The team isolated and characterized three Caudoviricetes cyanophages, named A-Lf14, A-Alj1, and A-Hlh1, each of which specifically infects the model nitrogen-fixing cyanobacterium Anabaena sp. PCC 7120. This filamentous strain has long served as a laboratory workhorse for studying cyanobacterial cell differentiation, because it forms specialized nitrogen-fixing cells called heterocysts along its chains of photosynthetic vegetative cells. Choosing such a multicellular host meant the researchers could ask questions that cannot be addressed in single-celled systems: does a phage sweep uniformly along a filament, or do neighboring cells progress through infection at different rates? The answer, obtained through super-resolution microscopy, turned out to be strikingly heterogeneous.

Using advanced imaging to localize phage DNA inside Anabaena filaments, the researchers observed that adjacent cells within the same filament displayed markedly different infection outcomes. Some cells harbored abundant viral DNA and progressed toward lysis, while their immediate neighbors remained largely unaffected or showed only early signs of infection. Cryo-scanning electron microscopy captured successive stages of phage-induced cell destruction, from surface alterations to complete collapse of cellular architecture. Together, these observations demonstrate that infection by these cyanophages proceeds asynchronously at the single-cell level, a finding with implications for how viral lysis propagates through the multicellular filaments that form the structural backbone of freshwater and wetland microbial communities.

The genomic analysis uncovered an equally surprising structural feature. Comparative genomics revealed a conserved invertible region of roughly 30 kilobases in the phage genomes, flanked by inverted repeats. Such invertible segments, which can flip their orientation through site-specific recombination, create two distinct genomic isomers within a single viral population. Although genomic inversion is well documented in some bacteriophages, precedent among cyanophages was limited. The team validated the coexistence of the two isomeric configurations using three independent approaches: diagnostic polymerase chain reaction, Sanger sequencing, and mapping of high-throughput sequencing reads onto the reference genomes. Both orientations were stably maintained in the phage populations, indicating that genomic isomerism represents a genuine and recurring source of structural plasticity in freshwater cyanophages rather than a sequencing artifact.

Beyond genome architecture, the study highlighted how auxiliary genes can tie phage infection strategies to the nutritional conditions of the environment. Unlike the previously characterized cyanophage A-1(L), which also infects Anabaena sp. PCC 7120, the three new isolates encode a non-cyanobacterial alkaline phosphatase of the PhoD family. Alkaline phosphatases liberate inorganic phosphate from organic phosphorus compounds, and the gene in question most likely arrived in the phage genome through horizontal transfer from an unrelated organism. During infection, expression of phoD was strongly upregulated, suggesting that the virus actively boosts the host cell’s capacity to scavenge phosphorus, a frequently limiting nutrient in freshwater systems. In effect, the phage appears to reprogram its host’s nutrient acquisition machinery to support viral replication, a mechanism that could also accelerate phosphorus turnover in natural microbial communities.

The comparison with A-1(L) also revealed what the new phages lack. A-1(L) carries an early-expressed tnpB endonuclease gene, whereas the three novel isolates do not. Instead, infection by the new phages modestly induced the transcription of five identical tnpB copies encoded in the host genome. In A-1(L) infection, by contrast, transcription of the phage-encoded tnpB was strongly upregulated from both strands of the gene, designated tnpB_fwd and tnpB_rev. TnpB is of particular interest to the broader life sciences because it is a miniature relative of Cas12, the effector protein of CRISPR-Cas12 systems, and has become a foundation for emerging genome-editing tools. Understanding how tnpB behaves during phage infection therefore carries relevance well beyond cyanobacterial ecology.

Perhaps the most unusual discovery concerns the architecture of that tnpB locus. In a highly atypical genetic arrangement, both strands of the A-1(L) tnpB gene are protein-coding over their entire length. The reverse strand, tnpB_rev, encodes a protein of 347 amino acid residues. Overlapping genes on opposite strands are known in compact viral genomes, but complete bidirectional coding across the full length of a gene of this size is rare. The finding demonstrates that a phage gene can be protein-coding on both strands simultaneously, expanding the known repertoire of genome organization strategies in viruses and raising questions about how such an arrangement evolved and is maintained without deleterious mutations accumulating on either reading frame.

