Genomic Analysis of the Recent Viral Isolate vB_BthP-Goe4 Reveals Increased Diversity of φ29-Like Phages

Acinetobacter baumannii satellite phage Aci01-2-Phanie depends on a helper myophage for its multiplication

We report the discovery of a satellite-helper phage system with a novel type of dependence on a tail donor. The Acinetobacter baumannii satellite podovirus Aci01-2-Phanie (short name Phanie) uses a phage phi29-like DNA replication and packaging mode. Its linear 11,885 bp dsDNA genome bears 171 bp inverted terminal repeats (ITR). Phanie is related to phage DU-PP-III from Pectobacterium and to members of the Astrithrvirus from Salmonella enterica. Together, they form a new clade of phages with 27% to 30% identity over the whole genome. Detailed 3D protein structure prediction and mass spectrometry analyses demonstrate that Phanie encodes its capsid structural genes and genes necessary to form a short tail. However, our study reveals that Phanie virions are non-infectious unless they associate with the contractile tail of an unrelated phage, Aci01-1, to produce chimeric myoviruses. Following the coinfection of Phanie with myovirus Aci01-1, hybrid viral particles composed of Phanie capsids and Aci01-1 contractile tails are assembled together with Phanie and Aci01-1 particles.IMPORTANCEThere are few reported cases of satellite-helper phage interactions but many more may be yet undiscovered. Here we describe a new mode of satellite phage dependence on a helper phage. Phanie, like phage phi29, replicates its linear dsDNA by a protein primed-mechanism and protects it inside podovirus-like particles. However, these particles are defective, requiring the acquisition of the tail from a myovirus helper for production of infectious virions. The formation of chimeras between a phi29-like podovirus and a helper contractile tail reveals an unexpected association between very different bacterial viruses.

Bacillus phage phi18-2 is a novel temperate virus with an unintegrated genome present in the cytoplasm of lysogenic cells as a linear phage-plasmid

Bacillus subtilis is a Gram-positive bacterium that is widely used in fermentation and in the pharmaceutical industry. Phage contamination occasionally occurs in various fermentation processes and causes significant economic loss. Here, we report the isolation and characterization of a temperate B. subtilis phage, termed phi18-2, from spore powder manufactured in a fermentation plant. Transmission electron microscopy showed that phi18-2 has a symmetrical polyhedral head and a long noncontractile tail. Receptor analysis showed that phi18-2 recognizes wall teichoic acid (WTA) for infection. The phage virions have a linear double-stranded DNA genome of 64,467 bp with identical direct repeat sequences of 309 bp at each end of the genome. In lysogenic cells, the phage genome was found to be present in the cytoplasm without integration into the host cell chromosome, and possibly as a linear phage-plasmid with unmodified ends. Our data may provide some insight into the molecular basis of the unique lysogenic cycle of phage phi18-2.

Flexible structural arrangement and DNA-binding properties of protein p6 from Bacillus subtillis phage φ29

The genome-organizing protein p6 of Bacillus subtilis bacteriophage φ29 plays an essential role in viral development by activating the initiation of DNA replication and participating in the early-to-late transcriptional switch. These activities require the formation of a nucleoprotein complex in which the DNA adopts a right-handed superhelix wrapping around a multimeric p6 scaffold, restraining positive supercoiling and compacting the viral genome. Due to the absence of homologous structures, prior attempts to unveil p6's structural architecture failed. Here, we employed AlphaFold2 to engineer rational p6 constructs yielding crystals for three-dimensional structure determination. Our findings reveal a novel fold adopted by p6 that sheds light on its self-association mechanism and its interaction with DNA. By means of protein-DNA docking and molecular dynamic simulations, we have generated a comprehensive structural model for the nucleoprotein complex that consistently aligns with its established biochemical and thermodynamic parameters. Besides, through analytical ultracentrifugation, we have confirmed the hydrodynamic properties of the nucleocomplex, further validating in solution our proposed model. Importantly, the disclosed structure not only provides a highly accurate explanation for previously experimental data accumulated over decades, but also enhances our holistic understanding of the structural and functional attributes of protein p6 during φ29 infection.

