Genomic islands are dynamic, ancient integrative elements in bacterial evolution

Exploring Mobile Genetic Elements in Vibrio cholerae

Members of the bacterial species Vibrio cholerae are known both as prominent constituents of marine environments and as the causative agents of cholera, a severe diarrheal disease. While strains responsible for cholera have been extensively studied over the past century, less is known about their environmental counterparts, despite their contributions to the species' pangenome. This study analyzed the genome compositions of 46 V. cholerae strains, including pandemic and nonpandemic, toxigenic, and environmental variants, to investigate the diversity of mobile genetic elements (MGEs), embedded bacterial defense systems, and phage-associated signatures. Our findings include both conserved and novel MGEs across strains, pointing to shared evolutionary pathways and ecological niches. The defensome analysis revealed a wide array of antiphage/antiplasmid mechanisms, extending well beyond the traditional CRISPR-Cas and restriction-modification systems. This underscores the dynamic arms race between V. cholerae and MGEs and suggests that nonpandemic strains may act as reservoirs for emerging defense strategies. Moreover, the study showed that MGEs are integrated into genomic hotspots, which may serve as critical platforms for the exchange of defense systems, thereby enhancing V. cholerae's adaptive capabilities against phage attacks and other invading MGEs. Overall, this research offers new insights into V. cholerae's genetic complexity and potential adaptive strategies, offering a better understanding of the differences between environmental strains and their pandemic counterparts, as well as the possible evolutionary pathways that led to the emergence of pandemic strains.

Evolution of the pheV-tRNA integrated genomic island in Escherichia coli

Escherichia coli exhibit extensive genetic diversity at the genome level, particularly within their accessory genome. The tRNA integrated genomic islands (GIs), a part of the E. coli accessory genome, play an important role in pathogenicity. However, studies examining the evolution of GIs have been challenging due to their large size, considerable gene content variation and fragmented assembly in draft genomes. Here we examined the evolution of the GI integrated at pheV-tRNA (GI-pheV), with a primary focus on uropathogenic E. coli (UPEC) and the globally disseminated multidrug resistant ST131 clone. We show the gene content of GI-pheV is highly diverse and arranged in a modular configuration, with the P4 integrase encoding gene intP4 the only conserved gene. Despite this diversity, the GI-pheV gene content displayed conserved features among strains from the same pathotype. In ST131, GI-pheV corresponding to the reference strain EC958 (EC958_GI-pheV) was found in ~90% of strains. Phylogenetic analyses suggested that GI-pheV in ST131 has evolved together with the core genome, with the loss/gain of specific modules (or the entire GI) linked to strain specific events. Overall, we show GI-pheV exhibits a dynamic evolutionary pathway, in which modules and genes have evolved through multiple events including insertions, deletions and recombination.

Hijackers, hitchhikers, or co-drivers? The mysteries of mobilizable genetic elements

Mobile genetic elements shape microbial gene repertoires and populations. Recent results reveal that many, possibly most, microbial mobile genetic elements require helpers to transfer between genomes, which we refer to as Hitcher Genetic Elements (hitchers or HGEs). They may be a large fraction of pathogenicity and resistance genomic islands, whose mechanisms of transfer have remained enigmatic for decades. Together with their helper elements and their bacterial hosts, hitchers form tripartite networks of interactions that evolve rapidly within a parasitism-mutualism continuum. In this emerging view of microbial genomes as communities of mobile genetic elements many questions arise. Which elements are being moved, by whom, and how? How often are hitchers costly hyper-parasites or beneficial mutualists? What is the evolutionary origin of hitchers? Are there key advantages associated with hitchers' lifestyle that justify their unexpected abundance? And why are hitchers systematically smaller than their helpers? In this essay, we start answering these questions and point ways ahead for understanding the principles, origin, mechanisms, and impact of hitchers in bacterial ecology and evolution.

Discovering genomic islands in unannotated bacterial genomes using sequence embedding

Motivation

Genomic islands (GEIs) are clusters of genes in bacterial genomes that are typically acquired by horizontal gene transfer. GEIs play a crucial role in the evolution of bacteria by rapidly introducing genetic diversity and thus helping them adapt to changing environments. Specifically of interest to human health, many GEIs contain pathogenicity and antimicrobial resistance genes. Detecting GEIs is, therefore, an important problem in biomedical and environmental research. There have been many previous studies for computationally identifying GEIs. Still, most of these studies rely on detecting anomalies in the unannotated nucleotide sequences or on a fixed set of known features on annotated nucleotide sequences.

Results

Here, we present TreasureIsland, which uses a new unsupervised representation of DNA sequences to predict GEIs. We developed a high-precision boundary detection method featuring an incremental fine-tuning of GEI borders, and we evaluated the accuracy of this framework using a new comprehensive reference dataset, Benbow. We show that TreasureIsland's accuracy rivals other GEI predictors, enabling efficient and faster identification of GEIs in unannotated bacterial genomes.

Availability and implementation

TreasureIsland is available under an MIT license at: https://github.com/FriedbergLab/GenomicIslandPrediction.

Genomic and environmental controls on Castellaniella biogeography in an anthropogenically disturbed subsurface

Castellaniella species have been isolated from a variety of mixed-waste environments including the nitrate and multiple metal-contaminated subsurface at the Oak Ridge Reservation (ORR). Previous studies examining microbial community composition and nitrate removal at ORR during biostimulation efforts reported increased abundances of members of the Castellaniella genus concurrent with increased denitrification rates. Thus, we asked how genomic and abiotic factors control the Castellaniella biogeography at the site to understand how these factors may influence nitrate transformation in an anthropogenically impacted setting. We report the isolation and characterization of several Castellaniella strains from the ORR subsurface. Five of these isolates match at 100% identity (at the 16S rRNA gene V4 region) to two Castellaniella amplicon sequence variants (ASVs), ASV1 and ASV2, that have persisted in the ORR subsurface for at least 2 decades. However, ASV2 has consistently higher relative abundance in samples taken from the site and was also the dominant blooming denitrifier population during a prior biostimulation effort. We found that the ASV2 representative strain has greater resistance to mixed metal stress than the ASV1 representative strains. We attribute this resistance, in part, to the large number of unique heavy metal resistance genes identified on a genomic island in the ASV2 representative genome. Additionally, we suggest that the relatively lower fitness of ASV1 may be connected to the loss of the nitrous oxide reductase (nos) operon (and associated nitrous oxide reductase activity) due to the insertion at this genomic locus of a mobile genetic element carrying copper resistance genes. This study demonstrates the value of integrating genomic, environmental, and phenotypic data to characterize the biogeography of key microorganisms in contaminated sites.

A novel conjugative transposon carrying an autonomously amplified plasmid

Tetracyclines serve as broad-spectrum antibiotics to treat bacterial infections. The discovery of new tetracycline resistance genes has led to new questions about the underlying mechanisms of resistance, gene transfer, and their relevance to human health. We tracked changes in the abundance of a 55-kbp conjugative transposon (CTn214) carrying tetQ, a tetracycline resistance gene, within a Bacteroides fragilis metagenome-assembled genome derived from shotgun sequencing of microbial DNA extracted from the ileal pouch of a patient with ulcerative colitis. The mapping of metagenomic reads to CTn214 revealed the multi-copy nature of a 17,044-nt region containing tetQ in samples collected during inflammation and uninflamed visits. B. fragilis cultivars isolated from the same patient during periods of inflammation harbored CTn214 integrated into the chromosome or both a circular, multi-copy, extrachromosomal region of the CTn214 containing tetQ and the corresponding integrated form. The tetracycline-dependent mechanism for the transmission of CTn214 is nearly identical to a common conjugative transposon found in the genome of B. fragilis (CTnDOT), but the autonomously amplified nature of a circular 17,044-nt region of CTn214 that codes for tetQ and the integration of the same sequence in the linear chromosome within the same cell is a novel observation. Genome and transcriptome sequencing of B. fragilis cultivars grown under different concentrations of tetracycline and ciprofloxacin indicates that tetQ in strains containing the circular form remains actively expressed regardless of treatment, while the expression of tetQ in strains containing the linear form increases only in the presence of tetracycline.IMPORTANCEThe exchange of antibiotic production and resistance genes between microorganisms can lead to the emergence of new pathogens. In this study, short-read mapping of metagenomic samples taken over time from the illeal pouch of a patient with ulcerative colitis to a Bacteroides fragilis metagenome-assembled genome revealed two distinct genomic arrangements of a novel conjugative transposon, CTn214, that encodes tetracycline resistance. The autonomous amplification of a plasmid-like circular form from CTn214 that includes tetQ potentially provides consistent ribosome protection against tetracycline. This mode of antibiotic resistance offers a novel mechanism for understanding the emergence of pathobionts like B. fragilis and their persistence for extended periods of time in patients with inflammatory bowel disease.

