Plasmid macro-evolution: selection of deletions during adaptation in a nutrient-limited environment

The generation game: Toward the generational genetic stability of continuous culture

Fed-batch bioprocesses are typically deployed to convert renewable feedstocks to bio-based products using metabolically engineered microorganisms. However, for low-value chemicals, fed-batch cultures provide insufficient volumetric productivity to yield commercially viable products. The greater overall volumetric productivity of continuous culture holds techno-economic promise, but the genetic instability of engineered strains has prevented commercial deployment. This study demonstrated the continuous bioproduction of citramalate (CMA) for over 1,000 h at a productivity of 0.32 gCMA gDCW -1 h-1. Plasmid segregational stability was ensured via infA-complementation, and structural stability was obtained under phosphate limitation in the chemostat. By contrast, glucose limitation promoted structural plasmid instability. Cost-prohibitive inducers were also avoided by using a constitutive promoter for gene expression. Plasmid-borne expression of CMA synthase delivered enhanced productivity compared to a chromosomal integrant strain also developed in this study. This work advances the techno-economic feasibility of sustainable chemicals manufacturing from renewable feedstocks by engineered strains in microbial cell culture.

Genomics, Transcriptomics, and Metabolomics Reveal That Minimal Modifications in the Host Are Crucial for the Compensatory Evolution of ColE1-Like Plasmids

Plasmid-mediated antimicrobial resistance is one of the major threats to public health worldwide. The mechanisms involved in the plasmid/host coadaptation are still poorly characterized, and their understanding is crucial to comprehend the genesis and evolution of multidrug-resistant bacteria. With this purpose, we designed an experimental evolution using Haemophilus influenzae RdKW20 as the model strain carrying the ColE1-like plasmid pB1000. Five H. influenzae populations adapted previously to the culture conditions were transformed with pB1000 and subsequently evolved to compensate for the plasmid-associated fitness cost. Afterward, we performed an integrative multiomic analysis combining genomics, transcriptomics, and metabolomics to explore the molecular mechanisms involved in the compensatory evolution of the plasmid. Our results demonstrate that minimal modifications in the host are responsible for plasmid adaptation. Among all of them, the most enriched process was amino acid metabolism, especially those pathways related to serine, tryptophan, and arginine, eventually related to the genesis and resolution of plasmid dimers. Additional rearrangements occurred during the plasmid adaptation, such as an overexpression of the ribonucleotide reductases and metabolic modifications within specific membrane phospholipids. All these findings demonstrate that the plasmid compensation occurs through the combination of diverse host-mediated mechanisms, of which some are beyond genomic and transcriptomic modifications. IMPORTANCE The ability of bacteria to horizontally transfer genetic material has turned antimicrobial resistance into one of the major sanitary crises of the 21st century. Plasmid conjugation is considered the main mechanism responsible for the mobilization of resistance genes, and its understanding is crucial to tackle this crisis. It is generally accepted that the acquisition and maintenance of mobile genetic elements entail a fitness cost to its host, which is susceptible to be alleviated through a coadaptation process or compensatory evolution. Notwithstanding, despite recent major efforts, the underlying mechanisms involved in this adaptation remain poorly characterized. Analyzing the plasmid/host coadaptation from a multiomic perspective sheds light on the physiological processes involved in the compensation, providing a new understanding on the genesis and evolution of plasmid-mediated antimicrobial-resistant bacteria.

Bacterial plasmid-associated and chromosomal proteins have fundamentally different properties in protein interaction networks

Plasmids facilitate horizontal gene transfer, which enables the diversification of pathogens into new anatomical and environmental niches, implying that plasmid-encoded genes can cooperate well with chromosomal genes. We hypothesise that such mobile genes are functionally different to chromosomal ones due to this ability to encode proteins performing non-essential functions like antimicrobial resistance and traverse distinct host cells. The effect of plasmid-driven gene gain on protein-protein interaction network topology is an important question in this area. Moreover, the extent to which these chromosomally- and plasmid-encoded proteins interact with proteins from their own groups compared to the levels with the other group remains unclear. Here, we examined the incidence and protein-protein interactions of all known plasmid-encoded proteins across representative specimens from most bacteria using all available plasmids. We found that plasmid-encoded genes constitute ~ 0.65% of the total number of genes per bacterial sample, and that plasmid genes are preferentially associated with different species but had limited taxonomical power beyond this. Surprisingly, plasmid-encoded proteins had both more protein-protein interactions compared to chromosomal proteins, countering the hypothesis that genes with higher mobility rates should have fewer protein-level interactions. Nonetheless, topological analysis and investigation of the protein-protein interaction networks' connectivity and change in the number of independent components demonstrated that the plasmid-encoded proteins had limited overall impact in > 96% of samples. This paper assembled extensive data on plasmid-encoded proteins, their interactions and associations with diverse bacterial specimens that is available for the community to investigate in more detail.

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.

