Microbial Evolution Is in the Cards: Horizontal Gene Transfer in the Classroom

ABSTRACT Bacteriophages infect an estimated 10 23 to 10 25 bacterial cells each second, many of which carry physiologically relevant plasmids (e.g., those encoding antibiotic resistance). However, even though phage-plasmid interactions occur on a massive scale and have potentially significant evolutionary, ecological, and biomedical implications, plasmid fate upon phage infection and lysis has not been investigated to date. Here we show that a subset of the natural lytic phage population, which we dub “superspreaders,” releases substantial amounts of intact, transformable plasmid DNA upon lysis, thereby promoting horizontal gene transfer by transformation. Two novel Escherichia coli phage superspreaders, SUSP1 and SUSP2, liberated four evolutionarily distinct plasmids with equal efficiency, including two close relatives of prominent antibiotic resistance vectors in natural environments. SUSP2 also mediated the extensive lateral transfer of antibiotic resistance in unbiased communities of soil bacteria from Maryland and Wyoming. Furthermore, the addition of SUSP2 to cocultures of kanamycin-resistant E. coli and kanamycin-sensitive Bacillus sp. bacteria resulted in roughly 1,000-fold more kanamycin-resistant Bacillus sp. bacteria than arose in phage-free controls. Unlike many other lytic phages, neither SUSP1 nor SUSP2 encodes homologs to known hydrolytic endonucleases, suggesting a simple potential mechanism underlying the superspreading phenotype. Consistent with this model, the deletion of endonuclease IV and the nucleoid-disrupting protein ndd from coliphage T4, a phage known to extensively degrade chromosomal DNA, significantly increased its ability to promote plasmid transformation. Taken together, our results suggest that phage superspreaders may play key roles in microbial evolution and ecology but should be avoided in phage therapy and other medical applications. IMPORTANCE Bacteriophages (phages), viruses that infect bacteria, are the planet’s most numerous biological entities and kill vast numbers of bacteria in natural environments. Many of these bacteria carry plasmids, extrachromosomal DNA elements that frequently encode antibiotic resistance. However, it is largely unknown whether plasmids are destroyed during phage infection or released intact upon phage lysis, whereupon their encoded resistance could be acquired and manifested by other bacteria (transformation). Because phages are being developed to combat antibiotic-resistant bacteria and because transformation is a principal form of horizontal gene transfer, this question has important implications for biomedicine and microbial evolution alike. Here we report the isolation and characterization of two novel Escherichia coli phages, dubbed “superspreaders,” that promote extensive plasmid transformation and efficiently disperse antibiotic resistance genes. Our work suggests that phage superspreaders are not suitable for use in medicine but may help drive bacterial evolution in natural environments.

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Transductomics: sequencing-based detection and analysis of transduced DNA in pure cultures and microbial communities

Microbiome ◽

10.1186/s40168-020-00935-5 ◽

2020 ◽

Vol 8 (1) ◽

Author(s):

Manuel Kleiner ◽

Brian Bushnell ◽

Kenneth E. Sanderson ◽

Lora V. Hooper ◽

Breck A. Duerkop

Keyword(s):

Gene Transfer ◽

Horizontal Gene Transfer ◽

Microbial Communities ◽

Microbial Evolution ◽

Model Systems ◽

Intestinal Microbiome ◽

Mobile Dna ◽

Pure Cultures ◽

Taxonomic Range ◽

Taxonomic Groups

Abstract Background Horizontal gene transfer (HGT) plays a central role in microbial evolution. Our understanding of the mechanisms, frequency, and taxonomic range of HGT in polymicrobial environments is limited, as we currently rely on historical HGT events inferred from genome sequencing and studies involving cultured microorganisms. We lack approaches to observe ongoing HGT in microbial communities. Results To address this knowledge gap, we developed a DNA sequencing-based “transductomics” approach that detects and characterizes microbial DNA transferred via transduction. We validated our approach using model systems representing a range of transduction modes and show that we can detect numerous classes of transducing DNA. Additionally, we show that we can use this methodology to obtain insights into DNA transduction among all major taxonomic groups of the intestinal microbiome. Conclusions The transductomics approach that we present here allows for the detection and characterization of genes that are potentially transferred between microbes in complex microbial communities at the time of measurement and thus provides insights into real-time ongoing horizontal gene transfer. This work extends the genomic toolkit for the broader study of mobile DNA within microbial communities and could be used to understand how phenotypes spread within microbiomes.