Core defense hotspots within Pseudomonas aeruginosa are a consistent and rich source of anti-phage defense systems

Bacteria use a diverse arsenal of anti-phage immune systems, including CRISPR-Cas and restriction enzymes. Recent advances in anti-phage system discovery and annotation tools have unearthed many unique systems, often encoded in horizontally transferred defense islands, which can be horizontally transferred. Here, we developed Hidden Markov Models (HMMs) for defense systems and queried microbial genomes on the NCBI database. Out of the 30 species with >200 completely sequenced genomes, our analysis found Pseudomonas aeruginosa exhibits the greatest diversity of anti-phage systems, as measured by Shannon entropy. Using network analysis to identify the common neighbors of anti-phage systems, we identified two core defense hotspot loci (cDHS1 and cDHS2). cDHS1 is up to 224 kb (median: 26 kb) with varied arrangements of more than 30 distinct immune systems across isolates, while cDHS2 has 24 distinct systems (median: 6 kb). Both cDHS regions are occupied in a majority of P. aeruginosa isolates. Most cDHS genes are of unknown function potentially representing new anti-phage systems, which we validated by identifying a novel anti-phage system (Shango) commonly encoded in cDHS1. Identifying core genes flanking immune islands could simplify immune system discovery and may represent popular landing spots for diverse MGEs carrying anti-phage systems.

Genome Study of α-, β-, and γ-Carbonic Anhydrases from the Thermophilic Microbiome of Marine Hydrothermal Vent Ecosystems

Carbonic anhydrases (CAs) are metalloenzymes that can help organisms survive in hydrothermal vents by hydrating carbon dioxide (CO₂). In this study, we focus on alpha (α), beta (β), and gamma (γ) CAs, which are present in the thermophilic microbiome of marine hydrothermal vents. The coding genes of these enzymes can be transferred between hydrothermal-vent organisms via horizontal gene transfer (HGT), which is an important tool in natural biodiversity. We performed big data mining and bioinformatics studies on α-, β-, and γ-CA coding genes from the thermophilic microbiome of marine hydrothermal vents. The results showed a reasonable association between thermostable α-, β-, and γ-CAs in the microbial population of the hydrothermal vents. This relationship could be due to HGT. We found evidence of HGT of α- and β-CAs between Cycloclasticus sp., a symbiont of Bathymodiolus heckerae, and an endosymbiont of Riftia pachyptila via Integrons. Conversely, HGT of β-CA genes from the endosymbiont Tevnia jerichonana to the endosymbiont Riftia pachyptila was detected. In addition, Hydrogenovibrio crunogenus SP-41 contains a β-CA gene on genomic islands (GIs). This gene can be transferred by HGT to Hydrogenovibrio sp. MA2-6, a methanotrophic endosymbiont of Bathymodiolus azoricus, and a methanotrophic endosymbiont of Bathymodiolus puteoserpentis. The endosymbiont of R. pachyptila has a γ-CA gene in the genome. If α- and β-CA coding genes have been derived from other microorganisms, such as endosymbionts of T. jerichonana and Cycloclasticus sp. as the endosymbiont of B. heckerae, through HGT, the theory of the necessity of thermostable CA enzymes for survival in the extreme ecosystem of hydrothermal vents is suggested and helps the conservation of microbiome natural diversity in hydrothermal vents. These harsh ecosystems, with their integral players, such as HGT and endosymbionts, significantly impact the enrichment of life on Earth and the carbon cycle in the ocean.

Cholera Dynamics and the Emergence of Pandemic Vibrio cholerae

Cholera is a severe diarrheal disease caused by the aquatic bacterium Vibrio cholerae. Interestingly, to date, only one major clade has emerged to cause pandemic disease in humans: the clade that encompasses the strains from the O1 and O139 serogroups. In this chapter, we provide a comprehensive perspective on the virulence factors and mobile genetic elements (MGEs) associated with the emergence of pandemic V. cholerae strains and highlight novel findings such as specific genomic background or interactions between MGEs that explain their confined distribution. Finally, we discuss pandemic cholera dynamics contextualizing them within the evolution of the bacterium.

Novel integrative elements and genomic plasticity in ocean ecosystems

Horizontal gene transfer accelerates microbial evolution. The marine picocyanobacterium Prochlorococcus exhibits high genomic plasticity, yet the underlying mechanisms are elusive. Here, we report a novel family of DNA transposons-"tycheposons"-some of which are viral satellites while others carry cargo, such as nutrient-acquisition genes, which shape the genetic variability in this globally abundant genus. Tycheposons share distinctive mobile-lifecycle-linked hallmark genes, including a deep-branching site-specific tyrosine recombinase. Their excision and integration at tRNA genes appear to drive the remodeling of genomic islands-key reservoirs for flexible genes in bacteria. In a selection experiment, tycheposons harboring a nitrate assimilation cassette were dynamically gained and lost, thereby promoting chromosomal rearrangements and host adaptation. Vesicles and phage particles harvested from seawater are enriched in tycheposons, providing a means for their dispersal in the wild. Similar elements are found in microbes co-occurring with Prochlorococcus, suggesting a common mechanism for microbial diversification in the vast oligotrophic oceans.

PRAWNS: compact pan-genomic features for whole-genome population genomics

MOTIVATION

Scientists seeking to understand the genomic basis of bacterial phenotypes, such as antibiotic resistance, today have access to an unprecedented number of complete and nearly complete genomes. Making sense of these data requires computational tools able to perform multiple-genome comparisons efficiently, yet currently available tools cannot scale beyond several tens of genomes.

RESULTS

We describe PRAWNS, an efficient and scalable tool for multiple-genome analysis. PRAWNS defines a concise set of genomic features (metablocks), as well as pairwise relationships between them, which can be used as a basis for large-scale genotype-phenotype association studies. We demonstrate the effectiveness of PRAWNS by identifying genomic regions associated with antibiotic resistance in Acinetobacter baumannii.

AVAILABILITY AND IMPLEMENTATION

PRAWNS is implemented in C++ and Python3, licensed under the GPLv3 license, and freely downloadable from GitHub (https://github.com/KiranJavkar/PRAWNS.git).

SUPPLEMENTARY INFORMATION

Supplementary data are available at Bioinformatics online.

High-quality pan-genome of Escherichia coli generated by excluding confounding and highly similar strains reveals an association between unique gene clusters and genomic islands

The pan-genome analysis of bacteria provides detailed insight into the diversity and evolution of a bacterial population. However, the genomes involved in the pan-genome analysis should be checked carefully, as the inclusion of confounding strains would have unfavorable effects on the identification of core genes, and the highly similar strains could bias the results of the pan-genome state (open versus closed). In this study, we found that the inclusion of highly similar strains also affects the results of unique genes in pan-genome analysis, which leads to a significant underestimation of the number of unique genes in the pan-genome. Therefore, these strains should be excluded from pan-genome analysis at the early stage of data processing. Currently, tens of thousands of genomes have been sequenced for Escherichia coli, which provides an unprecedented opportunity as well as a challenge for pan-genome analysis of this classical model organism. Using the proposed strategies, a high-quality E. coli pan-genome was obtained, and the unique genes was extracted and analyzed, revealing an association between the unique gene clusters and genomic islands from a pan-genome perspective, which may facilitate the identification of genomic islands.

Overview of Salmonella Genomic Island 1-Related Elements Among Gamma-Proteobacteria Reveals Their Wide Distribution Among Environmental Species

The aim of this study was to perform an in silico analysis of the available whole-genome sequencing data to detect syntenic genomic islands (GIs) having homology to Salmonella genomic island 1 (SGI1), analyze the genetic variations of their backbone, and determine their relatedness. Eighty-nine non-redundant SGI1-related elements (SGI1-REs) were identified among gamma-proteobacteria. With the inclusion of the thirty-seven backbones characterized to date, seven clusters were identified based on integrase homology: SGI1, PGI1, PGI2, AGI1 clusters, and clusters 5, 6, and 7 composed of GIs mainly harbored by waterborne or marine bacteria, such as Vibrio, Shewanella, Halomonas, Idiomarina, Marinobacter, and Pseudohongiella. The integrase genes and the backbones of SGI1-REs from clusters 6 and 7, and from PGI1, PGI2, and AGI1 clusters differed significantly from those of the SGI1 cluster, suggesting a different ancestor. All backbones consisted of two parts: the part from attL to the origin of transfer (oriT) harbored the DNA recombination, transfer, and mobilization genes, and the part from oriT to attR differed among the clusters. The diversity of SGI1-REs resulted from the recombination events between GIs of the same or other families. The oriT appeared to be a high recombination site. The multi-drug resistant (MDR) region was located upstream of the resolvase gene. However, most SGI1-REs in Vibrio, Shewanella, and marine bacteria did not harbor any MDR region. These strains could constitute a reservoir of SGI1-REs that could be potential ancestors of SGI1-REs encountered in pathogenic bacteria. Furthermore, four SGI1-REs did not harbor a resolvase gene and therefore could not acquire an integron. The presence of mobilization genes and AcaCD binding sites indicated that their conjugative transfer could occur with helper plasmids. The plasticity of SGI1-REs contributes to bacterial adaptation and evolution. We propose a more relevant classification to categorize SGI1-REs into different clusters based on their integrase gene similarity.