Insights into the persistence and phenotypic effects of the endogenous and cryptic plasmid pMF1 in its host strain Myxococcus fulvus 124B02

Many endogenous plasmids carry no noticeable benefits for their bacterial hosts, and the persistence of these 'cryptic plasmids' and their functional impacts are mostly unclear. In this study, we investigated these uncertainties using the social bacterium Myxococcus fulvus 124B02 and its endogenous plasmid pMF1. pMF1 possesses diverse genes that originated from myxobacteria, suggesting a longstanding co-existence of the plasmid with various myxobacterial species. The curing of pMF1 from 124B02 had almost no phenotypic effects on the host. Laboratory evolution experiments showed that the 124B02 strain retained pMF1 when subcultured on dead Escherichia coli cells but lost pMF1 when subcultured on living E. coli cells or on casitone medium; these results indicated that the persistence of pMF1 in 124B02 was environment-dependent. Curing pMF1 caused the mutant to lose the ability to predate and develop fruiting bodies more quickly than the pMF1-containing strain after they were subcultured on dead E. coli cells, which indicated that the presence of pMF1 in M. fulvus 124B02 has some long-term effects on its host. The results provide some new insights into the persistence and impacts of cryptic plasmids in their natural bacterial cells.

Evolution of satellite plasmids can prolong the maintenance of newly acquired accessory genes in bacteria

Transmissible plasmids spread genes encoding antibiotic resistance and other traits to new bacterial species. Here we report that laboratory populations of Escherichia coli with a newly acquired IncQ plasmid often evolve 'satellite plasmids' with deletions of accessory genes and genes required for plasmid replication. Satellite plasmids are molecular parasites: their presence reduces the copy number of the full-length plasmid on which they rely for their continued replication. Cells with satellite plasmids gain an immediate fitness advantage from reducing burdensome expression of accessory genes. Yet, they maintain copies of these genes and the complete plasmid, which potentially enables them to benefit from and transmit the traits they encode in the future. Evolution of satellite plasmids is transient. Cells that entirely lose accessory gene function or plasmid mobility dominate in the long run. Satellite plasmids also evolve in Snodgrassella alvi colonizing the honey bee gut, suggesting that this mechanism may broadly contribute to the importance of IncQ plasmids as agents of bacterial gene transfer in nature.

Mobile Compensatory Mutations Promote Plasmid Survival

The global dissemination of plasmids encoding antibiotic resistance represents an urgent issue for human health and society. While the fitness costs for host cells associated with plasmid acquisition are expected to limit plasmid dissemination in the absence of positive selection of plasmid traits, compensatory evolution can reduce this burden. Experimental data suggest that compensatory mutations can be located on either the chromosome or the plasmid, and these are likely to have contrasting effects on plasmid dynamics. Whereas chromosomal mutations are inherited vertically through bacterial fission, plasmid mutations can be inherited both vertically and horizontally and potentially reduce the initial cost of the plasmid in new host cells. Here we show using mathematical models and simulations that the dynamics of plasmids depends critically on the genomic location of the compensatory mutation. We demonstrate that plasmid-located compensatory evolution is better at enhancing plasmid persistence, even when its effects are smaller than those provided by chromosomal compensation. Moreover, either type of compensatory evolution facilitates the survival of resistance plasmids at low drug concentrations. These insights contribute to an improved understanding of the conditions and mechanisms driving the spread and the evolution of antibiotic resistance plasmids. IMPORTANCE Understanding the evolutionary forces that maintain antibiotic resistance genes in a population, especially when antibiotics are not used, is an important problem for human health and society. The most common platform for the dissemination of antibiotic resistance genes is conjugative plasmids. Experimental studies showed that mutations located on the plasmid or the bacterial chromosome can reduce the costs plasmids impose on their hosts, resulting in antibiotic resistance plasmids being maintained even in the absence of antibiotics. While chromosomal mutations are only vertically inherited by the daughter cells, plasmid mutations are also provided to bacteria that acquire the plasmid through conjugation. Here we demonstrate how the mode of inheritance of a compensatory mutation crucially influences the ability of plasmids to spread and persist in a bacterial population.

Temporal dynamics of bacteria-plasmid coevolution under antibiotic selection

Horizontally acquired genes can be costly to express even if they encode useful traits, such as antibiotic resistance. We previously showed that when selected with tetracycline, Escherichia coli carrying the tetracycline-resistance plasmid RK2 evolved mutations on both replicons that together provided increased tetracycline resistance at reduced cost. Here we investigate the temporal dynamics of this intragenomic coevolution. Using genome sequencing we show that the order of adaptive mutations was highly repeatable across three independently evolving populations. Each population first gained a chromosomal mutation in ompF which shortened lag phase and increased tetracycline resistance. This was followed by mutations impairing the plasmid-encoded tetracycline efflux pump, and finally, additional resistance-associated chromosomal mutations. Thus, reducing the cost of the horizontally acquired tetracycline resistance was contingent on first evolving a degree of chromosomally encoded resistance. We conclude therefore that the trajectory of bacteria-plasmid coevolution was constrained to a single repeatable path.

