In a groundbreaking study published in npj Viruses, researchers have unveiled two remarkably stable bacteriophages, PIN1 and PIN2, which challenge longstanding assumptions in virology by exhibiting characteristic features of flagellotropic phages—viruses that typically infect motile bacteria via their flagella—while uniquely targeting immotile bacterial hosts. This discovery opens an exciting new chapter in our understanding of phage-host interactions, viral stability, and microbial ecology, pushing the boundaries of how bacteriophages navigate and exploit their bacterial environments.
Bacteriophages, or phages, are viruses that specifically infect bacteria. Traditionally, phages studied in detail fall into a few general categories based on their infection mechanisms and host preferences. One such category, flagellotropic phages, is known for the targeted infection of bacteria possessing flagella—long, whip-like appendages used by many bacteria to move through their environments. These phages leverage the mechanical movement and molecular signatures of flagella to locate and anchor themselves to their hosts before initiating infection. The newly described phages PIN1 and PIN2, however, defy this archetypal behavior by infecting bacteria that lack motility altogether.
The research team led by Jati et al. conducted an extensive characterization of PIN1 and PIN2, analyzing their genetic makeup, structural features, and host ranges. Both phages exhibit hallmark molecular and morphological traits aligned with flagellotropic classification. Electron microscopy images reveal tail fibers and attachment structures finely tuned for interacting with bacterial flagella. Yet, intriguingly, their confirmed bacterial hosts are immotile species devoid of functional flagella, suggesting the presence of alternative infection strategies or binding mechanisms that mimic flagella interactions without requiring actual motile appendages.
This paradoxical finding is significant because it challenges the textbook definition of flagellotropic phages and hints at a more nuanced, flexible evolutionary trajectory. Phages and their bacterial hosts are locked in a perpetual arms race, driving the evolution of highly specialized viral recognition systems and bacterial defense mechanisms. PIN1 and PIN2 appear to represent a new evolutionary intermediate or a distinct adaptation pathway, where phages retain structural motifs associated with flagellar targeting but have developed the ability to circumvent the need for bacterial motility.
Moreover, the stability of these phages under a variety of environmental conditions is exceptional. Experiments in the study demonstrated that PIN1 and PIN2 maintain infectivity across a broad range of temperatures, pH levels, and ionic strengths, far surpassing the robustness typically observed in other phages with similar genome sizes. This robust stability is likely a critical factor enabling their persistence in diverse ecological niches where host motility may be restricted or absent.
The implications of PIN1 and PIN2’s stability extend well beyond fundamental science. Phages have been widely explored as alternatives or supplements to antibiotics in combating bacterial infections, particularly in the face of rising antimicrobial resistance. Stability is a prized trait for therapeutic phages, as it enhances shelf life, efficacy, and delivery options. The discovery of highly stable phages capable of infecting immotile bacteria, which often form biofilms or exist in dormant states, suggests novel applications in phage therapy and biotechnology that could improve treatment outcomes.
Detailed genomic analyses uncovered that PIN1 and PIN2 possess gene clusters similar to those found in classical flagellotropic phages. These genes encode for tail fibers, baseplates, and receptor-binding proteins, yet subtle mutations and structural variations imply specialization for alternative host receptors. This suggests a fascinating molecular mimicry or convergent evolution, whereby phage components structurally resemble machinery used for flagellar attachment but engage different, possibly conserved motifs on non-motile bacterial surfaces.
The team also investigated the co-evolutionary dynamics between PIN phages and their hosts through experimental evolution assays. Over successive bacterial generations, no detectable resistance developed against PIN phages, contrasting with many known phage-host pairings where resistance arises rapidly. This may stem from the unique attachment and infection mechanisms employed by these phages, perhaps targeting essential bacterial structures that are less prone to mutational escape due to their critical functional roles, or by engaging multiple receptor sites simultaneously.
