Negative feedback and transcriptional overshooting in a regulatory network for horizontal gene transfer

The winding journey of conjugative plasmids toward a novel host cell

Horizontal transfer of plasmids by conjugation is a fundamental mechanism driving the widespread dissemination of drug resistance among bacterial populations. The successful colonization of a new host cell necessitates the plasmid to navigate through a series of sequential steps, each dependent on specific plasmid or host factors. This review explores recent advancements in comprehending the cellular and molecular mechanisms that govern plasmid transmission, establishment, and long-term maintenance. Adopting a plasmid-centric perspective, we describe the critical steps and bottlenecks in the plasmid's journey toward a new host cell, encompassing exploration and contact initiation, invasion, establishment and control, and assimilation.

Antimicrobial resistance level and conjugation permissiveness shape plasmid distribution in clinical enterobacteria

Conjugative plasmids play a key role in the dissemination of antimicrobial resistance (AMR) genes across bacterial pathogens. AMR plasmids are widespread in clinical settings, but their distribution is not random, and certain associations between plasmids and bacterial clones are particularly successful. For example, the globally spread carbapenem resistance plasmid pOXA-48 can use a wide range of enterobacterial species as hosts, but it is usually associated with a small number of specific Klebsiella pneumoniae clones. These successful associations represent an important threat for hospitalized patients. However, knowledge remains limited about the factors determining AMR plasmid distribution in clinically relevant bacteria. Here, we combined in vitro and in vivo experimental approaches to analyze pOXA-48-associated AMR levels and conjugation dynamics in a collection of wild-type enterobacterial strains isolated from hospitalized patients. Our results revealed significant variability in these traits across different bacterial hosts, with Klebsiella spp. strains showing higher pOXA-48-mediated AMR and conjugation frequencies than Escherichia coli strains. Using experimentally determined parameters, we developed a simple mathematical model to interrogate the contribution of AMR levels and conjugation permissiveness to plasmid distribution in bacterial communities. The simulations revealed that a small subset of clones, combining high AMR levels and conjugation permissiveness, play a critical role in stabilizing the plasmid in different polyclonal microbial communities. These results help to explain the preferential association of plasmid pOXA-48 with K. pneumoniae clones in clinical settings. More generally, our study reveals that species- and strain-specific variability in plasmid-associated phenotypes shape AMR evolution in clinically relevant bacterial communities.

Dissemination of IncI plasmid encoding bla CTX-M-1 is not hampered by its fitness cost in the pig's gut

Multiresistance plasmids belonging to the IncI incompatibility group have become one of the most pervasive plasmid types in extended-spectrum beta-lactamase-producing Escherichia coli of animal origin. The extent of the burden imposed on the bacterial cell by these plasmids seems to modulate the emergence of "epidemic" plasmids. However, in vivo data in the natural environment of the strains are scarce. Here, we investigated the cost of a bla CTX-M-1-IncI1 epidemic plasmid in a commensal E. coli animal strain, UB12-RC, before and after oral inoculation of 15 6- to 8-week- old specific-pathogen-free pigs. Growth rate in rich medium was determined on (i) UB12-RC and derivatives, with or without plasmid, in vivo and/or in vitro evolved, and (ii) strains that acquired the plasmid in the gut during the experiment. Although bla CTX-M-1-IncI1 plasmid imposed no measurable burden on the recipient strain after conjugation and during the longitudinal carriage in the pig's gut, we observed a significant difference in the bacterial growth rate between IncI1 plasmid-carrying and plasmid-free isolates collected during in vivo carriage. Only a few mutations on the chromosome of the UB12-RC derivatives were detected by whole-genome sequencing. RNA-Seq analysis of a selected set of these strains showed that transcriptional responses to the bla CTX-M-1-IncI1 acquisition were limited, affecting metabolism, stress response, and motility functions. Our data suggest that the effect of IncI plasmid on host cells is limited, fitness cost being insufficient to act as a barrier to IncI plasmid spread among natural population of E. coli in the gut niche.

Toward Microbiome Engineering: Expanding the Repertoire of Genetically Tractable Members of the Human Gut Microbiome

Genetic manipulation is necessary to interrogate the functions of microbes in their environments, such as the human gut microbiome. Yet, the vast majority of human gut microbiome species are not genetically tractable. Here, we review the hurdles to seizing genetic control of more species. We address the barriers preventing the application of genetic techniques to gut microbes and report on genetic systems currently under development. While methods aimed at genetically transforming many species simultaneously in situ show promise, they are unable to overcome many of the same challenges that exist for individual microbes. Unless a major conceptual breakthrough emerges, the genetic tractability of the microbiome will remain an arduous task. Increasing the list of genetically tractable organisms from the human gut remains one of the highest priorities for microbiome research and will provide the foundation for microbiome engineering.

Evaporation-induced hydrodynamics control plasmid transfer during surface-associated microbial growth

Droplet evaporation is a general process in unsaturated environments that results in micro-scale hydrodynamic flows which in turn determine the spatial distributions of microbial cells across surfaces. These spatial distributions can have significant effects on the development and functioning of surface-associated microbial communities, with consequences for important processes such as the spread of plasmids. Here, we experimentally quantified how evaporation-induced hydrodynamic processes modulate the initial deposition patterns of microbial cells (via the coffee ring effect and Marangoni convection) and how these patterns control the spread of an antibiotic resistance-encoding plasmid during surface-associated growth. We found that plasmid spread is a function of the initial density of cells deposited along the droplet periphery, which is a manifestation of the coffee ring effect. Using an individual-based model, we systematically linked how the different initial cell deposition patterns caused by the relative strengths of the coffee ring effect and Marangoni convection determine the extent of plasmid transfer during surface-associated growth. Our study demonstrates that evaporation-induced hydrodynamic processes that are common in nature can alter crucial ecological properties of surface-associated microbial communities and control the proliferation of plasmids, with consequences on the spread of antibiotic resistance and other plasmid-encoded traits.

Atypical low-copy number plasmid segregation systems, all in one?

Plasmid families harbor different maintenances functions, depending on their size and copy number. Low copy number plasmids rely on active partition systems, organizing a partition complex at specific centromere sites that is actively positioned using NTPase proteins. Some low copy number plasmids lack an active partition system, but carry atypical intracellular positioning systems using a single protein that binds to the centromere site but without an associated NTPase. These systems have been studied in the case of the Escherichia coli R388 and of the Staphylococcus aureus pSK1 plasmids. Here we review these two systems, which appear to be unrelated but share common features, such as their distribution on plasmids of medium size and copy number, certain activities of their centromere-binding proteins, StbA and Par, respectively, as well as their mode of action, which may involve dynamic interactions with the nucleoid-packed chromosome of their hosts.

