The Floating (Pathogenicity) Island: A Genomic Dessert

Multi-layered genome defences in bacteria

Bacteria have evolved a variety of defence mechanisms to protect against mobile genetic elements, including restriction-modification systems and CRISPR-Cas. In recent years, dozens of previously unknown defence systems (DSs) have been discovered. Notably, diverse DSs often coexist within the same genome, and some co-occur at frequencies significantly higher than would be expected by chance, implying potential synergistic interactions. Recent studies have provided evidence of defence mechanisms that enhance or complement one another. Here, we review the interactions between DSs at the mechanistic, regulatory, ecological and evolutionary levels.

Genomic islands and their role in fitness traits of two key sepsis-causing bacterial pathogens

To survive and establish a niche for themselves, bacteria constantly evolve. Toward that, they not only insert point mutations and promote illegitimate recombinations within their genomes but also insert pieces of 'foreign' deoxyribonucleic acid, which are commonly referred to as 'genomic islands' (GEIs). The GEIs come in several forms, structures and types, often providing a fitness advantage to the harboring bacterium. In pathogenic bacteria, some GEIs may enhance virulence, thus altering disease burden, morbidity and mortality. Hence, delineating (i) the GEIs framework, (ii) their encoded functions, (iii) the triggers that help them move, (iv) the mechanisms they exploit to move among bacteria and (v) identification of their natural reservoirs will aid in superior tackling of several bacterial diseases, including sepsis. Given the vast array of comparative genomics data, in this short review, we provide an overview of the GEIs, their types and the compositions therein, especially highlighting GEIs harbored by two important pathogens, viz. Acinetobacter baumannii and Klebsiella pneumoniae, which prominently trigger sepsis in low- and middle-income countries. Our efforts help shed some light on the challenges these pathogens pose when equipped with GEIs. We hope that this review will provoke intense research into understanding GEIs, the cues that drive their mobility across bacteria and the ways and means to prevent their transfer, especially across pathogenic bacteria.

Phage Transduction of Staphylococcus aureus

Bacteriophage transduction is the major mechanism of horizontal gene transfer (HGT) among many bacteria. In Staphylococcus aureus, the phage-mediated acquisition of mobile genetic elements (MGEs) that encode virulence and antibiotic resistance genes largely contribute to its evolutionary adaptation and genetic plasticity. In molecular biology, generalized transduction is routinely used as a technique to manipulate and construct bacterial strains. Here, we describe optimized protocols for generalized transduction, applicable for the transfer of plasmid or chromosomal deoxyribonucleic acid (DNA) from donor to recipient S. aureus strains.

Dual pathogenicity island transfer by piggybacking lateral transduction

Lateral transduction (LT) is the process by which temperate phages mobilize large sections of bacterial genomes. Despite its importance, LT has only been observed during prophage induction. Here, we report that superantigen-carrying staphylococcal pathogenicity islands (SaPIs) employ a related but more versatile and complex mechanism of gene transfer to drive chromosomal hypermobility while self-transferring with additional virulence genes from the host. We found that after phage infection or prophage induction, activated SaPIs form concatamers in the bacterial chromosome by switching between parallel genomic tracks in replication bubbles. This dynamic life cycle enables SaPIbov1 to piggyback its LT of staphylococcal pathogenicity island vSaα, which encodes an array of genes involved in host-pathogen interactions, allowing both islands to be mobilized intact and transferred in a single infective particle. Our findings highlight previously unknown roles of pathogenicity islands in bacterial virulence and show that their evolutionary impact extends beyond the genes they carry.

The defense island repertoire of the Escherichia coli pan-genome

It has become clear in recent years that anti-phage defense systems cluster non-randomly within bacterial genomes in so-called "defense islands". Despite serving as a valuable tool for the discovery of novel defense systems, the nature and distribution of defense islands themselves remain poorly understood. In this study, we comprehensively mapped the defense system repertoire of >1,300 strains of Escherichia coli, the most widely studied organism for phage-bacteria interactions. We found that defense systems are usually carried on mobile genetic elements including prophages, integrative conjugative elements and transposons, which preferentially integrate at several dozens of dedicated hotspots in the E. coli genome. Each mobile genetic element type has a preferred integration position but can carry a diverse variety of defensive cargo. On average, an E. coli genome has 4.7 hotspots occupied by defense system-containing mobile elements, with some strains possessing up to eight defensively occupied hotspots. Defense systems frequently co-localize with other systems on the same mobile genetic element, in agreement with the observed defense island phenomenon. Our data show that the overwhelming majority of the E. coli pan-immune system is carried on mobile genetic elements, explaining why the immune repertoire varies substantially between different strains of the same species.

OUP accepted manuscript

Horizontal gene transfer (HGT) in the microbiome has profound consequences for human health and disease. The spread of antibiotic resistance genes, virulence, and pathogenicity determinants predominantly occurs by way of HGT. Evidence exists of extensive horizontal transfer in the human gut microbiome. Phage transduction is a type of HGT event in which a bacteriophage transfers non-viral DNA from one bacterial host cell to another. The abundance of tailed bacteriophages in the human gut suggests that transduction could act as a significant mode of HGT in the gut microbiome. Here we review in detail the known mechanisms of phage-mediated HGT, namely specialized and generalized transduction, lateral transduction, gene-transfer agents, and molecular piracy, as well as methods used to detect phage-mediated HGT, and discuss its potential implications for the human gut microbiome.

