Pathways for horizontal gene transfer in bacteria revealed by a global map of their plasmids

Unraveling the role of microplastics in antibiotic resistance: Insights from long-read metagenomics on ARG mobility and host dynamics

As two emerging pollutants, microplastics (MPs) potentially serve as vectors for antibiotic resistance genes (ARGs) in aquatic environments, but the mechanisms driving ARG enrichment remain unclear. This study used long-read metagenomics to investigate ARG mobility and hosts dynamics within the biofilms of MPs and rocks in different water environments. We identified distinct enrichment patterns for microbial communities and ARGs, highlighting the significant role of horizontal gene transfer in ARG enrichment. Specifically, plasmid-encoded ARGs varied significantly among MP biofilms, rock biofilms, and water samples, while chromosome-encoded ARGs remained consistent across these environments, emphasizing the impact of plasmids on ARG enrichment. Despite this, 55.1 % of ARGs were on chromosomes, indicating that host organisms also play a crucial role. The related mechanisms driving ARG enrichment included enhanced cell adhesion, increased transmembrane transporter activity, and responses to environmental stressors, which led to an increased presence of plasmid-encoded ARGs on MP biofilms, facilitating more frequent horizontal gene transfer. Additionally, the diversity of hosts on MPs was notably lower compared to the water column, with specific bacteria, including Herbaspirillu, Limnohabitans, Polaromonas, Variovorax, Rubrivivax, and Thauera significantly driving ARG enrichment. This study highlights key mechanisms and bacterial taxa involved in ARG dynamics on MPs.

Genomic characterization of plasmids harboring blaNDM-1, blaNDM-5, and blaNDM-7 carbapenemase alleles in clinical Klebsiella pneumoniae in Pakistan

Klebsiella pneumoniae is notorious for causing healthcare-associated infections, which become more complicated by the acquisition of blaNDM genes via mobile genetic elements. Although Pakistan is a well-established hot spot of blaNDM-positive K. pneumoniae, detailed molecular descriptions of blaNDM-carrying plasmids are scarce. Seven K. pneumoniae isolates harboring blaNDM were recovered from clinical sample sources during a 6 month period and tested for antimicrobial susceptibility. A long-read approach was used for whole-genome sequencing to obtain circularized plasmids and chromosomes for typing, annotation, and comparative analysis. The isolates were susceptible to colistin and tigecycline only among the tested antibiotics. We identified five sequence types (STs): ST11, ST16, ST716, ST464, and ST2856. Notably, three strains possessed the hypervirulent capsule KL2, while five were classified as O locus type O2a. Evidence of genetic diversity was further highlighted by the presence of four IncC plasmids harboring blaNDM-1, two IncX3 plasmids harboring blaNDM-5, and a single hybrid IncFIB/IncHI1B plasmid harboring blaNDM-7. These plasmids also carried additional antimicrobial resistance (AMR) genes conferring resistance to aminoglycosides, cephalosporins, and fluoroquinolones. We identified the plasmidome of the K. pneumoniae isolates and characterized the New Delhi metallo-beta-lactamase (NDM)-carrying plasmids. Genetic analysis confirmed the presence of blaNDM-1 and blaNDM-5 on broad host range plasmids and blaNDM-7 in a previously unreported hybrid plasmid backbone. We emphasized the critical role of plasmids in spreading blaNDM in the clinical setting in Pakistan. Hence, we stressed the urgent need for enhanced surveillance, not least in low-middle income countries, infection control measures, and adherence to the "Access," "Watch," and "Reserve" guidelines in antibiotics use.

IMPORTANCE

Infections caused by NDM-producing Klebsiella pneumoniae are a significant challenge to treat and represent a crucial health burden in low- and middle-income countries (LMICs). Most of the blaNDM are located on plasmids that promote horizontal gene transfer. However, there is a lack of comprehensive information on the genetic context of the NDM-carrying plasmids in Pakistan. This study presents a detailed analysis of seven NDM-plasmids in clinical K. pneumoniae isolates, shedding light on their high-risk sequence types and multiple resistance determinants. We also describe the plasmid-bearing NDM alleles (blaNDM-1, blaNDM-5, and blaNDM-7). Notably, we are the first to report blaNDM-7 on the hybrid IncFIB/IncHI1B backbone in Pakistan, a plasmid that has rarely been reported previously globally. Understanding the plasmid genomic landscape is paramount to comprehensively understanding the AMR scenario in this LMIC.

Plasmid-bacteria associations in the clinical context

Antimicrobial resistance (AMR) is one of the most pressing global health problems, with plasmids playing a central role in its evolution and dissemination. Over the past decades, many studies have investigated the ecoevolutionary dynamics between plasmids and their bacterial hosts. However, what drives the epidemiological success of certain plasmid-bacterium associations remains unclear. In this opinion article, we review which factors influence these associations and underline that studying plasmid-host interactions of clinical relevance is critical for understanding the evolution and spread of AMR. We also highlight the increasing importance of integrating experimental research with bioinformatics and machine learning tools to study plasmid-bacteria dynamics. This combined approach will assist researchers to dissect the molecular mechanisms underlying successful plasmid-host associations and to design strategies to prevent and predict future high-risk associations.

Antimicrobial Resistance and Phylogenetic Analysis of Multidrug-Resistant Non-Typhoidal Salmonella Isolates from Different Sources in Southern Vietnam

Background/Objectives: Non-typhoidal Salmonella (NTS) is one of the most common causative agents of food poisoning in Vietnam, and contaminated livestock meat poses a major risk to human health. The present study aims to provide the genetic characteristics of NTS with a particular focus on antimicrobial resistance and determine phylogenetic relationships between isolates from different sources in Southern Vietnam based on whole-genome sequencing (WGS) data. Methods: A total of 49 NTS isolates from pork/broiler meat, pigs, chickens, and humans were collected in Ho Chi Minh City and four provinces of Southern Vietnam. Phenotypic antimicrobial susceptibility testing (AST) and WGS for all isolates were performed. Results: As a result, 14 different serotypes were identified, among which S. Typhimurium and its monophasic variant were the dominant serotypes for human and pig sources. All chicken samples belonged to S. Indiana, whereas S. Infantis predominated in broiler meat. AST results revealed that 98% of isolates were multidrug resistant. NTS strains isolated from poultry and pigs exhibited resistance to the highest priority antimicrobials-quinolones and polymyxin, as well as to β-lactams, aminoglycosides, tetracycline, and sulfonamide, which are considered to be critical for the treatment of severe diseases. Conclusions: The results highlight the utmost importance of issues related to the selection, spreading, and transmission of multi-resistant strains from animals to humans.

cAMP-Mediated Biofilm eDNA Transfer Facilitates the Resilience of Soil Microbiome to Agrochemical Stress

Soil microorganisms utilize extracellular DNA (eDNA)-based biofilms as a defense against xenobiotics. However, the specific effects and transfer pathways of eDNA under persistent agrochemical exposure remain unclear. This study examined the transfer dynamics of carbofuran-hydrolase gene pchA from Pseudomonas stutzeri PS21. During biofilm formation, pchA was released from eDNA, leading to an enrichment of beneficial microorganisms such as Acidobacteria and Elusimicrobia, which enhanced organic compound metabolism and improved soil microbiome resilience. An increase in the pchA-associated mobile genetic elements and the colocalization of pchA with other bacterial species indicated the potential horizontal gene transfer (HGT) under carbofuran exposure. Additionally, carbofuran triggered a cAMP-dependent apoptotic pathway, leading to a 59.6% increase in pchA copy number, which suggested that cAMP played a role in initiating HGT. In conclusion, the cAMP-mediated interspecific transfer of pchA could enhance microbial coadaptation to carbofuran contamination, thereby strengthening the collective defense of soil microbiome against agrochemical stress.

Plasmidome analyses of Klebsiella pneumoniae co-producing blaKPC-2 and blaNDM-1 in Southern Brazil: characterisation of mobile genetic elements

OBJECTIVE

Infections due to carbapenemase-producing Enterobacterales harbouring more than one carbapenemase-encoding gene spread mainly by plasmid and transposon mobilisation. The objective of this study was to analyse the mobile genetic elements carrying blaKPC and blaNDM of Klebsiella pneumoniae carbapenemase co-producers (KpKN).

METHODS

K. pneumoniae isolates with reduced susceptibility to carbapenems were obtained between 2016 and 2023. To evaluate the genetic environment of KpKN, 22 isolates were selected for antimicrobial susceptibility testing and whole-genome sequencing.