Tradeoff between lag time and growth rate drives the plasmid acquisition cost

Conjugative plasmids drive genetic diversity and evolution in microbial populations. Despite their prevalence, plasmids can impose long-term fitness costs on their hosts, altering population structure, growth dynamics, and evolutionary outcomes. In addition to long-term fitness costs, acquiring a new plasmid introduces an immediate, short-term perturbation to the cell. However, due to the transient nature of this plasmid acquisition cost, a quantitative understanding of its physiological manifestations, overall magnitudes, and population-level implications, remains unclear. To address this, here we track growth of single colonies immediately following plasmid acquisition. We find that plasmid acquisition costs are primarily driven by changes in lag time, rather than growth rate, for nearly 60 conditions covering diverse plasmids, selection environments, and clinical strains/species. Surprisingly, for a costly plasmid, clones exhibiting longer lag times also achieve faster recovery growth rates, suggesting an evolutionary tradeoff. Modeling and experiments demonstrate that this tradeoff leads to counterintuitive ecological dynamics, whereby intermediate-cost plasmids outcompete both their low and high-cost counterparts. These results suggest that, unlike fitness costs, plasmid acquisition dynamics are not uniformly driven by minimizing growth disadvantages. Moreover, a lag/growth tradeoff has clear implications in predicting the ecological outcomes and intervention strategies of bacteria undergoing conjugation.

Structural basis of direct and inverted DNA sequence repeat recognition by helix-turn-helix transcription factors

Some transcription factors bind DNA motifs containing direct or inverted sequence repeats. Preference for each of these DNA topologies is dictated by structural constraints. Most prokaryotic regulators form symmetric oligomers, which require operators with a dyad structure. Binding to direct repeats requires breaking the internal symmetry, a property restricted to a few regulators, most of them from the AraC family. The KorA family of transcriptional repressors, involved in plasmid propagation and stability, includes members that form symmetric dimers and recognize inverted repeats. Our structural analyses show that ArdK, a member of this family, can form a symmetric dimer similar to that observed for KorA, yet it binds direct sequence repeats as a non-symmetric dimer. This is possible by the 180° rotation of one of the helix-turn-helix domains. We then probed and confirmed that ArdK shows affinity for an inverted repeat, which, surprisingly, is also recognized by a non-symmetrical dimer. Our results indicate that structural flexibility at different positions in the dimerization interface constrains transcription factors to bind DNA sequences with one of these two alternative DNA topologies.

Characterization of the DNA Binding Domain of StbA, A Key Protein of A New Type of DNA Segregation System

Low-copy-number plasmids require sophisticated genetic devices to achieve efficient segregation of plasmid copies during cell division. Plasmid R388 uses a unique segregation mechanism, based on StbA, a small multifunctional protein. StbA is the key protein in a segregation system not involving a plasmid-encoded NTPase partner, it regulates the expression of several plasmid operons, and it is the main regulator of plasmid conjugation. The mechanisms by which StbA, together with the centromere-like sequence stbS, achieves segregation, is largely uncharacterized. To better understand the molecular basis of R388 segregation, we determined the crystal structure of the conserved N-terminal domain of StbA to 1.9 Å resolution. It folds into an HTH DNA-binding domain, structurally related to that of the PadR subfamily II of transcriptional regulators. StbA is organized in two domains. Its N-terminal domain carries the specific stbS DNA binding activity. A truncated version of StbA, deleted of its C-terminal domain, displays only partial activities in vivo, indicating that the non-conserved C-terminal domain is required for efficient segregation and subcellular plasmid positioning. The structure of StbA DNA-binding domain also provides some insight into how StbA monomers cooperate to repress transcription by binding to the stbDR and to form the segregation complex with stbS.

Evolution of horizontal transmission in antimicrobial resistance plasmids

Mobile genetic elements (MGEs) are one of the main vectors for the spread of antimicrobial resistance (AMR) across bacteria, due to their ability to move horizontally between bacterial lineages. Horizontal transmission of AMR can increase AMR prevalence at multiple scales, from increasing the prevalence of infections by resistant bacteria to pathogen epidemics and worldwide spread of AMR across species. Among MGEs, conjugative plasmids are the main contributors to the spread of AMR. This review discusses the selective pressures acting on MGEs and their hosts to promote or limit the horizontal transmission of MGEs, the mechanisms by which transmission rates can evolve, and their implications for limiting the spread of AMR, with a focus on AMR plasmids.

PixR, a Novel Activator of Conjugative Transfer of IncX4 Resistance Plasmids, Mitigates the Fitness Cost of mcr-1 Carriage in Escherichia coli

The emergence of the plasmid-borne colistin resistance gene mcr-1 threatens public health. IncX4-type plasmids are one of the most epidemiologically successful vehicles for spreading mcr-1 worldwide. Since MCR-1 is known for imposing a fitness cost to its host bacterium, the successful spread of mcr-1-bearing plasmids might be linked to high conjugation frequency, which would enhance the maintenance of the plasmid in the host without antibiotic selection. However, the mechanism of IncX4 plasmid conjugation remains unclear. In this study, we used high-density transposon mutagenesis to identify factors required for IncX4 plasmid transfer. Eighteen essential transfer genes were identified, including five with annotations unrelated to conjugation. Cappable-seq, transcriptome sequencing (RNA-seq), electrophoretic mobility shift assay, and β-galactosidase assay confirmed that a novel transcriptional regulator gene, pixR, directly regulates the transfer of IncX4 plasmids by binding the promoter of 13 essential transfer genes to increase their transcription. PixR is not active under nonmating conditions, while the expression of the pixR, pilX3-4, and pilX11 genes increased 3- to 6-fold upon contact with recipient Escherichia coli C600. Plasmid invasion and coculture competition assays revealed the essentiality of pixR for spreading and persistence of mcr-1-bearing IncX4 plasmids in bacterial populations. Effective conjugation is crucial for alleviating the fitness cost exerted by mcr-1 carriage. The existence of the IncX4-specific pixR gene increases plasmid transmissibility while promoting the invasion and persistence of mcr-1-bearing plasmids in bacterial populations, which helps explain their global prevalence. IMPORTANCE The spread of clinically relevant antibiotic resistance genes is often linked to the dissemination of epidemic plasmids. However, the underlying molecular mechanisms contributing to the successful spread of epidemic plasmids remain unclear. In this report, we shine a light on the transfer activation of IncX4 plasmids. We show how conjugation promotes the invasion and persistence of IncX4 plasmids within a bacterial population. The dissection of the regulatory network of conjugation helps explain the rapid spread of epidemic plasmids in nature. It also reveals potential targets for the development of conjugation inhibitors.