The researchers also searched the phage genomes for anti-CRISPR genes, proteins that phages deploy to neutralize host immune defenses, with algorithmic support from collaborators, and compiled extensive catalogs of functional open reading frames, gene clusters, and CRISPR arrays detected across the cyanophages included in the analysis. Environmental sampling information and physicochemical parameters of the freshwater sites used for phage isolation were documented, anchoring the genomic findings in the ecological contexts from which the viruses were recovered. Phylogenetic placement of the new isolates among known cyanophages situates them within the diversity of Caudoviricetes, the broad class of tailed double-stranded DNA phages that dominates cultured bacteriophage diversity.

Methodologically, the study illustrates the value of combining complementary techniques. Super-resolution microscopy and electron microscopy provided spatial and ultrastructural evidence of asynchronous infection, while comparative genomics, targeted sequencing, and read-mapping converged on the reality of genomic isomerism. Transcriptomic measurements linked individual auxiliary genes to infection biology, showing not merely that phages carry metabolic genes but that those genes are actively and specifically expressed during the infection cycle. This integration of scales, from nucleotide-level genome structure to the behavior of entire multicellular filaments, offers a template for future work on phage-host interactions in other filamentous microbes.

Ecologically, the findings carry weight for understanding nutrient cycling in freshwater, wetland, and agricultural ecosystems, where nitrogen-fixing cyanobacteria serve as primary producers and their viral lysis releases fixed nitrogen and phosphorus back into the food web. The discovery that cyanophages can modulate phosphorus acquisition through phoD suggests that viruses may directly influence how limiting nutrients flow through these environments. The demonstration of stable genomic isomerism adds a layer of structural variability that could affect how phage genomes are annotated, compared, and tracked in metagenomic surveys, since a single viral population may harbor two sequence configurations. As more cyanophages infecting nitrogen-fixing cyanobacteria are characterized, the mechanisms uncovered here, from invertible DNA to dual-strand coding, are likely to prove part of a broader strategy by which these viruses diversify their genomes and adapt to the nutritional and defensive landscapes of their hosts.

Subject of Research: Comparative genomics and single-cell imaging of cyanophages infecting the nitrogen-fixing cyanobacterium Anabaena sp. PCC 7120

Article Title: Comparative genomics and super-resolution microscopy of novel cyanophages reveals genomic isomerism, auxiliary genes, and a non-synchronous infection process

Article References: Shi, Y., Ziemann, M., Zhao, Y., Reimann, V., Zhu, T., Hess, W. R., & Lu, X. (2026). Comparative genomics and super-resolution microscopy of novel cyanophages reveals genomic isomerism, auxiliary genes, and a non-synchronous infection process. Microbiome. https://doi.org/10.1186/s40168-026-02551-1

Image Credits: AI Generated

DOI: 10.1186/s40168-026-02551-1

Keywords: cyanophages, Anabaena sp. PCC 7120, genomic isomerism, phoD, tnpB, super-resolution microscopy, auxiliary metabolic genes, Caudoviricetes, nitrogen fixation, phosphorus acquisition, Microbiome, comparative genomics

The research team collected water samples from three sites along the Haridwar stretch of the river, all chosen for their high levels of human activity: the Daksheshwar Mahadev temple, the famous bathing ghat of Har ki Pauri, and Chandi Devi Ghat along the Eastern Ganga canal road. Samples from all three locations were gathered on the same day, transported at 4 degrees Celsius in sterile centrifuge tubes, and pooled in equal proportions before being stored at minus 20 degrees Celsius. From this pooled sample, the researchers extracted both metagenomic DNA and RNA using commercial kits, verifying quality by gel electrophoresis and spectrophotometry before constructing sequencing libraries.

The laboratory workflow followed established metagenomic protocols. DNA and RNA-derived cDNA were fragmented into roughly 600 base-pair segments using the KAPA HyperPlus kit, end-repaired, A-tailed, and ligated to sequencing adapters. After library amplification with Illumina primers and purification with Ampure beads, the libraries were quantified with a Qubit high-sensitivity assay and sequenced on an Illumina HiSeq 4000 platform. The run produced 22.4 million paired-end sequences of 151 base pairs each, with a GC content of 59 percent and low duplication rates of 4.9 percent for forward reads and 2.5 percent for reverse reads, indicating a robust and technically consistent sequencing depth suitable for downstream analysis.