RESULTS

The blaKPC-2 gene was carried mainly by IncN/IncFIB, a novel co-integrated plasmid in the Tn4401b transposon. blaNDM-1 was disseminated in the only two KpKN isolates recovered prior to 2020 by the IncHI1B/IncFIB plasmid type within the Tn3000 transposon. Significantly, isolates obtained since 2020 showed the blaNDM-1 gene carried by IncA/C in an IS26-flanked pseudo-composite transposon containing ISCR1, which also had genes that conferred resistance to sulfonamides, aminoglycosides, macrolides, quinolones, amphenicols, tetracyclines, rifampicin, sulfonamide, and trimethoprim. The isolates belonged mainly to ST11 and ST16.

CONCLUSIONS

Plasmid and transposon changes during different periods could be related to higher dissemination of blaNDM-1, and the large number of resistance genes present in the IS26-flanked transposon may have increased co-selection of this plasmid through the wide use of antimicrobials during the pandemic.

Contaminated drinking water facilitates Escherichia coli strain-sharing within households in urban informal settlements

Identifying bacterial transmission pathways is crucial to inform strategies that limit the spread of pathogenic and antibiotic-resistant bacteria. Here we assessed Escherichia coli strain-sharing and overlap of antibiotic resistance genes (ARGs) across humans, poultry, canines, soil, and drinking water within and between households in urban informal settlements in Nairobi, Kenya. We collected 321 samples from 50 households with half having access to chlorinated water. We performed Pooling Isolated Colonies-seq, which sequences pools of up to five E. coli colonies per sample to capture strain diversity. Pooling Isolated Colonies-seq captured 1,516 colonies and identified 154 strain-sharing events, overcoming limitations of single-isolate sequencing and metagenomics. Within households, strain-sharing rates and resistome similarities across sample types were strongly correlated, suggesting clonal transmission of ARGs. E. coli isolated from the environment carried clinically relevant ARGs. Strain-sharing was rare between animals and humans but frequent between humans and drinking water. E. coli-contaminated stored drinking water was associated with higher human-human strain-sharing within households. These results suggest that contaminated drinking water facilitates human to human strain-sharing, and water treatment can disrupt transmission.

Harmony in diversity: Reorganizing the families within the order Pseudomonadales

An accurate and coherent bacterial taxonomy is essential for studying the ecological aspects of microorganisms and for understanding microbial communities and their dynamics. The order Pseudomonadales is of particular importance in biological research due to its ability to interact with eukaryotic hosts, including taxa of clinical relevance (e.g.: Pseudomonas, Moraxella, Acinetobacter), or due to its functions in soil and water ecosystems. Despite their relevance, we have identified several inconsistencies in the organisation of genera within families in this order. Here, we perform comprehensive phylogenetic and phylogenomic analyses to reorganise these taxa. Average amino acid identity (AAI) values shared within and between families support our reclassifications. We propose seven new families, including new ecologically relevant families (e.g.: Oceanobacteraceae fam. nov.), as well as several taxonomic emendations. Our results also support the inclusion of Cellvibrionales and Oceanospirillales within Pseudomonadales. This revised organisation provides a robust delineation of these taxa into families, characterised by AAI values ranging from 60% to 77%. AAI distances between families are predominantly below 60%. This reclassification contributes to establishment of a more reliable taxonomic framework within Gammaproteobacteria, providing the basis for a more comprehensive understanding of their evolution.

Genomic Characterization of Escherichia coli O157:H7 Associated with Multiple Sources, United States

In the United States, Shiga toxin-producing Escherichia coli (STEC) outbreaks cause >265,000 infections and cost $280 million annually. We investigated REPEXH01, a persistent strain of STEC O157:H7 associated with multiple sources, including romaine lettuce and recreational water, that has caused multiple outbreaks since emerging in late 2015. By comparing the genomes of 729 REPEXH01 isolates with those of 2,027 other STEC O157:H7 isolates, we identified a highly conserved, single base pair deletion in espW that was strongly linked to REPEXH01 membership. The biological consequence of that deletion remains unclear; further studies are needed to elucidate its role in REPEXH01. Additional analyses revealed that REPEXH01 isolates belonged to Manning clade 8; possessed the toxins stx2a, stx2c, or both; were predicted to be resistant to several antimicrobial compounds; and possessed a diverse set of plasmids. Those factors underscore the need to continue monitoring REPEXH01 and clarify aspects contributing to its emergence and persistence.

Mapping antimicrobial resistance landscape at a city scale sewage network

Wastewater is a valuable source for monitoring contaminants of biotic or abiotic origin. Antimicrobial resistance (AMR) has emerged as a public health threat that consists of the ability of microorganisms to resist the effects of antimicrobial compounds, rendering them very difficult or impossible to eradicate in case of infection. Considering the dissemination of antimicrobial resistance genes (ARGs) to a wide number of ecosystems, there is a need for the identification of hotspots that concentrate antimicrobial resistance determinants. A comprehensive investigation conducted at a city-scale in Sabadell (Barcelona, Spain) has integrated both phenotypic and genotypic methodologies, including metagenomics and culture-based techniques coupled with whole-genome sequencing (WGS), to monitor ARG presence in seven different spots of the sewage system. Metagenomics approach identified 262 ARG variants across analyzed sampling sites, grouped into 15 resistance categories. The most prevalent ARGs were macrolides-lincosamides-class B streptogramins (MLSB) (35.1 %) and beta-lactams (28.7 %), including carbapenems (5.9 %) and cephalosporins (5.3 %). MLSB resistance featured dominant msr(E) and mph(E) genes, the most abundant ARGs in our study. ARGs conferring resistance to beta-lactam were dominated by blaOXA-464, blaOXA-491, and blaNPS. Key genes for carbapenem (blaOXA-372, blaKPC-2) and cephalosporin (blaOXA-10, blaOXA-1) resistance were identified. The hospital sector exhibited the highest relative abundance of ARGs, dominated by beta-lactams, MLSB, and aminoglycosides. Wastewater treatment plant (WWTP) entrance points and residential areas displayed similar ARG profiles, while WWTP effluent and industrial zones had the lowest ARG levels. WWTP significantly reduced ARG presence (93.3 %). The characterization of antibiotic-resistant bacterial isolates found that most abundant ARGs were predominantly plasmid-borne, favoring ARG spread across bacterial genera. This finding confirmed the significant role of plasmids in ARG dissemination, increasing both diversity and prevalence within waterborne bacterial communities. City-scale surveillance programs can play a pivotal role in guiding effective measures to reduce the dissemination of AMR and mitigate their environmental impact.

Unveiling plasmid diversity and functionality in pristine groundwater

BACKGROUND

Plasmids are key in creating a dynamic reservoir of genetic diversity, yet their impact on Earth's continental subsurface-an important microbial reservoir-remains unresolved. We analyzed 32 metagenomic samples from six groundwater wells within a hillslope aquifer system to assess the genetic and functional diversity of plasmids and to evaluate the role of these plasmids in horizontal gene transfer (HGT).

RESULTS

Our results revealed 4,609 non-redundant mobile genetic elements (MGEs), with 14% (664) confidently classified as plasmids. These plasmids displayed well-specific populations, with fewer than 15% shared across wells. Plasmids were linked to diverse microbial phyla, including Pseudomonadota (42.17%), Nitrospirota (3.31%), Candidate Phyla Radiation (CPR) bacteria (2.56%), and Omnitrophota (2.11%). The presence of plasmids in the dominant CPR bacteria is significant, as this group remains underexplored in this context. Plasmid composition strongly correlated with well-specific microbial communities, suggesting local selection pressures. Functional analyses highlighted that conjugative plasmids carry genes crucial for metabolic processes, such as cobalamin biosynthesis and hydrocarbon degradation. Importantly, we found no evidence of high confidence emerging antibiotic resistance genes, contrasting with findings from sewage and polluted groundwater.

CONCLUSIONS

Overall, our study emphasizes the diversity, composition, and eco-evolutionary role of plasmids in the groundwater microbiome. The absence of known antibiotic resistance genes highlights the need to preserve groundwater in its pristine state to safeguard its unique genetic and functional landscape.

