Genome-based identification of active prophage regions by next generation sequencing in Bacillus licheniformis DSM13

Isolation and characterization of novel acetogenic strains of the genera Terrisporobacter and Acetoanaerobium

Due to their metabolic versatility in substrate utilization, acetogenic bacteria represent industrially significant production platforms for biotechnological applications such as syngas fermentation, microbial electrosynthesis or transformation of one-carbon substrates. However, acetogenic strains from the genera Terrisporobacter and Acetoanaerobium remained poorly investigated for biotechnological applications. We report the isolation and characterization of four acetogenic Terrisporobacter strains and one Acetoanaerobium strain. All Terrisporobacter isolates showed a characteristic growth pattern under a H2 + CO2 atmosphere. An initial heterotrophic growth phase was followed by a stationary growth phase, where continuous acetate production was indicative of H2-dependent acetogenesis. One of the novel Terrisporobacter isolates obtained from compost (strain COMT) additionally produced ethanol besides acetate in the stationary growth phase in H2-supplemented cultures. Genomic and physiological characterizations showed that strain COMT represented a novel Terrisporobacter species and the name Terrisporobacter vanillatitrophus is proposed (=DSM 116160T = CCOS 2104T). Phylogenomic analysis of the novel isolates and reference strains implied the reclassification of the T. petrolearius/T. hibernicus phylogenomic cluster to the species T. petrolearius and of the A. noterae/A. sticklandii phylogenomic cluster to the species A. sticklandii. Furthermore, we provide first insights into active prophages of acetogens from the genera Terrisporobacter and Acetoanaerobium.

Reversible conjugation of a CBASS nucleotide cyclase regulates bacterial immune response to phage infection

Prokaryotic antiviral defence systems are frequently toxic for host cells and stringent regulation is required to ensure survival and fitness. These systems must be readily available in case of infection but tightly controlled to prevent activation of an unnecessary cellular response. Here we investigate how the bacterial cyclic oligonucleotide-based antiphage signalling system (CBASS) uses its intrinsic protein modification system to regulate the nucleotide cyclase. By integrating a type II CBASS system from Bacillus cereus into the model organism Bacillus subtilis, we show that the protein-conjugating Cap2 (CBASS associated protein 2) enzyme links the cyclase exclusively to the conserved phage shock protein A (PspA) in the absence of phage. The cyclase-PspA conjugation is reversed by the deconjugating isopeptidase Cap3 (CBASS associated protein 3). We propose a model in which the cyclase is held in an inactive state by conjugation to PspA in the absence of phage, with conjugation released upon infection, priming the cyclase for activation.

Novel insights into phage biology of the pathogen Clostridioides difficile based on the active virome

The global pathogen Clostridioides difficile is a well-studied organism, and researchers work on unraveling its fundamental virulence mechanisms and biology. Prophages have been demonstrated to influence C. difficile toxin expression and contribute to the distribution of advantageous genes. All these underline the importance of prophages in C. difficile virulence. Although several C. difficile prophages were sequenced and characterized, investigations on the entire active virome of a strain are still missing. Phages were mainly isolated after mitomycin C-induction, which does not resemble a natural stressor for C. difficile. We examined active prophages from different C. difficile strains after cultivation in the absence of mitomycin C by sequencing and characterization of particle-protected DNA. Phage particles were collected after standard cultivation, or after cultivation in the presence of the secondary bile salt deoxycholate (DCA). DCA is a natural stressor for C. difficile and a potential prophage-inducing agent. We also investigated differences in prophage activity between clinical and non-clinical C. difficile strains. Our experiments demonstrated that spontaneous prophage release is common in C. difficile and that DCA presence induces prophages. Fourteen different, active phages were identified by this experimental procedure. We could not identify a definitive connection between clinical background and phage activity. However, one phage exhibited distinctively higher activity upon DCA induction in the clinical strain than in the corresponding non-clinical strain, although the phage is identical in both strains. We recorded that enveloped DNA mapped to genome regions with characteristics of mobile genetic elements other than prophages. This pointed to mechanisms of DNA mobility that are not well-studied in C. difficile so far. We also detected phage-mediated lateral transduction of bacterial DNA, which is the first described case in C. difficile. This study significantly contributes to our knowledge of prophage activity in C. difficile and reveals novel aspects of C. difficile (phage) biology.