Translational demand is not a major source of plasmid-associated fitness costs

Plasmids are key drivers of bacterial evolution because they are crucial agents for the horizontal transfer of adaptive traits, such as antibiotic resistance. Most plasmids entail a metabolic burden that reduces the fitness of their host if there is no selection for plasmid-encoded genes. It has been hypothesized that the translational demand imposed by plasmid-encoded genes is a major mechanism driving the fitness cost of plasmids. Plasmid-encoded genes typically present a different codon usage from host chromosomal genes. As a consequence, the translation of plasmid-encoded genes might sequestrate ribosomes on plasmid transcripts, overwhelming the translation machinery of the cell. However, the pervasiveness and origins of the translation-derived costs of plasmids are yet to be assessed. Here, we systematically altered translation efficiency in the host cell to disentangle the fitness effects produced by six natural antibiotic resistance plasmids. We show that limiting translation efficiency either by reducing the number of available ribosomes or their processivity does not increase plasmid costs. Overall, our results suggest that ribosomal paucity is not a major contributor to plasmid fitness costs. This article is part of the theme issue 'The secret lives of microbial mobile genetic elements'.

Harmful behaviour through plasmid transfer: a successful evolutionary strategy of bacteria harbouring conjugative plasmids

Conjugative plasmids are extrachromosomal mobile genetic elements pervasive among bacteria. Plasmids' acquisition often lowers cells' growth rate, so their ubiquity has been a matter of debate. Chromosomes occasionally mutate, rendering plasmids cost-free. However, these compensatory mutations typically take hundreds of generations to appear after plasmid arrival. By then, it could be too late to compete with fast-growing plasmid-free cells successfully. Moreover, arriving plasmids would have to wait hundreds of generations for compensatory mutations to appear in the chromosome of their new host. We hypothesize that plasmid-donor cells may use the plasmid as a 'weapon' to compete with plasmid-free cells, particularly in structured environments. Cells already adapted to plasmids may increase their inclusive fitness through plasmid transfer to impose a cost to nearby plasmid-free cells and increase the replication opportunities of nearby relatives. A mathematical model suggests conditions under which the proposed hypothesis works, and computer simulations tested the long-term plasmid maintenance. Our hypothesis explains the maintenance of conjugative plasmids not coding for beneficial genes. This article is part of the theme issue 'The secret lives of microbial mobile genetic elements'.

Comment on "Indirect Fitness Benefits Enable the Spread of Host Genes Promoting Costly Transfer of Beneficial Plasmids"

Plasmid transfer contributes significantly to bacterial evolution, but the forces selecting such generosity are poorly understood; this Formal Comment revisits a study which examined these forces both analytically and experimentally, making a correction to the algebra and reaching strikingly different results.

Ecological and evolutionary solutions to the plasmid paradox

The 'plasmid paradox' arises because, although plasmids are common features of bacterial genomes, theoretically they should not exist: rates of conjugation were believed insufficient to allow plasmids to persist by infectious transmission, whereas the costs of plasmid maintenance meant that plasmids should be purged by negative selection regardless of whether they encoded beneficial accessory traits because these traits should eventually be captured by the chromosome, enabling the loss of the redundant plasmid. In the decade since the plasmid paradox was described, new data and theory show that a range of ecological and evolutionary mechanisms operate in bacterial populations and communities to explain the widespread distribution and stable maintenance of plasmids. We conclude, therefore, that multiple solutions to the plasmid paradox are now well understood. The current challenge for the field, however, is to better understand how these solutions operate in natural bacterial communities to explain and predict the distribution of plasmids and the dynamics of the horizontal gene transfer that they mediate in bacterial (pan)genomes.

Monitoring Bacterial Conjugation by Optical Microscopy

Bacterial conjugation is the main mechanism for horizontal gene transfer, conferring plasticity to the genome repertoire. This process is also the major instrument for the dissemination of antibiotic resistance genes. Hence, gathering primary information of the mechanism underlying this genetic transaction is of a capital interest. By using fluorescent protein fusions to the ATPases that power conjugation, we have been able to track the localization of these proteins in the presence and absence of recipient cells. Moreover, we have found that more than one copy of the conjugative plasmid is transferred during mating. Altogether, these findings provide new insights into the mechanism of such an important gene transfer device.

The Evolution of Plasmid Transfer Rate in Bacteria and Its Effect on Plasmid Persistence

AbstractPlasmids are extrachromosomal segments of DNA that can transfer genes between bacterial cells. Many plasmid genes benefit bacteria but cause harm to human health by granting antibiotic resistance to pathogens. Transfer rate is a key parameter for predicting plasmid dynamics, but observed rates are highly variable, and the effects of selective forces on their evolution are unclear. We apply evolutionary analysis to plasmid conjugation models to investigate selective pressures affecting plasmid transfer rate, emphasizing host versus plasmid control, the costs of plasmid transfer, and the role of recipient cells. Our analyses show that plasmid-determined transfer rates can be predicted with three parameters (host growth rate, plasmid loss rate, and the cost of plasmid transfer on growth) under some conditions. We also show that low-frequency genetic variation in transfer rate can accumulate, facilitating rapid adaptation to changing conditions. Furthermore, reduced transfer rates due to host control have limited effects on plasmid prevalence until low enough to prevent plasmid persistence. These results provide a framework to predict plasmid transfer rate evolution in different environments and demonstrate the limited impact of host mechanisms to control the costs incurred when plasmids are present.

Frequency-preference response in covalent modification cycles under substrate sequestration conditions

Covalent modification cycles (CMCs) are basic units of signaling systems and their properties are well understood. However, their behavior has been mostly characterized in situations where the substrate is in excess over the modifying enzymes. Experimental data on protein abundance suggest that the enzymes and their target proteins are present in comparable concentrations, leading to substrate sequestration by the enzymes. In this enzyme-in-excess regime, CMCs have been shown to exhibit signal termination, the ability of the product to return to a stationary value lower than its peak in response to constant stimulation, while this stimulation is still active, with possible implications for the ability of systems to adapt to environmental inputs. We characterize the conditions leading to signal termination in CMCs in the enzyme-in-excess regime. We also demonstrate that this behavior leads to a preferred frequency response (band-pass filters) when the cycle is subjected to periodic stimulation, whereas the literature reports that CMCs investigated so far behave as low-pass filters. We characterize the relationship between signal termination and the preferred frequency response to periodic inputs and we explore the dynamic mechanism underlying these phenomena. Finally, we describe how the behavior of CMCs is reflected in similar types of responses in the cascades of which they are part. Evidence of protein abundance in vivo shows that enzymes and substrates are present in comparable concentrations, thus suggesting that signal termination and frequency-preference response to periodic inputs are also important dynamic features of cell signaling systems, which have been overlooked.