Application of CRISPR-Cas System to Mitigate Superbug Infections

Multidrug resistance in bacterial strains known as superbugs is estimated to cause fatal infections worldwide. Migration and urbanization have resulted in overcrowding and inadequate sanitation, contributing to a high risk of superbug infections within and between different communities. The CRISPR-Cas system, mainly type II, has been projected as a robust tool to precisely edit drug-resistant bacterial genomes to combat antibiotic-resistant bacterial strains effectively. To entirely opt for its potential, advanced development in the CRISPR-Cas system is needed to reduce toxicity and promote efficacy in gene-editing applications. This might involve base-editing techniques used to produce point mutations. These methods employ designed Cas9 variations, such as the adenine base editor (ABE) and the cytidine base editor (CBE), to directly edit single base pairs without causing DSBs. The CBE and ABE could change a target base pair into a different one (for example, G-C to A-T or C-G to A-T). In this review, we addressed the limitations of the CRISPR/Cas system and explored strategies for circumventing these limitations by applying diverse base-editing techniques. Furthermore, we also discussed recent research showcasing the ability of base editors to eliminate drug-resistant microbes.

A Multimodal Approach towards Genomic Identification of Protein Inhibitors of Uracil-DNA Glycosylase

DNA-mimicking proteins encoded by viruses can modulate processes such as innate cellular immunity. An example is Ung-family uracil-DNA glycosylase inhibition, which prevents Ung-mediated degradation via the stoichiometric protein blockade of the Ung DNA-binding cleft. This is significant where uracil-DNA is a key determinant in the replication and distribution of virus genomes. Unrelated protein folds support a common physicochemical spatial strategy for Ung inhibition, characterised by pronounced sequence plasticity within the diverse fold families. That, and the fact that relatively few template sequences are biochemically verified to encode Ung inhibitor proteins, presents a barrier to the straightforward identification of Ung inhibitors in genomic sequences. In this study, distant homologs of known Ung inhibitors were characterised via structural biology and structure prediction methods. A recombinant cellular survival assay and in vitro biochemical assay were used to screen distant variants and mutants to further explore tolerated sequence plasticity in motifs supporting Ung inhibition. The resulting validated sequence repertoire defines an expanded set of heuristic sequence and biophysical signatures shared by known Ung inhibitor proteins. A computational search of genome database sequences and the results of recombinant tests of selected output sequences obtained are presented here.

Evidence of a Set of Core-Function Genes in 16 Bacillus Podoviral Genomes with Considerable Genomic Diversity

Bacteriophage genomes represent an enormous level of genetic diversity and provide considerable potential to acquire new insights about viral genome evolution. In this study, the genome sequences of sixteen Bacillus-infecting bacteriophages were explored through comparative genomics approaches to reveal shared and unique characteristics. These bacteriophages are in the Salasmaviridae family with small (18,548-27,206 bp) double-stranded DNA genomes encoding 25-46 predicted open reading frames. We observe extensive nucleotide and amino acid sequence divergence among a set of core-function genes that present clear synteny. We identify two examples of sequence directed recombination within essential genes, as well as explore the expansion of gene content in these genomes through the introduction of novel open reading frames. Together, these findings highlight the complex evolutionary relationships of phage genomes that include old, common origins as well as new components introduced through mosaicism.

Brevundimonas and Serratia as host systems for assessing associated environmental viromes and phage diversity by complementary approaches

Focusing on visible plaques for phage isolation leaves the question if we miss the diversity of non-plaque forming phages. We addressed this question through direct plaque-based isolation by employing the new hosts Brevundimonas pondensis LVF1 and Serratia marcescens LVF3 dsDNA, ssDNA, dsRNA, and ssRNA host-associated metavirome analysis. Of the 25 distinctive dsDNA phage isolates, 14 were associated with Brevundimonas and 11 with Serratia. TEM analysis revealed that 6 were myoviruses, 18 siphoviruses and 1 podovirus, while phages infecting Brevundimonas belonged all to siphoviruses. The associated viromes suggested a higher phage diversity in summer than in winter, and dsDNA phages were the dominant group. Isolation of vB_SmaP-Kaonashi was possible after investigating the viromes associated with Serratia, demonstrating the great potential of accompanying host-associated metavirome analysis. The ssDNA virome analysis showed that the B. pondensis LVF1 host is associated with Microviridae and Inoviridae phages, although none of them were isolated. The results demonstrated that the classical isolation technique is not exhausted, leading to the isolation of new dsDNA phages. It can be further improved by combination with metavirome techniques, which revealed further diversity.