Effect of subinhibitory concentrations on the spreading of the ampicillin resistance gene blaCMY-2 in an activated sludge microcosm

As the problem of multi-resistant bacteria grows a better understanding of the spread of antibiotic resistance genes is of utmost importance for society. Wastewater treatment plants contain subinhibitory concentrations of antibiotics and are thought to be hotspots for antibiotic resistance gene propagation. Here we evaluate the influence of sub-minimum inhibitory concentrations of antibiotics on the spread of resistance genes within the bacterial community in activated sludge laboratory-scale sequencing batch reactors. The mixed communities were fed two different ampicillin concentrations (500 and 5000 µg/L) and the reactors were run and monitored for 30 days. During the experiment the β-lactamase resistance gene blaCMY-2 was monitored via qPCR and DNA samples were taken to monitor the effect of ampicillin on the microbial community. The relative copy number of blaCMY-2 in the reactor fed with the sub-minimum inhibitory concentration of 500 µg/L ampicillin was spread out over a wider range of values than the control and 5000 µg/L ampicillin reactors indicating more variability of gene number in the 500 µg/L reactor. This result emphasises the problem of sub-minimum inhibitory concentrations of antibiotics in wastewater. High-throughput sequencing showed that continuous exposure to ampicillin caused a shift from a Bacteroidetes to Proteobacteria in the bacterial community. The combined use of qPCR and high-throughput sequencing showed that ampicillin stimulates the spread of resistance genes and leads to the propagation of microbial populations which are resistant to it.

The Spread of Antibiotic Resistance Is Driven by Plasmids Among the Fastest Evolving and of Broadest Host Range

Microorganisms endure novel challenges for which other microorganisms in other biomes may have already evolved solutions. This is the case of nosocomial bacteria under antibiotic therapy because antibiotics are of ancient natural origin and resistances to them have previously emerged in environmental bacteria. In such cases, the rate of adaptation crucially depends on the acquisition of genes by horizontal transfer of plasmids from distantly related bacteria in different biomes. We hypothesized that such processes should be driven by plasmids among the most mobile and evolvable. We confirmed these predictions by showing that plasmid species encoding antibiotic resistance are very mobile, have broad host ranges, while showing higher rates of homologous recombination and faster turnover of gene repertoires than the other plasmids. These characteristics remain outstanding when we remove resistance plasmids from our dataset, suggesting that antibiotic resistance genes are preferentially acquired and carried by plasmid species that are intrinsically very mobile and plastic. Evolvability and mobility facilitate the transfer of antibiotic resistance, and presumably of other phenotypes, across distant taxonomic groups and biomes. Hence, plasmid species, and possibly those of other mobile genetic elements, have differentiated and predictable roles in the spread of novel traits.

MOSTPLAS: a self-correction multi-label learning model for plasmid host range prediction

MOTIVATION

Plasmids play an essential role in horizontal gene transfer, aiding their host bacteria in acquiring beneficial traits like antibiotic and metal resistance. There exist some plasmids that can transfer, replicate, or persist in multiple organisms. Identifying the relatively complete host range of these plasmids provides insights into how plasmids promote bacterial evolution. To achieve this, we can apply multi-label learning models for plasmid host range prediction. However, there are no databases providing the detailed and complete host labels of these broad-host-range plasmids. Without adequate well-annotated training samples, learning models can fail to extract discriminative feature representations for plasmid host prediction.

RESULTS

To address this problem, we propose a self-correction multi-label learning model called MOSTPLAS. We design a pseudo label learning algorithm and a self-correction asymmetric loss to facilitate the training of multi-label learning model with samples containing some unknown missing labels. We conducted a series of experiments on the NCBI RefSeq plasmid database, the PLSDB 2025 database, plasmids with experimentally determined host labels, the Hi-C dataset, and the DoriC dataset. The benchmark results against other plasmid host range prediction tools demonstrated that MOSTPLAS recognized more host labels while keeping a high precision.

AVAILABILITY AND IMPLEMENTATION

MOSTPLAS is implemented with Python, which can be downloaded at https://github.com/wzou96/MOSTPLAS. All relevant data we used in the experiments can be found at https://zenodo.org/doi/10.5281/zenodo.14708999.

Piggybacking on nature: exploring the multifaceted world of porcine β-defensins

Porcine β-defensins (pBDs) are cationic peptides that are classically associated with the innate immune system. These molecules yield both antimicrobial and immunomodulatory properties, as evidenced by various in vitro and animal trials. Researchers have revealed that enhancing pBD expression can be achieved through dietary components and gene editing techniques in pigs and porcine cell models. This state-of-the-art review aims to encapsulate the pivotal findings and progress made in the field of pBD over recent decades, with a specific emphasis on the biological role of pBD in infection control and its usage in clinical trials, thereby offering a new landscape of opportunities for research aimed at identifying prophylactic and therapeutic alternatives for both swine medicine and translational purposes.

Global dissemination of Klebsiella pneumoniae in surface waters: Genomic insights into drug resistance, virulence, and clinical relevance

The aquatic environment is a major pathway for the spread of antibiotic resistance (AR) among microorganisms. Among these, Klebsiella pneumoniae reveals high genome plasticity, adaptability, and the ability to colonize humans, animals, and the natural environment, awarding it a significant role in the spread of AR. This work presents an in-depth analysis of the whole sequences of 149 K. pneumoniae genomes isolated from surface waters available in databases. The sequences were obtained from 20 countries in five continents. The analyses showed a high genomic diversity of isolates, classifying them into 94 unique sequence types. The isolates carried numerous virulence and drug resistance determinants in their genomes, including genes for carbapenem and colistin resistance. The critical resistance genes were located on plasmids, indicating their high mobility and ease of access in water environments. Sublineage 258 members, in particular ST11, have been identified as important carriers of both important drug resistance determinants and key virulence factors, thus posing a substantial threat to human health. Our analysis revealed the direct transmission of drug-resistant and virulent clinical strains to the natural environment, highlighting the role of K. pneumoniae in the dissemination of drug resistance within the "One Health" framework. Surface waters represent an environment conducive to the spread and evolution of drug resistance, and K. pneumoniae plays a significant role in this process by providing clinically-significant antibiotic resistance genes to environmental recipients.

The role of bacterial metabolism in antimicrobial resistance

The relationship between bacterial metabolism and antibiotic treatment is complex. On the one hand, antibiotics leverage cell metabolism to function. On the other hand, increasing research has highlighted that the metabolic state of the cell also impacts all aspects of antibiotic biology, from drug efficacy to the evolution of antimicrobial resistance (AMR). Given that AMR is a growing threat to the current global antibiotic arsenal and ability to treat infectious diseases, understanding these relationships is key to improving both public and human health. However, quantifying the contribution of metabolism to antibiotic activity and subsequent bacterial evolution has often proven challenging. In this Review, we discuss the complex and often bidirectional relationships between metabolism and the various facets of antibiotic treatment and response. We first summarize how antibiotics leverage metabolism for their function. We then focus on the converse of this relationship by specifically delineating the unique contribution of metabolism to three distinct but related arms of antibiotic biology: antibiotic efficacy, AMR evolution and AMR mechanisms. Finally, we note the relevance of metabolism in clinical contexts and explore the future of metabolic-based strategies for personalized antimicrobial therapies. A deeper understanding of these connections is crucial for the broader scientific community to address the growing crisis of AMR and develop future effective therapeutics.

Intracellular competition shapes plasmid population dynamics

Conflicts between levels of biological organization are central to evolution, from populations of multicellular organisms to selfish genetic elements in microbes. Plasmids are extrachromosomal, self-replicating genetic elements that underlie much of the evolutionary flexibility of bacteria. Evolving plasmids face selective pressures on their hosts, but also compete within the cell for replication, making them an ideal system for studying the joint dynamics of multilevel selection. While theory indicates that within-cell selection should matter for plasmid evolution, experimental measurement of within-cell plasmid fitness and its consequences has remained elusive. Here we measure the within-cell fitness of competing plasmids and characterize drift and selective dynamics. We achieve this by the controlled splitting of synthetic plasmid dimers to create balanced competition experiments. We find that incompatible plasmids co-occur for longer than expected due to methylation-based plasmid eclipsing. During this period of co-occurrence, less transcriptionally active plasmids display a within-cell selective advantage over their competing plasmids, leading to preferential fixation of silent plasmids. When the transcribed gene is beneficial to the cell, for example an antibiotic resistance gene, there is a cell-plasmid fitness tradeoff mediated by the dominance of the beneficial trait. Surprisingly, more dominant plasmid-encoded traits are less likely to fix but more likely to initially invade than less dominant traits. Taken together, our results show that plasmid evolution is driven by dynamics at two levels, with a transient, but critical, contribution of within-cell fitness.