Convergent Evolution in Intracellular Elements: Plasmids as Model Endosymbionts

Endosymbionts are organisms that live inside the cells of other species. This lifestyle is ubiquitous across the tree of life and is featured by unicellular eukaryotes, prokaryotes, and by extrachromosomal genetic elements such as plasmids. Given that all of these elements dwell in the cytoplasm of their host cell, they should be subject to similar selection pressures. Here we show that strikingly similar features have evolved in both bacterial endosymbionts and plasmids. Since host and endosymbiont are often metabolically tightly intertwined, they are difficult to disentangle experimentally. We propose that using plasmids as tractable model systems can help to solve this problem, thus allowing fundamental questions to be experimentally addressed about the ecology and evolution of endosymbiotic interactions.

Emerging patterns of plasmid-host coevolution that stabilize antibiotic resistance

Multidrug resistant bacterial pathogens have become a serious global human health threat, and conjugative plasmids are important drivers of the rapid spread of resistance to last-resort antibiotics. Whereas antibiotics have been shown to select for adaptation of resistance plasmids to their new bacterial hosts, or vice versa, a general evolutionary mechanism has not yet emerged. Here we conducted an experimental evolution study aimed at determining general patterns of plasmid-bacteria evolution. Specifically, we found that a large conjugative resistance plasmid follows the same evolutionary trajectories as its non-conjugative mini-replicon in the same and other species. Furthermore, within a single host-plasmid pair three distinct patterns of adaptive evolution led to increased plasmid persistence: i) mutations in the replication protein gene (trfA1); ii) the acquisition by the resistance plasmid of a transposon from a co-residing plasmid encoding a putative toxin-antitoxin system; iii) a mutation in the host's global transcriptional regulator gene fur. Since each of these evolutionary solutions individually have been shown to increase plasmid persistence in other plasmid-host pairs, our work points towards common mechanisms of plasmid stabilization. These could become the targets of future alternative drug therapies to slow down the spread of antibiotic resistance.

Evolutionary Ecology of Prokaryotic Immune Mechanisms

Bacteria have a range of distinct immune strategies that provide protection against bacteriophage (phage) infections. While much has been learned about the mechanism of action of these defense strategies, it is less clear why such diversity in defense strategies has evolved. In this review, we discuss the short- and long-term costs and benefits of the different resistance strategies and, hence, the ecological conditions that are likely to favor the different strategies alone and in combination. Finally, we discuss some of the broader consequences, beyond resistance to phage and other genetic elements, resulting from the operation of different immune strategies.

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.

Evolved plasmid-host interactions reduce plasmid interference cost

Antibiotic selection drives adaptation of antibiotic resistance plasmids to new bacterial hosts, but the molecular mechanisms are still poorly understood. We previously showed that a broad-host-range plasmid was poorly maintained in Shewanella oneidensis, but rapidly adapted through mutations in the replication initiation gene trfA1. Here we examined if these mutations reduced the fitness cost of TrfA1, and whether this was due to changes in interaction with the host's DNA helicase DnaB. The strains expressing evolved TrfA1 variants showed a higher growth rate than those expressing ancestral TrfA1. The evolved TrfA1 variants showed a lower affinity to the helicase than ancestral TrfA1 and were no longer able to activate the helicase at the oriV without host DnaA. Moreover, persistence of the ancestral plasmid was increased upon overexpression of DnaB. Finally, the evolved TrfA1 variants generated higher plasmid copy numbers than ancestral TrfA1. The findings suggest that ancestral plasmid instability can at least partly be explained by titration of DnaB by TrfA1. Thus under antibiotic selection resistance plasmids can adapt to a novel bacterial host through partial loss of function mutations that simultaneously increase plasmid copy number and decrease unfavorably high affinity to one of the hosts' essential proteins.

Rapid compensatory evolution promotes the survival of conjugative plasmids

Conjugative plasmids play a vital role in bacterial adaptation through horizontal gene transfer. Explaining how plasmids persist in host populations however is difficult, given the high costs often associated with plasmid carriage. Compensatory evolution to ameliorate this cost can rescue plasmids from extinction. In a recently published study we showed that compensatory evolution repeatedly targeted the same bacterial regulatory system, GacA/GacS, in populations of plasmid-carrying bacteria evolving across a range of selective environments. Mutations in these genes arose rapidly and completely eliminated the cost of plasmid carriage. Here we extend our analysis using an individual based model to explore the dynamics of compensatory evolution in this system. We show that mutations which ameliorate the cost of plasmid carriage can prevent both the loss of plasmids from the population and the fixation of accessory traits on the bacterial chromosome. We discuss how dependent the outcome of compensatory evolution is on the strength and availability of such mutations and the rate at which beneficial accessory traits integrate on the host chromosome.