vRhyme enables binning of viral genomes from metagenomes

Genome binning has been essential for characterization of bacteria, archaea, and even eukaryotes from metagenomes. Yet, few approaches exist for viruses. We developed vRhyme, a fast and precise software for construction of viral metagenome-assembled genomes (vMAGs). vRhyme utilizes single- or multi-sample coverage effect size comparisons between scaffolds and employs supervised machine learning to identify nucleotide feature similarities, which are compiled into iterations of weighted networks and refined bins. To refine bins, vRhyme utilizes unique features of viral genomes, namely a protein redundancy scoring mechanism based on the observation that viruses seldom encode redundant genes. Using simulated viromes, we displayed superior performance of vRhyme compared to available binning tools in constructing more complete and uncontaminated vMAGs. When applied to 10,601 viral scaffolds from human skin, vRhyme advanced our understanding of resident viruses, highlighted by identification of a Herelleviridae vMAG comprised of 22 scaffolds, and another vMAG encoding a nitrate reductase metabolic gene, representing near-complete genomes post-binning. vRhyme will enable a convention of binning uncultivated viral genomes and has the potential to transform metagenome-based viral ecology.

Deciphering Active Prophages from Metagenomes

Temperate phages (prophages) are ubiquitous in nature and persist as dormant components of host cells (lysogenic stage) before activating and lysing the host (lytic stage). Actively replicating prophages contribute to central community processes, such as enabling bacterial virulence, manipulating biogeochemical cycling, and driving microbial community diversification. Recent advances in sequencing technology have allowed for the identification and characterization of diverse phages, yet no approaches currently exist for identifying if a prophage has activated. Here, we present PropagAtE (Prophage Activity Estimator), an automated software tool for estimating if a prophage is in the lytic or lysogenic stage of infection. PropagAtE uses statistical analyses of prophage-to-host read coverage ratios to decipher actively replicating prophages, irrespective of whether prophages were induced or spontaneously activated. We demonstrate that PropagAtE is fast, accurate, and sensitive, regardless of sequencing depth. Application of PropagAtE to prophages from 348 complex metagenomes from human gut, murine gut, and soil environments identified distinct spatial and temporal prophage activation signatures, with the highest proportion of active prophages in murine gut samples. In infants treated with antibiotics or infants without treatment, we identified active prophage populations correlated with specific treatment groups. Within time series samples from the human gut, 11 prophage populations, some encoding the sulfur metabolism gene cysH or a rhuM-like virulence factor, were consistently present over time but not active. Overall, PropagAtE will facilitate accurate representations of viruses in microbiomes by associating prophages with their active roles in shaping microbial communities in nature.

IMPORTANCE Viruses that infect bacteria are key components of microbiomes and ecosystems. They can kill and manipulate microorganisms, drive planetary-scale processes and biogeochemical cycling, and influence the structures of entire food networks. Prophages are viruses that can exist in a dormant state within the genome of their host (lysogenic stage) before activating in order to replicate and kill the host (lytic stage). Recent advances have allowed for the identification of diverse viruses in nature, but no approaches exist for characterizing prophages and their stages of infection (prophage activity). We develop and benchmark an automated approach, PropagAtE, to identify the stages of infection of prophages from genomic data. We provide evidence that active prophages vary in identity and abundance across multiple environments and scales. Our approach will enable accurate and unbiased analyses of viruses in microbiomes and ecosystems.

Down in the pond: Isolation and characterization of a new Serratia marcescens strain (LVF3) from the surface water near frog's lettuce (Groenlandia densa)

Serratia marcescens is a species that belongs to the family of Yersiniaceae. This family comprises taxa representing opportunistic human- and phytopathogens but also plant growth-promoting rhizobacteria (PGPR). This study describes a novel Gram-negative strain (LVF3R) of the species Serratia marcescens. The strain was characterized genomically, morphologically, and physiologically. In addition, the potential of the isolate to act as a host strain to assess the diversity of Serratia associated phages in environmental samples was explored. Average nucleotide identity analysis revealed that LVF3R belongs to the species Serratia marcescens. In silico analysis and ProphageSeq data resulted in the identification of one prophage, which is capable of viral particle formation. Electron microscopy showed cells of a rod-shaped, flagellated morphotype. The cells revealed a length and width of 1-1.6 μm and 0.8 μm, respectively. LVF3R showed optimal growth at 30 C and in the presence of up to 2% (w/v) NaCl. It exhibited resistances to ampicillin, erythromycin, oxacillin, oxytetracycline, rifampicin, tetracycline, and vancomycin. Genome data indicate that strain S. marcescens LVF3R is a potential PGPR strain. It harbors genes coding for indole acetic acid (IAA) biosynthesis, siderophore production, plant polymer degradation enzymes, acetoin synthesis, flagellar proteins, type IV secretion system, chemotaxis, phosphorous solubilization, and biofilm formation.