In terms of ecology, PIN1 and PIN2’s ability to infect immotile bacteria with typical flagellotropic phage architecture could reflect adaptations to microenvironments where bacterial motility is suppressed or energetically unfavorable. For example, within biofilms, bacteria often downregulate flagellar production, entering sessile modes to optimize resource use and collective resilience. Phages like PIN1 and PIN2 might represent an evolutionary solution to sustain viral propagation under such conditions, adding complexity to microbial community dynamics and virus-mediated horizontal gene transfer.
Structurally, cryo-electron microscopy provided high-resolution visualization of PIN1 and PIN2 virions, illuminating their capsid geometries and tail assembly. Both phages exhibit a contractile tail sheath consistent with the Myoviridae family, known for potent injection mechanisms, which could facilitate penetration of bacterial cell envelopes that differ from motile species’ outer surfaces. The researchers speculate that the structural flexibility inherent in these tails allows targeting of alternative receptors while preserving infection efficiency.
Another notable aspect of the PIN phages is their genomic compactness combined with genetic robustness. The genomes, consisting of linear double-stranded DNA, include a repertoire of genes for DNA replication, structural proteins, and host lysis, but lack accessory genes commonly implicated in host manipulation or motility-specific interactions. This lean genomic design might reflect an optimized infection cycle tailored to stable yet selective host targeting without unnecessary metabolic burden.
Future avenues for research stemming from this study are manifold. Discovering the precise molecular receptors and binding patterns that allow PIN1 and PIN2 to infect immotile bacteria remains a priority. Such information could reveal novel bacterial surface molecules as phage receptors, broadening the spectrum of known host-phage interactions and facilitating the design of phage-based antibacterial agents with finely tuned host specificities.
In addition, exploring the environmental distribution of PIN-like phages could shed light on their ecological roles, prevalence, and influence within natural and clinical settings. Given their high stability, PIN phages may persist in harsh or fluctuating environments, acting as critical agents in bacterial population control and gene exchange, with possible impacts on microbial community structure and function.
The findings also raise fundamental questions about the evolutionary origins of flagellotropic phages. Are PIN1 and PIN2 remnants of ancestral phages that originally co-evolved with motile hosts but subsequently adapted to immotile species? Or do they represent an independent lineage that co-opted flagellotropic features for entirely different infection strategies? Resolving this will require phylogenomic comparisons against a broad database of phage sequences and functional assays to understand adaptation trajectories.
In therapeutic contexts, the unique properties of PIN1 and PIN2 suggest practical benefits. Their exceptional stability could facilitate storage and transportation logistics, overcoming significant hurdles faced by phage therapy products. Moreover, their targeting of immotile bacteria broadens the range of pathogenic species amenable to phage treatment, notably those forming chronic infections where bacteria adopt sessile lifestyles resistant to many antibiotics.
The study by Jati and colleagues thereby not only deepens the mechanistic understanding of phage biology but also provides a blueprint for exploring viral diversity beyond classical paradigms. It highlights the remarkable evolutionary ingenuity of bacteriophages, capable of adapting infection strategies to exploit even seemingly unfavorable host traits, such as the absence of motility structures.
This work resonates with the broader shift in microbiology and virology toward appreciating the vast, largely untapped diversity of viruses in nature. Advances in sequencing, microscopy, and bioinformatics now enable the uncovering of such extraordinary viral phenotypes that redefine established biological concepts and unlock new technological and therapeutic potentials.
As we continue to unravel the complexity of phage-host interactions through studies like this, we edge closer to harnessing these microbial predators effectively for human benefit, ecological management, and biotechnology innovation. PIN1 and PIN2 stand as compelling models for future research aiming to decode the intricate molecular dance between viruses and their bacterial hosts in all their astonishing variety.
Subject of Research: Characterization of bacteriophages PIN1 and PIN2 exhibiting features of flagellotropic phages but infecting immotile bacteria.
Article Title: Highly stable bacteriophages PIN1 and PIN2 have hallmarks of flagellotropic phages but infect immotile bacteria.
Article References:
Jati, A., Li, Y., Mu, A. et al. Highly stable bacteriophages PIN1 and PIN2 have hallmarks of flagellotropic phages but infect immotile bacteria. npj Viruses 3, 56 (2025). https://doi.org/10.1038/s44298-025-00139-4
Image Credits: AI Generated