Metagenomic analysis of pristine oil sheds new light on the global distribution of microbial genetic repertoire in hydrocarbon-associated ecosystems

Oil reservoirs are society's primary source of hydrocarbons. While microbial communities in industrially exploited oil reservoirs have been investigated in the past, pristine microbial communities in untapped oil reservoirs are little explored, as are distribution patterns of respective genetic signatures. Here, we show that a pristine oil sample contains a complex community consisting of bacteria and fungi for the degradation of hydrocarbons. We identified microorganisms and their pathways for the degradation of methane, n-alkanes, mono-aromatic, and polycyclic aromatic compounds in a metagenome retrieved from biodegraded petroleum encountered in a subsurface reservoir in the Barents Sea. Capitalizing on marker genes from metagenomes and public data mining, we compared the prokaryotes, putative viruses, and putative plasmids of the sampled site to those from 10 other hydrocarbon-associated sites, revealing a shared network of species and genetic elements across the globe. To test for the potential dispersal of the microbes and predicted elements via seawater, we compared our findings to the Tara Ocean dataset, resulting in a broad distribution of prokaryotic and viral signatures. Although frequently shared between hydrocarbon-associated sites, putative plasmids, however, showed little coverage in the Tara Oceans dataset, suggesting an undiscovered mode of transfer between hydrocarbon-affected ecosystems. Based on our analyses, genetic information is globally shared between oil reservoirs and hydrocarbon-associated sites, and we propose that currents and other physical occurrences within the ocean along with deep aquifers are major distributors of prokaryotes and viruses into these subsurface ecosystems.

Microbial functional diversity and redundancy: moving forward

Microbial functional ecology is expanding as we can now measure the traits of wild microbes that affect ecosystem functioning. Here, we review techniques and advances that could be the bedrock for a unified framework to study microbial functions. These include our newfound access to environmental microbial genomes, collections of microbial traits, but also our ability to study microbes' distribution and expression. We then explore the technical, ecological, and evolutionary processes that could explain environmental patterns of microbial functional diversity and redundancy. Next, we suggest reconciling microbiology with biodiversity-ecosystem functioning studies by experimentally testing the significance of microbial functional diversity and redundancy for the efficiency, resistance, and resilience of ecosystem processes. Such advances will aid in identifying state shifts and tipping points in microbiomes, enhancing our understanding of how and where will microbes guide Earth's biomes in the context of a changing planet.

The PLSDB 2025 update: enhanced annotations and improved functionality for comprehensive plasmid research

Plasmids are extrachromosomal DNA molecules in bacteria and archaea, playing critical roles in horizontal gene transfer, antibiotic resistance, and pathogenicity. Since its first release in 2018, our database on plasmids, PLSDB, has significantly grown and enhanced its content and scope. From 34 513 records contained in the 2021 version, PLSDB now hosts 72 360 entries. Designed to provide life scientists with convenient access to extensive plasmid data and to support computer scientists by offering curated datasets for artificial intelligence (AI) development, this latest update brings more comprehensive and accurate information for plasmid research, with interactive visualization options. We enriched PLSDB by refining the identification and classification of plasmid host ecosystems and host diseases. Additionally, we incorporated annotations for new functional structures, including protein-coding genes and biosynthetic gene clusters. Further, we enhanced existing annotations, such as antimicrobial resistance genes and mobility typing. To accommodate these improvements and to host the increase plasmid sets, the webserver architecture and underlying data structures of PLSDB have been re-reconstructed, resulting in decreased response times and enhanced visualization of features while ensuring that users have access to a more efficient and user-friendly interface. The latest release of PLSDB is freely accessible at https://www.ccb.uni-saarland.de/plsdb2025.

Detection of rare plasmid hosts using a targeted Hi-C approach

Despite the significant role plasmids play in microbial evolution, there is limited knowledge of their ecology, evolution, and transfer in microbial communities. This is partly due to the limitations of current methods in associating a plasmid with its host in microbiomes. To address this knowledge gap, we developed and implemented a novel approach to identify rare plasmid hosts by combining Hi-C, a proximity ligation method, with enrichment for plasmid-specific DNA. We hereafter refer to this approach as Hi-C+. We applied Hi-C and Hi-C+ to soil microbial communities in which we mimicked increasingly rare transfer of an antimicrobial resistance plasmid from a donor to a recipient. This was achieved by inoculating agricultural soil with mixtures of known plasmid-containing and plasmid-free cells at different proportions. We demonstrated that Hi-C can link a plasmid to its host in soil when the relative abundance of that plasmid-host pair is as low as 0.001%. Hi-C+ further improved the detection limit of Hi-C 100-fold and allowed the identification of plasmid hosts at the genus level. As a culture-independent approach, Hi-C+ will significantly improve our understanding of the range and frequency of spread of antibiotic resistance and other genes that are introduced into soil and other microbiomes.

New, Complete Circularized Genomes of Xanthomonas citri pv. mangiferaeindicae Produced from Short- and Long-Read Co-Assembly Shed Light on Strains that Emerged a Decade Ago on Mango and Cashew in Burkina Faso

We report high-quality genomes of three strains of Xanthomonas citri pv. mangiferaeindicae, the causal agent of mango bacterial canker disease, including the pathotype strain of this pathovar and two strains from Burkina Faso that emerged a decade ago. These strains hosted two to three plasmids of sizes ranging from 19 to 86 kb. Genome mining revealed the presence of several secretion systems and effectors involved in the virulence of xanthomonads with (i) a type I secretion system of the hlyDB group; (ii) xps and xcs type II secretion systems; (iii) a type III secretion system with several type III effectors, including transcription activator-like effectors; (iv) several type IV secretion systems associated with plasmid or integrative conjugative elements mobility; (v) three type V secretion system subclasses (Va, Vb, and Vc); and (vi) a single i3* type VI secretion system. The two strains isolated in Burkina Faso from mango (Mangifera indica) and cashew (Anacardium occidentale) differed by only 14 single-nucleotide polymorphisms and shared identical secretion systems and type III effector repertoires. Several transcription activator-like effectors were identified in each strain, some of which may target plant genes previously found implicated in disease development in other xanthomonad-associated pathosystems. These results support the emergence in Burkina Faso a decade ago of very closely related strains that became epidemic on mango and cashew (i.e., two distinct host genera of a same plant family). These new genomic resources will contribute to better understanding the biology and evolution of this agriculturally major crop pathogen.

Transmission and control strategies of antimicrobial resistance from the environment to the clinic: A holistic review

The environment serves as a significant reservoir of antimicrobial resistance (AMR) microbes and genes and is increasingly recognized as key source of clinical AMR. Modern human activities impose an additional burden on environmental AMR, promoting its transmission to clinical setting and posing a serious threat to human health and welfare. Therefore, a comprehensive review of AMR transmission from the environment to the clinic, along with proposed effective control strategies, is crucial. This review systematically summarized current research on the transmission of environmental AMR to clinical settings. Furthermore, the transmission pathways, horizontal gene transfer (HGT) mechanisms, as well as the influential drivers including triple planetary crisis that may facilitate AMR transfer from environmental species to clinical pathogens are highlighted. In response to the growing trend of AMR transmission, we propose insightful mitigation strategies under the One Health framework, integrating advanced surveillance and tracking technologies, interdisciplinary knowledge, multisectoral interventions, alongside multiple antimicrobial use and stewardship approaches to tacking development and spread of AMR.

Mechanisms and influencing factors of horizontal gene transfer in composting system: A review

Organic solid wastes such as livestock manure and sewage sludge are important sources and repositories of antibiotic resistance genes (ARGs). Composting, a solid waste treatment technology, has demonstrated efficacy in degrading various antibiotics and reducing ARGs. However, some recalcitrant ARGs (e.g., sul1, sul2) will enrich during the composting maturation period. These ARGs persist in compost products and spread through horizontal gene transfer (HGT). We analyzed the reasons behind the increase of ARGs during the maturation phase. It was found that the proliferation of ARG-host bacteria and HGT process play an important role. This article revealed that microbial physiological responses, environmental factors, pollutants, and quorum sensing (QS) can all influence the HGT process in composting systems. We examined the influence of these factors on HGT in the compost system and summarized potential mechanisms by analyzing the alterations in microbial communities. We comprehensively summarized the HGT hazards that these factors may present in composting systems. Finally, we summarized methods to inhibit HGT in compost, such as using additives, quorum sensing inhibitors (QSIs), microbial inoculation, and predicting HGT events. Overall, the HGT mechanism and driving force in complex composting systems are still insufficiently studied. In view of the current situation, using predictions to assess the risk of HGT in composting may be advisable.