Evolutionary Paths That Expand Plasmid Host-Range: Implications for Spread of Antibiotic Resistance

The World Health Organization has declared the emergence of antibiotic resistance to be a global threat to human health. Broad-host-range plasmids have a key role in causing this health crisis because they transfer multiple resistance genes to a wide range of bacteria. To limit the spread of antibiotic resistance, we need to gain insight into the mechanisms by which the host range of plasmids evolves. Although initially unstable plasmids have been shown to improve their persistence through evolution of the plasmid, the host, or both, the means by which this occurs are poorly understood. Here, we sought to identify the underlying genetic basis of expanded plasmid host-range and increased persistence of an antibiotic resistance plasmid using a combined experimental-modeling approach that included whole-genome resequencing, molecular genetics and a plasmid population dynamics model. In nine of the ten previously evolved clones, changes in host and plasmid each slightly improved plasmid persistence, but their combination resulted in a much larger improvement, which indicated positive epistasis. The only genetic change in the plasmid was the acquisition of a transposable element from a plasmid native to the Pseudomonas host used in these studies. The analysis of genetic deletions showed that the critical genes on this transposon encode a putative toxin-antitoxin (TA) and a cointegrate resolution system. As evolved plasmids were able to persist longer in multiple naïve hosts, acquisition of this transposon also expanded the plasmid's host range, which has important implications for the spread of antibiotic resistance.

Parallel compensatory evolution stabilizes plasmids across the parasitism-mutualism continuum

Plasmids drive genomic diversity in bacteria via horizontal gene transfer [1, 2]; nevertheless, explaining their survival in bacterial populations is challenging [3]. Theory predicts that irrespective of their net fitness effects, plasmids should be lost: when parasitic (costs outweigh benefits), plasmids should decline due to purifying selection [4-6], yet under mutualism (benefits outweigh costs), selection favors the capture of beneficial accessory genes by the chromosome and loss of the costly plasmid backbone [4]. While compensatory evolution can enhance plasmid stability within populations [7-15], the propensity for this to occur across the parasitism-mutualism continuum is unknown. We experimentally evolved Pseudomonas fluorescens and its mercury resistance mega-plasmid, pQBR103 [16], across an environment-mediated parasitism-mutualism continuum. Compensatory evolution stabilized plasmids by rapidly ameliorating the cost of plasmid carriage in all environments. Genomic analysis revealed that, in both parasitic and mutualistic treatments, evolution repeatedly targeted the gacA/gacS bacterial two-component global regulatory system while leaving the plasmid sequence intact. Deletion of either gacA or gacS was sufficient to completely ameliorate the cost of plasmid carriage. Mutation of gacA/gacS downregulated the expression of ∼17% of chromosomal and plasmid genes and appears to have relieved the translational demand imposed by the plasmid. Chromosomal capture of mercury resistance accompanied by plasmid loss occurred throughout the experiment but very rarely invaded to high frequency, suggesting that rapid compensatory evolution can limit this process. Compensatory evolution can explain the widespread occurrence of plasmids and allows bacteria to retain horizontally acquired plasmids even in environments where their accessory genes are not immediately useful.

Bacteriophages limit the existence conditions for conjugative plasmids

Bacteriophages are a major cause of bacterial mortality and impose strong selection on natural bacterial populations, yet their effects on the dynamics of conjugative plasmids have rarely been tested. We combined experimental evolution, mathematical modeling, and individual-based simulations to explain how the ecological and population genetics effects of bacteriophages upon bacteria interact to determine the dynamics of conjugative plasmids and their persistence. The ecological effects of bacteriophages on bacteria are predicted to limit the existence conditions for conjugative plasmids, preventing persistence under weak selection for plasmid accessory traits. Experiments showed that phages drove faster extinction of plasmids in environments where the plasmid conferred no benefit, but they also revealed more complex effects of phages on plasmid dynamics under these conditions, specifically, the temporary maintenance of plasmids at fixation followed by rapid loss. We hypothesized that the population genetic effects of bacteriophages, specifically, selection for phage resistance mutations, may have caused this. Further mathematical modeling and individual-based simulations supported our hypothesis, showing that conjugative plasmids may hitchhike with phage resistance mutations in the bacterial chromosome.

Conjugative plasmids are infectious loops of DNA capable of transmitting DNA between bacterial cells and between species. Because plasmids often carry extra genes that allow bacteria to live in otherwise-inhospitable environments, their dynamics are central to understanding bacterial adaptive evolution. The plasmid-bacterium interaction has typically been studied in isolation, but in natural bacterial communities, bacteriophages, viruses that infect bacteria, are ubiquitous. Using experiments, mathematical models, and computer simulations we show that bacteriophages drive plasmid dynamics through their ecological and evolutionary effects on bacteria and ultimately limit the conditions allowing plasmid existence. These results advance our understanding of bacterial adaptation and show that bacteriophages could be used to select against plasmids carrying undesirable traits, such as antibiotic resistance.

Positive selection and compensatory adaptation interact to stabilize non-transmissible plasmids

Plasmids are important drivers of bacterial evolution, but it is challenging to understand how plasmids persist over the long term because plasmid carriage is costly. Classical models predict that horizontal transfer is necessary for plasmid persistence, but recent work shows that almost half of plasmids are non-transmissible. Here we use a combination of mathematical modelling and experimental evolution to investigate how a costly, non-transmissible plasmid, pNUK73, can be maintained in populations of Pseudomonas aeruginosa. Compensatory adaptation increases plasmid stability by eliminating the cost of plasmid carriage. However, positive selection for plasmid-encoded antibiotic resistance is required to maintain the plasmid by offsetting reductions in plasmid frequency due to segregational loss. Crucially, we show that compensatory adaptation and positive selection reinforce each other’s effects. Our study provides a new understanding of how plasmids persist in bacterial populations, and it helps to explain why resistance can be maintained after antibiotic use is stopped.