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Horizontal gene transfer: essentiality and evolvability in prokaryotes, and roles in evolutionary transitions

F1000Research ◽

10.12688/f1000research.8737.1 ◽

2016 ◽

Vol 5 ◽

pp. 1805 ◽

Cited By ~ 81

Author(s):

Eugene V. Koonin

Keyword(s):

Gene Transfer ◽

Horizontal Gene Transfer ◽

Phylogenetic Trees ◽

Point Mutations ◽

Theoretical Models ◽

Microbial Evolution ◽

Gene Gain ◽

Gene Trees ◽

Ratchet Effect ◽

Evolutionary Transitions

The wide spread of gene exchange and loss in the prokaryotic world has prompted the concept of ‘lateral genomics’ to the point of an outright denial of the relevance of phylogenetic trees for evolution. However, the pronounced coherence congruence of the topologies of numerous gene trees, particularly those for (nearly) universal genes, translates into the notion of a statistical tree of life (STOL), which reflects a central trend of vertical evolution. The STOL can be employed as a framework for reconstruction of the evolutionary processes in the prokaryotic world. Quantitatively, however, horizontal gene transfer (HGT) dominates microbial evolution, with the rate of gene gain and loss being comparable to the rate of point mutations and much greater than the duplication rate. Theoretical models of evolution suggest that HGT is essential for the survival of microbial populations that otherwise deteriorate due to the Muller’s ratchet effect. Apparently, at least some bacteria and archaea evolved dedicated vehicles for gene transfer that evolved from selfish elements such as plasmids and viruses. Recent phylogenomic analyses suggest that episodes of massive HGT were pivotal for the emergence of major groups of organisms such as multiple archaeal phyla as well as eukaryotes. Similar analyses appear to indicate that, in addition to donating hundreds of genes to the emerging eukaryotic lineage, mitochondrial endosymbiosis severely curtailed HGT. These results shed new light on the routes of evolutionary transitions, but caution is due given the inherent uncertainty of deep phylogenies.

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The Composition of the Cell Envelope Affects Conjugation in Bacillus subtilis

Journal of Bacteriology ◽

10.1128/jb.01044-15 ◽

2016 ◽

Vol 198 (8) ◽

pp. 1241-1249 ◽

Cited By ~ 6

Author(s):

Christopher M. Johnson ◽

Alan D. Grossman

Keyword(s):

Bacillus Subtilis ◽

Gene Transfer ◽

Horizontal Gene Transfer ◽

Cell Envelope ◽

Microbial Evolution ◽

Major Type ◽

Content Type ◽

New Genes ◽

Mutant Cells ◽

Integrative And Conjugative Element

ABSTRACTConjugation in bacteria is the contact-dependent transfer of DNA from one cell to another via donor-encoded conjugation machinery. It is a major type of horizontal gene transfer between bacteria. Conjugation of the integrative and conjugative element ICEBs1intoBacillus subtilisis affected by the composition of phospholipids in the cell membranes of the donor and recipient. We found that reduction (or elimination) of lysyl-phosphatidylglycerol caused by loss ofmprFcaused a decrease in conjugation efficiency. Conversely, alterations that caused an increase in lysyl-phosphatidylglycerol, including loss ofugtPor overproduction ofmprF, caused an increase in conjugation efficiency. In addition, we found that mutations that alter production of other phospholipids, e.g., loss ofclsAandyfnI, also affected conjugation, apparently without substantively altering levels of lysyl-phosphatidylglycerol, indicating that there are multiple pathways by which changes to the cell envelope affect conjugation. We found that the contribution ofmprFto conjugation was affected by the chemical environment. Wild-type cells were generally more responsive to addition of anions that enhanced conjugation, whereasmprFmutant cells were more sensitive to combinations of anions that inhibited conjugation at pH 7. Our results indicate thatmprFand lysyl-phosphatidylglycerol allow cells to maintain relatively consistent conjugation efficiencies under a variety of ionic conditions.IMPORTANCEHorizontal gene transfer is a driving force in microbial evolution, enabling cells that receive DNA to acquire new genes and phenotypes. Conjugation, the contact-dependent transfer of DNA from a donor to a recipient by a donor-encoded secretion machine, is a prevalent type of horizontal gene transfer. Although critically important, it is not well understood how the recipient influences the success of conjugation. We found that the composition of phospholipids in the membranes of donors and recipients influences the success of transfer of the integrative and conjugative element ICEBs1inBacillus subtilis. Specifically, the presence of lysyl-phosphatidylglycerol enables relatively constant conjugation efficiencies in a range of diverse chemical environments.

