Researchers employed cutting-edge whole-genome sequencing to explore the relationship between bacterial genetic variation and antimicrobial tolerance. By analyzing an extensive dataset of 1.3 million M. abscessus unitigs, which are sequence fragments capturing diverse genomic variations, the team mapped these to phenotypic profiles of drug resistance and tolerance. Using linear mixed models, which account for complex genetic relationships and environmental factors, they could carefully dissect the fraction of phenotypic variance attributable to genetic variance—a measure known as heritability.

The most striking revelation from their analysis was the high heritability of tolerance phenotypes across various antibiotics. Contrary to prior assumptions of tolerance being primarily a plastic, non-genetic feature, the data indicate that for many drugs, genetic determinants account for between 32% and an astonishing 97% of the variability in tolerance levels between isolates. This far exceeds the minimal 1.1% heritability expected by chance, underscoring the heritable and strain-specific nature of drug killing phenotypes.

The team further contrasted heritability estimates between drug resistance, measured as minimum inhibitory concentrations (MICs), and tolerance, assessed via the area under the killing curve (AUC), highlighting that while resistance to some antibiotics such as macrolides was strongly genetically determined, others like imipenem and cefoxitin showed low heritability. This likely reflects the interplay of drug chemical properties and biological variability affecting phenotypic measurements, providing critical insights into heterogeneity in resistance and tolerance mechanisms.

Beyond quantifying heritability, the researchers integrated these data with detailed phylogenetic analyses of over 350 M. abscessus isolates. This evolutionary perspective enabled them to characterize how tolerance traits have emerged and been conserved across bacterial lineages. Strikingly, both convergent evolution and clade-specific inheritance patterns were evident. For example, distinct high- or low-tolerance phenotypes have evolved independently multiple times—a phenomenon known as homoplasy—while other traits are inherited within closely related clades.

One particularly noteworthy finding was the identification of a low tigecycline tolerance clade nested within the dominant circulating clone of M. abscessus massiliense. This clade also harbors high-level mutational resistance to aminoglycosides and macrolides and is associated with increased virulence, highlighting a paradox where high genetic drug resistance coincides with vulnerabilities in drug tolerance. The low tolerance to tigecycline within this clade could represent an exploitable therapeutic weakness, offering new avenues to improve treatment outcomes for infections notoriously difficult to manage.

The implications of this study extend far beyond mere academic interest. Understanding that tolerance, like resistance, has a strong genetic basis challenges established dogma and opens new research pathways. Therapeutic strategies could be refined considering not only resistance profiles but also tolerance genotypes, enabling more precise combination therapies that prevent both survival and proliferation of pathogenic strains.

Equally remarkable is the study’s demonstration that large-scale phenotypic screens coupled with whole genome sequencing and sophisticated statistical modeling provide a powerful lens to map the complex genotype-phenotype landscape in microorganisms. This approach serves as a blueprint for dissecting genetic contributions to other complex traits in diverse infectious agents, potentially revolutionizing antimicrobial stewardship and drug development.

Moreover, the heterogeneity observed in both resistance and tolerance suggests that treatment failures and relapses in mycobacterial infections may stem as much from genetically encoded tolerance as from resistance mutations. Clinical microbiology diagnostics may need to incorporate tolerance assessments, enhancing predictive precision for therapeutic success and reducing the mounting burden of chronic infections.

This research also spotlights the nuanced relationships between genetic variation, bacterial physiology, and antimicrobial lethality, emphasizing that phenotypic assays alone cannot capture the full biology of tolerance. Comprehensively integrating high-resolution genotype data enables identification of subtle genetic variants controlling tolerance across populations, which could be missed by conventional methods.

By mapping killing phenotypes onto the bacterial phylogeny, the study reveals how evolutionary pressures shape drug response strategies in bacterial populations. These dynamics of clonal inheritance and repeated emergence of similar traits underscore evolutionary constraints and plasticity in antimicrobial survival mechanisms, encouraging deeper evolutionary-informed drug design.

Ultimately, this work represents a paradigm shift with wide-reaching consequences for clinicians, microbiologists, and pharmacologists. The discovery that drug tolerance is not simply a transient phenotypic state but is robustly genetically encoded gives actionable insight into combatting mycobacterial infections with higher lethality rates and poorer clinical outcomes. A refined understanding of the genetic landscape controlling tolerance holds promise for enhanced diagnostics, targeted therapeutics, and improved patient prognoses worldwide.

As multidrug-resistant infections continue to jeopardize global health, deciphering the genetic architecture of tolerance in pathogens like M. abscessus emerges as an urgent priority. This seminal study lays vital groundwork for future investigations and therapeutic innovations that can transform our ability to outmaneuver antimicrobials evasion.

Subject of Research: Genetic determinants of antimicrobial tolerance and resistance in Mycobacterium abscessus.

Article Title: Large-scale testing of antimicrobial lethality at single-cell resolution predicts mycobacterial infection outcomes.

