Evolutionary, ecological and biotechnological perspectives on plasmids resident in the human gut mobile metagenome

Incorporating plasmid biology and metagenomics into a holistic model of the human gut microbiome

The human gut microbiome is often described as the collection of bacteria, archaea, fungi, protists, and viruses associated with an individual, with no acknowledgement of the plasmid constituents. However, like viruses, plasmids are autonomous intracellular replicating entities that can influence the genotype and phenotype of their host and mediate trans-kingdom interactions. Plasmids are frequently noted as vehicles for horizontal gene transfer and for spreading antibiotic resistance, yet their multifaceted contribution to mutualistic and antagonistic interactions within the human microbiome and impact on human health is overlooked. In this review, we highlight the importance of plasmids and their biological properties as overlooked components of microbiomes. Subsequent human microbiome studies should include dedicated analyses of plasmids, particularly as a holistic understanding of human-microbial interactions is required before effective and safe interventions can be implemented to improve human well-being.

A plasmid network from the gut microbiome of semi-isolated human groups reveals unique and shared metabolic and virulence traits

The plasmids in gut microbiomes have the potential to contribute to the microbiome community, as well as human health and physiology. Nevertheless, this niche remains poorly explored. In general, most microbiome studies focus on urban-industrialized groups, but here, we studied semi-isolated groups from South America and Africa, which would represent a link between ancestral and modern human groups. Based on open metagenomic data, we characterized the set of plasmids, including their genes and functions, from the gut microbiome of the Hadza, Matses, Tunapuco, and Yanomami, semi-isolated groups with a hunter, gather or subsistence lifestyle. Unique plasmid clusters and gene functions for each human group were identified. Moreover, a dozen plasmid clusters circulating in other niches worldwide are shared by these distinct groups. In addition, novel and unique plasmids harboring resistance (encompassing six antibiotic classes and multiple metals) and virulence (as type VI secretion systems) genes were identified. Functional analysis revealed pathways commonly associated with urban-industrialized groups, such as lipopolysaccharide biosynthesis that was characterized in the Hadza gut plasmids. These results demonstrate the richness of plasmids in semi-isolated human groups’ gut microbiome, which represents an important source of information with biotechnological/pharmaceutical potential, but also on the spread of resistance/virulence genes to semi-isolated groups.

Metagenomic assembled plasmids of the human microbiome vary across disease cohorts

We compiled a human metagenome assembled plasmid (MAP) database and interrogated differences across multiple studies that were originally designed to investigate the composition of the human microbiome across various lifestyles, life stages and events. This was performed as plasmids enable bacteria to rapidly expand their functional capacity through mobilisation, yet their contribution to human health and disease is poorly understood. We observed that inter-sample β-diversity differences of plasmid content (plasmidome) could distinguish cohorts across a multitude of conditions. We also show that reduced intra-sample plasmidome α-diversity is consistent amongst patients with inflammatory bowel disease (IBD) and Clostridioides difficile infections. We also show that faecal microbiota transplants can restore plasmidome diversity. Overall plasmidome diversity, specific plasmids, and plasmid-encoded functions can all potentially act as biomarkers of IBD or its severity. The human plasmidome is an overlooked facet of the microbiome and should be integrated into investigations regarding the role of the microbiome in promoting health or disease. Including MAP databases in analyses will enable a greater understanding of the roles of plasmid-encoded functions within the gut microbiome and will inform future human metagenome analyses.