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CRISPR-Cas-Mediated Phage Resistance Enhances Horizontal Gene Transfer by Transduction

mBio ◽

10.1128/mbio.02406-17 ◽

2018 ◽

Vol 9 (1) ◽

Cited By ~ 38

Author(s):

Bridget N. J. Watson ◽

Raymond H. J. Staals ◽

Peter C. Fineran

Keyword(s):

Gene Transfer ◽

Horizontal Gene Transfer ◽

Adaptive Immunity ◽

Microbial Evolution ◽

Phage Infection ◽

Phage Resistance ◽

Bacterial Dna ◽

Associated Proteins ◽

Mixed Populations ◽

Type Phage

ABSTRACTA powerful contributor to prokaryotic evolution is horizontal gene transfer (HGT) through transformation, conjugation, and transduction, which can be advantageous, neutral, or detrimental to fitness. Bacteria and archaea control HGT and phage infection through CRISPR-Cas (clustered regularly interspaced short palindromic repeats–CRISPR-associated proteins) adaptive immunity. Although the benefits of resisting phage infection are evident, this can come at a cost of inhibiting the acquisition of other beneficial genes through HGT. Despite the ability of CRISPR-Cas to limit HGT through conjugation and transformation, its role in transduction is largely overlooked. Transduction is the phage-mediated transfer of bacterial DNA between cells and arguably has the greatest impact on HGT. We demonstrate that inPectobacterium atrosepticum, CRISPR-Cas can inhibit the transduction of plasmids and chromosomal loci. In addition, we detected phage-mediated transfer of a large plant pathogenicity genomic island and show that CRISPR-Cas can inhibit its transduction. Despite these inhibitory effects of CRISPR-Cas on transduction, its more common role in phage resistance promotes rather than diminishes HGT via transduction by protecting bacteria from phage infection. This protective effect can also increase transduction of phage-sensitive members of mixed populations. CRISPR-Cas systems themselves display evidence of HGT, but little is known about their lateral dissemination between bacteria and whether transduction can contribute. We show that, through transduction, bacteria can acquire an entire chromosomal CRISPR-Cas system, includingcasgenes and phage-targeting spacers. We propose that the positive effect of CRISPR-Cas phage immunity on enhancing transduction surpasses the rarer cases where gene flow by transduction is restricted.IMPORTANCEThe generation of genetic diversity through acquisition of DNA is a powerful contributor to microbial evolution and occurs through transformation, conjugation, and transduction. Of these, transduction, the phage-mediated transfer of bacterial DNA, is arguably the major route for genetic exchange. CRISPR-Cas adaptive immune systems control gene transfer by conjugation and transformation, but transduction has been mostly overlooked. Our results indicate that CRISPR-Cas can impede, but typically enhances the transduction of plasmids, chromosomal genes, and pathogenicity islands. By limiting wild-type phage replication, CRISPR-Cas immunity increases transduction in both phage-resistant and -sensitive members of mixed populations. Furthermore, we demonstrate mobilization of a chromosomal CRISPR-Cas system containing phage-targeting spacers by generalized transduction, which might partly account for the uneven distribution of these systems in nature. Overall, the ability of CRISPR-Cas to promote transduction reveals an unexpected impact of adaptive immunity on horizontal gene transfer, with broader implications for microbial evolution.

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Tracking microbial evolution in the human gut using Hi-C

10.1101/594903 ◽

2019 ◽

Cited By ~ 2

Author(s):

Eitan Yaffe ◽

David A. Relman

Keyword(s):

Gene Transfer ◽

Horizontal Gene Transfer ◽

Gut Microbiota ◽

Adaptive Evolution ◽

High Throughput ◽

Microbial Evolution ◽

Natural Environments ◽

Human Gut ◽

Natural Communities

Despite the importance of horizontal gene transfer for rapid bacterial evolution, reliable assignment of mobile genetic elements to their microbial hosts in natural communities such as the human gut microbiota remains elusive. We used Hi-C (High-throughput chromosomal conformation capture), coupled with probabilistic modeling of experimental noise, to resolve 88 strain-level genomes of distal gut bacteria from two subjects, including 12,251 accessory elements. Comparisons of 2 samples collected 10 years apart for each of the subjects revealed extensive in situ exchange of accessory elements, as well as evidence of adaptive evolution in core genomes. Accessory elements were predominantly promiscuous and prevalent in the distal gut metagenomes of 218 adult subjects. This work provides a foundation and approach for studying microbial evolution in natural environments.