Chronic release of tailless phage particles from Lactococcus lactis

Lactococcus lactis strains residing in the microbial community of a complex dairy starter culture named “Ur” are hosts to prophages belonging to the family Siphoviridae. L. lactis strains (TIFN1 to TIFN7) showed detectable spontaneous phage production and release (10⁹ to 10¹⁰ phage particles/ml) and up to 10-fold increases upon prophage induction, while in both cases we observed no obvious cell lysis typically described for the lytic life cycle of Siphoviridae phages. Intrigued by this phenomenon, we investigated the host-phage interaction using strain TIFN1 (harboring prophage proPhi1) as a representative. We confirmed that during the massive phage release, all bacterial cells remain viable. Further, by monitoring phage replication in vivo, using a green fluorescence protein reporter combined with flow cytometry, we demonstrated that the majority of the bacterial population (over 80%) is actively producing phage particles when induced with mitomycin C. The released tailless phage particles were found to be engulfed in lipid membranes, as evidenced by electron microscopy and lipid staining combined with chemical lipid analysis. Based on the collective observations, we propose a model of phage-host interaction in L. lactis TIFN1 where the phage particles are engulfed in membranes upon release, thereby leaving the producing host intact. Moreover, we discuss possible mechanisms of chronic, or nonlytic, release of LAB Siphoviridae phages and its impact on the bacterial host.

IMPORTANCE In complex microbial consortia such as fermentation starters, bacteriophages can alter the dynamics and diversity of microbial communities. Bacteriophages infecting Lactococcus lactis are mostly studied for their detrimental impact on industrial dairy fermentation processes. In this study, we describe a novel form of phage-bacterium interaction in an L. lactis strain isolated from a complex dairy starter culture: when the prophages harbored in the L. lactis genome are activated, the phage particles are engulfed in lipid membranes upon release, leaving the producing host intact. Findings from this study provide additional insights into the diverse manners of phage-bacterium interactions and coevolution, which are essential for understanding the population dynamics in complex microbial communities like fermentation starters.

Prophage Tracer: precisely tracing prophages in prokaryotic genomes using overlapping split-read alignment

The life cycle of temperate phages includes a lysogenic cycle stage when the phage integrates into the host genome and becomes a prophage. However, the identification of prophages that are highly divergent from known phages remains challenging. In this study, by taking advantage of the lysis-lysogeny switch of temperate phages, we designed Prophage Tracer, a tool for recognizing active prophages in prokaryotic genomes using short-read sequencing data, independent of phage gene similarity searching. Prophage Tracer uses the criterion of overlapping split-read alignment to recognize discriminative reads that contain bacterial (attB) and phage (attP) att sites representing prophage excision signals. Performance testing showed that Prophage Tracer could predict known prophages with precise boundaries, as well as novel prophages. Two novel prophages, dsDNA and ssDNA, encoding highly divergent major capsid proteins, were identified in coral-associated bacteria. Prophage Tracer is a reliable data mining tool for the identification of novel temperate phages and mobile genetic elements. The code for the Prophage Tracer is publicly available at https://github.com/WangLab-SCSIO/Prophage_Tracer.