Carbapenem Resistance Determinants Acquired through Novel Chromosomal Integrations in Extensively Drug-Resistant Pseudomonas aeruginosa

Two novel bla_DIM-1- or bla_IMP-1-containing genomic islands (GIs) were discovered by whole-genome sequence analyses in four extensively drug-resistant (XDR) Pseudomonas aeruginosa isolates from inpatients at a tertiary hospital in Ghana. The strains were of sequence type 234 (ST234) and formed a phylogenetic clade together with ST111, which is recognized as a global high-risk clone. Their carbapenem resistance was encoded by two Tn402-type integrons, In1592 (bla_DIM-1) and In1595 (bla_IMP-1), both carrying complete tni mobilization modules. In1595 was bound by conserved 25-bp inverted repeats (IRs) flanked by 5-bp direct repeats (DRs) associated with target site duplication. The integrons were embedded in two GIs that contained cognate integrases and were distinguished by a lower GC content than the chromosomal average. PAGI-97A (52.659 bp; In1592), which encoded a P4-type site-specific integrase of the tyrosine recombinase family in its 3′ border, was integrated into tRNA-Pro(ggg) and bracketed by a 49-bp perfect DR created by 3′-end target duplication. GIs with the same structural features, but diverse genetic content, were identified in 41/226 completed P. aeruginosa genomes. PAGI-97B (22,636 bp; In1595), which encoded an XerC/D superfamily integrase in its 5′ border, was inserted into the small RNA (sRNA) PrrF1/PrrF2 locus. Specific insertions into this highly conserved locus involved in iron-dependent regulation, all leaving PrrF1 intact, were identified in an additional six phylogenetically unrelated P. aeruginosa genomes. Our molecular analyses unveiled a hospital-associated clonal dissemination of carbapenem-resistant ST234 P. aeruginosa in which the XDR phenotype resulted from novel insertions of two GIs into specific chromosomal sites.

Sequence analysis of tyrosine recombinases allows annotation of mobile genetic elements in prokaryotic genomes

Mobile genetic elements (MGEs) sequester and mobilize antibiotic resistance genes across bacterial genomes. Efficient and reliable identification of such elements is necessary to follow resistance spreading. However, automated tools for MGE identification are missing. Tyrosine recombinase (YR) proteins drive MGE mobilization and could provide markers for MGE detection, but they constitute a diverse family also involved in housekeeping functions. Here, we conducted a comprehensive survey of YRs from bacterial, archaeal, and phage genomes and developed a sequence-based classification system that dissects the characteristics of MGE-borne YRs. We revealed that MGE-related YRs evolved from non-mobile YRs by acquisition of a regulatory arm-binding domain that is essential for their mobility function. Based on these results, we further identified numerous unknown MGEs. This work provides a resource for comparative analysis and functional annotation of YRs and aids the development of computational tools for MGE annotation. Additionally, we reveal how YRs adapted to drive gene transfer across species and provide a tool to better characterize antibiotic resistance dissemination.

The Age of Phage: Friend or Foe in the New Dawn of Therapeutic and Biocontrol Applications?

Extended overuse and misuse of antibiotics and other antibacterial agents has resulted in an antimicrobial resistance crisis. Bacteriophages, viruses that infect bacteria, have emerged as a legitimate alternative antibacterial agent with a wide scope of applications which continue to be discovered and refined. However, the potential of some bacteriophages to aid in the acquisition, maintenance, and dissemination of negatively associated bacterial genes, including resistance and virulence genes, through transduction is of concern and requires deeper understanding in order to be properly addressed. In particular, their ability to interact with mobile genetic elements such as plasmids, genomic islands, and integrative conjugative elements (ICEs) enables bacteriophages to contribute greatly to bacterial evolution. Nonetheless, bacteriophages have the potential to be used as therapeutic and biocontrol agents within medical, agricultural, and food processing settings, against bacteria in both planktonic and biofilm environments. Additionally, bacteriophages have been deployed in developing rapid, sensitive, and specific biosensors for various bacterial targets. Intriguingly, their bioengineering capabilities show great promise in improving their adaptability and effectiveness as biocontrol and detection tools. This review aims to provide a balanced perspective on bacteriophages by outlining advantages, challenges, and future steps needed in order to boost their therapeutic and biocontrol potential, while also providing insight on their potential role in contributing to bacterial evolution and survival.

Genomic Characterization Provides an Insight into the Pathogenicity of the Poplar Canker Bacterium Lonsdalea populi

An emerging poplar canker caused by the gram-negative bacterium, Lonsdalea populi, has led to high mortality of hybrid poplars Populus × euramericana in China and Europe. The molecular bases of pathogenicity and bark adaptation of L. populi have become a focus of recent research. This study revealed the whole genome sequence and identified putative virulence factors of L. populi. A high-quality L. populi genome sequence was assembled de novo, with a genome size of 3,859,707 bp, containing approximately 3434 genes and 107 RNAs (75 tRNA, 22 rRNA, and 10 ncRNA). The L. populi genome contained 380 virulence-associated genes, mainly encoding for adhesion, extracellular enzymes, secretory systems, and two-component transduction systems. The genome had 110 carbohydrate-active enzyme (CAZy)-coding genes and putative secreted proteins. The antibiotic-resistance database annotation listed that L. populi was resistant to penicillin, fluoroquinolone, and kasugamycin. Analysis of comparative genomics found that L. populi exhibited the highest homology with the L. britannica genome and L. populi encompassed 1905 specific genes, 1769 dispensable genes, and 1381 conserved genes, suggesting high evolutionary diversity and genomic plasticity. Moreover, the pan genome analysis revealed that the N-5-1 genome is an open genome. These findings provide important resources for understanding the molecular basis of the pathogenicity and biology of L. populi and the poplar-bacterium interaction.

Potential Rhodopsin- and Bacteriochlorophyll-Based Dual Phototrophy in a High Arctic Glacier

Over the course of evolution for billions of years, bacteria that are capable of light-driven energy production have occupied every corner of surface Earth where sunlight can reach. Only two general biological systems have evolved in bacteria to be capable of net energy conservation via light harvesting: one is based on the pigment of (bacterio-)chlorophyll and the other is based on proton-pumping rhodopsin. There is emerging genomic evidence that these two rather different systems can coexist in a single bacterium to take advantage of their contrasting characteristics in the number of genes involved, biosynthesis cost, ease of expression control, and efficiency of energy production and thus enhance the capability of exploiting solar energy. Our data provide the first clear-cut evidence that such dual phototrophy potentially exists in glacial bacteria. Further public genome mining suggests this understudied dual phototrophic mechanism is possibly more common than our data alone suggested.

Conserving additional energy from sunlight through bacteriochlorophyll (BChl)-based reaction center or proton-pumping rhodopsin is a highly successful life strategy in environmental bacteria. BChl and rhodopsin-based systems display contrasting characteristics in the size of coding operon, cost of biosynthesis, ease of expression control, and efficiency of energy production. This raises an intriguing question of whether a single bacterium has evolved the ability to perform these two types of phototrophy complementarily according to energy needs and environmental conditions. Here, we report four Tardiphaga sp. strains (Alphaproteobacteria) of monophyletic origin isolated from a high Arctic glacier in northeast Greenland (81.566° N, 16.363° W) that are at different evolutionary stages concerning phototrophy. Their >99.8% identical genomes contain footprints of horizontal operon transfer (HOT) of the complete gene clusters encoding BChl- and xanthorhodopsin (XR)-based dual phototrophy. Two strains possess only a complete XR operon, while the other two strains have both a photosynthesis gene cluster and an XR operon in their genomes. All XR operons are heavily surrounded by mobile genetic elements and are located close to a tRNA gene, strongly signaling that a HOT event of the XR operon has occurred recently. Mining public genome databases and our high Arctic glacial and soil metagenomes revealed that phylogenetically diverse bacteria have the metabolic potential of performing BChl- and rhodopsin-based dual phototrophy. Our data provide new insights on how bacteria cope with the harsh and energy-deficient environment in surface glacier, possibly by maximizing the capability of exploiting solar energy.