Azithromycin-resistant mph(A)-positive Salmonella enterica serovar Typhi in the United States

OBJECTIVES

The United States Centers for Disease Control and Prevention (CDC) conducts active surveillance for typhoid fever cases caused by Salmonella enterica serovar Typhi (Typhi). Here we describe the characteristics of the first two cases of mph(A)-positive azithromycin-resistant Typhi identified through US surveillance.

METHODS

Isolates were submitted to public health laboratories, sequenced, and screened for antimicrobial resistance determinants and plasmids, as part of CDC PulseNet's routine genomic surveillance. Antimicrobial susceptibility testing and long-read sequencing were also performed. Basic case information (age, sex, travel, outcome) was collected through routine questionnaires; additional epidemiological data was requested through follow-up patient interviews.

RESULTS

The patients are related and both reported travel to India (overlapping travel dates) before illness onset. Both Typhi genomes belong to the GenoTyphi lineage 4.3.1.1 and carry the azithromycin-resistance gene mph(A) on a PTU-FE (IncFIA/FIB/FII) plasmid. These strains differ genetically from mph(A)-positive Typhi genomes recently reported from Pakistan, suggesting independent emergence of azithromycin resistance in India.

CONCLUSIONS

Cases of typhoid fever caused by Typhi strains resistant to all available oral treatment options are cause for concern and support the need for vaccination of travellers to Typhi endemic regions. US genomic surveillance serves as an important global sentinel for detection of strains with known and emerging antimicrobial resistance profiles, including strains from areas where routine surveillance is not conducted.

Expanding the diversity of origin of transfer-containing sequences in mobilizable plasmids

Conjugative plasmids are important drivers of bacterial evolution. Most plasmids lack genes for conjugation and characterized origins of transfer (oriT), which has hampered our understanding of plasmid mobility. Here we used bioinformatic analyses to characterize occurrences of known oriT families across 38,057 plasmids, confirming that most conjugative and mobilizable plasmids lack identifiable oriTs. Recognizable oriT sequences tend to be intergenic, upstream of relaxase genes and specifically associated with relaxase types. We used these criteria to develop a computational method to search for and identify 21 additional families of oriT-containing sequences in plasmids from the pathogens Escherichia coli, Klebsiella pneumoniae and Acinetobacter baumannii. Sequence analyses found 3,072 occurrences of these oriT-containing sequences across 2,976 plasmids, many of which encoded antimicrobial resistance genes. Six candidate oriT-containing sequences were validated experimentally and were shown to facilitate conjugation in E. coli. These findings expand our understanding of plasmid mobility.

Unveiling host-parasite relationships through conserved MITEs in prokaryote and viral genomes

Transposable elements (TEs) play a pivotal role in the evolution of genomes across all life domains. 'Miniature Inverted-repeat Transposable-Elements' (MITEs) are non-autonomous TEs mainly located in intergenic regions, relying on external transposases for mobilization. The extent of MITEs' mobilome was explored across nearly 1700 prokaryotic genera, 183 232 genomes, revealing a broad distribution. MITEs were identified in 56.5% of genomes, totaling over 1.4 million cMITEs (cellular MITEs). Cluster analysis revealed that 97.4% of cMITEs were specific within genera boundaries, with up to 23% being species-specific. Subsequently, this genus-specificity was evaluated as a method to link microbial host to their viruses. A total of 51 655 cMITEs had counterparts in viral sequences, termed vMITEs (viral MITEs), resulting in the identification of 2500 viral sequences with them. Among these, 1501 sequences were positively assigned to a previously known host (41.8% were isolated viruses and 12.3% were assigned through CRISPR data), while 379 new host-virus associations were predicted. Deeper analysis in Neisseria and Bacteroidota groups allowed the association of 242 and 530 new viral sequences, respectively. MITEs are proposed as a novel approach to establishing valid virus-host relationships.

KleTy: integrated typing scheme for core genome and plasmids reveals repeated emergence of multi-drug resistant epidemic lineages in Klebsiella worldwide

BACKGROUND

Clinically important lineages in Klebsiella, especially those expressing multi-drug resistance (MDR), pose severe threats to public health worldwide. They arose from the co-evolution of the vertically inherited core genome and horizontal gene transfers by plasmids, which has not been systematically explored.

METHODS

We designed KleTy, which consists of dedicated typing schemes for both the core genome and plasmids in Klebsiella. We compared the performance of KleTy with many state-of-the-art pipelines using both simulated and real data.

RESULTS

Employing KleTy, we genotyped 33,272 Klebsiella genomes, categorizing them into 1773 distinct populations and predicting the presence of 87,410 plasmids from 837 clusters (PCs). Notably, Klebsiella is the center of the plasmid-exchange network within Enterobacteriaceae. Our results associated the international emergence of prevalent Klebsiella populations with only four carbapenem-resistance (CR) PCs, two hypervirulent PCs, and two hvCR-PCs encoding both carbapenemase and hypervirulence. Furthermore, we observed the ongoing international emergence of blaNDM, accompanied by the replacement of the previously dominant population, blaKPC-encoding HC1360_8 (CC258), during 2003-2018, with the emerging blaNDM-encoding HC1360_3 (CC147) thereafter. Additionally, expansions of hypervirulent carbapenem-resistant Klebsiella pneumoniae (hvCRKP) were evidenced in both populations, driven by plasmids of MDR-hypervirulence convergences.

CONCLUSIONS

The study illuminates how the global genetic landscape of Klebsiella has been shaped by the co-evolution of both the core genome and the plasmids, underscoring the importance of surveillance and control of the dissemination of plasmids for curtailing the emergence of hvCRKPs.

The role of bacteriophages in facilitating the horizontal transfer of antibiotic resistance genes in municipal wastewater treatment plants

Bacteriophages play integral roles in the ecosystem; however, their precise involvement in horizontal gene transfer and the spread of antibiotic resistance genes (ARGs) are not fully understood. In this study, a coculture system involving consortia of bacteriophages and multidrug-resistant bacteria from an aerobic tank in a municipal wastewater treatment plant (WWTP) was established to investigate the functions of bacteriophages in ARG transfer and spread. The results of the cocultivation of the MRB and bacteriophage consortia indicated that the bacterial community remained stable throughout the whole process, but the addition of bacteriophages significantly increased ARG abundance, especially in bacteriophage DNA. Nine out of the 11 identified ARGs significantly increased, indicating that more bacteriophage particles carried ARGs in the system after cocultivation. In addition, 686 plasmids were detected during cocultivation, of which only 3.36 % were identified as conjugative plasmids, which is significantly lower than the proportion found among previously published plasmids (25.2 %, totaling 14,029 plasmids). Our findings revealed that bacteriophages may play important roles in the horizontal transfer of ARGs through both bacteriophage-mediated conduction and an increase in extracellular ARGs; however, conjugative transfer may not be the main mechanism by which multidrug-resistant bacteria acquire and spread ARGs. Unlike in most previous reports, a coculture system of diverse bacteria and bacteriophages was established in this study to assess bacteriophage functions in ARG transfer and dissemination in the environment, overcoming the limitations associated with the isolation of bacteria and bacteriophages, as well as the specificity of bacteriophage hosts.

CRISPR spacers acquired from plasmids primarily target backbone genes, making them valuable for predicting potential hosts and host range

In recent years, there has been a surge in metagenomic studies focused on identifying plasmids in environmental samples. Although these studies have unearthed numerous novel plasmids, enriching our understanding of their environmental roles, a significant gap remains: the scarcity of information regarding the bacterial hosts of these newly discovered plasmids. Furthermore, even when plasmids are identified within bacterial isolates, the reported host is typically limited to the original isolate, with no insights into alternative hosts or the plasmid's potential host range. Given that plasmids depend on hosts for their existence, investigating plasmids without the knowledge of potential hosts offers only a partial perspective. This study introduces a method for identifying potential hosts and host ranges for plasmids through alignment with CRISPR spacers. To validate the method, we compared the PLSDB plasmids database with the CRISPR spacers database, yielding host predictions for 46% of the plasmids. When compared with reported hosts, our predictions achieved 84% concordance at the family level and 99% concordance at the phylum level. Moreover, the method frequently identified multiple potential hosts for a plasmid, thereby enabling predictions of alternative hosts and the host range. Notably, we found that CRISPR spacers predominantly target plasmid backbone genes while sparing functional genes, such as those linked to antibiotic resistance, aligning with our hypothesis that CRISPR spacers are acquired from plasmid-specific regions rather than insertion elements from diverse sources. Finally, we illustrate the network of connections among different bacterial taxa through plasmids, revealing potential pathways for horizontal gene transfer.IMPORTANCEPlasmids are notorious for their role in distributing antibiotic resistance genes, but they may also carry and distribute other environmentally important genes. Since plasmids are not free-living entities and rely on host bacteria for survival and propagation, predicting their hosts is essential. This study presents a method for predicting potential hosts for plasmids and offers insights into the potential paths for spreading functional genes between different bacteria. Understanding plasmid-host relationships is crucial for comprehending the ecological and clinical impact of plasmids and implications for various biological processes.