Bioinformatic processing began with quality assessment using FastQC, followed by trimming and filtering with fastp and Trim Galore. Taxonomic classification was then performed with Kraken 2, a k-mer-based classifier that matches short sequence fragments against a reference database of complete bacterial, archaeal and viral genomes from NCBI RefSeq. Low-abundance taxa represented by fewer than ten reads or less than 0.01 percent of classified reads were excluded to remove likely false positives. Phylogenetic relationships among the identified phages were reconstructed using ETE 3 for visualization and VICTOR, the genome-based virus classification service of the Leibniz Institute DSMZ. The raw sequence data have been deposited in the NCBI Sequence Read Archive under BioProject accession PRJNA1207791.

The results revealed a strikingly diverse viral community. In total, the sample contained 6,543 distinct viral species, from which 549,515 sequence reads were obtained. Seventeen viral phyla were identified, dominated overwhelmingly by Uroviricota, the phylum that encompasses tailed bacteriophages, with 3,575 hits. Negarnaviricota and Pisuviricota followed with 474 and 460 reads respectively, while Artverviricota and Kitrinoviricota contributed 361 and 338. At the class level, Caudoviricetes, the tailed phages, stood out dramatically with 3,575 bacteriophages, far ahead of Revtraviricetes at 360 and Pisoniviricetes at 287. Among classified orders, Crassvirales led with 867 hits and Kirjokansivirales followed with 795, but the single largest category was the unclassified group, containing 1,488 reads, a clear sign that a substantial fraction of the river’s virome remains scientifically undescribed.

At the family level, one group dominated the dataset to a remarkable degree. Casjensviridae accounted for 516,163 generated sequences, dwarfing every other phage family; Rountreeviridae followed with 10,094 sequences, then Peduoviridae with 5,734 and Autographiviridae with 2,614. Kyanoviridae, Herelleviridae, Schitoviridae, Demerecviridae, Mesyanzhinovviridae and Crevaviridae completed the top ten. Among genera, the unclassified category again led with 300,671 sequences, followed by Chivirus with 205,535 and Copernicusvirus with 8,607. At the species level, the Salmonella phage Chivirus iEPS5 was the most abundant organism in the entire sample with 300,245 reads, followed by the Vibrio phage Valbvirus ValB1MD2 at 124,213 reads and the Staphylococcus phage Andhravirus andhra at 68,758. Abundance fell steeply beyond these top few, indicating a strongly skewed phage population structure.

Many of the dominant phages have well-characterized host ranges documented in earlier studies, and their presence in the Ganga carries ecological significance. Chivirus iEPS5 is a flagellatropic Salmonella phage that depends on the counterclockwise-rotating flagellar filament of Salmonella enterica serovar Typhimurium for adsorption and injects its DNA through the filament during infection. ValB1MD2 kills Vibrio alginolyticus, an emerging foodborne pathogen linked to contaminated seafood. Andhravirus andhra, originally classified among the podoviruses, infects Staphylococcus epidermidis, and its proteins Andhra_gp10 and Andhra_gp14 have been shown to inhibit bacterial growth and disintegrate staphylococcal cell walls. Rosenblumvirus CSA13, another Staphylococcus phage, can prevent biofilm formation across multiple environments, marking it as a candidate biocontrol agent.

The phylogenetic analysis added further texture to the picture. Five Pandoravirus species appeared among the 25 most prevalent viral species, making it the most species-rich genus in that set. Chivirus iEPS5, Zhonglingvirus SAP012, Andhravirus andhra and Rosenblumvirus CSA13 all belong to the class Caudoviricetes, with the first two sharing the family Casjensviridae and thus a closer taxonomic relationship. Other caudoviricetes including Elvirus EL, Efquatrovirus LY0322, which infects the drug-resistant bacterium Enterococcus faecalis, the Agrobacterium phage Atu_ph07 and the Pseudomonas phage Psa21 formed related clusters. Psa21 has previously been recognized as a potential biocontrol agent against bacterial canker of kiwifruit, while Atu_ph07 specifically infects Agrobacterium tumefaciens, the plant pathogen responsible for crown gall disease.