Plasmids are important for bacterial evolution but the evolutionary mechanisms behind their maintenance are unclear. Here the authors show that the interplay between compensatory adaptation and positive selection for plasmid-encoded antibiotic resistance determines plasmid persistence in P. aeruginosa.

The ecology and evolution of tetracycline resistance

The use of tetracycline over the past few decades has been accompanied by a drastic increase in the frequency of tetracycline resistance in a wide range of bacterial species and genera. A diversity of resistance determinants is found in the microbial world, coding for markedly different mechanisms of resistance. The recent analysis of one family of resistance determinants provides evidence for intergenic and intragenic coevolutionary changes as well as for an unusual evolutionary history of duplication and divergence in function of domains within a single locus.

Plasmid-mediated horizontal gene transfer is a coevolutionary process

Conjugative plasmids are key agents of horizontal gene transfer (HGT) that accelerate bacterial adaptation by vectoring ecologically important traits between strains and species. However, although many conjugative plasmids carry beneficial traits, all plasmids exert physiological costs-of-carriage on bacteria. The existence of conjugative plasmids, therefore, presents a paradox because non-beneficial plasmids should be lost to purifying selection, whereas beneficial genes carried on plasmids should be integrated into the bacterial chromosome. Several ecological solutions to the paradox have been proposed, but none account for co-adaptation of bacteria and conjugative plasmids. Drawing upon evidence from experimental evolution, we argue that HGT via conjugation can only be fully understood in a coevolutionary framework.

Transfer of an Escherichia coli ST131 multiresistance cassette has created a Klebsiella pneumoniae-specific plasmid associated with a major nosocomial outbreak

OBJECTIVES

To characterize the complete sequence, horizontal spread and stability of the CTX-M-15-encoding multiresistance plasmid of a Klebsiella pneumoniae strain involved in a large nosocomial outbreak.

METHODS

The 220 kbp plasmid pUUH239.2 was completely sequenced using 454 technology. The conjugational host range, conjugation frequencies, plasmid stability and fitness cost of plasmid carriage were studied in vitro. Conjugational spread during the outbreak was assessed retrospectively by multiplex PCR screening, restriction fragment length polymorphism and PFGE.

RESULTS

Plasmid pUUH239.2 encodes resistance to β-lactams (bla(CTX-M-15), bla(TEM-1) and bla(OXA-1)), aminoglycosides [aac-(6')-1b-cr and aadA2], tetracyclines [tet(A) and tetR], trimethoprim (dhfrXII), sulphonamides (sul1), quaternary ammonium compounds (qacEΔ1), macrolides [mph(A)-mxr-mphR(A)] and heavy metal ions (silver, copper and arsenic). The plasmid consists of a backbone, highly similar to the K. pneumoniae plasmid pKPN3, and a 41 kbp resistance region, highly similar to the resistance regions of plasmids pEK499 and pC15-1a previously isolated from Escherichia coli strains belonging to the outbreak lineage ST131 (where ST stands for sequence type). The pUUH239.2 plasmid is stable in K. pneumoniae but unstable in E. coli and confers a fitness cost when introduced into a naive host cell. Transfer of pUUH239.2 from the outbreak K. pneumoniae clone to the E. coli of the patients' intestinal floras has occurred on multiple occasions during the outbreak.

CONCLUSIONS

The plasmid pUUH239.2 is a composite of the pKPN3 K. pneumoniae plasmid backbone and the bla(CTX-M-15)-encoding multiresistance cassette associated with the internationally recognized outbreak strain E. coli ST131. The resulting plasmid differs in stability between K. pneumoniae and E. coli, and this has probably limited the spread of this plasmid during the outbreak.

Selection pressure required for long-term persistence of blaCMY-2-positive IncA/C plasmids

Multidrug resistance blaCMY-2 plasmids that confer resistance to expanded-spectrum cephalosporins have been found in multiple bacterial species collected from different hosts worldwide. The widespread distribution of blaCMY-2 plasmids may be driven by antibiotic use that selects for the dissemination and persistence of these plasmids. Alternatively, these plasmids may persist and spread in bacterial populations in the absence of selection pressure if a balance exists among conjugative transfer, segregation loss during cell division, and fitness cost to the host. We conducted a series of experiments (both in vivo and in vitro) to study these mechanisms for three blaCMY-2 plasmids, peH4H, pAR060302, and pAM04528. Results of filter mating experiments showed that the conjugation efficiency of blaCMY-2 plasmids is variable, from <10(-7) for pAM04528 and peH4H to ∼10(-3) for pAR060302. Neither peH4H nor pAM04528 was transferred from Escherichia coli strain DH10B, but peH4H was apparently mobilized by the coresident trimethoprim resistance-encoding plasmid pTmpR. Competition studies showed that carriage of blaCMY-2 plasmids imposed a measurable fitness cost on the host bacteria both in vitro (0.095 to 0.25) and in vivo (dairy calf model). Long-term passage experiments in the absence of antibiotics demonstrated that plasmids with limited antibiotic resistance phenotypes arose, but eventually drug-sensitive, plasmid-free clones dominated the populations. Given that plasmid decay or loss is inevitable, we infer that some level of selection is required for the long-term persistence of blaCMY-2 plasmids in bacterial populations.