Genome Complexity Browser: Visualization and quantification of genome variability

Comparative genomics studies may be used to acquire new knowledge regarding genome architecture, which defines the rules for combining sets of genes in the genome of living organisms. Hundreds of thousands of prokaryotic genomes have been sequenced and assembled. However, computational tools capable of simultaneously comparing large numbers of genomes are lacking. We developed the Genome Complexity Browser, a tool that allows the visualization of gene contexts, in a graph-based format, and the quantification of variability for different segments of a genome. The graph-based visualization allows the inspection of changes in gene contents and neighborhoods across hundreds of genomes, simultaneously, which may facilitate the identification of conserved and variable segments of operons or the estimation of the overall variability associated with a particular genome locus. We introduced a measure called complexity, to quantify genome variability. Intraspecies and interspecies comparisons revealed that regions with high complexity values tended to be located in areas that are conserved across different strains and species.

The comparison of genomes among different bacteria and archaea species has revealed that many species frequently exchange genes. Occasionally, such horizontal gene transfer events result in the acquisition of pathogenic properties or antibiotic resistance in the recipient organism. Previously, the probabilities of gene insertions were found to vary, with unequal distributions along a chromosome. At some loci, referred to as hotspots, changes occur with much higher frequencies compared with other regions of the chromosome. We developed a computational method and a software tool, called Genome Complexity Browser, that allows the identification of genome variability hotspots and the visualization of changes. We compared the localization of various hotspots and revealed that some demonstrate conserved localizations, even across species, whereas others are transient. Our tool allows users to visually inspect the patterns of gene changes in graph-based format, which presents the visualization in a format that is both compact and informative.

Pathogenicity Factors of Genomic Islands in Intestinal and Extraintestinal Escherichia coli

Escherichia coli is a versatile bacterial species that includes both harmless commensal strains and pathogenic strains found in the gastrointestinal tract in humans and warm-blooded animals. The growing amount of DNA sequence information generated in the era of "genomics" has helped to increase our understanding of the factors and mechanisms involved in the diversification of this bacterial species. The pathogenic side of E. coli that is afforded through horizontal transfers of genes encoding virulence factors enables this bacterium to become a highly diverse and adapted pathogen that is responsible for intestinal or extraintestinal diseases in humans and animals. Many of the accessory genes acquired by horizontal transfers form syntenic blocks and are recognized as genomic islands (GIs). These genomic regions contribute to the rapid evolution, diversification and adaptation of E. coli variants because they are frequently subject to rearrangements, excision and transfer, as well as to further acquisition of additional DNA. Here, we review a subgroup of GIs from E. coli termed pathogenicity islands (PAIs), a concept defined in the late 1980s by Jörg Hacker and colleagues in Werner Goebel's group at the University of Würzburg, Würzburg, Germany. As with other GIs, the PAIs comprise large genomic regions that differ from the rest of the genome by their G + C content, by their typical insertion within transfer RNA genes, and by their harboring of direct repeats (at their ends), integrase determinants, or other mobility loci. The hallmark of PAIs is their contribution to the emergence of virulent bacteria and to the development of intestinal and extraintestinal diseases. This review summarizes the current knowledge on the structure and functional features of PAIs, on PAI-encoded E. coli pathogenicity factors and on the role of PAIs in host-pathogen interactions.

Molecular insights into the genome dynamics and interactions between core and acquired genomes of Vibrio cholerae

Bacterial species are hosts to horizontally acquired mobile genetic elements (MGEs), which encode virulence, toxin, antimicrobial resistance, and other metabolic functions. The bipartite genome of Vibrio cholerae harbors sporadic and conserved MGEs that contribute in the disease development and survival of the pathogens. For a comprehensive understanding of dynamics of MGEs in the bacterial genome, we engineered the genome of V. cholerae and examined in vitro and in vivo stability of genomic islands (GIs), integrative conjugative elements (ICEs), and prophages. Recombinant vectors carrying the integration module of these GIs, ICE and CTXΦ, helped us to understand the efficiency of integrations of MGEs in the V. cholerae chromosome. We have deleted more than 250 acquired genes from 6 different loci in the V. cholerae chromosome and showed contribution of CTX prophage in the essentiality of SOS response master regulator LexA, which is otherwise not essential for viability in other bacteria, including Escherichia coli In addition, we observed that the core genome-encoded RecA helps CTXΦ to bypass V. cholerae immunity and allow it to replicate in the host bacterium in the presence of similar prophage in the chromosome. Finally, our proteomics analysis reveals the importance of MGEs in modulating the levels of cellular proteome. This study engineered the genome of V. cholerae to remove all of the GIs, ICEs, and prophages and revealed important interactions between core and acquired genomes.

IHF stabilizes pathogenicity island I of uropathogenic Escherichia coli strain 536 by attenuating integrase I promoter activity

Pathogenicity islands (PAIs) represent horizontally acquired chromosomal regions and encode their cognate integrase, which mediates chromosomal integration and excision of the island. These site-specific recombination reactions have to be tightly controlled to maintain genomic stability, and their directionality depends on accessory proteins. The integration host factor (IHF) and the factor for inversion stimulation (Fis) are often involved in recombinogenic complex formation and controlling the directionality of the recombination reaction. We investigated the role of the accessory host factors IHF and Fis in controlling the stability of six PAIs in uropathogenic Escherichia coli strain 536. By comparing the loss of individual PAIs in the presence or absence of IHF or Fis, we showed that IHF specifically stabilized PAI I₅₃₆ and that in particular the IHFB subunit seems to be important for this function. We employed complex genetic studies to address the role of IHF in PAI I₅₃₆-encoded integrase (IntI) expression. Based on different YFP-reporter constructs and electrophoretic mobility shift assays we demonstrated that IntI acts a strong repressor of its own synthesis, and that IHF binding to the intI promoter region reduces the probability of intI promoter activation. Our results extend the current knowledge of the role of IHF in controlling directionality of site specific recombination reactions and thus PAI stability.

Co-Occurrence of the bla KPC-2 and Mcr-3.3 Gene in Aeromonas caviae SCAc2001 Isolated from Patients with Diarrheal Disease

Purpose

To characterize the genetic feature of a multi-drug-resistant Aeromonas caviae strain isolated from the diarrhea sample of a 45-year-old male patient with acute diarrhea.

Materials and Methods

Whole-genome of the A. caviae strain SCAc2001 was sequenced via the Illumina system, followed by a series of bioinformatic analyses to describe the genetic feature.

Results

The genome sequence of A. caviae SCAc2001 was assembled into 340 scaffolds (305 of them were > 1000 bp in length and 4,487,370 bp in total) with an average G+C content of 61.09%. Phylogenetic analysis showed that the A. caviae SCAc2001 strain was highly similar to the A. caviae strain R25-2 and T25-39. Resistome analysis identified that A. caviae SCAc2001 carried 13 antimicrobial resistance genes, including β-lactams (bla_KPC, bla_CTX-M-14, bla_TEM-1, bla_OXA-10, bla_OXA-427, bla_VEB-3 and bla_MOX-6), aminoglycosides (aadA1), fluoroquinolones (aac(6ʹ)-Ib-cr), phenicol resistance (catB3), sulfonamide (sul1), trimethoprim (dfrA5) and colistin resistance (mcr-3.3).And also, A. caviae ScAc2001 carried 54 putative virulence genes including the type IV pilus, fimbria, flagellarthe, and hemolysin A encoding genes, and 12 pathogen–host interactions (PHI) genes. There were also four genomic islands and eight prophages in the genome of A. caviae ScAc2001. In addition, A. caviae SCAc2001 also carried three secondary metabolism products coding clusters including nonribosomal peptide synthetases (nrps), hserlactone and bacteriocin.

Conclusion

A. caviae ScAc2001 carries many resistance genes, a variety of virulence factors, PHI genes and four genomic islands and eight prophages, which poses a severe threat to infectious diseases control strategies, diagnosis methods and clinical treatment.

Vibrio harveyi: A brief survey of general characteristics and recent epidemiological traits associated with climate change

Here we briefly review the major characteristics of the emerging pathogen Vibrio harveyi and discuss survival strategies and adaptation mechanisms underlying the capacity of this marine bacterium to thrive in natural and artificial aquatic settings. Recent studies suggest that some adaptation mechanisms can easily be acquired by V. harveyi and other members of the Vibrionaceae family owing to efficient horizontal gene transfer and elevated mutation rate. While discussing the main factors in charge of the expansion of Vibrio spp. habitats and concomitant spread of Vibrio-associated diseases under climate change, this review highlights the need for future studies able to address the joint impact of environmental and anthropogenic factors on the long-term dynamics and virulence of V. harveyi populations at the global scale.