Characterisation and Molecular Analysis of an Unusual Chimeric Methicillin Resistant Staphylococcus Aureus Strain and its Bacteriophages

In the context of microarray-based epidemiological typing of the clonal organism Staphylococcus aureus/MRSA, a strain was identified that did not belong to known clonal complexes. The molecular analysis by microarray-based typing yielded signals suggesting that it was a mosaic or hybrid strain of two lineages. To verify this result, the isolate was sequenced with both, short-read Illumina and long-read Nanopore technologies and analysed in detail. This supported the hypothesis that the genome of this strain, ST6610-MRSA-IVg comprised of segments originating from two different clonal complexes (CC). While the backbone of the strain’s genome, i.e., roughly 2 megabases, belongs to CC8, a continuous insert of 894 kb (approx. 30% of the genome) originated from CC140. Beside core genomic markers in the normal succession and orientation, this insert also included the mecA gene, coding for PbP2a and causing methicillin resistance, localised on an SCCmec IVg element. This particular SCCmec type was also previously observed in CC140 MRSA from African countries. A second conspicuous observation was the presence of the trimethoprim resistance gene dfrG within on a prophage that occupied an attachment site normally used by Panton-Valentine Leucocidin phages. This observation could indicate a role of large-scale chromosomal recombination in the evolution of S. aureus as well as a role of phages in the dissemination of antibiotic resistance genes.

Phage satellites and their emerging applications in biotechnology

The arms race between (bacterio)phages and their hosts is a recognised hot spot for genome evolution. Indeed, phages and their components have historically paved the way for many molecular biology techniques and biotech applications. Further exploration into their complex lifestyles has revealed that phages are often parasitised by distinct types of hyperparasitic mobile genetic elements. These so-called phage satellites exploit phages to ensure their own propagation and horizontal transfer into new bacterial hosts, and their prevalence and peculiar lifestyle has caught the attention of many researchers. Here, we review the parasite-host dynamics of the known phage satellites, their genomic organisation and their hijacking mechanisms. Finally, we discuss how these elements can be repurposed for diverse biotech applications, kindling a new catalogue of exciting tools for microbiology and synthetic biology.

Shape shifter: redirection of prolate phage capsid assembly by staphylococcal pathogenicity islands

Staphylococcus aureus pathogenicity islands (SaPIs) are molecular parasites that hijack helper phages for their transfer. SaPIbov5, the prototypical member of a family of cos type SaPIs, redirects the assembly of ϕ12 helper capsids from prolate to isometric. This size and shape shift is dependent on the SaPIbov5-encoded protein Ccm, a homolog of the ϕ12 capsid protein (CP). Using cryo-electron microscopy, we have determined structures of prolate ϕ12 procapsids and isometric SaPIbov5 procapsids. ϕ12 procapsids have icosahedral end caps with T_end = 4 architecture and a T_mid = 14 cylindrical midsection, whereas SaPIbov5 procapsids have T = 4 icosahedral architecture. We built atomic models for CP and Ccm, and show that Ccm occupies the pentameric capsomers in the isometric SaPIbov5 procapsids, suggesting that preferential incorporation of Ccm pentamers prevents the cylindrical midsection from forming. Our results highlight that pirate elements have evolved diverse mechanisms to suppress phage multiplication, including the acquisition of phage capsid protein homologs.

Phage-inducible chromosomal islands (PICIs) are a group of mobile genetic elements that hijack the replication and assembly machinery of helper bacteriophages. Here the authors describe a mechanism by which a group of PICIs from Staphylococcus aureus re-direct the assembly pathway of their helpers using a capsid protein homolog.

Evolutionary origin and ecological implication of a unique nif island in free-living Bradyrhizobium lineages

The alphaproteobacterial genus Bradyrhizobium has been best known as N₂-fixing members that nodulate legumes, supported by the nif and nod gene clusters. Recent environmental surveys show that Bradyrhizobium represents one of the most abundant free-living bacterial lineages in the world's soils. However, our understanding of Bradyrhizobium comes largely from symbiotic members, biasing the current knowledge of their ecology and evolution. Here, we report the genomes of 88 Bradyrhizobium strains derived from diverse soil samples, including both nif-carrying and non-nif-carrying free-living (nod free) members. Phylogenomic analyses of these and 252 publicly available Bradyrhizobium genomes indicate that nif-carrying free-living members independently evolved from symbiotic ancestors (carrying both nif and nod) multiple times. Intriguingly, the nif phylogeny shows that the vast majority of nif-carrying free-living members comprise an independent cluster, indicating that horizontal gene transfer promotes nif expansion among the free-living Bradyrhizobium. Comparative genomics analysis identifies that the nif genes found in free-living Bradyrhizobium are located on a unique genomic island of ~50 kb equipped with genes potentially involved in coping with oxygen tension. We further analyze amplicon sequencing data to show that Bradyrhizobium members presumably carrying this nif island are widespread in a variety of environments. Given the dominance of Bradyrhizobium in world's soils, our findings have implications for global nitrogen cycles and agricultural research.

Microbial driven genetic variation in holobionts

Genetic variation in holobionts, (host and microbiome), occurring by changes in both host and microbiome genomes, can be observed from two perspectives: observable variations and the processes that bring about the variation. The observable includes the enormous genetic diversity of prokaryotes, which gave rise to eukaryotic organisms. Holobionts then evolved a rich microbiome with a stable core containing essential genes, less so common taxa, and a more diverse non-core enabling considerable genetic variation. The result being that, the human gut microbiome, for example, contains 1,000 times more unique genes than are present in the human genome. Microbial driven genetic variation processes in holobionts include: (1) Acquisition of novel microbes from the environment, which bring in multiple genes in one step, (2) amplification/reduction of certain microbes in the microbiome, that contribute to holobiont` s adaptation to changing conditions, (3) horizontal gene transfer between microbes and between microbes and host, (4) mutation, which plays an important role in optimizing interactions between different microbiota and between microbiota and host. We suggest that invertebrates and plants, where microbes can live intracellularly, have a greater chance of genetic exchange between microbiota and host, thus a greater chance of vertical transmission and a greater effect of microbiome on evolution of host than vertebrates. However, even in vertebrates the microbiome can aid in environmental fluctuations by amplification/reduction and by acquisition of novel microorganisms.