Metagenomic profiling of raw wastewater in Portugal highlights microbiota and resistome signatures of public health interest beyond the usual suspects

In response to the rapid emergence and dissemination of antimicrobial resistant bacteria (ARB) and genes (ARGs), integrated surveillance systems are needed to address antimicrobial resistance (AMR) within the One Health Era. Wastewater analyses enable biomarker monitoring at the sewershed level, offering timely insights into pathogen circulation and ARB/ARGs trends originating from different compartments. During two consecutive epidemic waves of the COVID-19 pandemic in Portugal, taxonomic and functional composition of raw urban wastewater from two wastewater treatment plants (WWTPs) representing one million in equivalent population, located in the main urban areas of the country, were profiled by shotgun metagenomics. Hospital wastewater from two central hospitals located in the WWTPs catchment areas were also sequenced. The resistome and virulome were profiled using metagenomic assemblies without taxonomic constraint, and then specifically characterized for ESKAPE pathogens. Urban and hospital wastewater exhibited specific microbiota signatures, Pseudomonadota dominated in the first and Bacteroidota in the latter. Correlation network analyses highlighted 85 (out of top 100) genera co-occurring across samples. The most frequent ARGs were classified in the multidrug, tetracyclines, and Macrolides, Lincosamides, Streptogramins (MLS) classes. Links established between AMR determinants and bacterial hosts evidenced that the diversity and abundance of ARGs is not restricted to ESKAPE, being also highly predominant among emergent enteropathogens, like Aeromonas and Aliarcobacter, or in the iron (II) oxidizer Acidovorax. The Aliarcobacter genus accumulated high abundance of sulphonamides and polymyxins ARGs, while Acinetobacter and Aeromonas hosted the highest abundance of ARGs against beta-lactams. Other bacteria (e.g. Clostridioides, Francisella, Vibrio cholerae) and genes (e.g. vanA-type vancomycin resistance) of public health interest were detected, with targeted monitoring efforts being needed to establish informative baseline data. Altogether, results highlight that wastewater monitoring is a valuable component of pathogen and AMR surveillance in healthy populations, providing a community-representative snapshot of public health trends beyond priority pathogens.

Toxin-Antitoxin Systems Reflect Community Interactions Through Horizontal Gene Transfer

Bacterial evolution through horizontal gene transfer (HGT) reflects their community interactions. In this way, HGT networks do well at mapping community interactions, but offer little toward controlling them-an important step in the translation of synthetic strains into natural contexts. Toxin-antitoxin (TA) systems serve as ubiquitous and diverse agents of selection; however, their utility is limited by their erratic distribution in hosts. Here we examine the heterogeneous distribution of TAs as a consequence of their mobility. By systematically mapping TA systems across a 10,000 plasmid network, we find HGT communities have unique and predictable TA signatures. We propose these TA signatures arise from plasmid competition and have further potential to signal the degree to which plasmids, hosts, and phage interact. To emphasize these relationships, we construct an HGT network based solely on TA similarity, framing specific selection markers in the broader context of bacterial communities. This work both clarifies the evolution of TA systems and unlocks a common framework for manipulating community interactions through TA compatibility.

Applying rearrangement distances to enable plasmid epidemiology with pling

Plasmids are a key vector of antibiotic resistance, but the current bioinformatics toolkit is not well suited to tracking them. The rapid structural changes seen in plasmid genomes present considerable challenges to evolutionary and epidemiological analysis. Typical approaches are either low resolution (replicon typing) or use shared k-mer content to define a genetic distance. However, this distance can both overestimate plasmid relatedness by ignoring rearrangements, and underestimate by over-penalizing gene gain/loss. Therefore a model is needed which captures the key components of how plasmid genomes evolve structurally - through gene/block gain or loss, and rearrangement. A secondary requirement is to prevent promiscuous transposable elements (TEs) leading to over-clustering of unrelated plasmids. We choose the 'Double Cut and Join Indel' (DCJ-Indel) model, in which plasmids are studied at a coarse level, as a sequence of signed integers (representing genes or aligned blocks), and the distance between two plasmids is the minimum number of rearrangement events or indels needed to transform one into the other. We show how this gives much more meaningful distances between plasmids. We introduce a software workflow pling (https://github.com/iqbal-lab-org/pling), which uses the DCJ-Indel model, to calculate distances between plasmids and then cluster them. In our approach, we combine containment distances and DCJ-Indel distances to build a TE-aware plasmid network. We demonstrate superior performance and interpretability to other plasmid clustering tools on the 'Russian Doll' dataset and a hospital transmission dataset.

Unveiling Insights into the Whole Genome Sequencing of Mycobacterium spp. Isolated from Siamese Fighting Fish (Betta splendens)

This study aims to genomically elucidate six isolates of rapidly growing non-tuberculous mycobacteria (RGM) derived from Siamese fighting fish (Betta splendens). These isolates had previously undergone phenotypic and biochemical characterization, antibiotic susceptibility testing, and in vivo virulence assessment. Initial DNA barcoding using the 16S rRNA sequence assigned these six isolates to five different species, namely Mycobacterium chelonae (BN1983), M. cosmeticum (BN1984 and N041), M. farcinogenes (SNSK5), M. mucogenicum (BN1956), and M. senegalense (BN1985). However, the identification relied solely on the highest percent identity of the 16S rRNA gene, raising concerns about the taxonomic ambiguity of these species. Comprehensive whole genome sequencing (WGS) and extended genomic comparisons using multilocus sequence typing (MLST), average nucleotide identity (ANI), and digital DNA-DNA hybridization (dDDH) led to the reclassification of BN1985 and SNSK5 as M. conceptionense while confirming BN1983 as M. chelonae and BN1984 and N041 as M. cosmeticum. Notably, the analysis of the BN1956 isolate revealed a potential new species that is proposed here as M. mucogenicum subsp. phocaicum sp. nov. Common genes encoding "mycobacterial" virulence proteins, such as PE and PPE family proteins, MCE, and YrbE proteins, were detected in all six isolates. Two species, namely M. chelonae and M. cosmeticum, appear to have horizontally acquired T6SS-II (clpB), catalase (katA), GroEL (groel), and capsule (rmlb) from distantly related environmental bacteria such as Klebsiella sp., Neisseria sp., Clostridium sp., and Streptococcus sp. This study provides the first draft genome sequence of RGM isolates currently circulating in B. splendens and underscores the necessity of WGS for the identification and classification of mycobacterial species.

Prophage-encoded antibiotic resistance genes are enriched in human-impacted environments

The spread of antibiotic resistance genes (ARGs) poses a substantial threat to human health. Phage-mediated transduction could exacerbate ARG transmission. While several case studies exist, it is yet unclear to what extent phages encode and mobilize ARGs at the global scale and whether human impacts play a role in this across different habitats. Here, we combine 38,605 bacterial genomes, 1432 metagenomes, and 1186 metatranscriptomes across 12 contrasting habitats to explore the distribution of prophages and their cargo ARGs in natural and human-impacted environments. Worldwide, we observe a significant increase in the abundance, diversity, and activity of prophage-encoded ARGs in human-impacted habitats linked with relatively higher risk of past antibiotic exposure. This effect was driven by phage-encoded cargo ARGs that could be mobilized to provide increased resistance in heterologous E. coli host for a subset of analyzed strains. Our findings suggest that human activities have altered bacteria-phage interactions, enriching ARGs in prophages and making ARGs more mobile across habitats globally.

Modern microbiology: Embracing complexity through integration across scales

Microbes were the only form of life on Earth for most of its history, and they still account for the vast majority of life's diversity. They convert rocks to soil, produce much of the oxygen we breathe, remediate our sewage, and sustain agriculture. Microbes are vital to planetary health as they maintain biogeochemical cycles that produce and consume major greenhouse gases and support large food webs. Modern microbiologists analyze nucleic acids, proteins, and metabolites; leverage sophisticated genetic tools, software, and bioinformatic algorithms; and process and integrate complex and heterogeneous datasets so that microbial systems may be harnessed to address contemporary challenges in health, the environment, and basic science. Here, we consider an inevitably incomplete list of emergent themes in our discipline and highlight those that we recognize as the archetypes of its modern era that aim to address the most pressing problems of the 21st century.