Genomic insights into methicillin-resistant Staphylococcus pseudintermedius isolates from dogs and humans of the same sequence types reveals diversity in prophages and pathogenicity islands

Methicillin-resistant Staphylococcus pseudintermedius (MRSP) is an important opportunistic pathogenic bacterium of dogs that also occasionally colonize and infect humans. However, whether MRSP can adapt to human hosts is not clear and whole genome sequences of MRSP from humans are still limited. Genomic comparative analyses of 3 couples of isolates from dogs (n = 3) and humans (n = 3) belonging to ST45, ST112, and ST181, the dominant clones in Thailand were conducted to determine the degree of similarities between human and animal MRSP of a same ST. Among eight prophages, three prophages associated with the leucocidins genes (lukF/S-I), φVB88-Pro1, φVB16-Pro1 and φAP20-Pro1, were distributed in the human MRSPs, while their remnants, φAH18-Pro1, were located in the dog MRSPs. A novel composite pathogenicity island, named SpPI-181, containing two integrase genes was identified in the ST181 isolates. The distribution of the integrase genes of the eight prophages and SpPI-181 was also analysed by PCR in 77 additional MRSP isolates belonging to different STs. The PCR screen revealed diversity in prophage carriage, especially in ST45 isolates. Prophage φAK9-Pro1 was only observed in ST112 isolates from dogs and SpPI-181 was found associated with ST181 clonal lineage. Among the 3 couple of isolates, ST45 strains showed the highest number of single nucleotide polymorphisms (SNP) in their core genomes (3,612 SNPs). The genomic diversity of ST45 isolates suggested a high level of adaptation that may lead to different host colonization of successful clones. This finding provided data on the genomic differences of MRSP associated with colonization and adaption to different hosts.

Putative plasmid prophages of Bacillus cereus sensu lato may hold the key to undiscovered phage diversity

Bacteriophages are bacterial viruses and the most abundant biological entities on Earth. Temperate bacteriophages can form prophages stably maintained in the host population: they either integrate into the host genome or replicate as plasmids in the host cytoplasm. As shown, tailed temperate bacteriophages may form circular plasmid prophages in many bacterial species of the taxa Firmicutes, Gammaproteobacteria and Spirochaetes. The actual number of such prophages is thought to be underestimated for two main reasons: first, in bacterial whole genome-sequencing assemblies, they are difficult to distinguish from actual plasmids; second, there is an absence of experimental studies which are vital to confirm their existence. In Firmicutes, such prophages appear to be especially numerous. In the present study, we identified 23 genomes from species of the Bacillus cereus group that were deposited in GenBank as plasmids and may belong to plasmid prophages with little or no homology to known viruses. We consider these putative prophages worth experimental assays since it will broaden our knowledge of phage diversity and suggest that more attention be paid to such molecules in all bacterial sequencing projects as this will help in identifying previously unknown phages.

High throughput sequencing provides exact genomic locations of inducible prophages and accurate phage-to-host ratios in gut microbial strains

BACKGROUND

Temperate phages influence the density, diversity and function of bacterial populations. Historically, they have been described as carriers of toxins. More recently, they have also been recognised as direct modulators of the gut microbiome, and indirectly of host health and disease. Despite recent advances in studying prophages using non-targeted sequencing approaches, methodological challenges in identifying inducible prophages in bacterial genomes and quantifying their activity have limited our understanding of prophage-host interactions.

RESULTS

We present methods for using high-throughput sequencing data to locate inducible prophages, including those previously undiscovered, to quantify prophage activity and to investigate their replication. We first used the well-established Salmonella enterica serovar Typhimurium/p22 system to validate our methods for (i) quantifying phage-to-host ratios and (ii) accurately locating inducible prophages in the reference genome based on phage-to-host ratio differences and read alignment alterations between induced and non-induced prophages. Investigating prophages in bacterial strains from a murine gut model microbiota known as Oligo-MM¹² or sDMDMm2, we located five novel inducible prophages in three strains, quantified their activity and showed signatures of lateral transduction potential for two of them. Furthermore, we show that the methods were also applicable to metagenomes of induced faecal samples from Oligo-MM¹² mice, including for strains with a relative abundance below 1%, illustrating its potential for the discovery of inducible prophages also in more complex metagenomes. Finally, we show that predictions of prophage locations in reference genomes of the strains we studied were variable and inconsistent for four bioinformatic tools we tested, which highlights the importance of their experimental validation.

CONCLUSIONS

This study demonstrates that the integration of experimental induction and bioinformatic analysis presented here is a powerful approach to accurately locate inducible prophages using high-throughput sequencing data and to quantify their activity. The ability to generate such quantitative information will be critical in helping us to gain better insights into the factors that determine phage activity and how prophage-bacteria interactions influence our microbiome and impact human health. Video abstract.