A Peek into the Plasmidome of Global Sewage

Plasmids can provide a selective advantage for microorganisms to survive and adapt to new environmental conditions. Plasmid-encoded traits, such as antimicrobial resistance (AMR) or virulence, impact the ecology and evolution of bacteria and can significantly influence the burden of infectious diseases. Insight about the identity and functions encoded on plasmids on the global scale are largely lacking. Here, we investigate the plasmidome of 24 samples (22 countries, 5 continents) from the global sewage surveillance project. We obtained 105-Gbp Oxford Nanopore and 167-Gbp Illumina NextSeq DNA sequences from plasmid DNA preparations and assembled 165,302 contigs (159,322 circular). Of these, 58,429 carried genes encoding for plasmid-related and 11,222 for virus/phage-related proteins. About 90% of the circular DNA elements did not have any similarity to known plasmids. Those that exhibited similarity had similarity to plasmids whose hosts were previously detected in these sewage samples (e.g., Acinetobacter, Escherichia, Moraxella, Enterobacter, Bacteroides, and Klebsiella). Some AMR classes were detected at a higher abundance in plasmidomes (e.g., macrolide-lincosamide-streptogramin B, macrolide, and quinolone) compared to the respective complex sewage samples. In addition to AMR genes, a range of functions were encoded on the candidate plasmids, including plasmid replication and maintenance, mobilization, and conjugation. In summary, we describe a laboratory and bioinformatics workflow for the recovery of plasmids and other potential extrachromosomal DNA elements from complex microbiomes. Moreover, the obtained data could provide further valuable insight into the ecology and evolution of microbiomes, knowledge about AMR transmission, and the discovery of novel functions.

IMPORTANCE This is, to the best of our knowledge, the first study to investigate plasmidomes at a global scale using long read sequencing from complex untreated domestic sewage. Previous metagenomic surveys have detected AMR genes in a variety of environments, including sewage. However, it is unknown whether the AMR genes were present on the microbial chromosome or located on extrachromosomal elements, such as plasmids. Using our approach, we recovered a large number of plasmids, of which most appear novel. We identified distinct AMR genes that were preferentially located on plasmids, potentially contributing to their transmissibility. Overall, plasmids are of great importance for the biology of microorganisms in their natural environments (free-living and host-associated), as well as for molecular biology and biotechnology. Plasmidome collections may therefore be valuable resources for the discovery of fundamental biological mechanisms and novel functions useful in a variety of contexts.

Contagious Antibiotic Resistance: Plasmid Transfer among Bacterial Residents of the Zebrafish Gut

By characterizing the trajectories of antibiotic resistance gene transfer in bacterial communities such as the gut microbiome, we will better understand the factors that influence this spread of resistance. Our aim was to investigate the host network of a multidrug resistance broad-host-range plasmid in the culturable gut microbiome of zebrafish. This was done through in vitro and in vivo conjugation experiments with Escherichia coli as the donor of the plasmid pB10::gfp When this donor was mixed with the extracted gut microbiome, only transconjugants of Aeromonas veronii were detected. In separate matings between the same donor and four prominent isolates from the gut microbiome, the plasmid transferred to two of these four isolates, A. veronii and Plesiomonas shigelloides, but not to Shewanella putrefaciens and Vibrio mimicus When these A. veronii and P. shigelloides transconjugants were the donors in matings with the same four isolates, the plasmid now also transferred from A. veronii to S. putrefaciensP. shigelloides was unable to donate the plasmid, and V. mimicus was unable to acquire it. Finally, when the E. coli donor was added in vivo to zebrafish through their food, plasmid transfer was observed in the gut, but only to Achromobacter, a rare member of the gut microbiome. This work shows that the success of plasmid-mediated antibiotic resistance spread in a gut microbiome depends on the donor-recipient species combinations and therefore their spatial arrangement. It also suggests that rare gut microbiome members should not be ignored as potential reservoirs of multidrug resistance plasmids from food.IMPORTANCE To understand how antibiotic resistance plasmids end up in human pathogens, it is crucial to learn how, where, and when they are transferred and maintained in members of bacterial communities such as the gut microbiome. To gain insight into the network of plasmid-mediated antibiotic resistance sharing in the gut microbiome, we investigated the transferability and maintenance of a multidrug resistance plasmid among the culturable bacteria of the zebrafish gut. We show that the success of plasmid-mediated antibiotic resistance spread in a gut microbiome can depend on which species are involved, as some are important nodes in the plasmid-host network and others are dead ends. Our findings also suggest that rare gut microbiome members should not be ignored as potential reservoirs of multidrug resistance plasmids from food.

