The investigation focused on GXPs, which entomopathogenic bacteria use to kill insect prey. The GxpS genes contain repeated domains whose separate histories are difficult to reconstruct because whole genes can undergo duplication or horizontal transfer, while individual domains and modules can also be duplicated, deleted, inserted or recombined. Conventional gene trees can therefore conflate several events. To establish a reference history, the researchers used the terminal thioesterase domain together with the adjacent T domain. Recombination-aware analyses detected no recombination in these regions among the sampled GxpS sequences, allowing them to serve as an anchor for the whole gene and for the individual A- and TC-domain trees.

Analysis of 54 putative GxpS orthologues placed them in a monophyletic group and included the four-modular Xenotetrapeptide synthetase and an unknown six-modular non-ribosomal peptide synthetase within that group. Reconciliation of the TTE-domain gene tree with a species tree indicated that GxpS first arose in Photorhabdus and was subsequently transferred to Xenorhabdus twice. This conflict between gene and species histories was interpreted as evidence of horizontal gene transfer. The researchers also traced the separate operon that supplies p-aminophenylalanine and its methylated derivative, concluding that it was present in the last common ancestor of the analyzed bacteria, was lost twice in Xenorhabdus groups, and was reacquired by horizontal transfer in Xenorhabdus doucetiae. In that species, the operon contains mobile genetic elements and experiments indicated that it was probably inactive.

Disagreement among the trees of the other GxpS domains exposed extensive recombination. Some exchanges replaced entire adenylation domains, whereas others transferred short sections, including fragments inserted into regions already altered by an earlier event. The team predicted 17 A-domain recombinations and used a mixture of phylogenetic incongruence, local sequence similarity and recombination-detection analysis to resolve more complicated histories. Their workflow separated unrecombined sequence from donor-derived segments and identified the likely donor clade for each inferred event. Experimental validation then included expression in Escherichia coli, high-performance liquid chromatography coupled to mass spectrometry, feeding with isotopically labelled amino acids, and co-injection of chemically synthesized peptides.

Those experiments confirmed every recombination that had been predicted to create a new peptide. They also established functional consequences that could not have been read directly from sequence alone. In one example, an A2-domain fragment replaced the A3 domain of the synthetase from Xenorhabdus sp. PB62.4, changing incorporation of phenylalanine to leucine alone. Elsewhere, a fifth A-domain fragment was inserted into the middle of a third A-domain and shifted production toward a different major peptide. A five-modular enzyme produced rare cyclic tetrapeptides despite retaining five modules. Disabling its terminal T and thioesterase domains did not eliminate the product, while replacing the recombined C4 domain with an LCL-C2 domain changed the output to cyclic pentapeptides. These results showed that the recombined C4 domain enabled early release and cyclization of tetrapeptides.

Across the family, the researchers predicted at least 12 new main products and detected 49 compounds, including side products, during the experimental analyses. They identified ten unique GxpS variants with product spectra expected to differ from the canonical ortholog. Isotopic feeding and peptide co-injection supported the proposed structures and substrate incorporation. They also found that every tested phenylalanine-incorporating synthetase could incorporate p-aminophenylalanine or its methylated derivative; in almost all feeding tests, the resulting peptide was dominant under the conditions used. For Xenorhabdus species lacking the operon, the phenylalanine-containing GameXPeptide was the main product. The scale of the recovered family diversity therefore extended beyond the canonical peptide, even though the study examined synthetases from only two bacterial genera.

Ancestral sequence reconstruction offered another view of the diversification. The researchers reconstructed the last common ancestor of the analyzed GxpS family, assembled its domains into a complete synthetase and expressed it in Escherichia coli. The inferred ancestor produced a cyclic pentapeptide with a structure similar to that of the canonical modern peptide. A deliberately less probable reconstruction, differing at 244 amino-acid positions, produced the same major product. Mapping peptide structures onto the TTE-domain tree revealed extensive convergence: phenylalanine at the third position was independently replaced by leucine seven times, producing four synthetases with the same major peptide, while 12 independent recombinations introduced valine-specific domains and yielded five origins of another pentapeptide. Tetrapeptide production arose through several routes, including two independently evolved four-modular synthetases and five-modular enzymes that acquired an early-cyclization domain.

Laboratory feeding tests then showed that the structural differences among GXPs translated into different insecticidal activities. The researchers tested eight peptides against larvae of Spodoptera exigua, Plutella xylostella, Tenebrio molitor and Frankliniella occidentalis. Each treatment contained 10 larvae, each was conducted with three biological replicates, and mortality was recorded three days later. Comparison of five models by the Akaike information criterion favored the most complex model, which assigned a distinct potency to each peptide–insect combination. Peptide 36 was the most active against S. exigua but among the least effective against P. xylostella, whereas peptide 4 was most effective against P. xylostella. The experiment nevertheless covered only four insect species, and the authors cautioned against assuming that any one GXP exclusively targets a particular insect in nature.

