The central challenge the researchers tackled is one of scale. Traditional phage genetics proceeds gene by gene: a researcher deletes or mutates a candidate gene and observes the consequence. That approach is slow, biased toward genes with predictable functions, and poorly suited to the many phage genes of unknown purpose that dominate most viral genomes. Transposon-insertion sequencing, or Tn-seq, has transformed bacterial functional genomics by creating saturated libraries of mutants and reading out insertion sites with deep sequencing, but adapting the technique to phages has been difficult because phages cannot be transformed with plasmids and their replication cycles are brief and tightly regulated.

The Otago team solved the selection problem with an elegant trick borrowed from the ongoing evolutionary arms race between bacteria and viruses. Their system couples Tn5 transposon mutagenesis to anti-CRISPR-based counter-selection. Phages are first propagated in host bacteria that express the Tn5 transposase together with a transposon carrying an anti-CRISPR gene, in this case AcrVIA1, which inhibits the RNA-targeting nuclease Cas13a. Only phage genomes that have acquired the transposon, and therefore the anti-CRISPR gene, can replicate in a second host strain that expresses Cas13a loaded with spacers targeting the phage. Unmutated phages are destroyed or blocked by the CRISPR defense, while transposon-bearing mutants are selectively enriched. Deep sequencing of the enriched population then reveals, across the whole genome, where insertions survived and where they were eliminated.

The logic of the readout is straightforward: genes that tolerate no transposon insertions after enrichment are essential for phage replication, whereas genes riddled with insertions are dispensable. Applying the method to phage JS26, a T7-like virus of the bacterium Prodigiosinella confusarubida, the researchers generated a genome-wide essentiality map that agreed well with independent lines of evidence, including structural proteomics of purified virions and conservation of core genes across related phages. Genes encoding capsid and tail components, DNA replication machinery, and other core functions showed the expected depletion of insertions, while genes of unknown or auxiliary function were heavily disrupted.

A particularly demanding test was the nucleus-forming jumbo phage PCH45, a relative of the well-studied phiKZ-like viruses of Pseudomonas. These giant phages, with genomes exceeding 200 kilobases, build a nucleus-like proteinaceous compartment inside the infected cell that shields their replicating DNA from CRISPR-Cas nucleases. The team found that standard transposase worked poorly in this context, so they engineered a fusion between the transposase and UvsX, a phage recombination protein that localizes to the viral nucleus. This UvsX-TnpA fusion dramatically improved mutagenesis of PCH45, allowing the researchers to construct the first genome-scale essentiality map for a nucleus-forming jumbo phage and to distinguish essential core genes from the large complement of non-essential genes that characterize these remarkable viruses.

Beyond essentiality, the method extracts information from the biases inherent in transposon insertion. Because Tn5 inserts preferentially into certain sequence contexts and strand orientations, the distribution of insertions carries signals about genome organization. The researchers showed that insertion orientation biases allowed them to predict the direction of transcription of phage genes, and that insertion density correlated with expression levels, enabling identification of regions injected early into the host and genes expressed at high levels during infection. The insertion index across non-essential genes showed a moderate positive correlation with RNA-seq expression measurements, demonstrating that a single mutagenesis experiment can yield both functional and regulatory information.

The transposon itself proved to be a versatile delivery vehicle. By modifying the cargo carried within the transposon, the team generated phages labeled with fluorescent proteins within a few days, a task that conventional phage engineering approaches can take weeks to accomplish. They also created an orthogonal system based on an artificial intelligence-designed anti-CRISPR protein, allowing a second, distinguishable transposon to be delivered into phages that already carried the first. Using this approach they constructed phage double mutants carrying two different transposons, each conferring resistance to a different Cas13a variant, which permitted sequential counter-selection. The strategy worked across several diverse phages, including LC53 and Bas46, infecting different bacterial hosts.

One further technical hurdle was DNA modification. Many phages protect their genomes from host defenses with heavily modified bases, such as hypermodified cytosines, which can block enzymatic steps in genome engineering. The researchers demonstrated that transposon insertion was achievable even in phages with such hypermodified DNA, extending the reach of the method to viruses whose chemical biology has historically resisted manipulation. This breadth matters because modified DNA is common among phages that are attractive candidates for therapeutic development, precisely because modifications help them evade bacterial immune systems.

The implications extend in two directions. For basic science, phage Tn-seq provides an unbiased, high-throughput entry point into the functional genomics of viruses whose genes are largely unannotated, complementing recent parallel efforts such as CRISPRi-based phage screens and other transposon approaches reported for bacteriophages. Essentiality maps anchored by proteomics and conservation data give researchers a rational basis for assigning functions to the growing torrent of phage genome sequences generated by environmental sequencing. For applied science, the ability to insert cargo rapidly and at many genomic positions addresses a persistent bottleneck in phage therapy engineering, where researchers seek to equip therapeutic phages with traits such as reporter genes, altered host ranges, or additional anti-defense functions without laborious homologous recombination protocols.