Virulence Factors in Coagulase-Negative Staphylococci

Coagulase-negative staphylococci (CoNS) have emerged as major pathogens in healthcare-associated facilities, being S. epidermidis, S. haemolyticus and, more recently, S. lugdunensis, the most clinically relevant species. Despite being less virulent than the well-studied pathogen S. aureus, the number of CoNS strains sequenced is constantly increasing and, with that, the number of virulence factors identified in those strains. In this regard, biofilm formation is considered the most important. Besides virulence factors, the presence of several antibiotic-resistance genes identified in CoNS is worrisome and makes treatment very challenging. In this review, we analyzed the different aspects involved in CoNS virulence and their impact on health and food.

Genomic analysis of Helicobacter himalayensis sp. nov. isolated from Marmota himalayana

Background

Helicobacter himalayensis was isolated from Marmota himalayana in the Qinghai-Tibet Plateau, China, and is a new non-H. pylori species, with unclear taxonomy, phylogeny, and pathogenicity.

Results

A comparative genomic analysis was performed between the H. himalayensis type strain 80(YS1)^T and other the genomes of Helicobacter species present in the National Center for Biotechnology Information (NCBI) database to explore the molecular evolution and potential pathogenicity of H. himalayensis. H. himalayensis 80(YS1)^T formed a clade with H. cinaedi and H. hepaticus that was phylogenetically distant from H. pylori. The H. himalayensis genome showed extensive collinearity with H. hepaticus and H. cinaedi. However, it also revealed a low degree of genome collinearity with H. pylori. The genome of 80(YS1)^T comprised 1,829,936 bp, with a 39.89% GC content, a predicted genomic island, and 1769 genes. Comparatively, H. himalayensis has more genes for functions in “cell wall/membrane/envelope biogenesis” and “coenzyme transport and metabolism” sub-branches than the other compared helicobacters, and its genome contained 42 virulence factors genes, including that encoding cytolethal distending toxin (CDT).

Conclusions

We characterized the H. himalayensis 80(YS1)^T genome, its phylogenetic position, and its potential pathogenicity. However, further understanding of the pathogenesis of this potentially pathogenic bacterium is required, which might help to manage H. himalayensis-induced diseases.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-020-07245-y.

Competition between mobile genetic elements drives optimization of a phage-encoded CRISPR-Cas system: insights from a natural arms race

CRISPR-Cas systems function as adaptive immune systems by acquiring nucleotide sequences called spacers that mediate sequence-specific defence against competitors. Uniquely, the phage ICP1 encodes a Type I-F CRISPR-Cas system that is deployed to target and overcome PLE, a mobile genetic element with anti-phage activity in Vibrio cholerae. Here, we exploit the arms race between ICP1 and PLE to examine spacer acquisition and interference under laboratory conditions to reconcile findings from wild populations. Natural ICP1 isolates encode multiple spacers directed against PLE, but we find that single spacers do not interfere equally with PLE mobilization. High-throughput sequencing to assay spacer acquisition reveals that ICP1 can also acquire spacers that target the V. cholerae chromosome. We find that targeting the V. cholerae chromosome proximal to PLE is sufficient to block PLE and is dependent on Cas2-3 helicase activity. We propose a model in which indirect chromosomal spacers are able to circumvent PLE by Cas2-3-mediated processive degradation of the V. cholerae chromosome before PLE mobilization. Generally, laboratory-acquired spacers are much more diverse than the subset of spacers maintained by ICP1 in nature, showing how evolutionary pressures can constrain CRISPR-Cas targeting in ways that are often not appreciated through in vitro analyses. This article is part of a discussion meeting issue 'The ecology and evolution of prokaryotic CRISPR-Cas adaptive immune systems'.

Microbial and Viral Communities and Their Antibiotic Resistance Genes Throughout a Hospital Wastewater Treatment System

Antibiotic resistance poses a serious threat to global public health, and antibiotic resistance determinants can enter natural aquatic systems through discharge of wastewater effluents. Hospital wastewater in particular is expected to contain high abundances of antibiotic resistance genes (ARGs) compared to municipal wastewater because it contains human enteric bacteria that may include antibiotic-resistant organisms originating from hospital patients, and can also have high concentrations of antibiotics and antimicrobials relative to municipal wastewater. Viruses also play an important role in wastewater treatment systems since they can influence the bacterial community composition through killing bacteria, facilitating transduction of genetic material between organisms, and modifying the chromosomal content of bacteria as prophages. However, little is known about the fate and connections between ARGs, viruses, and their associated bacteria in hospital wastewater systems. To address this knowledge gap, we characterized the composition and persistence of ARGs, dsDNA viruses, and bacteria from influent to effluent in a pilot-scale hospital wastewater treatment system in Israel using shotgun metagenomics. Results showed that ARGs, including genes conferring resistance to antibiotics of high clinical relevance, were detected in all sampling locations throughout the pilot-scale system, with only 16% overall depletion of ARGs per genome equivalent between influent and effluent. The most common classes of ARGs detected throughout the system conferred resistance to aminoglycoside, cephalosporin, macrolide, penam, and tetracycline antibiotics. A greater proportion of total ARGs were associated with plasmid-associated genes in effluent compared to in influent. No strong associations between viral sequences and ARGs were identified in viral metagenomes from the system, suggesting that phage may not be a significant vector for ARG transfer in this system. The majority of viruses in the pilot-scale system belonged to the families Myoviridae, Podoviridae, and Siphoviridae. Gammaproteobacteria was the dominant class of bacteria harboring ARGs and the most common putative viral host in all samples, followed by Bacilli and Betaproteobacteria. In the total bacterial community, the dominant class was Betaproteobacteria for each sample. Overall, we found that a variety of different types of ARGs and viruses were persistent throughout this hospital wastewater treatment system, which can be released to the environment through effluent discharge.