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Novel integrative elements and genomic plasticity in ocean ecosystems

10.1101/2020.12.28.424599 ◽

2020 ◽

Author(s):

Thomas Hackl ◽

Raphaël Laurenceau ◽

Markus J. Ankenbrand ◽

Christina Bliem ◽

Zev Cariani ◽

...

Keyword(s):

Gene Transfer ◽

Horizontal Gene Transfer ◽

Niche Differentiation ◽

Mobile Genetic Elements ◽

Microbial Evolution ◽

Genomic Islands ◽

Trna Genes ◽

Marine Microbes ◽

Genomic Plasticity ◽

Genetic Elements

Horizontal gene transfer accelerates microbial evolution, promoting diversification and adaptation. The globally abundant marine cyanobacterium Prochlorococcus has a highly streamlined genome with frequent gene exchange reflected in its extensive pangenome. The source of its genomic variability, however, remains elusive since most cells lack the common mechanisms that enable horizontal gene transfer, including conjugation, transformation, plasmids and prophages. Examining 623 genomes, we reveal a diverse system of mobile genetic elements – cargo-carrying transposons we named tycheposons – that shape Prochlorococcus’ genomic plasticity. The excision and integration of tycheposons at seven tRNA genes drive the remodeling of larger genomic islands containing most of Prochlorococcus’ flexible genes. Most tycheposons carry genes important for niche differentiation through nutrient acquisition; others appear similar to phage parasites. Tycheposons are highly enriched in extracellular vesicles and phage particles in ocean samples, suggesting efficient routes for their dispersal, transmission and propagation. Supported by evidence for similar elements in other marine microbes, our work underpins the role of vesicle- and virus-mediated transfer of mobile genetic elements in the diversification and adaptation of microbes in dilute aquatic environments – adding a significant piece to the puzzle of what governs microbial evolution in the planet’s largest habitat.

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On Inferring Additive and Replacing Horizontal Gene Transfers Through Phylogenetic Reconciliation

10.1101/2020.03.27.010785 ◽

2020 ◽

Author(s):

Misagh Kordi ◽

Soumya Kundu ◽

Mukul S. Bansal

Keyword(s):

Gene Transfer ◽

Horizontal Gene Transfer ◽

Classification Accuracy ◽

Software Package ◽

Microbial Evolution ◽

Homologous Gene ◽

Gene Duplications ◽

New Gene ◽

Phylogenetic Reconciliation ◽

The Impact

AbstractHorizontal gene transfer is one of the most important mechanisms for microbial evolution and adaptation. It is well known that horizontal gene transfer can be either additive or replacing depending on whether the transferred gene adds itself as a new gene in the recipient genome or replaces an existing homologous gene. Yet, all existing phylogenetic techniques for the inference of horizontal gene transfer assume either that all transfers are additive or that all transfers are replacing. This limitation not only affects the applicability and accuracy of these methods but also makes it difficult to distinguish between additive and replacing transfers.Here, we address this important problem by formalizing a phylogenetic reconciliation framework that simultaneously models both additive and replacing transfer events. Specifically, we (1) introduce the DTRL reconciliation framework that explicitly models both additive and replacing transfer events, along with gene duplications and losses, (2) prove that the underlying computational problem is NP-hard, (3) perform the first experimental study to assess the impact of replacing transfer events on the accuracy of the traditional DTL reconciliation model (which assumes that all transfers are additive) and demonstrate that traditional DTL reconciliation remains highly robust to the presence of replacing transfers, (4) propose a simple heuristic algorithm for DTRL reconciliation based on classifying transfer events inferred through DTL reconciliation as being replacing or additive, and (5) evaluate the classification accuracy of the heuristic under a range of evolutionary conditions. Thus, this work lays the methodological and algorithmic foundations for estimating DTRL reconciliations and distinguishing between additive and replacing transfers.An implementation of our heuristic for DTRL reconciliation is freely available open-source as part of the RANGER-DTL software package from https://compbio.engr.uconn.edu/software/ranger-dtl/.

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