The study’s context stretches back more than a century. In 1896, the bacteriologist Ernest Hanbury Hankin reported antibacterial activity in the waters of the Ganga and Yamuna, noting that the activity was filterable and heat-labile and limited the spread of cholera, an observation made decades before the formal discovery of bacteriophages. Today, with an estimated 10 to the power of 31 viruses on Earth, phages are understood as key regulators of microbial populations and environmental processes. The authors note that the Ganga’s phages, each with particular host bacterial species, appear to reduce the potency and lethality of their pathogenic hosts, and that phage genes linked to bacterial virulence and antibiotic resistance underscore the ecological role of phages in shaping microbial dynamics and bacterial evolution in the river.

The findings arrive at a moment when phage therapy is regaining momentum as a response to multidrug-resistant bacteria, since lytic phages can selectively destroy infected bacterial cells and multiply within their hosts, advantages that become critical as antibiotic resistance genes spread through aquatic environments. The authors caution that their approach has limitations: Kraken 2 relies on exact k-mer matches and can therefore miss divergent or erroneous sequences, read-level classification offers limited resolution at the species level in complex metagenomes, and read counts as proxies for abundance are biased toward larger viral genomes that generate more mappable fragments. Even so, the work provides a genomic baseline for the phageome of the upper Ganga, a system never analyzed in this way before, and points toward phage-driven strategies for environmental monitoring, pollution management and the eventual development of bacteriophage-based therapeutic interventions rooted in the river’s own viral diversity.

Subject of Research: Bacteriophage diversity in the Ganga River ecosystem revealed by metagenomic sequencing

Article Title: Analysis of bacteriophage diversity in Ganga river ecosystem: insights from metagenome-based sequencing

Article References: Katara, A., Chand, S., Chaudhry, V., Vishwakarma, S., Dubey, R. C., Chandra, H., Shilbayeh, S. A. R., Khan, S.-U.-D., Vohra, S., Poddar, N. K., & Khan, S. (2026). Analysis of bacteriophage diversity in Ganga river ecosystem: insights from metagenome-based sequencing. International Microbiology. https://doi.org/10.1007/s10123-026-00901-5

Image Credits: AI Generated

DOI: 10.1007/s10123-026-00901-5

Keywords: bacteriophages, metagenomics, Ganga River, virome, Caudoviricetes, Casjensviridae, water microbiology, phage therapy, antibiotic resistance, viral ecology, freshwater ecosystem, Haridwar

The discovery began with mitomycin C induction, a standard technique for coaxing temperate phages out of the bacterial hosts they quietly inhabit. Temperate phages, unlike their purely lytic relatives, can integrate into a host cell’s genome and replicate along with it as a prophage, only switching to the lytic cycle under certain conditions. By treating seawater samples with mitomycin C, the team triggered prophages harbored within marine bacteria to enter their lytic programs and release viral particles, allowing Lyfv1 to be isolated and propagated in the laboratory from its host, S. equorum. That host choice itself is notable, because the ecological roles of S. equorum in marine environments, from open seawater to sediments and marine organisms, remain largely unexplored.

Under the transmission electron microscope, Lyfv1 displayed the classic architecture of a siphovirus: an icosahedral head measuring approximately 56 nanometers in diameter, capped by a long, non-contractile tail stretching roughly 300 nanometers in length. This morphology, long and flexible tail attached to a geometric protein shell, is one of the most common body plans among tailed phages and is well suited to recognizing and injecting DNA into Gram-positive hosts such as Staphylococcus. Morphology alone, however, tells only part of the story, and the researchers turned to the virus’s genome for a more definitive picture of its evolutionary position.

Sequencing revealed a double-stranded DNA genome of 41,602 base pairs encoding 62 genes. Among these were the regulatory elements that govern the lysogeny-lysis switch, the molecular decision point that determines whether an infected cell harbors the phage as a dormant prophage or is commandeered to produce new viral particles and burst. The presence of these lysogeny and lysis control genes confirmed Lyfv1’s temperate lifestyle, consistent with the mitomycin C method used to recover it. Temperate phages are ecologically important in the ocean because they can persist within host populations for extended periods, transferring genes between bacterial lineages and shaping the genetic makeup of microbial communities over time.

One gene in particular caught the researchers’ attention: a putative auxiliary metabolic gene, or AMG, encoding a MazG-like family protein. AMGs are phage-borne genes that are not required for viral replication per se but instead modulate host metabolism during infection, and they are considered key levers by which phages influence biogeochemical cycling in marine systems. MazG-like proteins are nucleoside triphosphate pyrophosphohydrolases known in other contexts to participate in stress responses and nucleotide pool maintenance. The authors suggest that Lyfv1’s MazG-like gene may be involved in reprogramming host metabolism during infection, a hypothesis that, if confirmed experimentally, would add another example of how marine phages carry metabolic toolkits borrowed from, and deployed upon, their bacterial hosts.