Shifts in the host range of a promiscuous plasmid through parallel evolution of its replication initiation protein

The ability of bacterial plasmids to adapt to novel hosts and thereby shift their host range is key to their long-term persistence in bacterial communities. Promiscuous plasmids of the incompatibility group P (IncP)-1 can colonize a wide range of hosts, but it is not known if and how they can contract, shift or further expand their host range. To understand the evolutionary mechanisms of host range shifts of IncP-1 plasmids, an IncP-1β mini-replicon was experimentally evolved in four hosts in which it was initially unstable. After 1000 generations in serial batch cultures under antibiotic selection for plasmid maintenance (kanamycin resistance), the stability of the mini-plasmid dramatically improved in all coevolved hosts. However, only plasmids evolved in Shewanella oneidensis showed improved stability in the ancestor, indicating that adaptive mutations had occurred in the plasmid itself. Complete genome sequence analysis of nine independently evolved plasmids showed seven unique plasmid genotypes that had various kinds of single mutations at one locus, namely, the N-terminal region of the replication initiation protein TrfA. Such parallel evolution indicates that this region was under strong selection. In five of the seven evolved plasmids, these trfA mutations resulted in a significantly higher plasmid copy number. Evolved plasmids were found to be stable in four other naive hosts, but could no longer replicate in Pseudomonas aeruginosa. This study shows that plasmids can specialize to a novel host through trade-offs between improved stability in the new host and the ability to replicate in a previously permissive host.

Adaptive plasmid evolution results in host-range expansion of a broad-host-range plasmid

Little is known about the range of hosts in which broad-host-range (BHR) plasmids can persist in the absence of selection for plasmid-encoded traits, and whether this "long-term host range" can evolve over time. Previously, the BHR multidrug resistance plasmid pB10 was shown to be highly unstable in Stenotrophomonas maltophilia P21 and Pseudomonas putida H2. To investigate whether this plasmid can adapt to such unfavorable hosts, we performed evolution experiments wherein pB10 was maintained in strain P21, strain H2, and alternatingly in P21 and H2. Plasmids that evolved in P21 and in both hosts showed increased stability and decreased cost in ancestral host P21. However, the latter group showed higher variability in stability patterns, suggesting that regular switching between distinct hosts hampered adaptive plasmid evolution. The plasmids evolved in P21 were also equally or more stable in other hosts compared to pB10, which suggested true host-range expansion. The complete genome sequences of four evolved plasmids with improved stability showed only one or two genetic changes. The stability of plasmids evolved in H2 improved only in their coevolved hosts, not in the ancestral host. Thus a BHR plasmid can adapt to an unfavorable host and thereby expand its long-term host range.

Stability of a promiscuous plasmid in different hosts: no guarantee for a long-term relationship

Broad-host-range (BHR) IncP-1 plasmids have the ability to transfer between and replicate in nearly all species of the Alpha-, Beta- and Gammaproteobacteria, but surprisingly few data are available on the stability of these plasmids in strains within their host range. Moreover, even though molecular interactions between the bacterial host and its plasmid(s) exist, no systematic study to date has compared the stability of the same plasmid among different hosts. The goal of this study was to examine whether the stability characteristics of an IncP-1 plasmid can be variable between strains within the host range of the plasmid. Therefore, 19 strains within the Alpha-, Beta- or Gammaproteobacteria carrying the IncP-1beta plasmid pB10 were serially propagated in non-selective medium and the fraction of segregants was monitored through replica-picking. Remarkably, a large variation in the stability of pB10 in different strains was found, even between strains within the same genus or species. Ten strains showed no detectable plasmid loss over about 200 generations, and in two strains plasmid-free clones were only sporadically observed. In contrast, three strains, Pseudomonas koreensis R28, Pseudomonas putida H2 and Stenotrophomonas maltophilia P21, exhibited rapid plasmid loss within 80 generations. Parameter estimation after mathematical modelling of these stability data suggested high frequencies of segregation (about 0.04 per generation) or high plasmid cost (i.e. a relative fitness decrease in plasmid-bearing cells of about 15 and 40 %), which was confirmed experimentally. The models also suggested that plasmid reuptake by conjugation only played a significant role in plasmid stability in one of the three strains. Four of the 19 strains lost the plasmid very slowly over about 600 generations. The erratic decrease of the plasmid-containing fraction and simulation of the data with a new mathematical model suggested that plasmid cost was variable over time due to compensatory mutations. The findings of this study demonstrate that the ability of a so-called 'BHR' plasmid to persist in a bacterial population is influenced by strain-specific traits, and therefore observations made for one strain should not be generalized for the entire species or genus.