Molecular Mechanisms Influencing Bacterial Conjugation in the Intestinal Microbiota

Bacterial conjugation is a widespread and particularly efficient strategy to horizontally disseminate genes in microbial populations. With a rich and dense population of microorganisms, the intestinal microbiota is often considered a fertile environment for conjugative transfer and a major reservoir of antibiotic resistance genes. In this mini-review, we summarize recent findings suggesting that few conjugative plasmid families present in Enterobacteriaceae transfer at high rates in the gut microbiota. We discuss the importance of mating pair stabilization as well as additional factors influencing DNA transfer efficiency and conjugative host range in this environment. Finally, we examine the potential repurposing of bacterial conjugation for microbiome editing.

A Framework for Implementing Metaheuristic Algorithms Using Intercellular Communication

Metaheuristics (MH) are Artificial Intelligence procedures that frequently rely on evolution. MH approximate difficult problem solutions, but are computationally costly as they explore large solution spaces. This work pursues to lay the foundations of general mappings for implementing MH using Synthetic Biology constructs in cell colonies. Two advantages of this approach are: harnessing large scale parallelism capability of cell colonies and, using existing cell processes to implement basic dynamics defined in computational versions. We propose a framework that maps MH elements to synthetic circuits in growing cell colonies to replicate MH behavior in cell colonies. Cell-cell communication mechanisms such as quorum sensing (QS), bacterial conjugation, and environmental signals map to evolution operators in MH techniques to adapt to growing colonies. As a proof-of-concept, we implemented the workflow associated to the framework: automated MH simulation generators for the gro simulator and two classes of algorithms (Simple Genetic Algorithms and Simulated Annealing) encoded as synthetic circuits. Implementation tests show that synthetic counterparts mimicking MH are automatically produced, but also that cell colony parallelism speeds up the execution in terms of generations. Furthermore, we show an example of how our framework is extended by implementing a different computational model: The Cellular Automaton.

Biosolids as a Source of Antibiotic Resistance Plasmids for Commensal and Pathogenic Bacteria

Antibiotic resistance (AR) is a threat to modern medicine, and plasmids are driving the global spread of AR by horizontal gene transfer across microbiomes and environments. Determining the mobile resistome responsible for this spread of AR among environments is essential in our efforts to attenuate the current crisis. Biosolids are a wastewater treatment plant (WWTP) byproduct used globally as fertilizer in agriculture. Here, we investigated the mobile resistome of biosolids that are used as fertilizer. This was done by capturing resistance plasmids that can transfer to human pathogens and commensal bacteria. We used a higher-throughput version of the exogenous plasmid isolation approach by mixing several ESKAPE pathogens and a commensal Escherichia coli with biosolids and screening for newly acquired resistance to about 10 antibiotics in these strains. Six unique resistance plasmids transferred to Salmonella typhimurium, Klebsiella aerogenes, and E. coli. All the plasmids were self-transferable and carried 3-6 antibiotic resistance genes (ARG) conferring resistance to 2-4 antibiotic classes. These plasmids-borne resistance genes were further embedded in genetic elements promoting intracellular recombination (i.e., transposons or class 1 integrons). The plasmids belonged to the broad-host-range plasmid (BHR) groups IncP-1 or PromA. Several of them were persistent in their new hosts when grown in the absence of antibiotics, suggesting that the newly acquired drug resistance traits would be sustained over time. This study highlights the role of BHRs in the spread of ARG between environmental bacteria and human pathogens and commensals, where they may persist. The work further emphasizes biosolids as potential vehicles of highly mobile plasmid-borne antibiotic resistance genes.

Conjugation dynamics depend on both the plasmid acquisition cost and the fitness cost

Plasmid conjugation is a major mechanism responsible for the spread of antibiotic resistance. Plasmid fitness costs are known to impact long-term growth dynamics of microbial populations by providing plasmid-carrying cells a relative (dis)advantage compared to plasmid-free counterparts. Separately, plasmid acquisition introduces an immediate, but transient, metabolic perturbation. However, the impact of these short-term effects on subsequent growth dynamics has not previously been established. Here, we observed that de novo transconjugants grew significantly slower and/or with overall prolonged lag times, compared to lineages that had been replicating for several generations, indicating the presence of a plasmid acquisition cost. These effects were general to diverse incompatibility groups, well-characterized and clinically captured plasmids, Gram-negative recipient strains and species, and experimental conditions. Modeling revealed that both fitness and acquisition costs modulate overall conjugation dynamics, validated with previously published data. These results suggest that the hours immediately following conjugation may play a critical role in both short- and long-term plasmid prevalence. This time frame is particularly relevant to microbiomes with high plasmid/strain diversity considered to be hot spots for conjugation.

Unique Properties of the Alpha-Helical DNA-Binding Protein KfrA Encoded by the IncU Incompatibility Group Plasmid RA3 and Its Host-Dependent Role in Plasmid Maintenance

KfrA, encoded on the broad-host-range RA3 plasmid, is an alpha-helical DNA-binding protein that acts as a transcriptional autoregulator. The KfrARA3 operator site overlaps the kfrA promoter and is composed of five 9-bp direct repeats (DRs). Here, the biological properties of KfrA were studied using both in vivo and in vitro approaches. Localization of the DNA-binding helix-turn-helix motif (HTH) was mapped to the N29-R52 region by protein structure modeling and confirmed by alanine scanning. KfrA repressor ability depended on the number and orientation of DRs in the operator, as well as the ability of the protein to oligomerize. The long alpha-helical tail from residues 54 to 355 was shown to be involved in self-interactions, whereas the region from residue 54 to 177 was involved in heterodimerization with KfrC, another RA3-encoded alpha-helical protein. KfrA also interacted with the segrosome proteins IncC (ParA) and KorB (ParB), representatives of the class Ia active partition systems. Deletion of the kfr genes from the RA3 stability module decreased the plasmid retention in diverse hosts in a species-dependent manner. The specific interactions of KfrA with DNA are essential not only for the transcriptional regulatory function but also for the accessory role of KfrA in stable plasmid maintenance.IMPORTANCE Alpha-helical coiled-coil KfrA-type proteins are encoded by various broad-host-range low-copy-number conjugative plasmids. The DNA-binding protein KfrA encoded on the RA3 plasmid, a member of the IncU incompatibility group, oligomerizes, forms a complex with another plasmid-encoded, alpha-helical protein, KfrC, and interacts with the segrosome proteins IncC and KorB. The unique mode of KfrA dimer binding to the repetitive operator is required for a KfrA role in the stable maintenance of RA3 plasmid in distinct hosts.