Comparative genome characterization of the periodontal pathogen Tannerella forsythia

BACKGROUND

Tannerella forsythia is a bacterial pathogen implicated in periodontal disease. Numerous virulence-associated T. forsythia genes have been described, however, it is necessary to expand the knowledge on T. forsythia's genome structure and genetic repertoire to further elucidate its role within pathogenesis. Tannerella sp. BU063, a putative periodontal health-associated sister taxon and closest known relative to T. forsythia is available for comparative analyses. In the past, strain confusion involving the T. forsythia reference type strain ATCC 43037 led to discrepancies between results obtained from in silico analyses and wet-lab experimentation.

RESULTS

We generated a substantially improved genome assembly of T. forsythia ATCC 43037 covering 99% of the genome in three sequences. Using annotated genomes of ten Tannerella strains we established a soft core genome encompassing 2108 genes, based on orthologs present in > = 80% of the strains analysed. We used a set of known and hypothetical virulence factors for comparisons in pathogenic strains and the putative periodontal health-associated isolate Tannerella sp. BU063 to identify candidate genes promoting T. forsythia's pathogenesis. Searching for pathogenicity islands we detected 38 candidate regions in the T. forsythia genome. Only four of these regions corresponded to previously described pathogenicity islands. While the general protein O-glycosylation gene cluster of T. forsythia ATCC 43037 has been described previously, genes required for the initiation of glycan synthesis are yet to be discovered. We found six putative glycosylation loci which were only partially conserved in other bacteria. Lastly, we performed a comparative analysis of translational bias in T. forsythia and Tannerella sp. BU063 and detected highly biased genes.

CONCLUSIONS

We provide resources and important information on the genomes of Tannerella strains. Comparative analyses enabled us to assess the suitability of T. forsythia virulence factors as therapeutic targets and to suggest novel putative virulence factors. Further, we report on gene loci that should be addressed in the context of elucidating T. forsythia's protein O-glycosylation pathway. In summary, our work paves the way for further molecular dissection of T. forsythia biology in general and virulence of this species in particular.

Structure of the host cell recognition and penetration machinery of a Staphylococcus aureus bacteriophage

Staphylococcus aureus is a common cause of infections in humans. The emergence of virulent, antibiotic-resistant strains of S. aureus is a significant public health concern. Most virulence and resistance factors in S. aureus are encoded by mobile genetic elements, and transduction by bacteriophages represents the main mechanism for horizontal gene transfer. The baseplate is a specialized structure at the tip of bacteriophage tails that plays key roles in host recognition, cell wall penetration, and DNA ejection. We have used high-resolution cryo-electron microscopy to determine the structure of the S. aureus bacteriophage 80α baseplate at 3.75 Å resolution, allowing atomic models to be built for most of the major tail and baseplate proteins, including two tail fibers, the receptor binding protein, and part of the tape measure protein. Our structure provides a structural basis for understanding host recognition, cell wall penetration and DNA ejection in viruses infecting Gram-positive bacteria. Comparison to other phages demonstrates the modular design of baseplate proteins, and the adaptations to the host that take place during the evolution of staphylococci and other pathogens.

How pirate phage interferes with helper phage: Comparison of the two distinct strategies

Pirate phages use the structural proteins encoded by unrelated helper phages to propagate. The best-studied example is the pirate P4 and helper P2 of coliphages, and it has been known that the Staphylococcus aureus pathogenicity islands (SaPIs) that can encode virulence factors act as pirate phages, too. When alone in the host, the pirate phages act as a prophage, but when the helper phage gene is also in the same host cell, the pirate phage has ability to exploit the helper phages structural proteins to produce pirate phage particles and spread, interfering with the helper phage production. The known helper phages in these systems are temperate phages. Interestingly, the interference of the pirate phage to the helper phage occurs in a different manner between the SaPI-helper system and the P4-P2 system. SaPIs cannot lyse a helper lysogen upon infection, while when a helper phage lyse a SaPI lysogen, most of the phage particles produced are the SaPI particles. On the contrary, in the P4-P2 system, a pirate phage P4 can lyse a helper P2 lysogen to produce mostly the P4 particles, while when P2 phage lyses a P4 lysogen, most of the produced phages are the P2 particles. Here, the consequences of these different strategies in the pirate and helper phage spreading among uninfected host is analyzed by using mathematical models. It is found that SaPI's strategy interferes with the helper phage spreading significantly more than the P4's strategy, because SaPI interferes with the helper phage's main reproduction step, while P4 interferes only by forcing the helper lysogens to lyse. However, the interference is found to be weaker in the spatially structured environment than in the well-mixed environment. This is because, in the spatial setting, the system tends to self-organize so that the helper phages take over the front of propagation due to the need of helper phage for the pirate phage spreading.

Molecular Piracy: Redirection of Bacteriophage Capsid Assembly by Mobile Genetic Elements

Horizontal transfer of mobile genetic elements (MGEs) is a key aspect of the evolution of bacterial pathogens. Transduction by bacteriophages is especially important in this process. Bacteriophages—which assemble a machinery for efficient encapsidation and transfer of genetic material—often transfer MGEs and other chromosomal DNA in a more-or-less nonspecific low-frequency process known as generalized transduction. However, some MGEs have evolved highly specific mechanisms to take advantage of bacteriophages for their own propagation and high-frequency transfer while strongly interfering with phage production—“molecular piracy”. These mechanisms include the ability to sense the presence of a phage entering lytic growth, specific recognition and packaging of MGE genomes into phage capsids, and the redirection of the phage assembly pathway to form capsids with a size more appropriate for the size of the MGE. This review focuses on the process of assembly redirection, which has evolved convergently in many different MGEs from across the bacterial universe. The diverse mechanisms that exist suggest that size redirection is an evolutionarily advantageous strategy for many MGEs.

