The structure of the KlcA and ArdB proteins reveals a novel fold and antirestriction activity against Type I DNA restriction systems in vivo but not in vitro

Advances in Diversity, Evolutionary Dynamics and Biotechnological Potential of Restriction-Modification Systems

Restriction-modification systems (RMS) are ubiquitous in prokaryotes and serve as primitive immune-like mechanisms that safeguard microbial genomes against foreign genetic elements. Beyond their well-known role in sequence-specific defense, RMS also contribute significantly to genomic stability, drive evolutionary processes, and mitigate the deleterious effects of mutations. This review provides a comprehensive synthesis of current insights into RMS, emphasizing their structural and functional diversity, ecological and evolutionary roles, and expanding applications in biotechnology. By integrating recent advances with an analysis of persisting challenges, we highlight the critical contributions of RMS to both fundamental microbiology and practical applications in biomedicine and industrial biotechnology. Furthermore, we discuss emerging research directions in RMS, particularly in light of novel technologies and the increasing importance of microbial genetics in addressing global health and environmental issues.

Pseudomonas Phage Lydia and the Evolution of the Mesyanzhinovviridae Family

Phage Lydia, a newly isolated siphovirus infecting Pseudomonas aeruginosa, was characterized with respect to its basic kinetic properties and subjected to comparative bioinformatic analysis with related phages. The phage exhibited a restricted host range, with lytic activity observed against 7 of 30 tested isolates. The genome of phage Lydia consists of a 61,986 bp dsDNA molecule and contains 89 predicted genes. Bioinformatic analysis suggests the presence of a DNA modification system, but no apparent genes associated with lysogeny or antibiotic resistance were identified. Taxonomic classification places Lydia within the Mesyanzhinovviridae family, Rabinowitzvirinae subfamily, and Yuavirus genus, with the closest relation to Pseudomonas virus M6. Comprehensive bioinformatic studies, including structural modelling and analysis of phage proteins, as well as comparative taxonomic, phylogenomic, and pangenomic analyses of the Mesyanzhinovviridae family, revealed relationships between proteins of Mesyanzhinovviridae phages, proteins from other phage groups, encapsulins, and a gene transfer agent (GTA) particle from Rhodobacter capsulatus. These analyses uncovered patterns of evolutionary history within the family, characterized by genetic exchange events alongside the maintenance of a common genomic architecture, leading to the emergence of new groups within the family.

Various plasmid strategies limit the effect of bacterial restriction-modification systems against conjugation

In bacteria, genes conferring antibiotic resistance are mostly carried on conjugative plasmids, mobile genetic elements that spread horizontally between bacterial hosts. Bacteria carry defence systems that defend them against genetic parasites, but how effective these are against plasmid conjugation is poorly understood. Here, we study to what extent restriction-modification (RM) systems-by far the most prevalent bacterial defence systems-act as a barrier against plasmids. Using 10 different RM systems and 13 natural plasmids conferring antibiotic resistance in Escherichia coli, we uncovered variation in defence efficiency ranging from none to 105-fold protection. Further analysis revealed genetic features of plasmids that explain the observed variation in defence levels. First, the number of RM recognition sites present on the plasmids generally correlates with defence levels, with higher numbers of sites being associated with stronger defence. Second, some plasmids encode methylases that protect against restriction activity. Finally, we show that a high number of plasmids in our collection encode anti-restriction genes that provide protection against several types of RM systems. Overall, our results show that it is common for plasmids to encode anti-RM strategies, and that, as a consequence, RM systems form only a weak barrier for plasmid transfer by conjugation.