Transcriptional Organization of the Stability Module of Broad-Host-Range Plasmid RA3, from the IncU Group

The broad-host-range (BHR) conjugative plasmids have developed diverse adaptive mechanisms defining the range of their promiscuity. The BHR conjugative RA3 plasmid, the archetype of the IncU group, can transfer between, replicate in, and be maintained in representatives of Alpha-, Beta-, and Gammaproteobacteria Its stability module encompasses ten open reading frames (ORFs) apparently organized into five operons, all transcribed in the same direction from several strong promoters that are tightly regulated either by autorepressors or by global plasmid-encoded regulators. In this paper, we demonstrate that owing to an efficient RNA polymerase (RNAP) read-through, the transcription from the first promoter, orf02p, may continue through the whole module. Moreover, an analysis of mRNA produced from the wild-type (WT) stability module and its deletion variants deprived of particular internal transcription initiation sites reveals that in fact each operon may be transcribed from any upstream promoter, giving rise to multicistronic transcripts of variable length and creating an additional level of gene expression control by transcript dosage adjustment. The gene expression patterns differ among various hosts, indicating that promoter recognition, regulation, and the RNAP read-through mechanisms are modulated in a species-specific manner.IMPORTANCE The efficiently disseminating conjugative or mobilizable BHR plasmids play key roles in the horizontal spread of genetic information between closely related and phylogenetically distant species, which can be harmful from the medical, veterinary, or industrial point of view. Understanding the mechanisms determining the plasmid's ability to function in diverse hosts is essential to help limit the spread of undesirable plasmid-encoded traits, e.g., antibiotic resistance. The range of a plasmid's promiscuity depends on the adaptations of its transfer, replication, and stability functions to the various hosts. IncU plasmids, with the archetype plasmid RA3, are considered to constitute a reservoir of antibiotic resistance genes in aquatic environments; however, the molecular mechanisms determining their adaptability to a broad range of hosts are rather poorly characterized. Here, we present the transcriptional organization of the stability module and show that the gene transcript dosage effect is an important determinant of the stable maintenance of RA3 in different hosts.

ArdC, a ssDNA-binding protein with a metalloprotease domain, overpasses the recipient hsdRMS restriction system broadening conjugation host range

Plasmids, when transferred by conjugation in natural environments, must overpass restriction-modification systems of the recipient cell. We demonstrate that protein ArdC, encoded by broad host range plasmid R388, was required for conjugation from Escherichia coli to Pseudomonas putida. Expression of ardC was required in the recipient cells, but not in the donor cells. Besides, ardC was not required for conjugation if the hsdRMS system was deleted in P. putida recipient cells. ardC was also required if the hsdRMS system was present in E. coli recipient cells. Thus, ArdC has antirestriction activity against the HsdRMS system and consequently broadens R388 plasmid host range. The crystal structure of ArdC was solved both in the absence and presence of Mn²⁺. ArdC is composed of a non-specific ssDNA binding N-terminal domain and a C-terminal metalloprotease domain, although the metalloprotease activity was not needed for the antirestriction function. We also observed by RNA-seq that ArdC-dependent conjugation triggered an SOS response in the P. putida recipient cells. Our findings give new insights, and open new questions, into the antirestriction strategies developed by plasmids to counteract bacterial restriction strategies and settle into new hosts.

Horizontal gene transfer is the main mechanism by which bacteria acquire and disseminate new traits, such as antibiotic resistance genes, that allow adaptation and evolution. Here we identified a gene, ardC, that enables a plasmid to increase its conjugative host range, and thus positively contributes to plasmid fitness. The crystal structure of the antirestriction protein ArdC revealed a fold different from other antirestriction proteins. Our results have wide implications for understanding how a gene enlarges the environments a plasmid can colonize and point to new targets to harness the bacterial DNA uptake control.

Genomic and Transcriptomic Changes that Mediate Increased Platinum Resistance in Cupriavidus metallidurans

The extensive anthropogenic use of platinum, a rare element found in low natural abundance in the Earth's continental crust and one of the critical raw materials in the EU innovation partnership framework, has resulted in increased concentrations in surface environments. To minimize its spread and increase its recovery from the environment, biological recovery via different microbial systems is explored. In contrast, studies focusing on the effects of prolonged exposure to Pt are limited. In this study, we used the metal-resistant Cupriavidus metallidurans NA4 strain to explore the adaptation of environmental bacteria to platinum exposure. We used a combined Nanopore⁻Illumina sequencing approach to fully resolve all six replicons of the C. metallidurans NA4 genome, and compared them with the C. metallidurans CH34 genome, revealing an important role in metal resistance for its chromid rather than its megaplasmids. In addition, we identified the genomic and transcriptomic changes in a laboratory-evolved strain, displaying resistance to 160 µM Pt4+. The latter carried 20 mutations, including a large 69.9 kb deletion in its plasmid pNA4_D (89.6 kb in size), and 226 differentially-expressed genes compared to its parental strain. Many membrane-related processes were affected, including up-regulation of cytochrome c and a lytic transglycosylase, down-regulation of flagellar and pili-related genes, and loss of the pNA4_D conjugative machinery, pointing towards a significant role in the adaptation to platinum.

Integrative analysis of fitness and metabolic effects of plasmids in Pseudomonas aeruginosa PAO1

Horizontal gene transfer (HGT) mediated by the spread of plasmids fuels evolution in prokaryotes. Although plasmids provide bacteria with new adaptive genes, they also produce physiological alterations that often translate into a reduction in bacterial fitness. The fitness costs associated with plasmids represent an important limit to plasmid maintenance in bacterial communities, but their molecular origins remain largely unknown. In this work, we combine phenomics, transcriptomics and metabolomics to study the fitness effects produced by a collection of diverse plasmids in the opportunistic pathogen Pseudomonas aeruginosa PAO1. Using this approach, we scan the physiological changes imposed by plasmids and test the generality of some main mechanisms that have been proposed to explain the cost of HGT, including increased biosynthetic burden, reduced translational efficiency, and impaired chromosomal replication. Our results suggest that the fitness effects of plasmids have a complex origin, since none of these mechanisms could individually provide a general explanation for the cost of plasmid carriage. Interestingly, our results also showed that plasmids alter the expression of a common set of metabolic genes in PAO1, and produce convergent changes in host cell metabolism. These surprising results suggest that there is a common metabolic response to plasmids in P. aeruginosa PAO1.