Transposon-encoded CRISPR-Cas systems direct RNA-guided DNA integration

Conventional CRISPR-Cas systems maintain genomic integrity by leveraging guide RNAs for the nuclease-dependent degradation of mobile genetic elements, including plasmids and viruses. Here we describe a notable inversion of this paradigm, in which bacterial Tn7-like transposons have co-opted nuclease-deficient CRISPR-Cas systems to catalyse RNA-guided integration of mobile genetic elements into the genome. Programmable transposition of Vibrio cholerae Tn6677 in Escherichia coli requires CRISPR- and transposon-associated molecular machineries, including a co-complex between the DNA-targeting complex Cascade and the transposition protein TniQ. Integration of donor DNA occurs in one of two possible orientations at a fixed distance downstream of target DNA sequences, and can accommodate variable length genetic payloads. Deep-sequencing experiments reveal highly specific, genome-wide DNA insertion across dozens of unique target sites. This discovery of a fully programmable, RNA-guided integrase lays the foundation for genomic manipulations that obviate the requirements for double-strand breaks and homology-directed repair.

A novel sulfonamide resistance mechanism by two-component flavin-dependent monooxygenase system in sulfonamide-degrading actinobacteria

Sulfonamide-degrading bacteria have been discovered in various environments, suggesting the presence of novel resistance mechanisms via drug inactivation. In this study, Microbacterium sp. CJ77 capable of utilizing various sulfonamides as a sole carbon source was isolated from a composting facility. Genome and proteome analyses revealed that a gene cluster containing a flavin-dependent monooxygenase and a flavin reductase was highly up-regulated in response to sulfonamides. Biochemical analysis showed that the two-component monooxygenase system was key enzymes for the initial cleavage of sulfonamides. Co-expression of the two-component system in Escherichia coli conferred decreased susceptibility to sulfamethoxazole, indicating that the genes encoding drug-inactivating enzymes are potential resistance determinants. Comparative genomic analysis revealed that the gene cluster containing sulfonamide monooxygenase (renamed as sulX) and flavin reductase (sulR) was highly conserved in a genomic island shared among sulfonamide-degrading actinobacteria, all of which also contained sul1-carrying class 1 integrons. These results suggest that the sulfonamide metabolism may have evolved in sulfonamide-resistant bacteria which had already acquired the class 1 integron under sulfonamide selection pressures. Furthermore, the presence of multiple insertion sequence elements and putative composite transposon structures containing the sulX gene cluster indicated potential mobilization. This is the first study to report that sulX responsible for both sulfonamide degradation and resistance is prevalent in sulfonamide-degrading actinobacteria and its genetic signatures indicate horizontal gene transfer of the novel resistance gene.

Carbohydrate metabolic systems present on genomic islands are lost and gained in Vibrio parahaemolyticus

BACKGROUND

Utilizing unique carbohydrates or utilizing them more efficiently help bacteria expand and colonize new niches. Horizontal gene transfer (HGT) of catabolic systems is a powerful mechanism by which bacteria can acquire new phenotypic traits that can increase survival and fitness in different niches. In this work, we examined carbon catabolism diversity among Vibrio parahaemolyticus, a marine species that is also an important human and fish pathogen.

RESULTS

Phenotypic differences in carbon utilization between Vibrio parahaemolyticus strains lead us to examine genotypic differences in this species and the family Vibrionaceae in general. Bioinformatics analysis showed that the ability to utilize D-galactose was present in all V. parahaemolyticus but at least two distinct transporters were present; a major facilitator superfamily (MFS) transporter and a sodium/galactose transporter (SGLT). Growth and genetic analyses demonstrated that SGLT was a more efficient transporter of D-galactose and was the predominant type among strains. Phylogenetic analysis showed that D-galactose gene galM was acquired multiples times within the family Vibrionaceae and was transferred between distantly related species. The ability to utilize D-gluconate was universal within the species. Deletion of eda (VP0065), which encodes aldolase, a key enzyme in the Entner-Doudoroff (ED) pathway, reached a similar biomass to wild type when grown on D-gluconate as a sole carbon source. Two additional eda genes were identified, VPA1708 (eda2) associated with a D-glucuronate cluster and VPA0083 (eda3) that clustered with an oligogalacturonide (OGA) metabolism cluster. EDA2 and EDA3 were variably distributed among the species. A metabolic island was identified that contained citrate fermentation, L-rhamnose and OGA metabolism clusters as well as a CRISPR-Cas system. Phylogenetic analysis showed that CitF and RhaA had a limited distribution among V. parahaemolyticus, and RhaA was acquired at least three times. Within V. parahaemolyticus, two different regions contained the gene for L-arabinose catabolism and most strains had the ability to catabolism this sugar.

CONCLUSION

Our data suggest that horizontal transfer of metabolic systems among Vibrionaceae is an important source of metabolic diversity. This work identified four EDA homologues suggesting that the ED pathway plays a significant role in metabolism. We describe previously uncharacterized metabolism islands that were hotspots for the gain and loss of functional modules likely mediated by transposons.

Fate of two strains of extended-spectrum beta-lactamase producing Escherichia coli in constructed wetland microcosm sediments: survival and change in antibiotic resistance profiles

Free water surface constructed wetlands (FWS CW) are efficient technologies to limit the transfer of antibiotic resistant bacteria (ARB) originating from urban effluents into the aquatic environment. However, the decrease in ARB from inflow to outflow through the FWS CW may be explained by their transfer from the water body to the sediment. To investigate the behavior of ARB in the sediment of a FWS CW, we inoculated three microcosms with two strains of extended-spectrum beta-lactamase producing Escherichia coli (ESBL E. coli) belonging to two genotypes. Microcosms were composed of two sediments collected at two locations of an FWS CW from which the strains were isolated. Phragmites were planted in one of the microcosms. The survival curves of the two strains were close regardless of the genotype and the type of sediment. After a rapid decline, both strains were able to survive at low level in the sediments for 50 days. Their fate was not affected by the presence of phragmites. Changes in the bla content and antibiotic resistance of the inoculated strains were observed after three weeks of incubation, indicating that FWS CW sediments are favorable environments for spread of antibiotic resistance genes and for the acquisition of new antibiotic resistance.

Multiple Recombination Events Drive the Current Genetic Structure of Xanthomonas perforans in Florida

Prior to the identification of Xanthomonas perforans associated with bacterial spot of tomato in 1991, X. euvesicatoria was the only known species in Florida. Currently, X. perforans is the Xanthomonas sp. associated with tomato in Florida. Changes in pathogenic race and sequence alleles over time signify shifts in the dominant X. perforans genotype in Florida. We previously reported recombination of X. perforans strains with closely related Xanthomonas species as a potential driving factor for X. perforans evolution. However, the extent of recombination across the X. perforans genomes was unknown. We used a core genome multilocus sequence analysis approach to identify conserved genes and evaluated recombination-associated evolution of these genes in X. perforans. A total of 1,356 genes were determined to be "core" genes conserved among the 58 X. perforans genomes used in the study. Our approach identified three genetic groups of X. perforans in Florida based on the principal component analysis (PCA) using core genes. Nucleotide variation in 241 genes defined these groups, that are referred as Phylogenetic-group Defining (PgD) genes. Furthermore, alleles of many of these PgD genes showed 100% sequence identity with X. euvesicatoria, suggesting that variation likely has been introduced by recombination at multiple locations throughout the bacterial chromosome. Site-specific recombinase genes along with plasmid mobilization and phage associated genes were observed at different frequencies in the three phylogenetic groups and were associated with clusters of recombinant genes. Our analysis of core genes revealed the extent, source, and mechanisms of recombination events that shaped the current population and genomic structure of X. perforans in Florida.

CRISPR-Cas systems are present predominantly on mobile genetic elements in Vibrio species

Background

Bacteria are prey for many viruses that hijack the bacterial cell in order to propagate, which can result in bacterial cell lysis and death. Bacteria have developed diverse strategies to counteract virus predation, one of which is the clustered regularly interspaced short palindromic repeat (CRISPR) and CRISPR associated (Cas) proteins immune defense system. Species within the bacterial family Vibrionaceae are marine organisms that encounter large numbers of phages. Our goal was to determine the significance of CRISPR-Cas systems as a mechanism of defense in this group by investigating their prevalence, phylogenetic distribution, and genome context.