Impact of antimicrobial usage on antimicrobial resistance in commensal Escherichia coli strains colonizing broiler chickens

Escherichia coli strains isolated from commercial broilers and an experimental flock of chickens were screened to determine phenotypic expression of antimicrobial resistance and carriage of drug resistance determinants. The goal of this study was to investigate the influence of oxytetracycline, sarafloxacin, and enrofloxacin administration on the distribution of resistance determinants and strain types among intestinal commensal E. coli strains isolated from broiler chickens. We detected a high prevalence of resistance to drugs such as tetracycline (36 to 97%), sulfonamides (50 to 100%), and streptomycin (53 to 100%) in E. coli isolates from treated and untreated flocks. These isolates also had a high prevalence of class 1 integron carriage, and most of them possessed the streptomycin resistance cassette, aadA1. In order to investigate the contribution of E. coli strain distribution to the prevalence of antimicrobial resistance and the resistance determinants, isolates from each flock were DNA fingerprinted by enterobacterial repetitive intergenic consensus sequence (ERIC) PCR. Although very diverse E. coli strain types were detected, four ERIC strain types were present on all of the commercial broiler farms, and two of the strains were also found in the experimental flocks. Each E. coli strain consisted of both susceptible and antimicrobial agent-resistant isolates. In some instances, isolates of the same E. coli strain expressed the same drug resistance patterns although they harbored different tet determinants or streptomycin resistance genes. Therefore, drug resistance patterns could not be explained solely by strain prevalence, indicating that mobile elements contributed significantly to the prevalence of resistance.

Combining mathematical models and statistical methods to understand and predict the dynamics of antibiotic-sensitive mutants in a population of resistant bacteria during experimental evolution

Temporarily discontinuing the use of antibiotics has been proposed as a means to eliminate resistant bacteria by allowing sensitive clones to sweep through the population. In this study, we monitored a tetracycline-sensitive subpopulation that emerged during experimental evolution of E. coli K12 MG1655 carrying the multiresistance plasmid pB10 in the absence of antibiotics. The fraction of tetracycline-sensitive mutants increased slowly over 500 generations from 0.1 to 7%, and loss of resistance could be attributed to a recombination event that caused deletion of the tet operon. To help understand the population dynamics of these mutants, three mathematical models were developed that took into consideration recurrent mutations, increased host fitness (selection), or a combination of both mechanisms (full model). The data were best explained by the full model, which estimated a high mutation frequency (lambda = 3.11 x 10(-5)) and a significant but small selection coefficient (sigma = 0.007). This study emphasized the combined use of experimental data, mathematical models, and statistical methods to better understand and predict the dynamics of evolving bacterial populations, more specifically the possible consequences of discontinuing the use of antibiotics.

Genomic changes arising in long-term stab cultures of Escherichia coli

Genomic scans of clones isolated from long-term stab cultures of Escherichia coli K-12 showed the loss of two large segments of the genome, with each lost segment being approximately 20 kb long. A detailed analysis of one of the deletions, located between 5.4 and 5.9 min, revealed that similar deletions had arisen in several other stab cultures. All deletions of this type exhibited a right terminus ending precisely at an IS5A element and a left terminus that varied over an approximately 5-kb range but was bordered in all but two cases by sequences belonging to the preferred consensus target sequence for IS5, YTAR. The ubiquity of such deletions in independent stab cultures and the increase in their frequency over time argue that they have a selective advantage. It is speculated that the loss of the crl locus is responsible for the selective advantage of the deletions.

A chemostat culture as a tool for the improvement of a recombinant E. coli strain over-producing penicillin G acylase

The recombinant strain RE3(pKA18) of Escherichia coli constitutively overproduces penicillin G acylase (PGA) from plasmid-borne gene pga. The host strain RE3 bears the same pga gene on its chromosome, the expression of which is controlled by the natural mechanism of induction with phenylacetic acid (PA). To evaluate the maximum biosynthetic capacity for PGA, induction of the chromosomal pga by PA was studied in a culture of the recombinant strain. PGA production by batch cultures of RE3(pKA18) and RE3 showed a different response to the addition of PA to the medium: while an addition of PA induces PGA in a culture of strain RE3 as expected, in recombinant cells it lowers the specific activity of PGA and a large amount of PGA is released into the culture medium. To improve the PGA production, the strain RE3(pKA18) was cultured in a carbon-limited chemostat and subjected to selection pressure in a medium supplemented with phenylacetic acid amide (PAA). Phenylacetic acid amide served as a source of nitrogen, an inducer of PGA and a factor exerting positive selection pressure on the maintenance of the recombinant plasmid. After 130 generations of growth in the presence of the inducer, no recombinant strain with constitutive expression of the chromosomal gene pga was detected in the prevailing P(+) subpopulation in the chemostat. Shake-flask experiments with the parent recombinant strain RE3(pKA18), host strain RE3, chemostat evolvant ERE3(epKA18), the cured host ERE3 alone, and its derivative after retransformation with ancestral plasmid ERE3(pKA18) showed that inactivation of the plasmid-borne pga by a frame-shift mutation (plasmid epKA18) occurred in the plasmid-bearing subpopulation accumulated in the chemostat. Marked adaptive changes evolved in the host ERE3 during a 130 generation culture: (1) the specific growth rate of the host increased by 30% in a medium without PA, (2) the copy number of plasmids pKA18 and epKA18 in the host cultured in PA-free medium dropped by about 40%, and (3) the leakage of PGA from the cell in the presence of PA found in strain RE3(pKA18) was not observed in strain ERE3(pKA18). This new recombinant strain with modified traits was constructed by means of retransformation of the evolved host ERE3 with ancestral plasmid pKA18.

