Host-specific factors determine the persistence of IncP-1 plasmids

Plasmid-driven strategies for clone success in Escherichia coli

Escherichia coli is the most widely studied microbe in history, but the population structure and evolutionary trends of its extrachromosomal elements known as plasmids remain poorly delineated. Here we used long-read technology to high-resolution sequence the entire plasmidome and the corresponding host chromosomes from an unbiased longitudinal survey covering two decades and over 2000 E. coli isolates. We find that some plasmids have persisted in lineages even for centuries, demonstrating strong plasmid-lineage associations. Our analysis provides a detailed map of recent vertical and horizontal evolutionary events involving plasmids with key antibiotic resistance, competition and virulence determinants. We present genomic evidence of both chromosomal and plasmid-driven success strategies adopted by distant lineages by independently inheriting the same genomic elements. Further, we use in vitro experiments to verify the importance of key bacteriocin-producing plasmids for clone success. Our study has general implications for understanding plasmid biology and bacterial evolutionary strategies.

Evolution of a Plasmid Regulatory Circuit Ameliorates Plasmid Fitness Cost

Plasmids promote adaptation of bacteria by facilitating horizontal transfer of diverse genes, notably those conferring antibiotic resistance. Some plasmids, like those of the incompatibility group IncP-1, are known to replicate and persist in a broad range of bacteria. We investigated a poorly understood exception, the IncP-1β plasmid pBP136 from a clinical Bordetella pertussis isolate, which quickly became extinct in laboratory Escherichia coli populations. Through experimental evolution, we found that the inactivation of a previously uncharacterized plasmid gene, upf31, drastically improved plasmid persistence in E. coli. The gene inactivation caused alterations in the plasmid regulatory system, including decreased transcription of the global plasmid regulators (korA, korB, and korC) and numerous genes in their regulons. This is consistent with our findings that Upf31 represses its own transcription. It also caused secondary transcriptional changes in many chromosomal genes. In silico analyses predicted that Upf31 interacts with the plasmid regulator KorB at its C-terminal dimerization domain (CTD). We showed experimentally that adding the CTD of upf31/pBP136 to the naturally truncated upf31 allele of the stable IncP-1β archetype R751 results in plasmid destabilization in E. coli. Moreover, mutagenesis showed that upf31 alleles encoded on nearly half of the sequenced IncP-1β plasmids also possess this destabilization phenotype. While Upf31 might be beneficial in many hosts, we show that in E. coli some alleles have harmful effects that can be rapidly alleviated with a single mutation. Thus, broad-host-range plasmid adaptation to new hosts can involve fine-tuning their transcriptional circuitry through evolutionary changes in a single gene.

Evolution of a Plasmid Regulatory Circuit Ameliorates Plasmid Fitness Cost

Plasmids play a major role in rapid adaptation of bacteria by facilitating horizontal transfer of diverse genes, most notably those conferring antibiotic resistance. While most plasmids that replicate in a broad range of bacteria also persist well in diverse hosts, there are exceptions that are poorly understood. We investigated why a broad-host range plasmid, pBP136, originally found in clinical Bordetella pertussis isolates, quickly became extinct in laboratory Escherichia coli populations. Through experimental evolution we found that inactivation of a previously uncharacterized plasmid gene, upf31, drastically improved plasmid maintenance in E. coli. This gene inactivation resulted in decreased transcription of the global plasmid regulators (korA, korB, and korC) and numerous genes in their regulons. It also caused transcriptional changes in many chromosomal genes primarily related to metabolism. In silico analyses suggested that the change in plasmid transcriptome may be initiated by Upf31 interacting with the plasmid regulator KorB. Expression of upf31 in trans negatively affected persistence of pBP136Δupf31 as well as the closely related archetypal IncP-1β plasmid R751, which is stable in E. coli and natively encodes a truncated upf31 allele. Our results demonstrate that while the upf31 allele in pBP136 might advantageously modulate gene expression in its original host, B. pertussis, it has harmful effects in E. coli. Thus, evolution of a single plasmid gene can change the range of hosts in which that plasmid persists, due to effects on the regulation of plasmid gene transcription.