CRISPR-Cas9 Shaped Viral Metagenomes Associated with Bacillus subtilis

Phages are viruses of bacteria and have been known for over a century. They do not have a metabolism or protein synthesis machinery and rely on host cells for replication. The model organism Bacillus subtilis has served as a host strain for decades and enabled the isolation of many unique viral strains. However, many viral species representatives remained orphans as no, or only a few, related phages were ever re-isolated.The presented protocol describes how a CRISPR-Cas9 system with an artificial CRISPR-array can be set up and used to discriminate abundant and well-known B. subtilis phage from a host-based metagenome enrichment. The obtained viral suspension can be used for metagenome sequencing and isolating new viral strains.

Chances and limitations when uncovering essential and non-essential genes of Bacillus subtilis phages with CRISPR-Cas9

Virulent bacterial viruses, also known as phages or bacteriophages, are considered as a potential option to fight antibiotic-resistant bacteria. However, their biology is still poorly understood, and only a fraction of phage genes is assigned with a function. To enable the first classification, we explored new options to test phage genes for their requirement on viral replication. As a model, we used the smallest known Bacillus subtilis phage Goe1, and the Cas9-based mutagenesis vector pRH030 as a genetic tool. All phage genes were specifically disrupted, and individual survival rates and mutant genotypes were investigated. Surviving phages relied on the genome integrity through host intrinsic non-homologues end joining system or a natural alteration of the Cas9 target sequence. Quantification of phage survivors and verifying the underlying genetic situation enables the classification of genes in essential or non-essential sets for viral replication. We also observed structural genes to hold more natural mutations than genes of the genome replication machinery.

Isolation and Characterisation of the Bundooravirus Genus and Phylogenetic Investigation of the Salasmaviridae Bacteriophages

Bacillus is a highly diverse genus containing over 200 species that can be problematic in both industrial and medical settings. This is mainly attributed to Bacillus sp. being intrinsically resistant to an array of antimicrobial compounds, hence alternative treatment options are needed. In this study, two bacteriophages, PumA1 and PumA2 were isolated and characterized. Genome nucleotide analysis identified the two phages as novel at the DNA sequence level but contained proteins similar to phi29 and other related phages. Whole genome phylogenetic investigation of 34 phi29-like phages resulted in the formation of seven clusters that aligned with recent ICTV classifications. PumA1 and PumA2 share high genetic mosaicism and form a genus with another phage named WhyPhy, more recently isolated from the United States of America. The three phages within this cluster are the only candidates to infect B. pumilus. Sequence analysis of B. pumilus phage resistant mutants revealed that PumA1 and PumA2 require polymerized and peptidoglycan bound wall teichoic acid (WTA) for their infection. Bacteriophage classification is continuously evolving with the increasing phages' sequences in public databases. Understanding phage evolution by utilizing a combination of phylogenetic approaches provides invaluable information as phages become legitimate alternatives in both human health and industrial processes.

Isolation and Characterization of Bacillus cereus Phage vB_BceP-DLc1 Reveals the Largest Member of the Φ29-Like Phages

Bacillus phage φ29 and its relatives have been considered as one of the most important model organisms for DNA replication, transcription, morphogenesis, DNA packaging studies, and nanotechnology applications. Here, we isolated and characterized a new member of the φ29-like phage, named Bacillus cereus phage vB_BceP-DLc1. This phage, with a unique inserted gene cluster, has the largest genome among known φ29-like phages. DLc1 can use the surface carbohydrate structures of the host cell as receptors and only infects the most related B. cereus strains, showing high host-specificity. The adsorption rate constant and life cycle of DLc1 under experimental conditions were also determined. Not only stable under temperatures below 55 °C and pH range from 5 to 11, the new phage also showed tolerance to high concentrations of NaCl, 75% ethanol, chloroform, and mechanical vortex, which is preferable for practical use in the food and pharmaceutical industries.

Genome sequencing and characterization of three Bacillus cereus-specific phages, DK1, DK2, and DK3

In the study, three Bacillus cereus-specific phages, named DK1, DK2 and DK3, belonging to the family Podoviridae, were isolated from Pearl River water and sludge in Guangzhou, China. The genomes of DK1, DK2 and DK3 were 27,180 bp, 26,357 bp, and 26,865 bp in length and contained 49, 45 and 46 open reading frames, respectively. Among the three phages, DK2 shared the highest genome sequence similarity (96% identity) with DK3. Genes encoding rRNA, tRNA, virulence factors and antibiotic resistance were absent in these phage genomes. In addition, comparative genomic and phylogenetic analysis revealed that they were novel phages of B. cereus. Each genome encoded a putative endolysin that might be of value for the control of the foodborne pathogen B. cereus.