Closely Related Vibrio alginolyticus Strains Encode an Identical Repertoire of Caudovirales-Like Regions and Filamentous Phages

Many filamentous vibriophages encode virulence genes that lead to the emergence of pathogenic bacteria. Most genomes of filamentous vibriophages characterized up until today were isolated from human pathogens. Despite genome-based predictions that environmental Vibrios also contain filamentous phages that contribute to bacterial virulence, empirical evidence is scarce. This study aimed to characterize the bacteriophages of a marine pathogen, Vibrio alginolyticus (Kiel-alginolyticus ecotype) and to determine their role in bacterial virulence. To do so, we sequenced the phage-containing supernatant of eight different V. alginolyticus strains, characterized the phages therein and performed infection experiments on juvenile pipefish to assess their contribution to bacterial virulence. We were able to identify two actively replicating filamentous phages. Unique to this study was that all eight bacteria of the Kiel-alginolyticus ecotype have identical bacteriophages, supporting our previously established theory of a clonal expansion of the Kiel-alginolyticus ecotype. We further found that in one of the two filamentous phages, two phage-morphogenesis proteins (Zot and Ace) share high sequence similarity with putative toxins encoded on the Vibrio cholerae phage CTXΦ. The coverage of this filamentous phage correlated positively with virulence (measured in controlled infection experiments on the eukaryotic host), suggesting that this phage contributes to bacterial virulence.

Prophage Hunter: an integrative hunting tool for active prophages

Identifying active prophages is critical for studying coevolution of phage and bacteria, investigating phage physiology and biochemistry, and engineering designer phages for diverse applications. We present Prophage Hunter, a tool aimed at hunting for active prophages from whole genome assembly of bacteria. Combining sequence similarity-based matching and genetic features-based machine learning classification, we developed a novel scoring system that exhibits higher accuracy than current tools in predicting active prophages on the validation datasets. The option of skipping similarity matching is also available so that there's higher chance for novel phages to be discovered. Prophage Hunter provides a one-stop web service to extract prophage genomes from bacterial genomes, evaluate the activity of the prophages, identify phylogenetically related phages, and annotate the function of phage proteins. Prophage Hunter is freely available at https://pro-hunter.bgi.com/.

Stability of potential prophages in commercial strain Lactobacillus plantarum NCU116 under various stressors

Genetic stability of bacterium as a starter culture is vital for product quality in fermentation industry. The commercial strain Lactobacillus plantarum NCU116 widely used in fruit and vegetable fermentation was induced with various stressors to investigate the stability of potential prophages. PHAge Search Tool (PHAST) identified three potential prophages in bacterial genome. By spectrophotometric analysis, mitomycin C (MMC), lactic acid, and bile salt were found to inhibit the growth of L. plantarum NCU116 while ethanol and hydrogen peroxide had no notable impacts. Transcriptions of four phage-synthesizing genes (phaR, phacap, phaada, phatail) and four phage-resistant genes (cas116, helR, hsd1, hsd2) under stressors were investigated by quantitative reverse transcription PCR. MMC was found to most significantly upregulated transcriptions of phage-synthesizing genes, followed by lactic acid and bile salt. By transmission electron microscopy, no virus particles from the lysates of strain NCU116 treated by MMC were observed, corresponding to the result that no phage nucleic acids could be extracted from the supernatants of strain NCU116 treated by MMC. This study suggested that no prophages could be induced from L. plantarum NCU116 by strong inducer MMC, indicating its genetic stability, which supports the comprehensive application of strain NCU116 in industry without causing fermentation failure.

Classifying the Unclassified: A Phage Classification Method

This work reports the method ClassiPhage to classify phage genomes using sequence derived taxonomic features. ClassiPhage uses a set of phage specific Hidden Markov Models (HMMs) generated from clusters of related proteins. The method was validated on all publicly available genomes of phages that are known to infect Vibrionaceae. The phages belong to the well-described phage families of Myoviridae, Podoviridae, Siphoviridae, and Inoviridae. The achieved classification is consistent with the assignments of the International Committee on Taxonomy of Viruses (ICTV), all tested phages were assigned to the corresponding group of the ICTV-database. In addition, 44 out of 58 genomes of Vibrio phages not yet classified could be assigned to a phage family. The remaining 14 genomes may represent phages of new families or subfamilies. Comparative genomics indicates that the ability of the approach to identify and classify phages is correlated to the conserved genomic organization. ClassiPhage classifies phages exclusively based on genome sequence data and can be applied on distinct phage genomes as well as on prophage regions within host genomes. Possible applications include (a) classifying phages from assembled metagenomes; and (b) the identification and classification of integrated prophages and the splitting of phage families into subfamilies.