Results

Herein, we describe all the CRISPR-Cas system types and their distribution within the family Vibrionaceae. In Vibrio cholerae genomes, we identified multiple variant type I-F systems, which were also present in 41 additional species. In a large number of Vibrio species, we identified a mini type I-F system comprised of tniQcas5cas7cas6f, which was always associated with Tn7-like transposons. The Tn7-like elements, in addition to the CRISPR-Cas system, also contained additional cargo genes such as restriction modification systems and type three secretion systems. A putative hybrid CRISPR-Cas system was identified containing type III-B genes followed by a type I-F cas6f and a type I-F CRISPR that was associated with a prophage in V. cholerae and V. metoecus strains. Our analysis identified CRISPR-Cas types I-C, I-E, I-F, II-B, III-A, III-B, III-D, and the rare type IV systems as well as cas loci architectural variants among 70 species. All systems described contained a CRISPR array that ranged in size from 3 to 179 spacers. The systems identified were present predominantly within mobile genetic elements (MGEs) such as genomic islands, plasmids, and transposon-like elements. Phylogenetic analysis of Cas proteins indicated that the CRISPR-Cas systems were acquired by horizontal gene transfer.

Conclusions

Our data show that CRISPR-Cas systems are phylogenetically widespread but sporadic in occurrence, actively evolving, and present on MGEs within Vibrionaceae.

Electronic supplementary material

The online version of this article (10.1186/s12864-019-5439-1) contains supplementary material, which is available to authorized users.

Antibiotics, Resistome and Resistance Mechanisms: A Bacterial Perspective

History of mankind is regarded as struggle against infectious diseases. Rather than observing the withering away of bacterial diseases, antibiotic resistance has emerged as a serious global health concern. Medium of antibiotic resistance in bacteria varies greatly and comprises of target protection, target substitution, antibiotic detoxification and block of intracellular antibiotic accumulation. Further aggravation to prevailing situation arose on observing bacteria gradually becoming resistant to different classes of antibiotics through acquisition of resistance genes from same and different genera of bacteria. Attributing bacteria with feature of better adaptability, dispersal of antibiotic resistance genes to minimize effects of antibiotics by various means including horizontal gene transfer (conjugation, transformation, and transduction), Mobile genetic elements (plasmids, transposons, insertion sequences, integrons, and integrative-conjugative elements) and bacterial toxin-antitoxin system led to speedy bloom of antibiotic resistance amongst bacteria. Proficiency of bacteria to obtain resistance genes generated an unpleasant situation; a grave, but a lot unacknowledged, feature of resistance gene transfer.

Comparative and phylogenetic analysis of a novel family of Enterobacteriaceae-associated genomic islands that share a conserved excision/integration module

Genomic Islands (GIs) are DNA regions acquired through horizontal gene transfer that encode advantageous traits for bacteria. Many GIs harbor genes that encode the molecular machinery required for their excision from the bacterial chromosome. Notably, the excision/integration dynamics of GIs may modulate the virulence of some pathogens. Here, we report a novel family of GIs found in plant and animal Enterobacteriaceae pathogens that share genes with those found in ROD21, a pathogenicity island whose excision is involved in the virulence of Salmonella enterica serovar Enteritidis. In these GIs we identified a conserved set of genes that includes an excision/integration module, suggesting that they are excisable. Indeed, we found that GIs within carbapenem-resistant Klebsiella pneumoniae ST258 KP35 and enteropathogenic Escherichia coli O127:H6 E2348/69 are excised from the bacterial genome. In addition to putative virulence factors, these GIs encode conjugative transfer-related proteins and short and full-length homologues of the global transcriptional regulator H-NS. Phylogenetic analyses suggest that the identified GIs likely originated in phytopathogenic bacteria. Taken together, our findings indicate that these GIs are excisable and may play a role in bacterial interactions with their hosts.

Redefinition and Unification of the SXT/R391 Family of Integrative and Conjugative Elements

Integrative and conjugative elements (ICEs) of the SXT/R391 family are key drivers of the spread of antibiotic resistance in Vibrio cholerae, the infectious agent of cholera, and other pathogenic bacteria. The SXT/R391 family of ICEs was defined based on the conservation of a core set of 52 genes and site-specific integration into the 5' end of the chromosomal gene prfC Hence, the integrase gene int has been intensively used as a marker to detect SXT/R391 ICEs in clinical isolates. ICEs sharing most core genes but differing by their integration site and integrase gene have been recently reported and excluded from the SXT/R391 family. Here we explored the prevalence and diversity of atypical ICEs in GenBank databases and their relationship with typical SXT/R391 ICEs. We found atypical ICEs in V. cholerae isolates that predate the emergence and expansion of typical SXT/R391 ICEs in the mid-1980s in seventh-pandemic toxigenic V. cholerae strains O1 and O139. Our analyses revealed that while atypical ICEs are not associated with antibiotic resistance genes, they often carry cation efflux pumps, suggesting heavy metal resistance. Atypical ICEs constitute a polyphyletic group likely because of occasional recombination events with typical ICEs. Furthermore, we show that the alternative integration and excision genes of atypical ICEs remain under the control of SetCD, the main activator of the conjugative functions of SXT/R391 ICEs. Together, these observations indicate that substitution of the integration/excision module and change of specificity of integration do not preclude atypical ICEs from inclusion into the SXT/R391 family.IMPORTANCEVibrio cholerae is the causative agent of cholera, an acute intestinal infection that remains to this day a world public health threat. Integrative and conjugative elements (ICEs) of the SXT/R391 family have played a major role in spreading antimicrobial resistance in seventh-pandemic V. cholerae but also in several species of Enterobacteriaceae Most epidemiological surveys use the integrase gene as a marker to screen for SXT/R391 ICEs in clinical or environmental strains. With the recent reports of closely related elements that carry an alternative integrase gene, it became urgent to investigate whether ICEs that have been left out of the family are a liability for the accuracy of such screenings. In this study, based on comparative genomics, we broaden the SXT/R391 family of ICEs to include atypical ICEs that are often associated with heavy metal resistance.

Exopolysaccharide production in Caulobacter crescentus: A resource allocation trade-off between protection and proliferation

Complex and interacting selective pressures can produce bacterial communities with a range of phenotypes. One measure of bacterial success is the ability of cells or populations to proliferate while avoiding lytic phage infection. Resistance against bacteriophage infection can occur in the form of a metabolically expensive exopolysaccharide capsule. Here, we show that in Caulobacter crescentus, presence of an exopolysaccharide capsule provides measurable protection against infection from a lytic paracrystalline S-layer bacteriophage (CR30), but at a metabolic cost that reduces success in a phage-free environment. Carbon flux through GDP-mannose 4,6 dehydratase in different catabolic and anabolic pathways appears to mediate this trade-off. Together, our data support a model in which diversity in bacterial communities may be maintained through variable selection on phenotypes utilizing the same metabolic pathway.

A DNA segment encoding the anticodon stem/loop of tRNA determines the specific recombination of integrative-conjugative elements in Acidithiobacillus species

Horizontal gene transfer is crucial for the adaptation of microorganisms to environmental cues. The acidophilic, bioleaching bacterium Acidithiobacillus ferrooxidans encodes an integrative-conjugative genetic element (ICEAfe1) inserted in the gene encoding a tRNA^Ala. This genetic element is actively excised from the chromosome upon induction of DNA damage. A similar genetic element (ICEAca_TY.2) is also found in an equivalent position in the genome of Acidithiobacillus caldus. The local genomic context of both mobile genetic elements is highly syntenous and the cognate integrases are well conserved. By means of site directed mutagenesis, target site deletions and in vivo integrations assays in the heterologous model Escherichia coli, we assessed the target sequence requirements for site-specific recombination to be catalyzed by these integrases. We determined that each enzyme recognizes a specific small DNA segment encoding the anticodon stem/loop of the tRNA as target site and that specific positions in these regions are well conserved in the target attB sites of orthologous integrases. Also, we demonstrate that the local genetic context of the target sequence is not relevant for the integration to take place. These findings shed new light on the mechanism of site-specific integration of integrative-conjugative elements in members of Acidithiobacillus genus.

The nitrate-reduction gene cluster components exert lineage-dependent contributions to optimization of Sinorhizobium symbiosis with soybeans

Receiving nodulation and nitrogen fixation genes does not guarantee rhizobia an effective symbiosis with legumes. Here, variations in gene content were determined for three Sinorhizobium species showing contrasting symbiotic efficiency on soybeans. A nitrate-reduction gene cluster absent in S. sojae was found to be essential for symbiotic adaptations of S. fredii and S. sp. III. In S. fredii, the deletion mutation of the nap (nitrate reductase), instead of nir (nitrite reductase) and nor (nitric oxide reductase), led to defects in nitrogen-fixation (Fix- ). By contrast, none of these core nitrate-reduction genes were required for the symbiosis of S. sp. III. However, within the same gene cluster, the deletion of hemN1 (encoding oxygen-independent coproporphyrinogen III oxidase) in both S. fredii and S. sp. III led to the formation of nitrogen-fixing (Fix+ ) but ineffective (Eff- ) nodules. These Fix+ /Eff- nodules were characterized by significantly lower enzyme activity of glutamine synthetase indicating rhizobial modulation of nitrogen-assimilation by plants. A distant homologue of HemN1 from S. sojae can complement this defect in S. fredii and S. sp. III, but exhibited a more pleotropic role in symbiosis establishment. These findings highlighted the lineage-dependent optimization of symbiotic functions in different rhizobial species associated with the same host.