Assessing the Role of Bacterial Innate and Adaptive Immunity as Barriers to Conjugative Plasmids

Plasmids are ubiquitous mobile genetic elements, that can be either costly or beneficial for their bacterial host. In response to constant viral threat, bacteria have evolved various immune systems, such as the prevalent restriction modification (innate immunity) and CRISPR-Cas systems (adaptive immunity). At the molecular level, both systems also target plasmids, but the consequences of these interactions for plasmid spread are unclear. Using a modeling approach, we show that restriction modification and CRISPR-Cas are effective as barriers against the spread of costly plasmids, but not against beneficial ones. Consequently, bacteria can profit from the selective advantages that beneficial plasmids confer even in the presence of bacterial immunity. While plasmids that are costly for bacteria may persist in the bacterial population for a certain period, restriction modification and CRISPR-Cas can eventually drive them to extinction. Finally, we demonstrate that the selection pressure imposed by bacterial immunity on costly plasmids can be circumvented through a diversity of escape mechanisms and highlight how plasmid carriage might be common despite bacterial immunity. In summary, the population-level outcome of interactions between plasmids and defense systems in a bacterial population is closely tied to plasmid cost: Beneficial plasmids can persist at high prevalence in bacterial populations despite defense systems, while costly plasmids may face extinction.

CRISPR-Cas3 and type I restriction-modification team up against blaKPC-IncF plasmid transfer in Klebsiella pneumoniae

OBJECTIVE

We explored whether the Clustered regularly interspaced short palindromic repeat (CRISPR)-Cas and restriction-modification (R-M) systems are compatible and act together to resist plasmid attacks.

METHODS

932 global whole-genome sequences from GenBank, and 459 K. pneumoniae isolates from six provinces of China, were collected to investigate the co-distribution of CRISPR-Cas, R-M systems, and blaKPC plasmid. Conjugation and transformation assays were applied to explore the anti-plasmid function of CRISPR and R-M systems.

RESULTS

We found a significant inverse correlation between the presence of CRISPR and R-M systems and blaKPC plasmids in K. pneumoniae, especially when both systems cohabited in one host. The multiple matched recognition sequences of both systems in blaKPC-IncF plasmids (97%) revealed that they were good targets for both systems. Furthermore, the results of conjugation assay demonstrated that CRISPR-Cas and R-M systems in K. pneumoniae could effectively hinder blaKPC plasmid invasion. Notably, CRISPR-Cas and R-M worked together to confer a 4-log reduction in the acquisition of blaKPC plasmid in conjugative events, exhibiting robust synergistic anti-plasmid immunity.

CONCLUSIONS

Our results indicate the synergistic role of CRISPR and R-M in regulating horizontal gene transfer in K. pneumoniae and rationalize the development of antimicrobial strategies that capitalize on the immunocompromised status of KPC-KP.

RecA-dependent or independent recombination of plasmid DNA generates a conflict with the host EcoKI immunity by launching restriction alleviation

Bacterial defence systems are tightly regulated to avoid autoimmunity. In Type I restriction-modification (R-M) systems, a specific mechanism called restriction alleviation (RA) controls the activity of the restriction module. In the case of the Escherichia coli Type I R-M system EcoKI, RA proceeds through ClpXP-mediated proteolysis of restriction complexes bound to non-methylated sites that appear after replication or reparation of host DNA. Here, we show that RA is also induced in the presence of plasmids carrying EcoKI recognition sites, a phenomenon we refer to as plasmid-induced RA. Further, we show that the anti-restriction behavior of plasmid-borne non-conjugative transposons such as Tn5053, previously attributed to their ardD loci, is due to plasmid-induced RA. Plasmids carrying both EcoKI and Chi sites induce RA in RecA- and RecBCD-dependent manner. However, inactivation of both RecA and RecBCD restores RA, indicating that there exists an alternative, RecA-independent, homologous recombination pathway that is blocked in the presence of RecBCD. Indeed, plasmid-induced RA in a RecBCD-deficient background does not depend on the presence of Chi sites. We propose that processing of random dsDNA breaks in plasmid DNA via homologous recombination generates non-methylated EcoKI sites, which attract EcoKI restriction complexes channeling them for ClpXP-mediated proteolysis.