Identification of an Integrase That Responsible for Precise Integration and Excision of Riemerella anatipestifer Genomic Island

Riemerella anatipestifer is a Gram-negative, pathogenic bacterium, which is harmful to poultry. However, the genomic islands (GIs) in R. anatipestifer are not well-studied. In this study, a 10K genomic island was predicted by the bioinformatics analysis of R. anatipestifer ATCC 11845, which is widely found in other R. anatipestifer genomes. We had first reported the genomic island integration and excision function in R. anatipestifer. We successfully constructed the integration plasmid by using the integrase and 53 bp attP elements. The 10K GI was found integrated at the 53 bp attB located in the Arg-tRNA of the R. anatipestifer RA-YM chromosome. We identified an integrase that helped in the precise integration and excision in R. anatipestifer and elucidated the molecular mechanism of the 10K genomic island integration and excision. Furthermore, we provided a new method for the gene expression and construction of complementary strain.

Pathogenicity Islands and Their Role in Staphylococcal Biology

Pathogenicity islands are members of a vast collection of genomic islands that encode important virulence, antibiotic resistance and other accessory functions and have a critical role in bacterial gene transfer. Staphylococcus aureus is host to a large family of such islands, known as SaPIs, which encode super antigen and other virulence determinants, are mobilized by helper phages and transferred at extremely high frequencies. They benefit their host cells by interfering with phage predation and enhancing horizontal gene transfer. This chapter describes their life cycle, the bases of their phage interference mechanisms, their transfer system and their conversion to antibacterial agents for treatment ofstaphylococcal infections.

Integrated mobile genetic elements in Thaumarchaeota

To explore the diversity of mobile genetic elements (MGE) associated with archaea of the phylum Thaumarchaeota, we exploited the property of most MGE to integrate into the genomes of their hosts. Integrated MGE (iMGE) were identified in 20 thaumarchaeal genomes amounting to 2 Mbp of mobile thaumarchaeal DNA. These iMGE group into five major classes: (i) proviruses, (ii) casposons, (iii) insertion sequence-like transposons, (iv) integrative-conjugative elements and (v) cryptic integrated elements. The majority of the iMGE belong to the latter category and might represent novel families of viruses or plasmids. The identified proviruses are related to tailed viruses of the order Caudovirales and to tailless icosahedral viruses with the double jelly-roll capsid proteins. The thaumarchaeal iMGE are all connected within a gene sharing network, highlighting pervasive gene exchange between MGE occupying the same ecological niche. The thaumarchaeal mobilome carries multiple auxiliary metabolic genes, including multicopper oxidases and ammonia monooxygenase subunit C (AmoC), and stress response genes, such as those for universal stress response proteins (UspA). Thus, iMGE might make important contributions to the fitness and adaptation of their hosts. We identified several iMGE carrying type I-B CRISPR-Cas systems and spacers matching other thaumarchaeal iMGE, suggesting antagonistic interactions between coexisting MGE and symbiotic relationships with the ir archaeal hosts.

Comprehensive Virulence Gene Profiling of Bovine Non-aureus Staphylococci Based on Whole-Genome Sequencing Data

Non-aureus staphylococci (NAS) are the most frequently isolated pathogens from milk in dairy cattle worldwide. The virulence factors (VFs) and mechanisms by which these bacteria cause udder infection are not fully known. We determined the distribution and associations of 191 VFs in 25 NAS species and investigated the relationship between VFs and disease. Although the overall number of VFs was not associated with disease severity, increasing numbers of toxin and host immune evasion genes specifically were associated with more severe disease outcomes. These findings suggest that the development of disease and the interactions of VFs with the host are complex and determined by the interplay of genes rather than just the presence of virulence genes. Together, our results provide foundational genetic knowledge to other researchers to design and conduct further experiments, focusing on understanding the synergy between VFs and roles of individual NAS species in IMI and characterizing species-specific effects on udder health.

Non-aureus staphylococci (NAS) are the most frequently isolated pathogens from intramammary infection (IMI) in dairy cattle. Virulence factors (VFs) and mechanisms by which NAS cause IMI are not fully known. Herein, we analyzed the distribution of 191 VFs in 441 genomes of 25 NAS species, after classifying VFs into functional categories: adherence (n = 28), exoenzymes (n = 21), immune evasion (n = 20), iron metabolism (n = 29), and toxins (n = 93). In addition to establishing VF gene profiles, associations of VF genes between and among functional categories were computed, revealing distinctive patterns of association among VFs for various NAS species. Associations were also computed for low, medium, and high somatic cell count (SCC) and clinical mastitis (CM) isolates, demonstrating distinctive patterns of associations for low SCC and CM isolates, but no differences between high SCC and CM isolates. To determine whether VF distributions had any association with SCC or CM, various clustering approaches, including complete linkages, Ward clustering, and t-distributed stochastic neighbor embedding, were applied. However, no clustering of isolates representing low SCC, medium SCC, or high SCC or CM was identified. Regression analysis to test for associations with individual VF functional categories demonstrated that each additional toxin and host immune evasion gene increased the odds of having high SCC or CM, although an overall increase in the number of VFs was not associated with increased SCC or occurrence of CM. In conclusion, we established comprehensive VF gene profiling, determined VF gene distributions and associations, calculated pathogenic potentials of all NAS species, and detected no clear link between VF genes and mastitis.

IMPORTANCE Non-aureus staphylococci (NAS) are the most frequently isolated pathogens from milk in dairy cattle worldwide. The virulence factors (VFs) and mechanisms by which these bacteria cause udder infection are not fully known. We determined the distribution and associations of 191 VFs in 25 NAS species and investigated the relationship between VFs and disease. Although the overall number of VFs was not associated with disease severity, increasing numbers of toxin and host immune evasion genes specifically were associated with more severe disease outcomes. These findings suggest that the development of disease and the interactions of VFs with the host are complex and determined by the interplay of genes rather than just the presence of virulence genes. Together, our results provide foundational genetic knowledge to other researchers to design and conduct further experiments, focusing on understanding the synergy between VFs and roles of individual NAS species in IMI and characterizing species-specific effects on udder health.