Piggybacking on Niche Adaptation Improves the Maintenance of Multidrug-Resistance Plasmids

The persistence of plasmids in bacterial populations represents a puzzling evolutionary problem with serious clinical implications due to their role in the ongoing antibiotic resistance crisis. Recently, major advancements have been made toward resolving this “plasmid paradox” but mainly in a nonclinical context. Here, we propose an additional explanation for the maintenance of multidrug‐resistance plasmids in clinical Escherichia coli strains. After coevolving two multidrug‐resistance plasmids encoding resistance to last resort carbapenems with an extraintestinal pathogenic E. coli strain, we observed that chromosomal media adaptive mutations in the global regulatory systems CCR (carbon catabolite repression) and ArcAB (aerobic respiration control) pleiotropically improved the maintenance of both plasmids. Mechanistically, a net downregulation of plasmid gene expression reduced the fitness cost. Our results suggest that global chromosomal transcriptional rewiring during bacterial niche adaptation may facilitate plasmid maintenance.

Plasmids persist in a microbial community by providing fitness benefit to multiple phylotypes

The current epidemic of antibiotic resistance has been facilitated by the wide and rapid horizontal dissemination of antibiotic resistance genes (ARGs) in microbial communities. Indeed, ARGs are often located on plasmids, which can efficiently shuttle genes across diverse taxa. While the existence conditions of plasmids have been extensively studied in a few model bacterial populations, their fate in complex bacterial communities is poorly understood. Here, we coupled plasmid transfer assays with serial growth experiments to investigate the persistence of the broad-host-range IncP-1 plasmid pKJK5 in microbial communities derived from a sewage treatment plant. The cultivation conditions combined different nutrient and oxygen levels, and were non-selective and non-conducive for liquid-phase conjugal transfer. Following initial transfer, the plasmid persisted in almost all conditions during a 10-day serial growth experiment (equivalent to 60 generations), with a transient transconjugant incidence up to 30%. By combining cell enumeration and sorting with amplicon sequencing, we mapped plasmid fitness effects across taxa of the microbial community. Unexpected plasmid fitness benefits were observed in multiple phylotypes of Aeromonas, Enterobacteriaceae, and Pseudomonas, which resulted in community-level plasmid persistence. We demonstrate, for the first time, that plasmid fitness effects across community members can be estimated in high-throughput without prior isolation. By gaining a fitness benefit when carrying plasmids, members within complex microbial communities might have a hitherto unrecognised potential to maintain plasmids for long-term community-wide access.

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.

IncP-1β Plasmids Are Important Carriers of Fitness Traits for Variovorax Species in the Mycosphere--Two Novel Plasmids, pHB44 and pBS64, with Differential Effects Unveiled

The Laccaria proxima mycosphere strongly selects Variovorax paradoxus cells. Fifteen independent V. paradoxus strains, isolated from mycospheres sampled at two occasions, were investigated with respect to the occurrence of plasmids of sizes <60-100 kb. Two V. paradoxus strains, HB44 and BS64, were found to contain such plasmids, which were coined pHB44 and pBS64. Replicon typing using a suite of plasmid-specific PCR systems indicated that both plasmids belong to the IncP-1β group. Also, both were able to mobilize selectable IncQ group plasmids into Escherichia coli as well as Pseudomonas fluorescens. Moreover, they showed stable replication in these organisms, confirming their broad host range. Strain BS64 was cured of pBS64 and plasmid pHB44 was subsequently moved into this cured strain by making use of the IncQ group tracer plasmid pSUP104, which was then removed at elevated temperature. Thus, both plasmids could be screened for their ability to confer a phenotype upon strain BS64. No evidence for the presence of genes for xenobiotic degradation and/or antibiotic or heavy metal resistances was found for either of the two plasmids. Remarkably, both could stimulate the production of biofilm material by strain BS64. Also, the population densities of pBS64-containing strain BS64 were temporarily raised in liquid as well as soil systems (versus the plasmid-cured strain), both in the presence of the fungal host Lyophyllum sp. strain Karsten. Strikingly, plasmid pHB44 significantly enhanced the fitness of strain BS64 in soil containing Lyophyllum sp. strain Karsten, but decreased its fitness in soil supplemented with extra FeCl3. The effect was noted both in separate (no inter-strain competition) and joint (competition) inoculations.