Article References:
Jovanovic, A., Bright, F.K., Sadeghi, A. et al. Large-scale testing of antimicrobial lethality at single-cell resolution predicts mycobacterial infection outcomes. Nat Microbiol (2026). https://doi.org/10.1038/s41564-025-02217-y

Image Credits: AI Generated

DOI: https://doi.org/10.1038/s41564-025-02217-y

Bacteriophages, or phages, are viruses that infect bacteria and have been of interest for over a century due to their potential therapeutic uses. However, the complexity and natural variability of phage genomes have historically hindered efforts to manipulate them systematically. Professor Hatfull’s team has overcome these challenges by synthesizing complete phage genomes from scratch, enabling researchers to customize their genetic makeup according to precise experimental requirements. This capability marks a transformative leap in the capacity to dissect phage function and regulation at an unprecedented level of detail.

The synthetic construction of phage genomes permits scientists to interrogate fundamental biological questions that have long remained elusive. For example, among phages that contain upwards of 100 genes, it has been unclear which genes are essential for infectivity, replication, or host interaction and which are redundant or auxiliary. With the ability to design and assemble synthetic phage genomes devoid of certain genes, researchers can now systematically delete or replace individual genetic elements and observe the resultant effects on phage viability and efficacy. This experimental flexibility accelerates discovery, paving the way to understand gene regulation and interaction networks within these viruses.

For their landmark study, Hatfull and his colleagues focused on mycobacteriophages—phages that infect mycobacteria, a genus that includes significant human pathogens such as Mycobacterium tuberculosis and Mycobacterium leprae, responsible for tuberculosis and leprosy respectively. By synthesizing and assembling genomes representative of two naturally occurring high G+C content mycobacteriophages, the team demonstrated that bespoke phage genomes could be “rebooted” or activated to create functioning viral particles in the laboratory. This synthetic rebooting confirms that phages retain their bactericidal properties even when entirely constructed from synthetic DNA.

The practical implications of this breakthrough extend deeply into the field of antimicrobial therapy. Antibiotic resistance poses a grave and escalating threat worldwide, with superbugs rendering many conventional treatments ineffective. Engineered phages, tailored to precisely target specific bacterial strains, offer a potent alternative to broad-spectrum antibiotics. The synthetic genome technique enables the design of phages with enhanced efficacy, specificity, and the ability to evade bacterial defense systems, potentially revitalizing therapeutic strategies against resistant infections.

Moreover, the ability to assemble artificial genomes brings synthetic biology principles into virology, enabling the design of novel phage variants with properties not found in nature. Researchers are no longer restricted to naturally occurring genetic combinations; they can now imagine and realize entirely new genomes that optimize infection mechanics, host range, and safety profiles. The phrase used by Professor Hatfull, “the sky’s the limit,” reflects the vast potential unlocked by this technology to create phages of significant therapeutic and research value.

This ambitious project was carried out in collaboration with two pioneering institutions in biotechnology: Ansa Biotech and New England Biolabs. These collaborations combined cutting-edge DNA synthesis and assembly technologies with decades of expertise in phage biology and mycobacterial research. The integration of synthetic genomics and classical phage biology methodologies ensured that the synthetic genomes were both functional and representative of complex natural phage systems, making this study a model for future interdisciplinary research.

Scaling synthetic phage engineering could also contribute to faster and more effective responses against emerging bacterial threats. By enabling rapid prototyping of phages with tailored genomes, laboratories can adapt to new bacterial variants or outbreaks more swiftly than ever before. Unlike traditional antibiotic development, which can take years, synthetic phage design and validation could be accelerated substantially using this platform, allowing for more agile public health interventions.

Furthermore, the detailed mechanistic insights gained from studying synthetic phage genomes could inform bioengineering efforts to enhance phage stability and delivery in clinical settings. Synthetic manipulation may optimize viral capsid structures, DNA packaging signals, or host recognition receptors, potentially leading to phages that remain active longer in the human body or target hard-to-reach bacterial reservoirs. This could vastly improve the therapeutic index of phage treatment, increasing its viability as a frontline medical tool.

Scientifically, this work also addresses fundamental questions about the modularity and evolution of viral genomes. Through synthetic assembly, researchers can experiment with genome rearrangements, gene insertions from other organisms, or even the creation of chimeric phages. Such experiments could reveal unknown genetic interactions and evolutionary constraints while expanding the molecular toolkit available for viral engineering.

The findings from this study will be published in the prestigious Proceedings of the National Academy of Sciences (PNAS), emphasizing the high impact and relevance of this research to multiple scientific disciplines. Importantly, the project is funded by the NIH and the Howard Hughes Medical Institute, highlighting its critical importance and potential to transform clinical microbiology and synthetic biology.

As the scientific community digests this revolutionary approach, the knock-on effects are expected to ripple across why we study viruses, treat bacterial diseases, and engineer synthetic biological systems. The innovative synthesis and rebooting of phages represent a milestone in both basic and applied research, providing a flexible platform for future innovations that could dramatically reshape bacterial infection management and further advance synthetic genomics.

Contacts for media inquiries and further information about this groundbreaking research are available at the University of Pittsburgh, ensuring that the exciting discoveries will be communicated broadly and promptly as developments progress.

Subject of Research: Cells
Article Title: Genome synthesis, assembly, and rebooting of therapeutically useful high G+C% mycobacteriophages
News Publication Date: 14-Nov-2025
Web References: 10.1073/pnas.2523871122
Keywords: Bacteriophages, Antibiotic resistance, Drug resistance, Artificial genomes, Synthetic biology