Giant Transposons in Eukaryotes: Is Bigger Better?

Transposable elements (TEs) are ubiquitous in both prokaryotes and eukaryotes, and the dynamic character of their interaction with host genomes brings about numerous evolutionary innovations and shapes genome structure and function in a multitude of ways. In traditional classification systems, TEs are often being depicted in simplistic ways, based primarily on the key enzymes required for transposition, such as transposases/recombinases and reverse transcriptases. Recent progress in whole-genome sequencing and long-read assembly, combined with expansion of the familiar range of model organisms, resulted in identification of unprecedentedly long transposable units spanning dozens or even hundreds of kilobases, initially in prokaryotic and more recently in eukaryotic systems. Here, we focus on such oversized eukaryotic TEs, including retrotransposons and DNA transposons, outline their complex and often combinatorial nature and closely intertwined relationship with viruses, and discuss their potential for participating in transfer of long stretches of DNA in eukaryotes.

Mobilisation Mechanism of Pathogenicity Islands by Endogenous Phages in Staphylococcus aureus clinical strains

Staphylococcus aureus pathogenicity islands (SaPIs) are a type of mobile genetic element that play a significant role in the pathogenesis and virulence of this microorganism. SaPIs are integrated in the chromosome under the control of the master repressor Stl, but they can be horizontally transferred at a high frequency due to certain bacteriophages. Thus, a phage protein can bind to the SaPI Stl and induce the SaPI cycle, spreading the SaPI virulence factors to other bacterial populations. We report the dissemination mechanism of SaPIs mediated by endogenous prophages in S. aureus clinical strains. We reveal the induction of SaPIs by a co-resident prophage in seven clinically relevant strains, and we further study this mechanism in MW2, a community-acquired methicillin-resistant S. aureus strain that contains two bacteriophages (ɸSa2mw and ɸSa3mw) and one SaPI (SaPImw2) encoding for three enterotoxins (sec, sel and ear). ɸSa2mw was identified as responsible for SaPImw2 induction, and the specific phage derepressor protein DUF3113 was determined. The Stl-DUF3113 protein interaction was demonstrated, along with the existence of variants of this protein in S. aureus phages with different abilities to induce SaPI. Both Stl and DUF3113 are present in other Staphylococcus species, which indicates that this is a generalised mechanism.

In silico Analysis Suggests Common Appearance of scaRNAs in Type II Systems and Their Association With Bacterial Virulence

In addition to its well-established defense function, CRISPR/Cas can also exhibit crucial non-canonical activity through endogenous gene expression regulation, which was found to mainly affect bacterial virulence. These non-canonical functions depend on scaRNA, which is a small RNA encoded outside of CRISPR array, that is typically flanked by a transcription start site (TSS) and a terminator, and is in part complementary to another small CRISPR/Cas-associated RNA (tracrRNAs). Identification of scaRNAs is however largely complicated by the scarcity of RNA-Seq data across different bacteria, so that they were identified only in a relatively rare CRISPR/Cas subtype (IIB), and the possibility of finding them in other Type II systems is currently unclear. This study presents the first effort toward systematic detection of small CRISPR/Cas-associated regulatory RNAs, where obtained predictions can guide future experiments. The core of our approach is ab initio detection of small RNAs from bacterial genome, which is based on jointly predicting transcription signals - TSS and terminators - and homology to CRISPR array repeat. Particularly, we employ our improved approach for detecting bacterial TSS, since accurate TSS detection is the main limiting factor for accurate small RNA prediction. We also explore how our predictions match to available RNA-Seq data and analyze their conservation across related bacterial species. In Type IIB systems, our predictions are consistent with experimental data, and we systematically identify scaRNAs throughout this subtype. Furthermore, we identify scaRNA:tracrRNA pairs in a number of IIA/IIC systems, where the appearance of scaRNAs co-occurs with the strains being pathogenic. RNA-Seq and conservation analysis show that our method is well suited for predicting CRISPR/Cas-associated small RNAs. We also find possible existence of a modified mechanism of CRISPR-associated small RNA action, which, interestingly, closely resembles the setup employed in biotechnological applications. Overall, our findings indicate that scaRNA:tracrRNA pairs are present in all subtypes of Type II systems, and point to an underlying connection with bacterial virulence. In addition to formulating these hypotheses, careful manual curation that we performed, makes an important first step toward fully automated predictor of CRISPR/Cas-associated small RNAs, which will allow their large scale analysis across diverse bacterial genomes.

Mobile Genetic Elements Associated with Antimicrobial Resistance

Strains of bacteria resistant to antibiotics, particularly those that are multiresistant, are an increasing major health care problem around the world. It is now abundantly clear that both Gram-negative and Gram-positive bacteria are able to meet the evolutionary challenge of combating antimicrobial chemotherapy, often by acquiring preexisting resistance determinants from the bacterial gene pool. This is achieved through the concerted activities of mobile genetic elements able to move within or between DNA molecules, which include insertion sequences, transposons, and gene cassettes/integrons, and those that are able to transfer between bacterial cells, such as plasmids and integrative conjugative elements. Together these elements play a central role in facilitating horizontal genetic exchange and therefore promote the acquisition and spread of resistance genes. This review aims to outline the characteristics of the major types of mobile genetic elements involved in acquisition and spread of antibiotic resistance in both Gram-negative and Gram-positive bacteria, focusing on the so-called ESKAPEE group of organisms (Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa, Enterobacter spp., and Escherichia coli), which have become the most problematic hospital pathogens.