Recombination statistics supplied evidence that selection affected some events. The researchers distinguished homotypic exchanges, which involved domains with the same substrate specificity and did not change the peptide’s composition, from heterotypic exchanges involving different specificities. Homotypic events served as a proxy for the neutral process. Heterotypic recombinations affected the specificity pocket of the A-domain significantly more often than expected from that proxy; Fisher’s exact tests gave P less than 0.05 in the evaluated tables. This pattern was consistent with rare, specificity-changing events being preferentially retained by selection. It was not a direct measurement of the fitness of ancient peptides, however, and the authors also described a possible non-selective explanation. The 27 synonymous recombination events identified, compared with 26 non-synonymous events, suggested that drift also played a substantial role in GxpS diversification. Likewise, the presence of several peptides with similar potency against each tested insect left room for chemically distinct variants to persist without a large measured activity difference.

A third influence emerged from donor bias. The actual historical A-domain donors were generally more similar at the nucleotide level to their recipients than alternative donors with the same specificity located elsewhere in the same genome. Candidate donors from other non-ribosomal peptide synthetase genes were significantly less similar, with P less than 10⁻⁴ in the relevant paired comparisons. Most A-domain donors also came from within GxpS itself. This pattern suggests a self-reinforcing restriction: recombination among similar domains within the gene would homogenize portions of the sequence and make later exchanges among those internal domains still more likely, while less-similar external domains would remain less accessible. The observed convergence may therefore reflect both the benefits of a particular peptide and the limited set of substitutions that biased recombination made readily available.

Together, the results portray GxpS evolution as the outcome of interacting forces rather than a purely adaptive or purely random process. Selection was consistent with preferential retention of some specificity-changing recombinations, drift accompanied many neutral and nearly neutral structural changes, and recombination bias restricted which amino-acid specificities could be accessed easily. The study also has clear limits. It examined one synthetase family, tested only eight peptides and four insect species, and used homotypic recombination as an approximation to neutrality. Synonymous events were harder to verify experimentally, while some could still have affected peptide production or enzyme folding. The authors further noted that every recombination overwrites part of the sequence record, meaning that some ancient history may be unrecoverable. Whether the same combination of forces shapes other non-ribosomal peptide families remains an open question that will require analyses of other synthetase systems.

Subject of Research: Evolutionary drivers of non-ribosomal peptide diversification in entomopathogenic bacteria

Article Title: Drift, selection and recombination bias drive the evolution of a non-ribosomal peptide

Article References: Kaiser, S., Shahmohammadi, N., Goel, T. S. M., Buchanan, C. J., Garg, S. G., Vidaurre-Barahona, D., Qian, Z., Lefèvre, C., Schindler, D., Kim, Y., Bode, H. B., & Hochberg, G. K. A. (2026). Drift, selection and recombination bias drive the evolution of a non-ribosomal peptide. Nature Ecology & Evolution. https://doi.org/10.1038/s41559-026-03214-y

Image Credits: AI Generated

DOI: 10.1038/s41559-026-03214-y

Keywords: non-ribosomal peptides, GameXPeptides, GxpS, bacterial evolution, genetic recombination, natural selection, genetic drift, convergent evolution, entomopathogenic bacteria, insecticidal peptides, Drift, selection

The study, published in BMC Biology, examines an apparent tension in host–parasite conflict. Mobile genetic elements, including viruses, plasmids and transposons, are major drivers of evolution in bacteria and archaea, and recent metagenomic research and functional annotation have revealed that many defense systems are located on these elements. Because mobile genetic elements are effectively genomic parasites, their frequent carriage of defense systems raises a question about why such protections are so prevalent and may sometimes exceed their prevalence on the host chromosome.

To address this question, the researchers developed a simple mathematical model representing the ecology of mobile genetic elements and the conflicts among them. The analysis identifies two qualitatively distinct regimes according to the basic reproductive number in mixed coinfections, with the strength of inter-element interference serving as the central determinant. In the weak-interference regime, a mobile element tends to carry a defense system whose cost is low. The modeled system enhances the survival of its host when the host encounters a more damaging mobile genetic element.

Under this weak-interference regime, the defense system may occasionally be transferred to the host, but it usually remains on the mobile element that carries it. The authors consequently describe weakly interfering mobile genetic elements as potential “defense banks”: reservoirs in which low-cost systems can be maintained and provide a host-level benefit. This designation is a model-based interpretation, not a directly observed state of natural microbial communities.