Graphite: painting genomes using a colored de Bruijn graph

The recent growth of microbial sequence data allows comparisons at unprecedented scales, enabling the tracking of strains, mobile genetic elements, or genes. Querying a genome against a large reference database can easily yield thousands of matches that are tedious to interpret and pose computational challenges. We developed Graphite that uses a colored de Bruijn graph (cDBG) to paint query genomes, selecting the local best matches along the full query length. By focusing on the best genomic match of each query region, Graphite reduces the number of matches while providing the most promising leads for sequence tracking or genomic forensics. When applied to hundreds of Campylobacter genomes we found extensive gene sharing, including a previously undetected C. coli plasmid that matched a C. jejuni chromosome. Together, genome painting using cDBGs as enabled by Graphite, can reveal new biological phenomena by mitigating computational hurdles.

Towards quantifying plasmid similarity

Plasmids are extrachromosomal replicons which can quickly spread resistance and virulence genes between clinical pathogens. From the tens of thousands of currently available plasmid sequences we know that overall plasmid diversity is structured, with related plasmids sharing a largely conserved 'backbone' of genes while being able to carry very different genetic cargo. Moreover, plasmid genomes can be structurally plastic and undergo frequent rearrangements. So, how can we quantify plasmid similarity? Answering this question requires practical efforts to sample natural variation as well as theoretical considerations of what defines a group of related plasmids. Here we consider the challenges of analysing and rationalising the current plasmid data deluge to define appropriate similarity thresholds.

Mobile genetic elements define the non-random structure of the Salmonella enterica serovar Typhi pangenome

Bacterial relatedness measured using select chromosomal loci forms the basis of public health genomic surveillance. While approximating vertical evolution through this approach has proven exceptionally valuable for understanding pathogen dynamics, it excludes a fundamental dimension of bacterial evolution-horizontal gene transfer. Incorporating the accessory genome is the logical remediation and has recently shown promise in expanding epidemiological resolution for enteric pathogens. Employing k-mer-based Jaccard index analysis, and a novel genome length distance metric, we computed pangenome (i.e., core and accessory) relatedness for the globally important pathogen Salmonella enterica serotype Typhi (Typhi), and graphically express both vertical (homology-by-descent) and horizontal (homology-by-admixture) evolutionary relationships in a reticulate network of over 2,200 U.S. Typhi genomes. This analysis revealed non-random structure in the Typhi pangenome that is driven predominantly by the gain and loss of mobile genetic elements, confirming and expanding upon known epidemiological patterns, revealing novel plasmid dynamics, and identifying avenues for further genomic epidemiological exploration. With an eye to public health application, this work adds important biological context to the rapidly improving ways of analyzing bacterial genetic data and demonstrates the value of the accessory genome to infer pathogen epidemiology and evolution.IMPORTANCEGiven bacterial evolution occurs in both vertical and horizontal dimensions, inclusion of both core and accessory genetic material (i.e., the pangenome) is a logical step toward a more thorough understanding of pathogen dynamics. With an eye to public, and indeed, global health relevance, we couple contemporary tools for genomic analysis with decades of research on mobile genetic elements to demonstrate the value of the pangenome, known and unknown, annotated, and hypothetical, for stratification of Salmonella enterica serovar Typhi (Typhi) populations. We confirm and expand upon what is known about Typhi epidemiology, plasmids, and antimicrobial resistance dynamics, and offer new avenues of exploration to further deduce Typhi ecology and evolution, and ultimately to reduce the incidence of human disease.

Melon: metagenomic long-read-based taxonomic identification and quantification using marker genes

Long-read sequencing holds great potential for characterizing complex microbial communities, yet taxonomic profiling tools designed specifically for long reads remain lacking. We introduce Melon, a novel marker-based taxonomic profiler that capitalizes on the unique attributes of long reads. Melon employs a two-stage classification scheme to reduce computational time and is equipped with an expectation-maximization-based post-correction module to handle ambiguous reads. Melon achieves superior performance compared to existing tools in both mock and simulated samples. Using wastewater metagenomic samples, we demonstrate the applicability of Melon by showing it provides reliable estimates of overall genome copies, and species-level taxonomic profiles.

Bacterial strain sharing between humans, animals, and the environment among urban households

Identifying bacterial transmission pathways is crucial to inform strategies aimed at curbing the spread of pathogenic and antibiotic-resistant bacteria, especially in rapidly urbanizing low- and middle-income countries. In this study, we assessed bacterial strain-sharing and dissemination of antibiotic resistance across humans, domesticated poultry, canines, household soil, and drinking water in urban informal settlements in Nairobi, Kenya. We collected 321 samples from 50 households and performed Pooling Isolated Colonies-seq (PIC-seq) by sequencing pools of up to five Escherichia coli colonies per sample to capture strain diversity, strain-sharing patterns, and overlap of antibiotic-resistant genes (ARGs). Bacterial strains isolated from the household environment carried clinically relevant ARGs, reinforcing the role of the environment in antibiotic resistance dissemination. Strain-sharing rates and resistome similarities across sample types were strongly correlated within households, suggesting clonal spread of bacteria is a main driver of dissemination of ARGs in the domestic urban environment. Within households, E. coli strain-sharing was rare between humans and animals but more frequent between humans and drinking water. E. coli contamination in stored drinking water was also associated with higher strain-sharing between humans in the same household. Our study demonstrates that contaminated drinking water facilitates human to human strain sharing and water treatment can disrupt transmission.

Horizontal plasmid transfer promotes antibiotic resistance in selected bacteria in Chinese frog farms

The emergence and dissemination of antibiotic resistance genes (ARGs) in the ecosystem are global public health concerns. One Health emphasizes the interconnectivity between different habitats and seeks to optimize animal, human, and environmental health. However, information on the dissemination of antibiotic resistance genes (ARGs) within complex microbiomes in natural habitats is scarce. We investigated the prevalence of antibiotic resistant bacteria (ARB) and the spread of ARGs in intensive bullfrog (Rana catesbeiana) farms in the Shantou area of China. Antibiotic susceptibilities of 361 strains, combined with microbiome analyses, revealed Escherichia coli, Edwardsiella tarda, Citrobacter and Klebsiella sp. as prevalent multidrug resistant bacteria on these farms. Whole genome sequencing of 95 ARB identified 250 large plasmids that harbored a wide range of ARGs. Plasmid sequences and sediment metagenomes revealed an abundance of tetA, sul1, and aph(3″)-Ib ARGs. Notably, antibiotic resistance (against 15 antibiotics) highly correlated with plasmid-borne rather than chromosome-borne ARGs. Based on sequence similarities, most plasmids (62%) fell into 32 distinct groups, indicating a potential for horizontal plasmid transfer (HPT) within the frog farm microbiome. HPT was confirmed in inter- and intra-species conjugation experiments. Furthermore, identical mobile ARGs, flanked by mobile genetic elements (MGEs), were found in different locations on the same plasmid, or on different plasmids residing in the same or different hosts. Our results suggest a synergy between MGEs and HPT to facilitate ARGs dissemination in frog farms. Mining public databases retrieved similar plasmids from different bacterial species found in other environmental niches globally. Our findings underscore the importance of HPT in mediating the spread of ARGs in frog farms and other microbiomes of the ecosystem.

Pseudogenes in plasmid genomes reveal past transitions in plasmid mobility

Evidence for gene non-functionalization due to mutational processes is found in genomes in the form of pseudogenes. Pseudogenes are known to be rare in prokaryote chromosomes, with the exception of lineages that underwent an extreme genome reduction (e.g. obligatory symbionts). Much less is known about the frequency of pseudogenes in prokaryotic plasmids; those are genetic elements that can transfer between cells and may encode beneficial traits for their host. Non-functionalization of plasmid-encoded genes may alter the plasmid characteristics, e.g. mobility, or their effect on the host. Analyzing 10 832 prokaryotic genomes, we find that plasmid genomes are characterized by threefold-higher pseudogene density compared to chromosomes. The majority of plasmid pseudogenes correspond to deteriorated transposable elements. A detailed analysis of enterobacterial plasmids furthermore reveals frequent gene non-functionalization events associated with the loss of plasmid self-transmissibility. Reconstructing the evolution of closely related plasmids reveals that non-functionalization of the conjugation machinery led to the emergence of non-mobilizable plasmid types. Examples are virulence plasmids in Escherichia and Salmonella. Our study highlights non-functionalization of core plasmid mobility functions as one route for the evolution of domesticated plasmids. Pseudogenes in plasmids supply insights into past transitions in plasmid mobility that are akin to transitions in bacterial lifestyle.