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.

Characterization of putative circular plasmids in sponge-associated bacterial communities using a selective multiply-primed rolling circle amplification

Plasmid transfers among bacterial populations can directly influence the ecological adaptation of these populations and their interactions with host species and environment. In this study, we developed a selective multiply-primed rolling circle amplification (smRCA) approach to enrich and characterize circular plasmid DNA from sponge microbial symbionts via high-throughput sequencing (HTS). DNA (plasmid and total community DNA) obtained from sponge (Cinachyrella sp.) samples and a bacterial symbiont (Vibrio sp. CyArs1) isolated from the same sponge species (carrying unknown plasmids) were used to develop and validate our methodology. The smRCA was performed during 16 hr with 141 plasmid-specific primers covering all known circular plasmid groups. The amplified products were purified and subjected to a reamplification with random hexamer primers (2 hr) and then sequenced using Illumina MiSeq. The developed method resulted in the successful amplification and characterization of the sponge plasmidome and allowed us to detect plasmids associated with the bacterial symbiont Vibrio sp. CyArs1 in the sponge host. In addition to this, a large number of small (<2 kbp) and cryptic plasmids were also amplified in sponge samples. Functional analysis identified proteins involved in the control of plasmid partitioning, maintenance and replication. However, most plasmids contained unknown genes, which could potentially serve as a resource of unknown genetic information and novel replication systems. Overall, our results indicate that the smRCA-HTS approach developed here was able to selectively enrich and characterize plasmids from bacterial isolates and sponge host microbial communities, including plasmids larger than 20 kbp.

Modulating the gut-liver axis and the pivotal role of the faecal microbiome in cirrhosis

Cirrhosis is associated with intestinal dysbiosis, with specific alterations in the gut microbiota linked to particular aetiologies and manifestations of the disease. We review the role of the gut microbiome and the importance of the intestinal barrier in cirrhosis, provide an overview of the terminology and techniques relevant to this emerging area, and discuss the latest developments in therapies targeting the gut-liver axis.

Antimicrobial resistance in chronic liver disease

High levels of antimicrobial drug resistance deleteriously affecting the outcome of treatment with antibacterial agents are causing increasing concern worldwide. This is particularly worrying in patients with cirrhosis with a depressed immune system and heightened susceptibility to infection. Antibiotics have to be started early before results of microbiological culture are available. Current guidelines for the empirical choice of antibiotics in this situation are not very helpful, and embracing antimicrobial stewardship including rapid de-escalation of therapy are not sufficiently emphasised. Multi-drug resistant organism rates to quinolone drugs of up to 40% are recorded in patients with spontaneous bacterial peritonitis on prophylactic antibiotics, leading to a break-through recurrence of intra-peritoneal infection. Also considered in this review is the value of rifaximin-α, non-selective beta-blockers, and concerns around proton pump inhibitor drug use. Fecal microbial transplantation and other gut-targeting therapies in lessening gut bacterial translocation are a promising approach, and new molecular techniques for determining bacterial sensitivity will allow more specific targeted therapy.

Long-read metagenomic exploration of extrachromosomal mobile genetic elements in the human gut

BACKGROUND

Elucidating the ecological and biological identity of extrachromosomal mobile genetic elements (eMGEs), such as plasmids and bacteriophages, in the human gut remains challenging due to their high complexity and diversity.

RESULTS

Here, we show efficient identification of eMGEs as complete circular or linear contigs from PacBio long-read metagenomic data. De novo assembly of PacBio long reads from 12 faecal samples generated 82 eMGE contigs (2.5~666.7-kb), which were classified as 71 plasmids and 11 bacteriophages, including 58 novel plasmids and six bacteriophages, and complete genomes of five diverse crAssphages with terminal direct repeats. In a dataset of 413 gut metagenomes from five countries, many of the identified plasmids were highly abundant and prevalent. The ratio of gut plasmids by our plasmid data is more than twice that in the public database. Plasmids outnumbered bacterial chromosomes three to one on average in this metagenomic dataset. Host prediction suggested that Bacteroidetes-associated plasmids predominated, regardless of microbial abundance. The analysis found several plasmid-enriched functions, such as inorganic ion transport, while antibiotic resistance genes were harboured mostly in low-abundance Proteobacteria-associated plasmids.