The Obscure World of Integrative and Mobilizable Elements, Highly Widespread Elements that Pirate Bacterial Conjugative Systems

Conjugation is a key mechanism of bacterial evolution that involves mobile genetic elements. Recent findings indicated that the main actors of conjugative transfer are not the well-known conjugative or mobilizable plasmids but are the integrated elements. This paper reviews current knowledge on “integrative and mobilizable elements” (IMEs) that have recently been shown to be highly diverse and highly widespread but are still rarely described. IMEs encode their own excision and integration and use the conjugation machinery of unrelated co-resident conjugative element for their own transfer. Recent studies revealed a much more complex and much more diverse lifecycle than initially thought. Besides their main transmission as integrated elements, IMEs probably use plasmid-like strategies to ensure their maintenance after excision. Their interaction with conjugative elements reveals not only harmless hitchhikers but also hunters that use conjugative elements as target for their integration or harmful parasites that subvert the conjugative apparatus of incoming elements to invade cells that harbor them. IMEs carry genes conferring various functions, such as resistance to antibiotics, that can enhance the fitness of their hosts and that contribute to their maintenance in bacterial populations. Taken as a whole, IMEs are probably major contributors to bacterial evolution.

Integrative and conjugative elements and their hosts: composition, distribution and organization

Conjugation of single-stranded DNA drives horizontal gene transfer between bacteria and was widely studied in conjugative plasmids. The organization and function of integrative and conjugative elements (ICE), even if they are more abundant, was only studied in a few model systems. Comparative genomics of ICE has been precluded by the difficulty in finding and delimiting these elements. Here, we present the results of a method that circumvents these problems by requiring only the identification of the conjugation genes and the species’ pan-genome. We delimited 200 ICEs and this allowed the first large-scale characterization of these elements. We quantified the presence in ICEs of a wide set of functions associated with the biology of mobile genetic elements, including some that are typically associated with plasmids, such as partition and replication. Protein sequence similarity networks and phylogenetic analyses revealed that ICEs are structured in functional modules. Integrases and conjugation systems have different evolutionary histories, even if the gene repertoires of ICEs can be grouped in function of conjugation types. Our characterization of the composition and organization of ICEs paves the way for future functional and evolutionary analyses of their cargo genes, composed of a majority of unknown function genes.

The chromosomal organization of horizontal gene transfer in bacteria

Bacterial adaptation is accelerated by the acquisition of novel traits through horizontal gene transfer, but the integration of these genes affects genome organization. We found that transferred genes are concentrated in only ~1% of the chromosomal regions (hotspots) in 80 bacterial species. This concentration increases with genome size and with the rate of transfer. Hotspots diversify by rapid gene turnover; their chromosomal distribution depends on local contexts (neighboring core genes), and content in mobile genetic elements. Hotspots concentrate most changes in gene repertoires, reduce the trade-off between genome diversification and organization, and should be treasure troves of strain-specific adaptive genes. Most mobile genetic elements and antibiotic resistance genes are in hotspots, but many hotspots lack recognizable mobile genetic elements and exhibit frequent homologous recombination at flanking core genes. Overrepresentation of hotspots with fewer mobile genetic elements in naturally transformable bacteria suggests that homologous recombination and horizontal gene transfer are tightly linked in genome evolution.

Horizontal gene transfer (HGT) is an important mechanism for genome evolution and adaptation in bacteria. Here, Oliveira and colleagues find HGT hotspots comprising  ~ 1% of the chromosomal regions in 80 bacterial species.

Four genes essential for recombination define GInts, a new type of mobile genomic island widespread in bacteria

Integrases are a family of tyrosine recombinases that are highly abundant in bacterial genomes, actively disseminating adaptive characters such as pathogenicity determinants and antibiotics resistance. Using comparative genomics and functional assays, we identified a novel type of mobile genetic element, the GInt, in many diverse bacterial groups but not in archaea. Integrated as genomic islands, GInts show a tripartite structure consisting of the ginABCD operon, a cargo DNA region from 2.5 to at least 70 kb, and a short AT-rich 3' end. The gin operon is characteristic of GInts and codes for three putative integrases and a small putative helix-loop-helix protein, all of which are essential for integration and excision of the element. Genes in the cargo DNA are acquired mostly from phylogenetically related bacteria and often code for traits that might increase fitness, such as resistance to antimicrobials or virulence. GInts also tend to capture clusters of genes involved in complex processes, such as the biosynthesis of phaseolotoxin by Pseudomonas syringae. GInts integrate site-specifically, generating two flanking direct imperfect repeats, and excise forming circular molecules. The excision process generates sequence variants at the element attachment site, which can increase frequency of integration and drive target specificity.

Fingerprint Analysis and Identification of Strains ST309 as a Potential High Risk Clone in a Pseudomonas aeruginosa Population Isolated from Children with Bacteremia in Mexico City

Pseudomonas aeruginosa is an opportunistic pathogen and is associated with nosocomial infections. Its ability to thrive in a broad range of environments is due to a large and diverse genome of which its accessory genome is part. The objective of this study was to characterize P. aeruginosa strains isolated from children who developed bacteremia, using pulse-field gel electrophoresis, and in terms of its genomic islands, virulence genes, multilocus sequence type, and antimicrobial susceptibility. Our results showed that P. aeruginosa strains presented the seven virulence genes: toxA, lasB, lecA, algR, plcH, phzA1, and toxR, a type IV pilin alleles (TFP) group I or II. Additionally, we detected a novel pilin and accessory gene, expanding the number of TFP alleles to group VI. All strains presented the PAPI-2 Island and the majority were exoU+ and exoS+ genotype. Ten percent of the strains were multi-drug resistant phenotype, 18% extensively drug-resistant, 68% moderately resistant and only 3% were susceptible to all the antimicrobial tested. The most prevalent acquired β-Lactamase was KPC. We identified a group of ST309 strains, as a potential high risk clone. Our finding also showed that the strains isolated from patients with bacteremia have important virulence factors involved in colonization and dissemination as: a TFP group I or II; the presence of the exoU gene within the PAPI-2 island and the presence of the exoS gene.

Genomic Analysis Reveals Novel Diversity among the 1976 Philadelphia Legionnaires' Disease Outbreak Isolates and Additional ST36 Strains

Legionella pneumophila was first recognized as a cause of severe and potentially fatal pneumonia during a large-scale outbreak of Legionnaires’ disease (LD) at a Pennsylvania veterans’ convention in Philadelphia, 1976. The ensuing investigation and recovery of four clinical isolates launched the fields of Legionella epidemiology and scientific research. Only one of the original isolates, “Philadelphia-1”, has been widely distributed or extensively studied. Here we describe the whole-genome sequencing (WGS), complete assembly, and comparative analysis of all Philadelphia LD strains recovered from that investigation, along with L. pneumophila isolates sharing the Philadelphia sequence type (ST36). Analyses revealed that the 1976 outbreak was due to multiple serogroup 1 strains within the same genetic lineage, differentiated by an actively mobilized, self-replicating episome that is shared with L. pneumophila str. Paris, and two large, horizontally-transferred genomic loci, among other polymorphisms. We also found a completely unassociated ST36 strain that displayed remarkable genetic similarity to the historical Philadelphia isolates. This similar strain implies the presence of a potential clonal population, and suggests important implications may exist for considering epidemiological context when interpreting phylogenetic relationships among outbreak-associated isolates. Additional extensive archival research identified the Philadelphia isolate associated with a non-Legionnaire case of “Broad Street pneumonia”, and provided new historical and genetic insights into the 1976 epidemic. This retrospective analysis has underscored the utility of fully-assembled WGS data for Legionella outbreak investigations, highlighting the increased resolution that comes from long-read sequencing and a sequence type-matched genomic data set.

ICEApl1, an Integrative Conjugative Element Related to ICEHin1056, Identified in the Pig Pathogen Actinobacillus pleuropneumoniae

ICEApl1 was identified in the whole genome sequence of MIDG2331, a tetracycline-resistant (MIC = 8 mg/L) serovar 8 clinical isolate of Actinobacillus pleuropneumoniae, the causative agent of porcine pleuropneumonia. PCR amplification of virB4, one of the core genes involved in conjugation, was used to identify other A. pleuropneumoniae isolates potentially carrying ICEApl1. MICs for tetracycline were determined for virB4 positive isolates, and shotgun whole genome sequence analysis was used to confirm presence of the complete ICEApl1. The sequence of ICEApl1 is 56083 bp long and contains 67 genes including a Tn10 element encoding tetracycline resistance. Comparative sequence analysis was performed with similar integrative conjugative elements (ICEs) found in other members of the Pasteurellaceae. ICEApl1 is most similar to the 59393 bp ICEHin1056, from Haemophilus influenzae strain 1056. Although initially identified only in serovar 8 isolates of A. pleuropneumoniae (31 from the UK and 1 from Cyprus), conjugal transfer of ICEApl1 to representative isolates of other serovars was confirmed. All isolates carrying ICEApl1 had a MIC for tetracycline of 8 mg/L. This is, to our knowledge, the first description of an ICE in A. pleuropneumoniae, and the first report of a member of the ICEHin1056 subfamily in a non-human pathogen. ICEApl1 confers resistance to tetracycline, currently one of the more commonly used antibiotics for treatment and control of porcine pleuropneumonia.