The structural machinery of the phage also yielded interesting details. The tail region of Lyfv1 contains both a polysaccharide deacetylase and tail spike proteins, enzymes and binding modules that in many phages help degrade or modify the carbohydrate-rich cell wall surfaces of Gram-positive bacteria. Using AlphaFold structural predictions combined with genomic-context analysis, the team proposed that gene product 19, or gp19, may function as the receptor-binding protein, the component that physically recognizes the host cell surface, while gp18 may act as a polysaccharide-modifying tail-associated protein. Together, these two proteins likely participate in host recognition and the earliest steps of phage-host interaction, determining which bacterial strains the virus can successfully infect.

Establishing that a virus represents a new genus requires more than an unusual appearance, and the researchers assembled a multi-pronged case. Protein-sharing network analysis, which clusters viruses according to the proteins they hold in common, placed Lyfv1 on a distinct lineage among known Staphylococcus phages. Phylogenetic analyses of hallmark genes pointed in the same direction. Quantitative measures reinforced the picture: intergenomic similarity, average amino acid identity, orthologous fraction, and whole-genome comparisons all showed that Lyfv1 shares less than 26 percent genomic similarity with any other characterized phage, a level of divergence consistent with genus-level novelty within the Caudoviricetes. Genome-BLAST Distance Phylogeny, a method that converts whole-genome distances into phylogenetic trees, further supported this placement.

Perhaps the most striking finding came from ecological rather than laboratory data. By searching for Lyfv1-related sequences across oceanic datasets, the researchers found that the phage has a wider ecological distribution than other Staphylococcus phages, with a particularly notable presence in polar zones. The team categorized its occurrence across viral ecological zones spanning the epipelagic, mesopelagic, and bathypelagic layers of the water column, as well as Arctic and Antarctic regions. This broad distribution, from sunlit surface waters to the deep ocean and into the frigid seas at both poles, suggests that Lyfv1 and its relatives are not rare curiosities but established members of the global marine virome, capable of persisting under dramatically different temperature, pressure, and nutrient regimes.

The study also carries implications for how scientists understand the genus Staphylococcus itself. Although often thought of as a human-associated pathogen group, Staphylococcus species are widely distributed across seawater, sediments, and marine organisms, where they contribute to biogeochemical cycling. Phages such as Lyfv1 that infect these bacteria are therefore not merely clinical curiosities; they are participants in ocean food webs and nutrient transformations. Temperate phages in particular can act as reservoirs of genetic diversity, and the carriage of an AMG like the MazG-like gene hints that Lyfv1 may influence the metabolic activities of its hosts in ways that ripple through microbial communities.

For the field of marine virology, Lyfv1 underscores a recurring lesson: even among well-studied bacterial host groups, the ocean continues to yield viruses that defy existing classification schemes. The combination of a temperate siphovirus architecture, a compact 62-gene genome, distinctive tail proteins, a putative metabolic gene, and a distribution spanning tropical surface waters to polar depths makes this phage a compelling subject for future work. Confirming the functions of gp18, gp19, and the MazG-like protein experimentally, and mapping the phage’s host range among marine Staphylococcus strains, will be natural next steps. In the meantime, Lyfv1 stands as evidence that the census of tailed phages in the sea remains far from complete, and that new branches of the viral tree of life are still waiting in a few liters of seawater.

Subject of Research: Genomics and ecological distribution of a novel temperate marine Staphylococcus equorum phage representing a new virus genus

Article Title: A novel widely distributed marine Staphylococcus phage representing a new virus genus

Article References: Li, Y., Feng, X., Yu, M., Geslin, C., & Jin, M. (2026). A novel widely distributed marine Staphylococcus phage representing a new virus genus. BMC Genomics. https://doi.org/10.1186/s12864-026-13362-x

Image Credits: AI Generated

DOI: 10.1186/s12864-026-13362-x

Keywords: bacteriophage, Staphylococcus equorum, marine virome, Caudoviricetes, temperate phage, auxiliary metabolic genes, MazG-like protein, tail spike protein, virus taxonomy, polar oceans, mitomycin C induction, phage-host interaction