CONCLUSIONS

Overall, long-read metagenomics provided an efficient approach for unravelling the complete structure of human gut eMGEs, particularly plasmids.

Tracking the Rules of Transmission and Introgression with Networks

Understanding how an animal organism and its gut microbes form an integrated biological organization, known as a holobiont, is becoming a central issue in biological studies. Such an organization inevitably involves a complex web of transmission processes that occur on different scales in time and space, across microbes and hosts. Network-based models are introduced in this chapter to tackle aspects of this complexity and to better take into account vertical and horizontal dimensions of transmission. Two types of network-based models are presented, sequence similarity networks and bipartite graphs. One interest of these networks is that they can consider a rich diversity of important players in microbial evolution that are usually excluded from evolutionary studies, like plasmids and viruses. These methods bring forward the notion of "gene externalization," which is defined as the presence of redundant copies of prokaryotic genes on mobile genetic elements (MGEs), and therefore emphasizes a related although distinct process from lateral gene transfer between microbial cells. This chapter introduces guidelines to the construction of these networks, reviews their analysis, and illustrates their possible biological interpretations and uses. The application to human gut microbiomes shows that sequences present in a higher diversity of MGEs have both biased functions and a broader microbial and human host range. These results suggest that an "externalized gut metagenome" is partly common to humans and benefits the gut microbial community. We conclude that testing relationships between microbial genes, microbes, and their animal hosts, using network-based methods, could help to unravel additional mechanisms of transmission in holobionts.

The human gut virome: form and function

Advances in next-generation sequencing technologies and the application of metagenomic approaches have fuelled an exponential increase in our understanding of the human gut microbiome. These approaches are now also illuminating features of the diverse and abundant collection of viruses (termed the virome) subsisting with the microbial ecosystems residing within the human holobiont. Here, we focus on the current and emerging knowledge of the human gut virome, in particular on viruses infecting bacteria (bacteriophage or phage), which are a dominant component of this viral community. We summarise current insights regarding the form and function of this 'human gut phageome' and highlight promising avenues for future research. In doing so, we discuss the potential for phage to drive ecological functioning and evolutionary change within this important microbial ecosystem, their contribution to modulation of host-microbiome interactions and stability of the community as a whole, as well as the potential role of the phageome in human health and disease. We also consider the emerging concepts of a 'core healthy gut phageome' and the putative existence of 'viral enterotypes' and 'viral dysbiosis'.

Association of the gut microbiota mobilome with hospital location and birth weight in preterm infants

BackgroundThe preterm infant gut microbiota is vulnerable to different biotic and abiotic factors. Although the development of this microbiota has been extensively studied, the mobilome-i.e. the mobile genetic elements (MGEs) in the gut microbiota-has not been considered. Therefore, the aim of this study was to investigate the association of the mobilome with birth weight and hospital location in the preterm infant gut microbiota.MethodsThe data set consists of fecal samples from 62 preterm infants with and without necrotizing enterocolitis (NEC) from three different hospitals. We analyzed the gut microbiome by using 16S rRNA amplicon sequencing, shot-gun metagenome sequencing, and quantitative PCR. Predictive models and other data analyses were performed using MATLAB and QIIME.ResultSThe microbiota composition was significantly different between NEC-positive and NEC-negative infants and significantly different between hospitals. An operational taxanomic unit (OTU) showed strong positive and negative correlation with NEC and birth weight, respectively, whereas none showed significance for mode of delivery. Metagenome analyses revealed high levels of conjugative plasmids with MGEs and virulence genes. Results from quantitative PCR showed that the plasmid signature genes were significantly different between hospitals and in NEC-positive infants.ConclusionOur results point toward an association of the mobilome with hospital location in preterm infants.