The second regime arises when interference among mobile genetic elements is strong. Here, elements can carry high-cost defense systems that harm both the host and the population as a whole. Despite those costs, the modeled defenses assist their carriers in spreading by actively replacing competing elements. The study identifies strongly interfering plasmids from the same incompatibility group as an example of this regime. Such elements may operate as proving grounds for emerging defense systems, allowing systems that mediate conflicts between mobile elements to be optimized by selection before they become cost-effective.

The results offer a testable mathematical account of how defense strategies could evolve within mobile genetic elements, but they do not establish a specific evolutionary trajectory in bacteria or archaea. The model distinguishes host-protective, low-cost defenses from carrier-benefiting, high-cost systems and links the distinction to the intensity of competition among mobile elements. Whether these predicted regimes, including the proposed roles of defense banks and proving grounds, occur at the stated frequencies or with the predicted effects in natural microbial populations remains to be tested empirically.

Subject of Research: Modeled evolution of defense systems carried by mobile genetic elements in bacteria and archaea

Article Title: Eco-evolutionary dynamics of defense systems carried by mobile genetic elements

Article References: Iranzo, J., Wolf, Y. I., & Koonin, E. V. (2026). Eco-evolutionary dynamics of defense systems carried by mobile genetic elements. BMC Biology. https://doi.org/10.1186/s12915-026-02760-y

Image Credits: AI Generated

DOI: 10.1186/s12915-026-02760-y

Keywords: mobile genetic elements, bacterial evolution, archaeal evolution, defense systems, host–parasite conflict, inter-element interference, plasmids, mathematical modeling, Eco-evolutionary, dynamics, defense, systems

The puzzle at the heart of the study is a broad empirical pattern that has been noted across bacterial genomes for years. When researchers compare pairs of bacterial strains that are only slightly diverged from one another, the measured dN/dS tends to be relatively high. As the comparison moves to more distantly related pairs, the ratio steadily declines. This downward trend has been reported in many bacterial lineages, yet the mechanisms responsible for it have remained contested. Some explanations invoke changes in the strength of natural selection over evolutionary time, while others point to saturation effects or the peculiar demographics of bacterial populations. What most existing models share is an assumption that the genetic differences being measured arise from de novo mutations, the fresh errors that accumulate as lineages split apart and drift away from each other.

Liu and Good argue that this assumption misses a major contributor to bacterial diversity. Bacteria do not simply inherit their genomes clonally from parent cells. Through horizontal gene transfer and homologous recombination, they continually import segments of DNA from related strains, replacing chunks of their own chromosomes with foreign versions of the same genes. In dense microbial communities such as the human gut, this traffic of genetic material can be extensive, meaning that a substantial fraction of the differences between any two strains may not be the product of new mutations at all but of older variants that were acquired from the wider population. Those older variants have had more time to be filtered by selection, and that simple fact, the researchers show, can reshape the entire pattern of dN/dS across a genome.

To disentangle these effects, the team built a phenomenological model of dN/dS in recombining bacterial populations. The model is designed to separate the contributions of two kinds of genomic regions: segments inherited clonally, which trace back through a straightforward line of descent, and segments acquired through recombination from other strains. By applying this framework to pairs of closely related strains, the researchers could ask a question that mutation-only models cannot address: does the mode of inheritance of a DNA segment change the selective signal it carries? The answer, according to their analysis, is a clear yes, and the difference is large enough to account for one of the most widespread patterns in bacterial genomics.

The central finding is strikingly consistent. Clonally inherited regions of the genome, those stretches of DNA passed down directly through the lineage without being swapped, exhibit consistently higher dN/dS ratios than recombined segments. The interpretation follows directly from the age of the variants involved. A freshly arisen mutation in a clonal segment has, on average, only existed briefly, so weakly deleterious changes have not yet been purged and still contribute to the nonsynonymous substitution count. A recombined segment, by contrast, carries variants that circulated in the population before being imported, giving selection a longer opportunity to remove the harmful ones. The result is that recombined DNA looks more constrained, even if the underlying selection pressures on the genes themselves have not changed at all.

Crucially, the researchers did not stop at a qualitative observation. They quantified how the accumulation of recombined segments over time affects the measured ratio, and found that this process can quantitatively explain the majority of the decline in dN/dS with divergence. In other words, much of the trend that has been interpreted as a signal of changing selection pressures across evolutionary time may instead be a bookkeeping artifact: as pairs of strains become more diverged, a growing share of their differences comes from recombined, pre-filtered DNA, dragging the apparent ratio downward. This reframing matters because dN/dS is used throughout microbiology to infer which genes are under selection, to identify adaptive changes, and to estimate the effective strength of natural selection in natural populations. If recombination systematically biases the metric, many of those inferences need to be re-examined.