Klebsiella pneumoniae Asparagine tDNAs Are Integration Hotspots for Different Genomic Islands Encoding Microcin E492 Production Determinants and Other Putative Virulence Factors Present in Hypervirulent Strains

Due to the developing of multi-resistant and invasive hypervirulent strains, Klebsiella pneumoniae has become one of the most urgent bacterial pathogen threats in the last years. Genomic comparison of a growing number of sequenced isolates has allowed the identification of putative virulence factors, proposed to be acquirable mainly through horizontal gene transfer. In particular, those related with synthesizing the antibacterial peptide microcin E492 (MccE492) and salmochelin siderophores were found to be highly prevalent among hypervirulent strains. The determinants for the production of both molecules were first reported as part of a 13-kbp segment of K. pneumoniae RYC492 chromosome, and were cloned and characterized in E. coli. However, the genomic context of this segment in K. pneumoniae remained uncharacterized. In this work, we provided experimental and bioinformatics evidence indicating that the MccE492 cluster is part of a highly conserved 23-kbp genomic island (GI) named GIE492, that was integrated in a specific asparagine-tRNA gene (asn-tDNA) and was found in a high proportion of isolates from liver abscesses sampled around the world. This element resulted to be unstable and its excision frequency increased after treating bacteria with mitomycin C and upon the overexpression of the island-encoded integrase. Besides the MccE492 genetic cluster, it invariably included an integrase-coding gene, at least seven protein-coding genes of unknown function, and a putative transfer origin that possibly allows this GI to be mobilized through conjugation. In addition, we analyzed the asn-tDNA loci of all the available K. pneumoniae assembled chromosomes to evaluate them as GI-integration sites. Remarkably, 73% of the strains harbored at least one GI integrated in one of the four asn-tDNA present in this species, confirming them as integration hotspots. Each of these tDNAs was occupied with different frequencies, although they were 100% identical. Also, we identified a total of 47 asn-tDNA-associated GIs that were classified into 12 groups of homology differing in theencoded functionalities but sharing with GIE492 a conserved recombination module and potentially its mobility features. Most of these GIs encoded factors with proven or potential role in pathogenesis, constituting a major reservoir of virulence factors in this species.

Computational methods for predicting genomic islands in microbial genomes

Clusters of genes acquired by lateral gene transfer in microbial genomes, are broadly referred to as genomic islands (GIs). GIs often carry genes important for genome evolution and adaptation to niches, such as genes involved in pathogenesis and antibiotic resistance. Therefore, GI prediction has gradually become an important part of microbial genome analysis. Despite inherent difficulties in identifying GIs, many computational methods have been developed and show good performance. In this mini-review, we first summarize the general challenges in predicting GIs. Then we group existing GI detection methods by their input, briefly describe representative methods in each group, and discuss their advantages as well as limitations. Finally, we look into the potential improvements for better GI prediction.

Genome sequencing and analysis of the first complete genome of Lactobacillus kunkeei strain MP2, an Apis mellifera gut isolate

Background. The honey bee (Apis mellifera) is the most important pollinator in agriculture worldwide. However, the number of honey bees has fallen significantly since 2006, becoming a huge ecological problem nowadays. The principal cause is CCD, or Colony Collapse Disorder, characterized by the seemingly spontaneous abandonment of hives by their workers. One of the characteristics of CCD in honey bees is the alteration of the bacterial communities in their gastrointestinal tract, mainly due to the decrease of Firmicutes populations, such as the Lactobacilli. At this time, the causes of these alterations remain unknown. We recently isolated a strain of Lactobacillus kunkeei (L. kunkeei strain MP2) from the gut of Chilean honey bees. L. kunkeei, is one of the most commonly isolated bacterium from the honey bee gut and is highly versatile in different ecological niches. In this study, we aimed to elucidate in detail, the L. kunkeei genetic background and perform a comparative genome analysis with other Lactobacillus species. Methods. L. kunkeei MP2 was originally isolated from the guts of Chilean A. mellifera individuals. Genome sequencing was done using Pacific Biosciences single-molecule real-time sequencing technology. De novo assembly was performed using Celera assembler. The genome was annotated using Prokka, and functional information was added using the EggNOG 3.1 database. In addition, genomic islands were predicted using IslandViewer, and pro-phage sequences using PHAST. Comparisons between L. kunkeei MP2 with other L. kunkeei, and Lactobacillus strains were done using Roary. Results. The complete genome of L. kunkeei MP2 comprises one circular chromosome of 1,614,522 nt. with a GC content of 36,9%. Pangenome analysis with 16 L. kunkeei strains, identified 113 unique genes, most of them related to phage insertions. A large and unique region of L. kunkeei MP2 genome contains several genes that encode for phage structural protein and replication components. Comparative analysis of MP2 with other Lactobacillus species, identified several unique genes of L. kunkeei MP2 related with metabolism, biofilm generation, survival under stress conditions, and mobile genetic elements (MGEs). Discussion. The presence of multiple mobile genetic elements, including phage sequences, suggest a high degree of genetic variability in L. kunkeei. Its versatility and ability to survive in different ecological niches (bee guts, flowers, fruits among others) could be given by its genetic capacity to change and adapt to different environments. L. kunkeei could be a new source of Lactobacillus with beneficial properties. Indeed, L. kunkeei MP2 could play an important role in honey bee nutrition through the synthesis of components as isoprenoids.

The Integron: Adaptation On Demand

The integron is a powerful system which, by capturing, stockpiling, and rearranging new functions carried by gene encoding cassettes, confers upon bacteria a rapid adaptation capability in changing environments. Chromosomally located integrons (CI) have been identified in a large number of environmental Gram-negative bacteria. Integron evolutionary history suggests that these sedentary CIs acquired mobility among bacterial species through their association with transposable elements and conjugative plasmids. As a result of massive antibiotic use, these so-called mobile integrons are now widespread in clinically relevant bacteria and are considered to be the principal agent in the emergence and rise of antibiotic multiresistance in Gram-negative bacteria. Cassette rearrangements are catalyzed by the integron integrase, a site-specific tyrosine recombinase. Central to these reactions is the single-stranded DNA nature of one of the recombination partners, the attC site. This makes the integron a unique recombination system. This review describes the current knowledge on this atypical recombination mechanism, its implications in the reactions involving the different types of sites, attC and attI, and focuses on the tight regulation exerted by the host on integron activity through the control of attC site folding. Furthermore, cassette and integrase expression are also highly controlled by host regulatory networks and the bacterial stress (SOS) response. These intimate connections to the host make the integron a genetically stable and efficient system, granting the bacteria a low cost, highly adaptive evolution potential "on demand".

A Site-Specific Integrative Plasmid Found in Pseudomonas aeruginosa Clinical Isolate HS87 along with A Plasmid Carrying an Aminoglycoside-Resistant Gene

Plasmids play critical roles in bacterial fitness and evolution of Pseudomonas aeruginosa. Here two plasmids found in a drug-resistant P. aeruginosa clinical isolate HS87 were completely sequenced. The pHS87b plasmid (11.2 kb) carries phage-related genes and function-unknown genes. Notably, pHS87b encodes an integrase and has an adjacent tRNA^Thr-associated attachment site. A corresponding integrated form of pHS87b at the tRNA^Thr locus was identified on the chromosome of P. aeruginosa, showing that pHS87b is able to site-specifically integrate into the 3’-end of the tRNA^Thr gene. The pHS87a plasmid (26.8 kb) displays a plastic structure containing a putative replication module, stability factors and a variable region. The RepA of pHS87a shows significant similarity to the replication proteins of pPT23A-family plasmids. pHS87a carries a transposon Tn6049, a truncated insertion sequence ΔIS1071 and a Tn402-like class 1 integron which contains an aacA4 cassette that may confer aminoglycoside resistance. Thus, pHS87b is a site-specific integrative plasmid whereas pHS87a is a plastic antibiotic resistance plasmid. The two native plasmids may promote the fitness and evolution of P. aeruginosa.