The human gut resistome

In recent decades, the emergence and spread of antibiotic resistance among bacterial pathogens has become a major threat to public health. Bacteria can acquire antibiotic resistance genes by the mobilization and transfer of resistance genes from a donor strain. The human gut contains a densely populated microbial ecosystem, termed the gut microbiota, which offers ample opportunities for the horizontal transfer of genetic material, including antibiotic resistance genes. Recent technological advances allow microbiota-wide studies into the diversity and dynamics of the antibiotic resistance genes that are harboured by the gut microbiota (‘the gut resistome’). Genes conferring resistance to antibiotics are ubiquitously present among the gut microbiota of humans and most resistance genes are harboured by strictly anaerobic gut commensals. The horizontal transfer of genetic material, including antibiotic resistance genes, through conjugation and transduction is a frequent event in the gut microbiota, but mostly involves non-pathogenic gut commensals as these dominate the microbiota of healthy individuals. Resistance gene transfer from commensals to gut-dwelling opportunistic pathogens appears to be a relatively rare event but may contribute to the emergence of multi-drug resistant strains, as is illustrated by the vancomycin resistance determinants that are shared by anaerobic gut commensals and the nosocomial pathogen Enterococcus faecium.

Escherichia coli Nissle 1917 enhances bioavailability of serotonin in gut tissues through modulation of synthesis and clearance

Accumulating evidence shows indigenous gut microbes can interact with the human host through modulation of serotonin (5-HT) signaling. Here we investigate the impact of the probiotic Escherichia coli Nissle 1917 (EcN) on 5-HT signalling in gut tissues. Ex-vivo mouse ileal tissue sections were treated with either EcN or the human gut commensal MG1655, and effects on levels of 5-HT, precursors, and metabolites, were evaluated using amperometry and high performance liquid chromatography with electrochemical detection (HPLC-EC). Exposure of tissue to EcN cells, but not MG1655 cells, was found to increase levels of extra-cellular 5-HT. These effects were not observed when tissues were treated with cell-free supernatant from bacterial cultures. In contrast, when supernatant recovered from untreated ileal tissue was pre-incubated with EcN, the derivative cell-free supernatant was able to elevate 5-HT overflow when used to treat fresh ileal tissue. Measurement of 5-HT precursors and metabolites indicated EcN also increases intracellular 5-HTP and reduces 5-HIAA. The former pointed to modulation of tryptophan hydroxylase-1 to enhance 5-HT synthesis, while the latter indicates an impact on clearance into enterocytes through SERT. Taken together, these findings show EcN is able to enhance 5-HT bioavailability in ileal tissues through interaction with compounds secreted from host tissues.

The human gut virome: a multifaceted majority

Here, we outline our current understanding of the human gut virome, in particular the phage component of this ecosystem, highlighting progress, and challenges in viral discovery in this arena. We reveal how developments in high-throughput sequencing technologies and associated data analysis methodologies are helping to illuminate this abundant 'biological dark matter.' Current evidence suggests that the human gut virome is a highly individual but temporally stable collective, dominated by phages exhibiting a temperate lifestyle. This viral community also appears to encode a surprisingly rich functional repertoire that confers a range of attributes to their bacterial hosts, ranging from bacterial virulence and pathogenesis to maintaining host-microbiome stability and community resilience. Despite the significant advances in our understanding of the gut virome in recent years, it is clear that we remain in a period of discovery and revelation, as new methods and technologies begin to provide deeper understanding of the inherent ecological characteristics of this viral ecosystem. As our understanding increases, the nature of the multi-partite interactions occurring between host and microbiome will become clearer, helping us to more rationally define the concepts and principles that will underpin approaches to using human gut virome components for medical or biotechnological applications.