That is precisely what Liu and Good did next. Armed with a model that accounts for recombination, they revisited the leading explanations for the dN/dS decline in human gut bacteria, including models of purifying selection and adaptive reversion. Adaptive reversion refers to a scenario in which slightly deleterious mutations that fix in small populations are later reversed by compensatory changes, a process that can also depress dN/dS in comparisons between diverged lineages. By fitting their framework to population genomic data from gut bacterial species sampled around the world, the team was able to weigh these competing explanations against the recombination-driven account and to extract new estimates of the selection pressures acting on protein-coding sites.

The re-analysis uncovered evidence for widespread weak selection operating at a large fraction of protein-coding sites in gut bacterial genomes. Weak selection occupies an awkward middle ground in evolutionary genetics: changes under it are not neutral enough to drift freely, yet not strong enough to be efficiently eliminated, particularly in populations whose effective sizes fluctuate. Detecting such selection is notoriously difficult, because its signature is subtle and easily confounded by demographic noise. The finding that a large share of coding sites in gut bacteria experience this regime suggests that the vast majority of the proteome is under some degree of selective constraint, even at positions that a naive analysis might classify as effectively neutral. It also implies that the classical categories of neutral, nearly neutral, and strongly selected sites may need recalibration in species where recombination is rampant.

The broader implications reach beyond the human gut. Horizontal gene transfer is a defining feature of bacterial evolution, shaping everything from antibiotic resistance to metabolic innovation, and recombination among close relatives occurs in many microbial communities, from ocean plankton to soil. If the exchange of older genetic variants systematically alters genome-wide measures of selection, then the standard toolkit of molecular evolution may require revision for any recombining species. The study’s authors frame their conclusions as raising new questions about the effectiveness of natural selection in complex bacterial populations, where the interplay of clonal inheritance, gene flow, and weak selection creates patterns that neither mutation-only nor recombination-free models can capture. For a field that has long treated the bacterial chromosome as a record of mutational history, the message is that the record has been repeatedly edited, and learning to read the edits is now essential for understanding what selection has actually done.

Subject of Research: The role of homologous recombination in shaping genome-wide patterns of selective constraint in human gut bacteria

Article Title: Recombination shapes the dynamics of selective constraint in global populations of human gut bacteria

Article References: Liu, Z., & Good, B. H. (2026). Recombination shapes the dynamics of selective constraint in global populations of human gut bacteria. PLOS Biology, 24(10), e3004052. https://doi.org/10.1371/journal.pbio.3004052

Image Credits: AI Generated

DOI: 10.1371/journal.pbio.3004052

Keywords: dN/dS, recombination, horizontal gene transfer, human gut microbiome, purifying selection, bacterial evolution, population genetics, selective constraint, molecular evolution, PLOS Biology, weak selection, genomic divergence

The research team, led by Matthew M. Carter, Zhiru Liu and Matthew R. Olm of Stanford University and the University of Colorado Boulder, together with Benjamin H. Good and Justin L. Sonnenburg, faced a formidable technical challenge. Bacterial genomes are constantly reshuffled by homologous recombination, a process in which strains exchange stretches of DNA with one another. This genetic mixing produces mosaic patterns of ancestry that cannot be captured by a single phylogenetic tree, which is why conventional evolutionary methods often fail when applied at the strain level. To get around this problem, the researchers developed population genetic analyses that explicitly account for recombination, allowing them to distinguish ancient co-migration from more recent microbial exchange driven by geography or lifestyle.

The study began with an extraordinary sequencing effort. The team performed deep metagenomic sequencing on 133 fecal samples collected from 85 Tsimane adults in 2009 and between 2012 and 2013, reaching a median depth of 31.9 gigabases per sample. From these data they recovered 12,746 metagenome-assembled genomes, or MAGs, representing 1,408 distinct bacterial and archaeal species. They then compared this collection with an analogous set of 32,034 MAGs previously obtained from 137 Hadza hunter-gatherers in Tanzania. The result was striking: 87.4 percent of the species found in the Tsimane cohort, a total of 1,231 species, were also present in the Hadza, despite the enormous geographic distance and the absence of any historical contact between the two groups.

Most of these shared species are not cosmopolitan gut dwellers. Roughly 60 percent of them, some 848 species, are rare in or completely absent from the fecal microbiomes of industrialized populations. A few, such as the starch-degrading bacterium Ruminococcus bromii, are found worldwide, but the majority belong to a category of microbes that previous work has labeled VANISH taxa, meaning they are volatile in prevalence or negatively associated with industrialized societies. The new study provides the strongest evidence yet that these vanishing species were not recent acquisitions by non-industrialized peoples but rather ancient companions of the human lineage that have been lost in the West.