Natural and Artificial Strategies To Control the Conjugative Transmission of Plasmids

Conjugative plasmids are the main carriers of transmissible antibiotic resistance (AbR) genes. For that reason, strategies to control plasmid transmission have been proposed as potential solutions to prevent AbR dissemination. Natural mechanisms that bacteria employ as defense barriers against invading genomes, such as restriction-modification or CRISPR-Cas systems, could be exploited to control conjugation. Besides, conjugative plasmids themselves display mechanisms to minimize their associated burden or to compete with related or unrelated plasmids. Thus, FinOP systems, composed of FinO repressor protein and FinP antisense RNA, aid plasmids to regulate their own transfer; exclusion systems avoid conjugative transfer of related plasmids to the same recipient bacteria; and fertility inhibition systems block transmission of unrelated plasmids from the same donor cell. Artificial strategies have also been designed to control bacterial conjugation. For instance, intrabodies against R388 relaxase expressed in recipient cells inhibit plasmid R388 conjugative transfer; pIII protein of bacteriophage M13 inhibits plasmid F transmission by obstructing conjugative pili; and unsaturated fatty acids prevent transfer of clinically relevant plasmids in different hosts, promoting plasmid extinction in bacterial populations. Overall, a number of exogenous and endogenous factors have an effect on the sophisticated process of bacterial conjugation. This review puts them together in an effort to offer a wide picture and inform research to control plasmid transmission, focusing on Gram-negative bacteria.

PifC and Osa, Plasmid Weapons against Rival Conjugative Coupling Proteins

Bacteria display a variety of mechanisms to control plasmid conjugation. Among them, fertility inhibition (FI) systems prevent conjugation of co-resident plasmids within donor cells. Analysis of the mechanisms of inhibition between conjugative plasmids could provide new alternatives to fight antibiotic resistance dissemination. In this work, inhibition of conjugation of broad host range IncW plasmids was analyzed in the presence of a set of co-resident plasmids. Strong FI systems against plasmid R388 conjugation were found in IncF/MOB_F12 as well as in IncI/MOB_P12 plasmids, represented by plasmids F and R64, respectively. In both cases, the responsible gene was pifC, known also to be involved in FI of IncP plasmids and Agrobacterium T-DNA transfer to plant cells. It was also discovered that the R388 gene osa, which affects T-DNA transfer, also prevented conjugation of IncP-1/MOB_P11 plasmids represented by plasmids RP4 and R751. Conjugation experiments of different mobilizable plasmids, helped by either FI-susceptible or FI-resistant transfer systems, demonstrated that the conjugative component affected by both PifC and Osa was the type IV conjugative coupling protein. In addition, in silico analysis of FI proteins suggests that they represent recent acquisitions of conjugative plasmids, i.e., are not shared by members of the same plasmid species. This implies that FI are rapidly-moving accessory genes, possibly acting on evolutionary fights between plasmids for the colonization of specific hosts.

Fitness Costs of Plasmids: a Limit to Plasmid Transmission

Plasmids mediate the horizontal transmission of genetic information between bacteria, facilitating their adaptation to multiple environmental conditions. An especially important example of the ability of plasmids to catalyze bacterial adaptation and evolution is their instrumental role in the global spread of antibiotic resistance, which constitutes a major threat to public health. Plasmids provide bacteria with new adaptive tools, but they also entail a metabolic burden that, in the absence of selection for plasmid-encoded traits, reduces the competitiveness of the plasmid-carrying clone. Although this fitness reduction can be alleviated over time through compensatory evolution, the initial cost associated with plasmid carriage is the main constraint on the vertical and horizontal replication of these genetic elements. The fitness effects of plasmids therefore have a crucial influence on their ability to associate with new bacterial hosts and consequently on the evolution of plasmid-mediated antibiotic resistance. However, the molecular mechanisms underlying plasmid fitness cost remain poorly understood. Here, we analyze the literature in the field and examine the potential fitness effects produced by plasmids throughout their life cycle in the host bacterium. We also explore the various mechanisms evolved by plasmids and bacteria to minimize the cost entailed by these mobile genetic elements. Finally, we discuss potential future research directions in the field.

The evolution of plasmid stability: Are infectious transmission and compensatory evolution competing evolutionary trajectories?

Conjugative plasmids are widespread and play an important role in bacterial evolution by accelerating adaptation through horizontal gene transfer. However, explaining the long-term stability of plasmids remains challenging because segregational loss and the costs of plasmid carriage should drive the loss of plasmids though purifying selection. Theoretical and experimental studies suggest two key evolutionary routes to plasmid stability: First, the evolution of high conjugation rates would allow plasmids to survive through horizontal transmission as infectious agents, and second, compensatory evolution to ameliorate the cost of plasmid carriage can weaken purifying selection against plasmids. How these two evolutionary strategies for plasmid stability interact is unclear. Here, we summarise the literature on the evolution of plasmid stability and then use individual based modelling to investigate the evolutionary interplay between the evolution of plasmid conjugation rate and cost amelioration. We find that, individually, both strategies promote plasmid stability, and that they act together to increase the likelihood of plasmid survival. However, due to the inherent costs of increasing conjugation rate, particularly where conjugation is unlikely to be successful, our model predicts that amelioration is the more likely long-term solution to evolving stable bacteria-plasmid associations. Our model therefore suggests that bacteria-plasmid relationships should evolve towards lower plasmid costs that may forestall the evolution of highly conjugative, 'infectious' plasmids.

Comparative Genomics of the Conjugation Region of F-like Plasmids: Five Shades of F

The F plasmid is the foremost representative of a large group of conjugative plasmids, prevalent in Escherichia coli, and widely distributed among the Enterobacteriaceae. These plasmids are of clinical relevance, given their frequent association with virulence determinants, colicins, and antibiotic resistance genes. Originally defined by their sensitivity to certain male-specific phages, IncF plasmids share a conserved conjugative system and regulatory circuits. In order to determine whether the genetic architecture and regulation circuits are preserved among these plasmids, we analyzed the natural diversity of F-like plasmids. Using the relaxase as a phylogenetic marker, we identified 256 plasmids belonging to the IncF/ MOB_F12group, present as complete DNA sequences in the NCBI database. By comparative genomics, we identified five major groups of F-like plasmids. Each shows a particular operon structure and alternate regulatory systems. Results show that the IncF/MOB_F12 conjugation gene cluster conforms a diverse and ancient group, which evolved alternative regulatory schemes in its adaptation to different environments and bacterial hosts.