Phage-inducible chromosomal islands are ubiquitous within the bacterial universe

Phage-inducible chromosomal islands (PICIs) are a recently discovered family of pathogenicity islands that contribute substantively to horizontal gene transfer, host adaptation and virulence in Gram-positive cocci. Here we report that similar elements also occur widely in Gram-negative bacteria. As with the PICIs from Gram-positive cocci, their uniqueness is defined by a constellation of features: unique and specific attachment sites, exclusive PICI genes, a phage-dependent mechanism of induction, conserved replication origin organization, convergent mechanisms of phage interference, and specific packaging of PICI DNA into phage-like infectious particles, resulting in very high transfer frequencies. We suggest that the PICIs represent two or more distinct lineages, have spread widely throughout the bacterial world, and have diverged much more slowly than their host organisms or their prophage cousins. Overall, these findings represent the discovery of a universal class of mobile genetic elements.

The Stl repressor from Staphylococcus aureus is an efficient inhibitor of the eukaryotic fruitfly dUTPase

DNA metabolism and repair is vital for the maintenance of genome integrity. Specific proteinaceous inhibitors of key factors in this process have high potential for deciphering pathways of DNA metabolism and repair. The dUTPase enzyme family is responsible for guarding against erroneous uracil incorporation into DNA. Here, we investigate whether the staphylococcal Stl repressor may interact with not only bacterial but also eukaryotic dUTPase. We provide experimental evidence for the formation of a strong complex between Stl and Drosophila melanogasterdUTPase. We also find that dUTPase activity is strongly diminished in this complex. Our results suggest that the dUTPase protein sequences involved in binding to Stl are at least partially conserved through evolution from bacteria to eukaryotes.

Staphylococcal pathogenicity islands-movers and shakers in the genomic firmament

The staphylococcal pathogenicity islands (SaPIs) are highly mobile 15kb genomic islands that carry superantigen genes and other virulence factors and are mobilized by helper phages. Helper phages counteract the SaPI repressor to induce the SaPI replication cycle, resulting in encapsidation in phage like particles, enabling high frequency transfer. The SaPIs split from a protophage lineage in the distant past, have evolved a variety of novel and salient features, and have become an invaluable component of the staphylococcal genome. This review focuses on recent studies describing three different mechanisms of SaPI interference with helper phage reproduction and other studies demonstrating that helper phage mutations to resistance against this interference impact phage evolution. Also described are recent results showing that SaPIs contribute in a major way to lateral transfer of host genes as well as enabling their own transfer. SaPI-like elements, readily identifiable in the bacterial genome, are widespread throughout the Gram-positive cocci, though functionality has thus far been demonstrated for only a single one of these.

Competing scaffolding proteins determine capsid size during mobilization of Staphylococcus aureus pathogenicity islands

Staphylococcus aureus pathogenicity islands (SaPIs), such as SaPI1, exploit specific helper bacteriophages, like 80α, for their high frequency mobilization, a process termed 'molecular piracy'. SaPI1 redirects the helper's assembly pathway to form small capsids that can only accommodate the smaller SaPI1 genome, but not a complete phage genome. SaPI1 encodes two proteins, CpmA and CpmB, that are responsible for this size redirection. We have determined the structures of the 80α and SaPI1 procapsids to near-atomic resolution by cryo-electron microscopy, and show that CpmB competes with the 80α scaffolding protein (SP) for a binding site on the capsid protein (CP), and works by altering the angle between capsomers. We probed these interactions genetically and identified second-site suppressors of lethal mutations in SP. Our structures show, for the first time, the detailed interactions between SP and CP in a bacteriophage, providing unique insights into macromolecular assembly processes.

Beyond the canonical strategies of horizontal gene transfer in prokaryotes

Efforts to identify and characterize strategies for horizontal gene transfer (HGT) in prokaryotes could have overlooked some mechanisms that do not entirely fit in with the canonical ones most often described (conjugation of plasmids, phage transduction and transformation). The difficulty in distinguishing the different HGT strategies could have contributed to underestimate their real extent. Here we review non classical HGT strategies: some that require mobile genetic elements (MGEs) and others independent of MGE. Among those strategies that require MGEs, there is a range of newly reported, hybrid and intermediate MGEs mobilizing only their own DNA, others that mobilize preferentially bacterial DNA, or both. Considering HGT strategies independent of MGE, a few are even not restricted to DNA transfer, but can also mobilize other molecules. This review considers those HGT strategies that are less commonly dealt with in the literature. The real impact of these elements could, in some conditions, be more relevant than previously thought.

Mobilizable genomic islands, different strategies for the dissemination of multidrug resistance and other adaptive traits

Mobile genetic elements are near ubiquitous DNA segments that revealed a surprising variety of strategies for their propagation among prokaryotes and between eukaryotes. In bacteria, conjugative elements were shown to be key drivers of evolution and adaptation by efficiently disseminating genes involved in pathogenicity, symbiosis, metabolic pathways, and antibiotic resistance. Conjugative plasmids of the incompatibility groups A and C (A/C) are important vehicles for the dissemination of antibiotic resistance and the consequent global emergence and spread of multi-resistant pathogenic bacteria. Beyond their own mobility, A/C plasmids were also shown to drive the mobility of unrelated non-autonomous mobilizable genomic islands, which may also confer further advantageous traits. In this commentary, we summarize the current knowledge on different classes of A/C-dependent mobilizable genomic islands and we discuss other DNA hitchhikers and their implication in bacterial evolution. Furthermore, we glimpse at the complex genetic network linking autonomous and non-autonomous mobile genetic elements, and at the associated flow of genetic information between bacteria.