To establish the antiquity of this partnership, the researchers calculated the average nucleotide identity, or ANI, between every pair of genomes within each of 636 species that had at least four MAGs in both populations. Because gut bacteria accumulate mutations at a rate of at least 10 to the power of minus 7 per site per year, the typical ANI of around 98 percent between Tsimane and Hadza genomes implies that they shared a common ancestor within the past 100,000 to 200,000 years. This timeframe places strong constraints on the evolutionary history of these microbes: the ancestors of the Tsimane did not acquire an entirely new suite of gut organisms during or after their migration into the Americas. Instead, they carried ancient African lineages with them.

The team then probed the high-similarity tail of the distribution, focusing on pairs of genomes sharing more than 99.75 percent average nucleotide identity. These nearly clonal strains contain large stretches of identical DNA that have not yet been overwritten by recombination, and their age can be estimated from the fraction of core genes with identical sequences. By calibrating this signal with known mutation rates, the researchers determined that genome pairs sharing more than 10 percent identical genes correspond to strains that diverged within roughly the past 5,000 years. Across all 636 species, 7.8 percent of genome pairs met this clonal criterion, but 93.8 percent of those pairs came from within the same human population. Moreover, 545 of the 636 species, or 85.6 percent, showed no recent strain sharing between the Tsimane and Hadza at all, indicating that very little microbial transmission has occurred between the two groups over the past five millennia.

There were telling exceptions. Species such as Akkermansia muciniphila and Bacteroides ovatus showed Tsimane-Hadza strain sharing rates comparable to their within-population baselines, and these same species tend to be enriched in industrialized populations. When the team expanded the analysis to include 8,855 additional MAGs from Europe, Asia and North America, these recently shared species turned out to have close relatives distributed globally, suggesting they were introduced into the Tsimane and Hadza through more recent contact with other human populations. The researchers also identified a third class of microbes, which they call StIL species, stable independent of lifestyle, that are prevalent across all populations yet still bear the genetic signature of ancient Tsimane-Hadza isolation, exemplified by common species such as Agathobacter rectalis and Ruminococcus bromii.

An independent line of evidence came from tracking horizontal gene transfer, the movement of DNA between bacterial strains. When two strains recombine, the transferred segment initially appears as a long tract of identical DNA that is gradually shortened by subsequent mutations, creating a predictable relationship between tract length and the age of the transfer event. The researchers measured the 99th percentile of identical tract lengths, called the L99 metric, for each species and found that between-population tracts were systematically shorter for the Tsimane and Hadza than within either population, a hallmark of genetic isolation. By contrast, the same comparisons between industrialized populations in Europe, Asia and North America showed much weaker barriers to gene flow, indicating that ongoing exchange in modern societies has blurred the historical signal.

Using these L99 lengths, the team estimated when the gut microbes of the two populations last exchanged DNA. The median split time for the Tsimane-Hadza comparison was 17,090 years, with a standard error of 863 years, dramatically older than the split times inferred between industrialized populations, which ranged from a median of 1,109 years for Europe-North America to 4,637 years for Asia-North America. A second, entirely orthogonal approach based on the distribution of single-nucleotide variant frequencies, applied through demographic modeling in 15 species, broadly confirmed these estimates. Notably, the inferred onset of microbial genetic isolation falls roughly within the window between the initial human migration out of Africa, around 60,000 to 70,000 years ago, and the settlement of the Americas, around 16,000 to 23,000 years ago, mirroring the timeline of human population splits.

The implications extend far beyond evolutionary history. The loss of VANISH taxa with industrialization has been linked to an increased risk of several chronic metabolic and inflammatory disorders, and the authors suggest that reintroducing these co-migrating species might one day help restore microbial functions that co-evolved with human biology. Such an endeavor, they caution, will require thoughtful and inclusive dialogue among researchers, ethicists and Indigenous communities, and the study itself was conducted under protocols approved by the Gran Consejo Tsimane and Bolivian institutions, with data governance guided by the CARE Principles for Indigenous Data Governance. As industrialized life continues to erode microbial diversity, the guts of the Tsimane and Hadza preserve a living archive of the ancient partnership between humans and their microbes, one that our species carried across continents and that modern societies are now quietly letting go.

Subject of Research: Prehistoric co-migration of human gut microbiome species traced through population genetics of the Tsimane and Hadza

Article Title: Prehistoric global migration of vanishing gut microbes with humans

Article References: Carter, M. M., Liu, Z., Olm, M. R., Martin, M., Sprockett, D. D., Ghadermazi, P., Trumble, B. C., Kaplan, H., Stieglitz, J., Rodriguez, D. E., Relman, D. A., Sonnenburg, E. D., Gurven, M., Good, B. H., & Sonnenburg, J. L. (2026). Prehistoric global migration of vanishing gut microbes with humans. Nature. https://doi.org/10.1038/s41586-026-11106-1

Image Credits: AI Generated

DOI: 10.1038/s41586-026-11106-1

Keywords: gut microbiome, Tsimane, Hadza, metagenomics, human migration, co-evolution, microbial diversity, horizontal gene transfer, VANISH taxa, industrialization, population genetics, bacterial evolution

Salmonella Typhimurium is among the leading causes of foodborne gastroenteritis worldwide, and its monophasic variant—distinguished serologically by the absence of one of its two flagellar phases—has surged in prevalence across Europe, the Americas and Asia over recent decades. The lineage, which belongs to sequence type 34 and typically displays the antigenic formula 4,[5],12:i:-, has repeatedly been implicated in outbreaks linked to pork, poultry and processed foods. Yet despite its clear epidemiological importance, the genetic determinants that allowed this clone to outcompete its biphasic ancestor, the previously prevalent ST19 lineage, had remained uncertain. The new study set out to resolve that question at unprecedented scale.