Survival and Evolution of a Large Multidrug Resistance Plasmid in New Clinical Bacterial Hosts

Large conjugative plasmids are important drivers of bacterial evolution and contribute significantly to the dissemination of antibiotic resistance. Although plasmid borne multidrug resistance is recognized as one of the main challenges in modern medicine, the adaptive forces shaping the evolution of these plasmids within pathogenic hosts are poorly understood. Here we study plasmid–host adaptations following transfer of a 73 kb conjugative multidrug resistance plasmid to naïve clinical isolates of Klebsiella pneumoniae and Escherichia coli. We use experimental evolution, mathematical modelling and population sequencing to show that the long-term persistence and molecular integrity of the plasmid is highly influenced by multiple factors within a 25 kb plasmid region constituting a host-dependent burden. In the E. coli hosts investigated here, improved plasmid stability readily evolves via IS26 mediated deletions of costly regions from the plasmid backbone, effectively expanding the host-range of the plasmid. Although these adaptations were also beneficial to plasmid persistence in a naïve K. pneumoniae host, they were never observed in this species, indicating that differential evolvability can limit opportunities of plasmid adaptation. While insertion sequences are well known to supply plasmids with adaptive traits, our findings suggest that they also play an important role in plasmid evolution by maintaining the plasticity necessary to alleviate plasmid–host constrains. Further, the observed evolutionary strategy consistently followed by all evolved E. coli lineages exposes a trade-off between horizontal and vertical transmission that may ultimately limit the dissemination potential of clinical multidrug resistance plasmids in these hosts.

Decomposition and tunability of expression noise in the presence of coupled feedbacks

Expression noise results in cell-to-cell variability in expression levels, and feedback regulation may complicate the tracing of sources of this noise. Using a representative model of gene expression with feedbacks, we analytically show that the expression noise (or the total noise) is decomposed into three parts: feedback-dependent promoter noise determined by a continuous approximation, birth-death noise determined by a simple Poisson process, and correlation noise induced by feedbacks. We clarify confused relationships between feedback and noise in previous studies, by showing that feedback-regulated noisy sources have different contributions to the total noise in different cases of promoter switching (it is an essential reason resulting in confusions). More importantly, we find that there is a tradeoff between response time and expression noise. In addition, we show that in contrast to single feedbacks, coupled positive and negative feedbacks can perform better in tuning expression noise, controlling expression levels, and maintaining response time. The overall analysis implies that living organisms would utilize coupled positive and negative feedbacks for better survival in complex and fluctuating environments.

Tanzawaic Acids, a Chemically Novel Set of Bacterial Conjugation Inhibitors

Bacterial conjugation is the main mechanism for the dissemination of multiple antibiotic resistance in human pathogens. This dissemination could be controlled by molecules that interfere with the conjugation process. A search for conjugation inhibitors among a collection of 1,632 natural compounds, identified tanzawaic acids A and B as best hits. They specially inhibited IncW and IncFII conjugative systems, including plasmids mobilized by them. Plasmids belonging to IncFI, IncI, IncL/M, IncX and IncH incompatibility groups were targeted to a lesser extent, whereas IncN and IncP plasmids were unaffected. Tanzawaic acids showed reduced toxicity in bacterial, fungal or human cells, when compared to synthetic conjugation inhibitors, opening the possibility of their deployment in complex environments, including natural settings relevant for antibiotic resistance dissemination.

Diversification of broad host range plasmids correlates with the presence of antibiotic resistance genes

The IncP-1ε subgroup is a recently identified phylogenetic clade within IncP-1 plasmids, which plays an important role in the spread of antibiotic resistance and degradation of xenobiotic pollutants. Here, four IncP-1ε plasmids were exogenously captured from a petroleum-contaminated habitat in China and compared phylogenetically and genomically with previously reported IncP-1ε and other IncP-1 plasmids. The IncP-1ε plasmids can be clearly subdivided into two subclades, designated as ε-I and ε-II, based on phylogenetic analysis of backbone proteins TraI and TrfA. This was further supported by comparison of concatenated backbone genes. Moreover, the two subclades differed in the transposon types, phenotypes and insertion locations of the accessory elements. The accessory genes on ε-I plasmids were inserted between parA and traC, and harbored ISPa17 and Tn402-like transposon modules, typically carrying antibiotic resistance genes. In contrast, the accessory elements on ε-II plasmids were typically located between trfA and oriV, and contained IS1071, which was commonly inserted within the Tn501-like transposon, typically harboring a cluster of genes encoding mercury resistance and/or catabolic pathways. Our study is one of the first to compare IncP-1 plasmid genomes from China, expands the available collection of IncP-1ε plasmids and enhances our understanding of their diversity, biogeography and evolutionary history.

Migration and horizontal gene transfer divide microbial genomes into multiple niches

Horizontal gene transfer is central to microbial evolution, because it enables genetic regions to spread horizontally through diverse communities. However, how gene transfer exerts such a strong effect is not understood. Here we develop an eco-evolutionary model and show how genetic transfer, even when rare, can transform the evolution and ecology of microbes. We recapitulate existing models, which suggest that asexual reproduction will overpower horizontal transfer and greatly limit its effects. We then show that allowing immigration completely changes these predictions. With migration, the rates and impacts of horizontal transfer are greatly increased, and transfer is most frequent for loci under positive natural selection. Our analysis explains how ecologically important loci can sweep through competing strains and species. In this way, microbial genomes can evolve to become ecologically diverse where different genomic regions encode for partially overlapping, but distinct, ecologies. Under these conditions ecological species do not exist, because genes, not species, inhabit niches.

Horizontal gene transfer is central to microbial evolution. Here, the authors develop an eco-evolutionary model and show that migration can greatly promote horizontal gene transfer, which explains how ecologically-important loci can sweep through the species in a microbial community.

The extended regulatory networks of SXT/R391 integrative and conjugative elements and IncA/C conjugative plasmids

Nowadays, healthcare systems are challenged by a major worldwide drug resistance crisis caused by the massive and rapid dissemination of antibiotic resistance genes and associated emergence of multidrug resistant pathogenic bacteria, in both clinical and environmental settings. Conjugation is the main driving force of gene transfer among microorganisms. This mechanism of horizontal gene transfer mediates the translocation of large DNA fragments between two bacterial cells in direct contact. Integrative and conjugative elements (ICEs) of the SXT/R391 family (SRIs) and IncA/C conjugative plasmids (ACPs) are responsible for the dissemination of a broad spectrum of antibiotic resistance genes among diverse species of Enterobacteriaceae and Vibrionaceae. The biology, diversity, prevalence and distribution of these two families of conjugative elements have been the subject of extensive studies for the past 15 years. Recently, the transcriptional regulators that govern their dissemination through the expression of ICE- or plasmid-encoded transfer genes have been described. Unrelated repressors control the activation of conjugation by preventing the expression of two related master activator complexes in both types of elements, i.e., SetCD in SXT/R391 ICEs and AcaCD in IncA/C plasmids. Finally, in addition to activating ICE- or plasmid-borne genes, these master activators have been shown to specifically activate phylogenetically unrelated mobilizable genomic islands (MGIs) that also disseminate antibiotic resistance genes and other adaptive traits among a plethora of pathogens such as Vibrio cholerae and Salmonella enterica.

