiotic-resistant bacterial infections. Bacteriophage therapy is considered as a possible alternative over antibiotics to treat antibiotic-resistant bacteria. However, bacteria can evolve resistance towards bacteriophages through antiphage defense mechanisms, which is a major limitation of phage therapy. The antiphage mechanisms target the phage life cycle, including adsorption, the injection of DNA, synthesis, the assembly of phage particles, and the release of progeny virions. The non-specific bacterial defense mechanisms include adsorption inhibition, superinfection exclusion, restriction-modification, and abortive infection systems. The antiphage defense mechanism includes a clustered regularly interspaced short palindromic repeats (CRISPR)–CRISPR-associated (Cas) system. At the same time, phages can execute a counterstrategy against antiphage defense mechanisms. However, the antibiotic susceptibility and antibiotic resistance in bacteriophage-resistant bacteria still remain unclear in terms of evolutionary trade-offs and trade-ups between phages and bacteria. Since phage resistance has been a major barrier in phage therapy, the trade-offs can be a possible approach to design effective bacteriophage-mediated intervention strategies. Specifically, the trade-offs between phage resistance and antibiotic resistance can be used as therapeutic models for promoting antibiotic susceptibility and reducing virulence traits, known as bacteriophage steering or evolutionary medicine. Therefore, this review highlights the synergistic application of bacteriophages and antibiotics in association with the pleiotropic trade-offs of bacteriophage resistance.

Effects of abiotic factors on the stability and infectivity of polyvalent coliphage

Bacteriophage has attracted growing interest as a promising therapeutic agent for pathogenic bacteria, especially for antibiotic-resistant bacteria. However, the various abiotic conditions could impact the stability of phages and further threat host-virus interactions. Here, we investigated the stability and lytic activity of virulent polyvalent coliphage (named PE1) by double-layer plaque assay. PE1 can efficiently infect both the drug-sensitive Escherichia coli K12 and multidrug-resistant E. coli NDM-1 even after prolonged storage at 4 °C for up to two months. Results showed that PE1 exhibits an outstanding stability to infect E. coli strains under a wide range of thermal (4 °C-60 °C) and pH (4-11) conditions, which covers the thermal and pH variations of most wastewater treatment plants. Moreover, PE1 exhibited high resistibility to heavy metals exposure including Cu²⁺, Cd²⁺, Co²⁺, and Cr³⁺ at the concentrations below 0.5 mM, and an excellent resistant ability to the variation of ionic strength, which still retained strong infectious ability even treated with saturated sodium chloride solution (350 g/L). This work shows that polyvalent phage PE1 has a strong adaptive capacity to various abiotic factors and should be a good candidate of being an antibacterial agent, especially for antibiotic-resistant bacteria control in sewage.

Improved bactericidal efficacy and thermostability of Staphylococcus aureus-specific bacteriophage SA3821 by repeated sodium pyrophosphate challenges

As antibiotic resistance is being a threat to public health worldwide, bacteriophages are re-highlighted as alternative antimicrobials to fight with pathogens. Various wild-type phages isolated from diverse sources have been tested, but potential mutant phages generated by genome engineering or random mutagenesis are drawing increasing attention. Here, we applied a chelating agent, sodium pyrophosphate, to the staphylococcal temperate Siphoviridae phage SA3821 to introduce random mutations. Through 30 sequential sodium pyrophosphate challenges and random selections, the suspected mutant phage SA3821^M was isolated. SA3821^M maintained an intact virion morphology, but exhibited better bactericidal activity against its host Staphylococcous aureus CCARM 3821 for up to 17 h and thermostability than its parent, SA3821. Sodium pyrophosphate-mediated mutations in SA3821^M were absent in lysogenic development genes but concentrated (83.9%) in genes related to the phage tail, particularly in the tail tape measure protein, indicating that changes in the tail module might have been responsible for the altered traits. This intentional random mutagenesis through controlled treatments with sodium pyrophosphate could be applied to other phages as a simple but potent method to improve their traits as alternative antimicrobials.

Exploring the diversity of bacteriophage specific to Oenococcus oeni and Lactobacillus spp and their role in wine production

The widespread existence of bacteriophage has been of great interest to the biological research community and ongoing investigations continue to explore their diversity and role. They have also attracted attention and in-depth research in connection to fermented food processing, in particular from the dairy and wine industries. Bacteriophage, mostly oenophage, may in fact be a 'double edged sword' for winemakers: whilst they have been implicated as a causal agent of difficulties with malolactic fermentation (although not proven), they are also beginning to be considered as alternatives to using sulphur dioxide to prevent wine spoilage. Investigation and characterisation of oenophage of Oenococcus oeni, the main species used in winemaking, are still limited compared to lactococcal bacteriophage of Lactococcus lactis and Lactiplantibacillus plantarum (formally Lactobacillus plantarum), the drivers of most fermented dairy products. Interestingly, these strains are also being used or considered for use in winemaking. In this review, the genetic diversity and life cycle of phage, together with the debate on the consequent impact of phage predation in wine, and potential control strategies are discussed. KEY POINTS: • Bacteriophage detected in wine are diverse. • Many lysogenic bacteriophage are found in wine bacteria. • Phage impact on winemaking can depend on the stage of the winemaking process. • Bacteriophage as potential antimicrobial agents against spoilage organisms.

Structure and mechanism of DNA delivery of a gene transfer agent

Alphaproteobacteria, which are the most abundant microorganisms of temperate oceans, produce phage-like particles called gene transfer agents (GTAs) that mediate lateral gene exchange. However, the mechanism by which GTAs deliver DNA into cells is unknown. Here we present the structure of the GTA of Rhodobacter capsulatus (RcGTA) and describe the conformational changes required for its DNA ejection. The structure of RcGTA resembles that of a tailed phage, but it has an oblate head shortened in the direction of the tail axis, which limits its packaging capacity to less than 4,500 base pairs of linear double-stranded DNA. The tail channel of RcGTA contains a trimer of proteins that possess features of both tape measure proteins of long-tailed phages from the family Siphoviridae and tail needle proteins of short-tailed phages from the family Podoviridae. The opening of a constriction within the RcGTA baseplate enables the ejection of DNA into bacterial periplasm.