The team’s comparative analysis of tens of thousands of genomes revealed a striking pattern: the expanding ST34 clone carried a frameshift mutation in dinB, the gene encoding a translesion DNA synthesis polymerase also known as DNA polymerase IV. The mutation, a deletion of a single thymine residue within a homopolymeric tract of seven thymidines near the 5′ end of the gene, introduces a premature stop codon that truncates the 39.7-kilodalton DinB protein to a nonfunctional fragment of roughly 1.9 kilodaltons. Because DinB is normally involved in bypassing damaged DNA templates and in generating genetic diversity under stress, its loss initially appears counterintuitive for a pathogen on the rise.

Functional experiments, however, demonstrated that the crippled polymerase has far-reaching downstream consequences. Transcriptomic profiling showed that the frameshift dinB mutation rewires the expression of flagellar genes, altering the balance between the two phase-one and phase-two flagellin systems that Salmonella uses to evade host immune detection. Specifically, the mutation led to transcriptional changes that favored retention of fliC, which encodes phase-one flagellin, and set the stage for the subsequent loss of fljB, the gene encoding phase-two flagellin. Western blot analyses confirmed that dinB-mutant strains displayed an increased proportion of cells expressing FliC, while in vitro motility assays showed that swimming ability was preserved despite the underlying genetic upheaval.

The physiological consequence of this reprogramming became apparent in infection models. Using transposon insertion sequencing and in vivo competition experiments in mice, the researchers found that the requirements for successful gut colonization shifted once fljB was lost. Genes that were dispensable, or even costly, in the ancestral background became essential or advantageous in the mutant context, and vice versa. The frameshift dinB strain carrying the fljB deletion outcompeted the previously prevalent biphasic ST19 lineage in the intestine, indicating that the combination of the polymerase defect and flagellin loss conferred a measurable competitive advantage within the host environment.

To move beyond correlation, the authors built a temporal evolutionary model that simulated the dynamics of four competing lineages: biphasic ST19, biphasic ST34, ST34 carrying the frameshift dinB, and monophasic ST34 carrying both the frameshift and the fljB deletion. The modeling demonstrated that the chronological order of the events matters critically. The dinB mutation had to precede the loss of fljB for the monophasic clone to expand as observed, and the simulated selection coefficients derived from the model were quantitatively consistent with selection estimates computed from real-world changes in genotype frequencies among human clinical isolates. Sensitivity analyses confirmed that the conclusion held regardless of the initial frequency of ST34, the presence or absence of spatial population structure, or the precise metric used to quantify selection.

This ordered dependency is what the researchers describe as a stepwise pathoadaptation model: a sequence of gene-loss events in which each deletion reshapes the genetic background so that the next loss becomes beneficial rather than deleterious. The concept echoes a growing recognition in microbial evolution that genome reduction can be a creative force. Recent work on Vibrio parahaemolyticus, for example, showed that wave succession within a pandemic clone was likewise driven by gene loss, and earlier studies documented how monophasic Salmonella Typhimurium microevolved during a prolonged epidemic in the United Kingdom. The new study, however, is notable for linking the ecological success of a zoonotic lineage to a specific, mechanistically validated chain of molecular events.

The broader implications extend to public health forecasting. If the emergence of major bacterial clones can depend on relatively simple, ordered mutational steps, then surveillance systems that monitor homopolymeric tracts and other mutation-prone loci may gain predictive power over which lineages are poised to expand. The study’s data resources—including genomic data deposited at the China National GeneBank Database, Enterobase and NCBI, transcriptomic data under accession GSE280131, proteomic data via PRIDE, and transposon-sequencing reads under BioProject PRJNA1347559—provide a substantial foundation for such efforts. The mathematical model code has also been made publicly available on GitHub, enabling other groups to test the generality of the sequential gene-loss framework in other pathogens.