Variations in antibiotic resistance profile in Enterobacteriaceae isolated from wild Australian mammals

We carried out a retrospective analysis of 946 strains of Enterobacteriaceae isolated from wild Australian mammals between 1993 and 1997. The prevalence of resistance to fixed concentrations of 32 antimicrobial agents was determined, and the respective roles that taxonomic family of the host, state of origin and bacterial species play in defining prevalence and range of resistance were investigated. Our results demonstrated a low but widespread prevalence of antimicrobial resistance in wild isolates. Only amikacin, ciprofloxacin, meropenem and gentamicin inhibited growth in all 946 samples. There was extensive variation in the combination of antibiotics to which isolates were resistant, and multiple antibiotic resistance was common. Geographical location and host group significantly influenced the antibiotic resistance profile of an isolate, whereas bacterial species influenced both the resistance profile of an isolate and the number of antibiotics it was resistant to. The role of these factors in determining observed antibiotic resistance profiles suggests that any study measuring resistance in wild isolates should include the broadest possible range of bacterial species, host species and sampling locations. As such, this study provides an important new baseline for future measurements of antibiotic resistance in the Australian environment.

The cost of antibiotic resistance from a bacterial perspective

The emergence, spread and stability of antibiotic resistance in a bacterial population will be determined by several factors including (a) the volume of drug use, (b) the rate of formation of resistant mutants, (c) the biological cost of resistance and (d) the rate and extent of the genetic compensation of the costs. Generally, resistance is associated with a cost, suggesting that the frequency of resistant bacteria might decline when the use of antibiotics is decreased. However, evolution to reduce these costs, without a concomitant loss of resistance, can occur and result in a stabilization of the resistant bacteria in the population. The rate and trajectory of this compensatory evolution is dependent on the bacterial species, the specific resistance mutation and the environmental conditions under which evolution occurs. Copyright 2000 Harcourt Publishers Ltd.

The biological cost of antibiotic resistance

The frequency and rates of ascent and dissemination of antibiotic resistance in bacterial populations are anticipated to be directly related to the volume of antibiotic use and inversely related to the cost that resistance imposes on the fitness of bacteria. The data available from recent laboratory studies suggest that most, but not all, resistance-determining mutations and accessory elements engender some fitness cost, but those costs are likely to be ameliorated by subsequent evolution.

Trade-off between segregational stability and metabolic burden: a mathematical model of plasmid ColE1 replication control

A model of ColE1 copy number control has been developed where molecular details of replication are connected both to segregational stability and metabolic burden. Efficient replication control reduces copy number variation and increases segregational stability for a given average copy number. Copy number variation is predicted to depend on the type of inhibition mechanism as well as RNA I and RNA II turnover rate constants. It is shown that when both RNA I and RNA II transcription frequencies and the rate constant for degradation of free RNA I are very large, a hyperbolic inhibition mechanism must compensate with a 1.4 times greater average copy number to obtain the same segregational stability as an exponential inhibition mechanism. How sensitively the replication frequency responds to changes in RNA I concentration depends on the type of inhibition mechanism and the number of attempts to form an RNA II replication primer per plasmid and cell cycle. If RNA I is too stable, it will not follow changes in plasmid concentration closely, and when the transcription frequency for RNA I is only slightly higher than for RNA II, RNA I concentration becomes randomized. In both these cases, the proportionality between the single cell RNA I and plasmid concentrations is lost and this impairs copy number control. Thresholds in the rate for degradation of free RNA I as well as in RNA I and RNA II transcription frequencies have been computed, where an increase in these rate constants has a negligible effect on segregational stability but a corresponding decrease leads to segregational disaster. This indicates that there exists a well defined optimal set of rate constants where the regulation system works well without excessive metabolic load. A number of new experiments are suggested to address features of particular importance for the evolution of ColE1 copy number control.

Genetic analysis of a plasmid-encoded, host genotype-specific enhancement of bacterial fitness

In the absence of antibiotics, carriage of pACYC184 reduces the competitive fitness of an Escherichia coli B genotype that was not previously selected for plasmid carriage, relative to that of an isogenic plasmid-free competitor. However, a host genotype propagated with the plasmid for 500 generations evolved an unexpected competitive advantage from plasmid carriage, relative to its own isogenic plasmid-free segregant. We manipulated the pACYC184 genome in order to identify the plasmid-encoded function that was required for the enhancement of the coevolved host genotype's competitive fitness. Inactivation of the plasmid-encoded tetracycline resistance gene, by deletion of either the promoter region or the entire gene, eliminated the beneficial effect of plasmid carriage for the coevolved host. This beneficial effect for the coevolved host was also manifest with pBR322, which contains a tetracycline resistance gene identical to that of pACYC184 but is otherwise heterologous.