The authors have made the underlying resources broadly available. Raw sequencing data are deposited in the NCBI Sequence Read Archive under BioProject PRJNA1381746, proteomics datasets are accessible through the PRIDE repository, and the plasmids developed for the system, including the transposon delivery and selection constructs, are deposited at Addgene. As phage-based antimicrobials move closer to clinical and agricultural deployment amid rising antibiotic resistance, tools that compress the design-build-test cycle for viral genomes are likely to become standard equipment in the phage engineering toolbox. Phage Tn-seq, by simultaneously mapping what a phage cannot live without and delivering what engineers want to add, offers both the map and the vehicle in a single experiment.

Subject of Research: Genome-wide essentiality mapping and transposon-based engineering of bacteriophages

Article Title: Defining the essential genome of diverse phages with phage Tn-seq

Article References: Kyte, N., Fuchs, M., Smith, L. M., & Fineran, P. C. (2026). Defining the essential genome of diverse phages with phage Tn-seq. Nature Microbiology. https://doi.org/10.1038/s41564-026-02486-1

Image Credits: AI Generated

DOI: 10.1038/s41564-026-02486-1

Keywords: bacteriophage, phage Tn-seq, transposon mutagenesis, gene essentiality, anti-CRISPR, jumbo phage, phage engineering, functional genomics, phage therapy, Cas13a, DNA modification, biotechnology

The Pseudomonas syringae complex is a phylogenetic group comprising at least fifteen bacterial species and more than sixty host-specialized pathovars that collectively cause disease in over 180 plant species, including virtually all economically relevant crops. In highbush blueberry (Vaccinium corymbosum), infections manifest as blight of leafy tissues or cankers of woody tissues, both of which reduce plant health and productivity. Disease development depends on the transition of the bacterium from an epiphytic phase, in which cells remain adhered to plant surfaces, to an endophytic phase involving colonization of internal structures and the release of tissue-destroying virulence factors. Environmental conditions common in British Columbia—cool temperatures, high humidity and frequent precipitation—favour the growth and motility of epiphytic populations and thereby increase the likelihood of endophytic infection, making strategies that reduce surface populations before infection establishes an attractive line of defence.

The economic stakes are considerable. Global blueberry production reached roughly 2.14 billion kilograms in 2024, and Canada is the fourth-largest producer worldwide, accounting for nearly one-sixth of global output. Canadian blueberry exports were valued at approximately CAD $695 million in 2024, representing more than 65 percent of all Canadian fruit exports. Nearly 94 percent of the country’s highbush blueberry production occurs in British Columbia, where conditions favourable to the crop simultaneously favour bacterial growth and dispersal on plant surfaces. Traditional management has relied on copper-based biocides, but this approach damages the natural environment, can stunt plant growth through interference with the rhizosphere microbiome, and has been losing efficacy due to the spread of plasmid-borne copper resistance genes. Mitigation is often limited to the costly and unsustainable practice of pruning or culling infected plants.

Against this backdrop, the research team isolated bacteriophages from wastewater samples and from Pseudomonas syringae complex-infected blueberry plants collected at sites in the lower mainland and Okanagan valley regions, ultimately obtaining nineteen relatively stable phage isolates. Because phage predation by individual phage species is generally restricted to a subset of target strains and readily selects for resistant subpopulations, cocktail formulations of multiple distinct phages are typically employed to expand host range, improve efficacy and forestall resistance. The researchers used a semiquantitative spotting analysis to score infection interactions between each phage and a panel of twenty bacterial strains on a scale from zero to four, then excluded seven isolates—deemed weak specialists—that fell below median thresholds for both the number and strength of interactions. Twelve candidate phages advanced to genomic and functional characterization.

Whole-genome sequencing revealed extensive phylogenomic diversity among the candidates. Three isolates proved to be variants of a species within the genus Unosvirus, three were clonal duplicates belonging to Pifdecavirus, and a further group of four—three clonal and one diverged—may represent a novel species within an as-yet-undefined genus most closely related to Pseudomonas phage QdP_Psa3. The most striking discovery was phage φCB10, a jumbo phage with a genome of approximately 310 kilobases encoding 476 predicted coding sequences, most of them of unknown function. φCB10 was unrelated to the other lineages, most closely resembled Pseudomonas jumbo phage Psa21 at only 53.2 percent genomic similarity, and may represent the founding member of a novel genus. All twelve candidates were predicted to be obligately lytic, and bioinformatic screening found no lysogeny-associated genes, antimicrobial resistance determinants or metal resistance cassettes—characteristics considered indispensable for safe agricultural biocontrol.