Fighting Fire with Fire: Phage Potential for the Treatment of E. coli O157 Infection

Hemolytic⁻uremic syndrome is a life-threating disease most often associated with Shiga toxin-producing microorganisms like Escherichia coli (STEC), including E. coli O157:H7. Shiga toxin is encoded by resident prophages present within this bacterium, and both its production and release depend on the induction of Shiga toxin-encoding prophages. Consequently, treatment of STEC infections tend to be largely supportive rather than antibacterial, in part due to concerns about exacerbating such prophage induction. Here we explore STEC O157:H7 prophage induction in vitro as it pertains to phage therapy-the application of bacteriophages as antibacterial agents to treat bacterial infections-to curtail prophage induction events, while also reducing STEC O157:H7 presence. We observed that cultures treated with strictly lytic phages, despite being lysed, produce substantially fewer Shiga toxin-encoding temperate-phage virions than untreated STEC controls. We therefore suggest that phage therapy could have utility as a prophylactic treatment of individuals suspected of having been recently exposed to STEC, especially if prophage induction and by extension Shiga toxin production is not exacerbated.

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

We present the recently isolated virus vB_BthP-Goe4 infecting Bacillus thuringiensis HD1. Morphological investigation via transmission electron microscopy revealed key characteristics of the genus Phi29virus, but with an elongated head resulting in larger virion particles of approximately 50 nm width and 120 nm height. Genome sequencing and analysis resulted in a linear phage chromosome of approximately 26 kb, harbouring 40 protein-encoding genes and a packaging RNA. Sequence comparison confirmed the relation to the Phi29virus genus and genomes of other related strains. A global average nucleotide identity analysis of all identified φ29-like viruses revealed the formation of several new groups previously not observed. The largest group includes Goe4 and may significantly expand the genus Phi29virus (Salasvirus) or the Picovirinae subfamily.

Characterization of Bacillus Subtilis Viruses vB_BsuM-Goe2 and vB_BsuM-Goe3

The Spounavirinae viruses are ubiquitous in nature and have an obligatory virulent lifestyle. They infect Firmicutes, a bacterial phylum containing an array of environmental non-pathogenic and pathogenic organisms. To expand the knowledge of this viral subfamily, new strains were isolated and investigated in this study. Here we present two new viruses, vB_BsuM-Goe2 and vB_BsuM-Goe3, isolated from raw sewage and infecting Bacillus species. Both were morphologically classified via transmission electron microscopy (TEM) as members of the Spounavirinae subfamily belonging to the Myoviridae family. Genomic sequencing and analyses allowed further affiliation of vB_BsuM-Goe2 to the SPO1-like virus group and vB_BsuM-Goe3 to the Bastille-like virus group. Experimentally determined adsorption constant, latency period, burst size and host range for both viruses revealed different survival strategies. Thus vB_BsuM-Goe2 seemed to rely on fewer host species compared to vB_BsuM-Goe3, but efficiently recruits those. Stability tests pointed out that both viruses are best preserved in LB-medium or TMK-buffer at 4 or 21 °C, whereas cryopreservation strongly reduced viability.

Phage vB_BsuP-Goe1: the smallest identified lytic phage of Bacillus subtilis

Currently, interest in phage science is on the rise again as this subject remains largely unexplored, and its potential diversity mainly untapped. Here, we present phage vB_BsuP-Goe1, a new isolate that infects Bacillus subtilis The phage forms round plaques with a matt outline on agar plates containing a B. subtilis Δ6 lawn. Transmission electron microscopy and genomic analyses revealed that phage vB_BsuP-Goe1 belongs to the Podoviridae family as a new member of the Phi29likevirus genus. The phage genome comprises 18 379 bp that encode for one RNA gene and for 24 open reading frames. Phage vB_BsuP-Goe1 clusters with phage B103 and Nf within the Phi29likevirus genus and is currently the smallest member of this genus. Genome comparison with members of the Phi29likevirus genus reveals a conserved gene set coding for essential replication and morphogenesis functions while the genome extremities are more prone to gene and genome organisation variability.