Derepression of SaPIbov1 Is Independent of φNM1 Type 2 dUTPase Activity and Is Inhibited by dUTP and dUMP

Staphylococcus aureus is an opportunistic human pathogen able to transfer virulence genes to other cells through the mobilization of S. aureus pathogenicity islands (SaPIs). SaPIs are derepressed and packaged into phage-like transducing particles by helper phages like 80α or φNM1. Phages 80α and φNM1 encode structurally distinct dUTPases, Dut_80α (type 1) and Dut_NM1 (type 2). Both dUTPases can interact with the SaPIbov1 Stl master repressor, leading to derepression and mobilization. That two structurally distinct dUTPases bind the same repressor led us to speculate that dUTPase activity may be important to the derepression process. In type 1 dUTPases, Stl binding is inhibited by dUTP. The purpose of this study was to assess the involvement of dUTP binding and dUTPase activity in derepression by Dut_NM1. Dut_NM1 activity mutants were created and tested for dUTPase activity using a novel NMR-based assay. We found that all Dut_NM1 null activity mutants interacted with the SaPIbov1 Stl C-terminal domain, formed Dut_NM1-Stl heterodimers, and caused the release of the P_str promoter. However, promoter release was inhibited in the presence of dUTP or dUMP. We tested two φNM1 mutant phages that had null enzyme activity and found that they could still mobilize SaPIbov1. These results show that only the apo form of Dut_NM1 is active in Stl derepression and that dUTPase activity is not necessary for the mobilization of SaPIbov1 by Dut_NM1.

Genomics Reveals a Unique Clone of Burkholderia cenocepacia Harboring an Actively Excising Novel Genomic Island

Burkholderia cenocepacia is a clinically dominant form among the other virulent species of Burkholderia cepacia complex (Bcc). In the present study, we sequenced and analyzed the genomes of seven nosocomial Bcc isolates, five of which were isolated from the bloodstream infections and two isolates were recovered from the hospital setting during the surveillance. Genome-based species identification of the Bcc isolates using a type strain explicitly identified the species as B. cenocepacia. Moreover, single nucleotide polymorphism analysis revealed that the six isolates were clonal and phylogenetically distinct from the other B. cenocepacia. Comparative genomics distinctly revealed the larger genome size of six clonal isolates as well as the presence of a novel 107 kb genomic island named as BcenGI15, which encodes putative pathogenicity-associated genes. We have shown that the BcenGI15 has an ability to actively excise from the genome and forming an extrachromosomal circular form suggesting its mobile nature. Surprisingly, a homolog of BcenGI15 was also present in the genome of a clinical isolate named Burkholderia pseudomallei strain EY1. This novel genetic element is present only in the variants of B. cenocepacia and B. pseudomallei isolates suggesting its interspecies existence in the main pathogenic species of the genus Burkholderia. In conclusion, the whole genome analysis of the genomically distinct B. cenocepacia clinical isolates has advanced our understanding of the epidemiology and evolution of this important nosocomial pathogen as well as its relatives.

Phage-inducible islands in the Gram-positive cocci

The SaPIs are a cohesive subfamily of extremely common phage-inducible chromosomal islands (PICIs) that reside quiescently at specific att sites in the staphylococcal chromosome and are induced by helper phages to excise and replicate. They are usually packaged in small capsids composed of phage virion proteins, giving rise to very high transfer frequencies, which they enhance by interfering with helper phage reproduction. As the SaPIs represent a highly successful biological strategy, with many natural Staphylococcus aureus strains containing two or more, we assumed that similar elements would be widespread in the Gram-positive cocci. On the basis of resemblance to the paradigmatic SaPI genome, we have readily identified large cohesive families of similar elements in the lactococci and pneumococci/streptococci plus a few such elements in Enterococcus faecalis. Based on extensive ortholog analyses, we found that the PICI elements in the four different genera all represent distinct but parallel lineages, suggesting that they represent convergent evolution towards a highly successful lifestyle. We have characterized in depth the enterococcal element, EfCIV583, and have shown that it very closely resembles the SaPIs in functionality as well as in genome organization, setting the stage for expansion of the study of elements of this type. In summary, our findings greatly broaden the PICI family to include elements from at least three genera of cocci.

Understanding the Streptococcus mutans Cid/Lrg System through CidB Function

The Streptococcus mutans lrgAB and cidAB operons have been previously described as a potential model system to dissect the complexity of biofilm development and virulence of S. mutans Herein, we have attempted to further characterize the Cid/Lrg system by focusing on CidB, which has been shown to be critical for the ability of S. mutans to survive and persist in a nonpreferred oxygen-enriched condition. We have found that the expression level of cidB is critical to oxidative stress tolerance of S. mutans, most likely by impacting lrg expression. Intriguingly, the impaired aerobic growth phenotype of the cidB mutant could be restored by the additional loss of either CidA or LrgA. Growth-dependent expression of cid and lrg was demonstrated to be tightly under the control of both CcpA and the VicKR two-component system (TCS), regulators known to play an essential role in controlling major catabolic pathways and cell envelope homeostasis, respectively. RNA sequencing (RNA-Seq) analysis revealed that mutation of cidB resulted in global gene expression changes, comprising major domains of central metabolism and virulence processes, particularly in those involved with oxidative stress resistance. Loss of CidB also significantly changed the expression of genes related to genomic islands (GI) TnSmu1 and TnSmu2, the CRISPR (clustered regularly interspaced short palindromic repeats)-Cas system, and toxin-antitoxin (T/A) modules. Taken together, these data show that CidB impinges on the stress response, as well as the fundamental cellular physiology of S. mutans, and further suggest a potential link between Cid/Lrg-mediated cellular processes, S. mutans pathogenicity, and possible programmed growth arrest and cell death mechanisms.

IMPORTANCE

The ability of Streptococcus mutans to survive a variety of harmful or stressful conditions and to emerge as a numerically significant member of stable oral biofilm communities are essential elements for its persistence and cariogenicity. In this study, the homologous cidAB and lrgAB operons, previously identified as being highly balanced and coordinated during S. mutans aerobic growth, were further characterized through the functional and transcriptomic analysis of CidB. Precise control of CidB levels is shown to impact the expression of lrg, oxidative stress tolerance, major metabolic domains, and the molecular modules linked to cell death and lysis. This study advances our understanding of the Cid/Lrg system as a key player in the integration of complex environmental signals (such as oxidative stress) into the regulatory networks that modulate S. mutans virulence and cell homeostasis.