Rethinking evolutionary individuality

This paper considers whether multispecies biofilms are evolutionary individuals. Numerous multispecies biofilms have characteristics associated with individuality, such as internal integrity, division of labor, coordination among parts, and heritable adaptive traits. However, such multispecies biofilms often fail standard reproductive criteria for individuality: they lack reproductive bottlenecks, are comprised of multiple species, do not form unified reproductive lineages, and fail to have a significant division of reproductive labor among their parts. If such biofilms are good candidates for evolutionary individuals, then evolutionary individuality is achieved through other means than frequently cited reproductive processes. The case of multispecies biofilms suggests that standard reproductive requirements placed on individuality should be reconsidered. More generally, the case of multispecies biofilms indicates that accounts of individuality that focus on single-species eukaryotes are too restrictive and that a pluralistic and open-ended account of evolutionary individuality is needed.

Analysis of a novel 8.9kb cryptic plasmid from Bacteroides uniformis, its long-term stability and spread within human microbiota

The analysis of plasmid content in dominant Bacteroidales order intestinal strains isolated from the same child at a 5 year interval identified a 8.9 kb plasmid in Bacteroides uniformis BUN24 strain isolated at age 6 and indistinguishably sized plasmids in the isolates of B. uniformis, B. vulgatus, B. intesinalis, and Parabacteroides distasonis at age 11. We sequenced a B. uniformis BUN24 plasmid, designated pBUN24, and using molecular surveys of diverse species we established that this 8944bp molecule (G+C content 43.5%) represents a novel family of small cryptic Bacteroidales plasmids. The replication region of pBUN24 was experimentally localized to a 1707-bp fragment that includes a putative repA gene, coding for a protein of Rep_3 superfamily of replication proteins of theta-type plasmids preceded by a putative iteron-containing origin of replication. The other open reading frames (ORFs) identified in pBUN24 sequence include a putative tad-ata-type toxin-antitoxin and mobA-mobB mobilization modules, as well as seven additional cryptic ORFs. The interaction of Tad and Ada components demonstrated by a pull-down assay and the toxicity of Tad in Escherichia coli host suggests the functionality of the plasmid addiction module. Re-sequencing of plasmids in two Bacteroides strains isolated at the age of 11 showed 100% nucleotide identity to pBUN24. This data supports the notion that this plasmid is transmissible to other Bacteroidales strains in the natural ecosystem. The possible roles of toxin-antitoxin system and other proteins encoded by pBUN24 in providing an apparent ecological advantage to the plasmid-harbouring strains of a bacterial symbiont in the human gut deserve further investigation.

Comparative (meta)genomic analysis and ecological profiling of human gut-specific bacteriophage φB124-14

Bacteriophage associated with the human gut microbiome are likely to have an important impact on community structure and function, and provide a wealth of biotechnological opportunities. Despite this, knowledge of the ecology and composition of bacteriophage in the gut bacterial community remains poor, with few well characterized gut-associated phage genomes currently available. Here we describe the identification and in-depth (meta)genomic, proteomic, and ecological analysis of a human gut-specific bacteriophage (designated φB124-14). In doing so we illuminate a fraction of the biological dark matter extant in this ecosystem and its surrounding eco-genomic landscape, identifying a novel and uncharted bacteriophage gene-space in this community. φB124-14 infects only a subset of closely related gut-associated Bacteroides fragilis strains, and the circular genome encodes functions previously found to be rare in viral genomes and human gut viral metagenome sequences, including those which potentially confer advantages upon phage and/or host bacteria. Comparative genomic analyses revealed φB124-14 is most closely related to φB40-8, the only other publically available Bacteroides sp. phage genome, whilst comparative metagenomic analysis of both phage failed to identify any homologous sequences in 136 non-human gut metagenomic datasets searched, supporting the human gut-specific nature of this phage. Moreover, a potential geographic variation in the carriage of these and related phage was revealed by analysis of their distribution and prevalence within 151 human gut microbiomes and viromes from Europe, America and Japan. Finally, ecological profiling of φB124-14 and φB40-8, using both gene-centric alignment-driven phylogenetic analyses, as well as alignment-free gene-independent approaches was undertaken. This not only verified the human gut-specific nature of both phage, but also indicated that these phage populate a distinct and unexplored ecological landscape within the human gut microbiome.