Rebooting the genome: The role of negative feedback in horizontal gene transfer

Horizontal Gene Transfer (HGT) is one of the key mechanisms driving bacterial evolution. Conjugative plasmids are fundamental vehicles for HGT in bacteria, playing an essential role in the spread of antibiotic resistances. Although the classical view has stressed the instrumental role of these mobile genetic elements in the dissemination of antibiotic resistance genes, plasmids contain a rich physiology devoted to horizontal and vertical reproduction. This particular lifestyle imposes specific constrains and trade-offs on plasmid physiology, and plasmids have evolved dedicated circuits to balance the opposing demands of vertical and horizontal reproduction. Recent studies on the transcriptional networks of IncW plasmids and other incompatibility groups have unveiled common architectures in the regulatory networks of different plasmid groups. Comparative studies show that negative feedback loops (NFLs) with strong gains are preferred, opening the question of a possible convergent evolution dictated by certain adaptive properties of this particular network motif. System analysis of NFLs with strong feedback gains indicate that this architecture exhibits transient overshooting after horizontal gene transfer. Since plasmid burden is dependent on the expression of plasmid functions, transcriptional overshooting results in a transient increase of the burden immediately after conjugation. We discuss the possible implications of this phenomenon on plasmid propagation, and the regulatory networks that plasmids have evolved to counteract the detrimental side effects of transient overshooting.

Public health evolutionary biology of antimicrobial resistance: priorities for intervention

The three main processes shaping the evolutionary ecology of antibiotic resistance (AbR) involve the emergence, invasion and occupation by antibiotic-resistant genes of significant environments for human health. The process of emergence in complex bacterial populations is a high-frequency, continuous swarming of ephemeral combinatory genetic and epigenetic explorations inside cells and among cells, populations and communities, expanding in different environments (migration), creating the stochastic variation required for evolutionary progress. Invasion refers to the process by which AbR significantly increases in frequency in a given (invaded) environment, led by external invaders local multiplication and spread, or by endogenous conversion. Conversion occurs because of the spread of AbR genes from an exogenous resistant clone into an established (endogenous) bacterial clone(s) colonizing the environment; and/or because of dissemination of particular resistant genetic variants that emerged within an endogenous clonal population. Occupation of a given environment by a resistant variant means a permanent establishment of this organism in this environment, even in the absence of antibiotic selection. Specific interventions on emergence influence invasion, those acting on invasion also influence occupation and interventions on occupation determine emergence. Such interventions should be simultaneously applied, as they are not simple solutions to the complex problem of AbR.

Plasmid Diversity and Adaptation Analyzed by Massive Sequencing of Escherichia coli Plasmids

Whole-genome sequencing is revolutionizing the analysis of bacterial genomes. It leads to a massive increase in the amount of available data to be analyzed. Bacterial genomes are usually composed of one main chromosome and a number of accessory chromosomes, called plasmids. A recently developed methodology called PLACNET (for pla smid c onstellation net works) allows the reconstruction of the plasmids of a given genome. Thus, it opens an avenue for plasmidome analysis on a global scale. This work reviews our knowledge of the genetic determinants for plasmid propagation (conjugation and related functions), their diversity, and their prevalence in the variety of plasmids found by whole-genome sequencing. It focuses on the results obtained from a collection of 255 Escherichia coli plasmids reconstructed by PLACNET. The plasmids found in E. coli represent a nonaleatory subset of the plasmids found in proteobacteria. Potential reasons for the prevalence of some specific plasmid groups will be discussed and, more importantly, additional questions will be posed.

The master activator of IncA/C conjugative plasmids stimulates genomic islands and multidrug resistance dissemination

Dissemination of antibiotic resistance genes occurs mostly by conjugation, which mediates DNA transfer between cells in direct contact. Conjugative plasmids of the IncA/C incompatibility group have become a substantial threat due to their broad host-range, the extended spectrum of antimicrobial resistance they confer, their prevalence in enteric bacteria and their very efficient spread by conjugation. However, their biology remains largely unexplored. Using the IncA/C conjugative plasmid pVCR94ΔX as a prototype, we have investigated the regulatory circuitry that governs IncA/C plasmids dissemination and found that the transcriptional activator complex AcaCD is essential for the expression of plasmid transfer genes. Using chromatin immunoprecipitation coupled with exonuclease digestion (ChIP-exo) and RNA sequencing (RNA-seq) approaches, we have identified the sequences recognized by AcaCD and characterized the AcaCD regulon. Data mining using the DNA motif recognized by AcaCD revealed potential AcaCD-binding sites upstream of genes involved in the intracellular mobility functions (recombination directionality factor and mobilization genes) in two widespread classes of genomic islands (GIs) phylogenetically unrelated to IncA/C plasmids. The first class, SGI1, confers and propagates multidrug resistance in Salmonella enterica and Proteus mirabilis, whereas MGIVmi1 in Vibrio mimicus belongs to a previously uncharacterized class of GIs. We have demonstrated that through expression of AcaCD, IncA/C plasmids specifically trigger the excision and mobilization of the GIs at high frequencies. This study provides new evidence of the considerable impact of IncA/C plasmids on bacterial genome plasticity through their own mobility and the mobilization of genomic islands.

Multidrug resistance is a major health concern that complicates treatments of even the most common infections caused by bacteria. In recent years, IncA/C plasmids have emerged and spread in bacteria infecting humans, food-producing animals and food products, driving at the same time the dissemination of a broad spectrum of antibiotic resistance genes in environmental and in clinical settings. In this study, we have characterized the regulatory pathway that governs IncA/C plasmid dissemination. We have found that AcaCD, the master activator complex encoded by these plasmids, is not only essential for the dissemination of IncA/C plasmids but also activates unrelated mobile genetic elements in bacterial genomes, thereby further promoting the interspecies propagation of multidrug resistance and other adaptive traits at a very high frequency.