Paramixta manurensis gen. nov., sp. nov., a novel member of the family Erwiniaceae producing indole-3-acetic acid isolated from mushroom compost

There are numerous species in the Erwiniaceae family that are important for agricultural and clinical purposes. Here we described the Erwiniaceae bacterium PD-1 isolated from mushroom (Pleurotus eryngii) compost. Comparative genomic and phylogenetic analyses showed that the strain PD-1 was assigned to a new genus and species, Paramixta manurensis gen. nov., sp. nov. in the family Erwiniaceae. From the average amino acid index, we identified the five AroBEKAC proteins in the shikimate pathway as a minimal set of molecular markers to reconstruct the phylogenetic tree of the Erwiniaceae species. The strain PD-1 containing annotated genes for ubiquinone and menaquinone produced a higher level of ubiquinone (Q8) than demethylmenaquinone (DMK8) and menaquinone (MK8) in anaerobic condition compared to aerobic condition, as similarly did the reference strains from the genera Mixta and Erwinia. Results from fatty acid methyl ester and numerical analyses of strain PD-1 showed a similarity to species of the genera Mixta and Winslowiella. This study revealed that the strain's ability to utilize polyols, such as glycerol, erythritol, and D-arabitol, distinguished the strain PD-1 from the nearest relative and other type strains. The analyzed genetic markers and biochemical properties of the strain PD-1 suggest its potential role in the process of mushroom compost through the degradation of carbohydrates and polysaccharides derived from fungi and plants. Additionally, it can produce a high concentration of indole-3-acetic acid as a plant growth-promoting agent.

The IncC and IncX1 resistance plasmids present in multi-drug resistant Escherichia coli strains isolated from poultry manure in Poland

The study describes the whole-genome sequencing of two antibiotic-resistant representative Escherichia coli strains, isolated from poultry manure in 2020. The samples were obtained from a commercial chicken meat production facility in Poland. The antibiotic resistance profile was characterized by co-resistance to β-lactam antibiotics, aminoglycosides, and fluoroquinolones. The three identified resistance plasmids (R-plasmids), pECmdr13.2, pECmdr13.3, and pECmdr14.1, harbored various genes conferring resistance to tetracyclines (tetR[A]) for, aminoglycoside (aph, aac, and aad families), β-lactam (blaCMY-2, blaTEM-176), sulfonamide (sul1, sul2), fluoroquinolone (qnrS1), and phenicol (floR). These plasmids, which have not been previously reported in Poland, were found to carry IS26 insertion elements, the intI1-integrase gene, and conjugal transfer genes, facilitating horizontal gene transfer. Plasmids pECmdr13.2 and pECmdr14.1 also possessed a mercury resistance gene operon related to transposon Tn6196; this promotes plasmid persistence even without antibiotic selection pressure due to co-selection mechanisms such as co-resistance. The chicken manure-derived plasmids belonged to the IncX1 (narrow host range) and IncC (broad host range) incompatibility groups. Similar plasmids have been identified in various environments, clinical isolates, and farm animals, including cattle, swine, and poultry. This study holds significant importance for the One Health approach, as it highlights the potential for antibiotic-resistant bacteria from livestock and food sources, particularly E. coli, to transfer through the food chain to humans and vice versa.

Phage-plasmids: missed links between mobile genetic elements

Phages and plasmids are discrete mobile genetic elements (MGEs) with critical roles in gene dissemination across bacteria but limited scope for exchanging DNA between them. By investigating recent gene-sharing events, Pfeifer and Rocha describe how the hybrid elements phage-plasmids (P-Ps) promote gene flow between MGE types and evolve into new ones.

Short-chain fatty acids inhibit bacterial plasmid transfer through conjugation in vitro and in ex vivo chicken tissue explants

The animal gut acts as a potent reservoir for spreading and maintaining conjugative plasmids that confer antimicrobial resistance (AMR), fitness, and virulence attributes. Interventions that inhibit the continued emergence and expansion of AMR and virulent strains in agricultural and clinical environments are greatly desired. This study aims to determine the presence and efficacy of short-chain fatty acids (SCFA) inhibitory effects on the conjugal transfer of AMR plasmids. In vitro broth conjugations were conducted between donor Escherichia coli strains carrying AMP plasmids and the plasmid-less Escherichia coli HS-4 recipient strain. Conjugations were supplemented with ddH2O or SCFAs at 1, 0.1, 0.01, or 0.001 molar final concentration. The addition of SCFAs completely inhibited plasmid transfer at 1 and 0.1 molar and significantly (p < 0.05) reduced transfer at 0.01 molar, regardless of SCFA tested. In explant models for the chicken ceca, either ddH2O or a final concentration of 0.025 M SCFAs were supplemented to the explants infected with donor and recipient E. coli. In every SCFA tested, significant decreases in transconjugant populations compared to ddH2O-treated control samples were observed with minimal effects on donor and recipient populations. Finally, significant reductions in transconjugants for plasmids of each incompatibility type (IncP1ε, IncFIβ, and IncI1) tested were detected. This study demonstrates for the first time the broad inhibition ability of SCFAs on bacterial plasmid transfer and eliminates AMR with minimal effect on bacteria. Implementing interventions that increase the concentrations of SCFAs in the gut may be a viable method to reduce the risk, incidence, and rate of AMR emergence in agricultural and human environments.

Wide distribution of plasmid mediated quinolone resistance gene, qnrS, among Salmonella spp. isolated from canal water in Thailand

AIMS

This research focused on assessing the prevalence of plasmid-mediated quinolone resistance (PMQR) determinants and antimicrobial susceptibility in Salmonella strains isolated from Thai canal water.

METHODS AND RESULTS

From 2016 to 2020, 333 water samples were collected from six canals across Bangkok, Thailand. Salmonella spp. was isolated, PMQR genes were detected through polymerase chain reactions, and the antimicrobial susceptibility was examined using the disk diffusion method. The results indicated a 92.2% prevalence of Salmonella spp. in canal water, being serogroups B and C the most frequently detected. Overall, 35.3% of isolates harbored PMQR genes, being qnrS the most prevalent gene (97.2%, n = 137/141). Other PMQR genes, including qnrB, qnrD, oqxAB, and aac(6')-Ib-cr, were detected. Notably, six isolates harbored multiple PMQR genes. Furthermore, 9.3% and 3.8% of the overall isolates were resistant to nalidixic acid (NAL) and ciprofloxacin (CIP), respectively. PMQR-positive isolates showed higher rates of non-susceptibility to both NAL (48.2%, n = 68/141) and CIP (92.2%, n = 130/141) compared to PMQR-negative isolates (NAL: 8.9%, n = 23/258; CIP: 11.2%, n = 30/258).

CONCLUSIONS

The high prevalence of Salmonella spp., significant PMQR-positive, and reduced susceptibility isolates in canal water is of public health concern in Bangkok.

A Broad-Spectrum Horizontal Transfer Inhibitor Prevents Transmission of Plasmids Carrying Multiple Antibiotic Resistance Genes

The dissemination of antimicrobial resistance (AMR) severely degrades the performance of antibiotics and constantly paralyzes the global health system. In particular, plasmid-mediated transfer of antibiotic resistance genes (ARGs) across bacteria is recognized as the primary driver. Therefore, antiplasmid transfer approaches are urgently warranted to resolve this intractable problem. Herein, we demonstrated the potential of azidothymidine (AZT), an FDA-approved anti-HIV drug, as a broad-spectrum horizontal transfer inhibitor to effectively prevent the transmission of multiple ARGs, including mcr-1, bla NDM-5, and tet(X4), both in vitro and in vivo. It was also noteworthy that the inhibitory effect of AZT was proved to be valid within and across bacterial genera under different mating conditions. Mechanistic studies revealed that AZT dissipated bacterial proton motive force, which was indispensable for ATP synthesis and flagellar motility. In addition, AZT downregulated bacterial secretion systems involving general and type IV secretion systems (T4SS). Furthermore, the thymidine kinase, which is associated with DNA synthesis, turned out to be the potential target of AZT. Collectively, our work demonstrates the broad inhibitory effect of AZT in preventing ARGs transmission, opening new horizons for controlling AMR.