Environmental stability assays revealed considerable heterogeneity among isolates. Exposure to ultraviolet radiation for one hour reduced viable titres of all phages, with the severity depending on wavelength: under UV-A at 365 nanometres, relevant to field conditions, only φCB16 fell below the detection limit, whereas UV-C at 254 nanometres drove the titres of most phages below detection. Freezing at −20 degrees Celsius produced a particularly polarizing split along predicted morphology: the three myovirus-like phages lost roughly 1.6 to 3.4 orders of magnitude in titre within a single day and became undetectable after 30 days, while podovirus-like isolates fared substantially better, with φCB02 and φCB05 showing no significant losses over the full month. This partitioning suggests that virion structural properties may broadly influence environmental persistence, a phenomenon the authors say warrants mechanistic investigation. Notably, some isolates appeared more stable at 22 degrees Celsius than at the conventional 4 degrees Celsius storage temperature, raising questions about the universality of phage storage conventions. Prolonged exposure to highly acidic conditions, potentially relevant given that blueberries thrive in soil at pH 4.0 to 5.5, was among the most consistently destabilizing stresses, although most phages retained functionality over biologically relevant timescales.

To quantify antibacterial activity, the team performed planktonic killing assays against eight virulent bacterial strains, summarizing 64 phage-host pairs with a Growth Reduction Coefficient derived from bacterial growth curves. Individual phage performance varied widely and depended strongly on the strain: the jumbo phage φCB10 achieved the highest median coefficient, 0.943, and was the only phage rated as strong across all targeted hosts, while roughly half of all single-phage interactions were deemed poor. The two most effective phages were variously combined with the four least effective to construct eight cocktails of three to five variants. Although cocktails did not significantly outperform individual phages overall, four formulations—φC_A, φC_B, φC_C and φC_D—achieved median coefficients above 0.75 and were rated as strong across the full host panel, and were carried forward to plant-based testing.

The decisive validation used a detached-leaf model in which young Draper-variety leaves were pretreated with phage cocktails and then inoculated with two highly virulent strains. Bacterial densities were substantially reduced in all pretreated leaves. At early timepoints corresponding to maximal bacterial densities in untreated leaves, the four cocktails induced reductions of 4.5 to 5.5 logs against strain B11 and 3.2 to 5.6 logs against strain B04. At the ten-day endpoint, two cocktails pushed B11 below the detection limit, and two others reduced B04 below detection. A variant of the Growth Reduction Coefficient calculated from bacterial density over time rated all four cocktails as strong against B11, with φC_D also strong against B04. Critically, ex planta performance correlated strongly with in vitro performance, suggesting that planktonic killing assays can serve as a useful proxy for more complex validation models in high-throughput screens.

Measurements of phage densities inside the leaves supported the conclusion that bacterial suppression was driven by ongoing lytic replication rather than a single contact-mediated kill. The mean in situ phage-to-bacterium ratio rose rapidly to a peak of roughly 1.8 × 10⁴ for treatments targeting B11 before decaying by day ten, and endpoint phage-to-bacterium ratios correlated positively with overall treatment efficacy. The authors interpret this as evidence that successful biocontrol depends not only on the killing efficiency of individual phages but also on their capacity to persist within the plant environment and maintain productive infections over extended periods.

The study establishes a framework for the systematic identification and evaluation of candidate phages for agricultural biocontrol, while flagging the questions that remain before field deployment. Detached-leaf assays cannot fully recapitulate whole-plant systems, and the authors emphasize the need to reproduce these results in larger-scale studies with intact plants and, ultimately, under field conditions, alongside assessments of any impacts on plant health or the native phyllosphere microbiome. The mechanisms governing phage susceptibility among bacterial strains also remain poorly understood. Meanwhile, φCB10—with its exceptional antibacterial performance, favourable stability, enormous coding capacity and potential to carry accessory functions capable of circumventing bacterial defence systems such as restriction-modification and CRISPR-Cas—stands out as a particularly compelling subject for future investigation, both as a biocontrol candidate and as a window into the biology of underexplored jumbo phages.

Subject of Research: Isolation and evaluation of bacteriophages for biocontrol of Pseudomonas syringae complex populations on highbush blueberry leaves

Article Title: Bacteriophages Control Epiphytic Pseudomonas syringae Populations in Highbush Blueberry Leaves

Article References: Ball, C., Lauman, P., Xu, T., Guy, T., Dadej, K., Richter, R., Lubberts, M., Cross, K., Ren, M., Latchman, S. R., Burlakoti, R., Deng, X., Fong, K., & Wang, S. (2026). Bacteriophages Control Epiphytic Pseudomonas syringae Populations in Highbush Blueberry Leaves. Microbial Biotechnology, 19(9), Article e70438. https://doi.org/10.1111/1751-7915.70438

Image Credits: AI Generated

DOI: 10.1111/1751-7915.70438

Keywords: bacteriophages, phage biocontrol, Pseudomonas syringae complex, highbush blueberry, Vaccinium corymbosum, plant pathology, epiphytic bacteria, jumbo phage, phage cocktails, agricultural biotechnology, British Columbia, leaf blight