At the same time, the work underscores how little of bacterial adaptation is driven by gene acquisition alone. Monophasic ST34 also carries traits such as copper resistance and heavy-metal tolerance that have been implicated in its persistence in livestock production environments, and antimicrobial resistance remains a defining feature of many circulating clones. But the new analysis places gene loss at the center of the lineage’s rise, reframing flagellar phase variation—a long-studied quirk of Salmonella biology—as a decisive axis of clonal competition. As the authors note, understanding the genetic factors facilitating the emergence of infectious diseases is critical, and this study delivers a concrete mechanistic answer for one of the most consequential zoonotic pathogens of the modern food system. The lesson is uncomfortable but clarifying: for bacteria, sometimes less is more, and the road to pandemic status can be paved by the genes a pathogen leaves behind.

Subject of Research: Stepwise gene loss driving the clonal expansion of monophasic Salmonella Typhimurium ST34

Article Title: Sequential gene loss promotes expansion of monophasic Salmonella Typhimurium ST34

Article References: Wu, X., Wang, T., Jia, C., Zhou, H., Li, Y., Baker, S., & Yue, M. (2026). Sequential gene loss promotes expansion of monophasic Salmonella Typhimurium ST34. Nature Microbiology. https://doi.org/10.1038/s41564-026-02483-4

Image Credits: AI Generated

DOI: 10.1038/s41564-026-02483-4

Keywords: Salmonella Typhimurium, monophasic ST34, gene loss, dinB, flagellin, fljB, pathoadaptation, bacterial evolution, genomic epidemiology, foodborne pathogen, zoonotic disease, Nature Microbiology

Integrons are genetic platforms that bacteria use to capture, stockpile and rearrange small mobile elements called gene cassettes. Each cassette typically carries a single gene and its own recombination site, and the cassettes sit in arrays that can be shuffled by an integrase enzyme in response to stress. This architecture is famously responsible for the rapid spread of antibiotic resistance genes among pathogens, but the same machinery also represents an evolutionary archive of functions that bacteria have recruited over millions of years, from metabolic enzymes to toxins and defensive systems. Until now, most efforts to mine this archive have relied on laborious one-cassette-at-a-time approaches or on sequence-based predictions that say little about what a gene actually does.

The new study, led by Filipa Trigo da Roza and José Antonio Escudero of the Universidad Complutense de Madrid, together with colleagues including Melanie Blokesch of the École Polytechnique Fédérale de Lausanne, introduces two tools with evocative names: the cassette gatherer and the cassette hunter. Both exploit a clever piece of molecular engineering in which a class 1 integron recombination site, known as attI1, is embedded inside a gene that acts as a counterselection marker. In the plasmid-based version, the attI1 site was inserted into the ccdB toxin gene from Vibrio fischeri; in the chromosomal version, it was placed inside the sacB gene from Bacillus subtilis.

The logic of the system is elegantly simple. In its empty state, the disrupted toxin gene kills or prevents growth of the host bacterium under selective conditions. But when the integrase catalyses the capture of a gene cassette at the embedded attI1 site, the cassette restores the reading frame of the marker, inactivating the counterselection and allowing the cell to survive. Only cells that have successfully captured a cassette form colonies, which means the selection is entirely independent of the sequence or predicted function of the captured gene. This sequence- and function-independence is what distinguishes the approach from earlier methods that depended on PCR primers or prior knowledge of cassette boundaries.

The researchers deployed the plasmid-based cassette gatherer and the chromosomal cassette hunter in a naturally competent strain of Vibrio cholerae from which the native superintegron had been removed. Taking advantage of the bacterium’s ability to take up DNA from its environment, a capacity triggered by growth on chitin, the team could deliver genomic libraries directly into cells where the engineered integron machinery awaited them. When applied to a panel of Vibrio species, including V. cholerae, V. vulnificus, V. mimicus and V. parahaemolyticus, the tools recovered hundreds of single cassettes per assay with more than 99 percent specificity, a capture rate that dwarfs what conventional cloning strategies could achieve.

High-throughput sequencing of the resulting libraries confirmed that the recovered cassettes faithfully represented the diversity of the source integrons. Correlation analyses showed that the abundance of each cassette in the recovered pools tracked reproducibly with its representation in the starting material, and that the same cassette repertoires were recovered by both the plasmid-based and chromosomal versions of the tool. The data also revealed the diversity of functions hidden in these arrays, with many cassettes encoding proteins of unknown function, a reminder of how much uncharacterised biology remains buried in bacterial genomes.

To demonstrate the discovery power of the approach, the researchers turned their cassette libraries against two very different bacteriophages: ICP2, a vibriophage that preys on pandemic V. cholerae, and the classic Escherichia coli phage T4. Screens of the recovered cassettes identified nine distinct phage-defence systems, five of which had never been described before. The result builds on a series of recent studies showing that mobile integrons and sedentary chromosomal integrons act as biobanks of anti-phage defence, and it provides the first general-purpose pipeline for converting that observation into a systematic, high-throughput inventory of defence genes.

The significance of the advance extends well beyond phage defence. Because integron cassettes are exchanged across bacterial lineages through horizontal gene transfer, they constitute a naturally curated collection of genes that have passed repeated tests of utility in diverse cellular contexts. Gene cassette PCR, developed two decades ago, allowed researchers to amplify cassette boundaries from environmental DNA, but the products were mixtures that resisted clean isolation. Bioinformatic surveys, including comprehensive scans of metagenomes, have catalogued millions of predicted cassettes, yet prediction alone cannot assign function. The gatherer and hunter tools close this gap by pairing unbiased physical recovery of individual cassettes with immediate amenability to functional screens, whether for antibiotic resistance, metabolic activities, antimicrobial compounds or industrial enzymes.

The technical groundwork for the study drew on decades of integron biology, from the discovery of the distinctive V. cholerae superintegron in 1998 to detailed dissections of how attC recombination sites fold into single-stranded hairpins that guide strand selection during recombination. The team also engineered the recipient strain to optimise natural transformation, deleting extracellular nucleases and tuning competence regulators so that incoming genomic DNA could recombine efficiently into the capture platform. Structural predictions generated with AlphaFold3 guided the placement of the attI1 site inside the counterselection markers, minimising disruption of protein folding while preserving the lethal phenotype needed for stringent selection.

The tools, their datasets and the analysis scripts have been made available through Zenodo and GitHub, and the underlying strains are covered by patent filings, signalling likely commercial interest in what amounts to a programmable gene-discovery platform. As sequencing continues to reveal integron cassettes in environments ranging from soil and ocean to the human gut, the ability to recover and test those genes at scale transforms a passive cataloguing exercise into an active search for function. For a field that has spent forty years documenting integrons as agents of bacterial evolution, the new work offers something rarer: a way to read, one cassette at a time, the full library of tricks that bacteria have been collecting all along.

The choice of Vibrio cholerae as the engineering chassis reflects the deep historical connection between integron research and this organism. The massive superintegron of V. cholerae, first described in 1998, carries well over a hundred cassettes and remains the archetype of the sedentary chromosomal integrons found across Vibrionaceae. Because the integrase of class 1 integrons and the V. cholerae superintegron integrase share overlapping recombination specificities at attC sites, the engineered platform can in principle process cassettes arriving from a wide range of donor integrons, which is precisely what the cross-species recovery experiments demonstrated.

The phage-defence findings also fit into a rapidly consolidating picture. Within the past two years, independent teams have reported that mobile integrons in clinical settings encode anti-phage systems, that sedentary chromosomal integrons function as biobanks of defence genes, and that V. parahaemolyticus integrons are particularly rich in such systems. The nine systems identified here, five of them entirely new, suggest that this enrichment is not a peculiarity of any single lineage but a general property of integron arrays, plausibly reflecting the intense phage pressure experienced by bacteria in aquatic environments where cassettes are most actively exchanged.

Another dimension worth noting concerns expression. Cassette arrays are transcribed from a single promoter positioned in the integrase gene region, and the translation rate of upstream cassettes shapes the expression of those downstream, meaning that a captured gene’s activity depends heavily on its position in the array. By recovering cassettes as individual entities, the new tools sidestep this positional context entirely, allowing each gene to be assayed under standardised conditions. This decoupling of capture from native expression is likely to be important for screens targeting enzymatic or antimicrobial activities that may be silent or weakly expressed in the donor organism.

The environmental dimension is equally significant. Most integron cassettes on Earth reside in so-called environmental integrons, which are not associated with mobile elements or clinical resistance and remain almost entirely uncharacterised. Extending the gatherer and hunter workflow to metagenomic DNA from sediments, biofilms or wastewater could grant functional access to this reservoir, complementing sequence-similarity approaches that struggle with the high proportion of novel genes. As antimicrobial resistance continues to mobilise cassettes into pathogens, understanding what these elements normally do in their native hosts may prove as consequential as the biotechnological applications that motivated the work.

Subject of Research: High-throughput recovery of integron gene cassettes for functional gene discovery and phage-defence screening.

Article Title: High-throughput recovery of integron cassettes for gene discovery screens

Article References: Trigo da Roza, F., Carvalho, A., Prieto, A., Blanco, P., Vergara, E., López-Igual, R., Redrejo-Rodríguez, M., Blokesch, M., & Escudero, J. A. (2026). High-throughput recovery of integron cassettes for gene discovery screens. Nature Microbiology. https://doi.org/10.1038/s41564-026-02474-5

Image Credits: AI Generated

DOI: 10.1038/s41564-026-02474-5

Keywords: integrons, gene cassettes, phage defence, antibiotic resistance, Vibrio cholerae, horizontal gene transfer, natural transformation, gene discovery, integrase, biotechnology, microbiology, bacterial evolution