Inhibitors of bacterial immune systems: discovery, mechanisms and applications

To contend with the diversity and ubiquity of bacteriophages and other mobile genetic elements, bacteria have developed an arsenal of immune defence mechanisms. Bacterial defences include CRISPR-Cas, restriction-modification and a growing list of mechanistically diverse systems, which constitute the bacterial 'immune system'. As a response, bacteriophages and mobile genetic elements have evolved direct and indirect mechanisms to circumvent or block bacterial defence pathways and ensure successful infection. Recent advances in methodological and computational approaches, as well as the increasing availability of genome sequences, have boosted the discovery of direct inhibitors of bacterial defence systems. In this Review, we discuss methods for the discovery of direct inhibitors, their diverse mechanisms of action and perspectives on their emerging applications in biotechnology and beyond.

Accumulation of defense systems in phage-resistant strains of Pseudomonas aeruginosa

Prokaryotes encode multiple distinct anti-phage defense systems in their genomes. However, the impact of carrying a multitude of defense systems on phage resistance remains unclear, especially in a clinical context. Using a collection of antibiotic-resistant clinical strains of Pseudomonas aeruginosa and a broad panel of phages, we demonstrate that defense systems contribute substantially to defining phage host range and that overall phage resistance scales with the number of defense systems in the bacterial genome. We show that many individual defense systems target specific phage genera and that defense systems with complementary phage specificities co-occur in P. aeruginosa genomes likely to provide benefits in phage-diverse environments. Overall, we show that phage-resistant phenotypes of P. aeruginosa with at least 19 phage defense systems exist in the populations of clinical, antibiotic-resistant P. aeruginosa strains.

Interrogating two extensively self-targeting Type I CRISPR-Cas systems in Xanthomonas albilineans reveals distinct anti-CRISPR proteins that block DNA degradation

CRISPR-Cas systems store fragments of invader DNA as spacers to recognize and clear those same invaders in the future. Spacers can also be acquired from the host's genomic DNA, leading to lethal self-targeting. While self-targeting can be circumvented through different mechanisms, natural examples remain poorly explored. Here, we investigate extensive self-targeting by two CRISPR-Cas systems encoding 24 self-targeting spacers in the plant pathogen Xanthomonas albilineans. We show that the native I-C and I-F1 systems are actively expressed and that CRISPR RNAs are properly processed. When expressed in Escherichia coli, each Cascade complex binds its PAM-flanked DNA target to block transcription, while the addition of Cas3 paired with genome targeting induces cell killing. While exploring how X. albilineans survives self-targeting, we predicted putative anti-CRISPR proteins (Acrs) encoded within the bacterium's genome. Screening of identified candidates with cell-free transcription-translation systems and in E. coli revealed two Acrs, which we named AcrIC11 and AcrIF12Xal, that inhibit the activity of Cas3 but not Cascade of the respective system. While AcrF12Xal is homologous to AcrIF12, AcrIC11 shares sequence and structural homology with the anti-restriction protein KlcA. These findings help explain tolerance of self-targeting through two CRISPR-Cas systems and expand the known suite of DNA degradation-inhibiting Acrs.

Isolation and draft genome sequence of Enterobacter asburiae strain i6 amenable to genetic manipulation

Enterobacter asburiae is a species of Gram-negative bacteria that is found in soil, water, and sewage. E. asburiae is generally considered to be an opportunistic pathogen, but has also been reported as a plant growth-promoting bacterium (PGPB), which may have beneficial effects on plant growth and development. However, genetic analysis of E. asburiae has been limited, possibly due to its redundant enzymes that digest exogenous DNA in the cell. Here, an E. asburiae strain i6 was isolated from soil in Nara, Japan. This strain was amenable to transformation and the one-step gene inactivation method based on λ Red recombinase. The transformation efficiency of the i6 strain with the 10 kb plasmid DNA pCF430 was at least four orders of magnitude higher than that of the previously sequenced E. asburiae strain ATCC 35953, which could not be transformed with the same plasmid DNA. A draft genome sequence of the i6 strain was determined and deposited into the database, allowing several factors that may determine transformation efficiency to be perturbed and tested. Together with the amenability of the i6 strain to genetic manipulation, the information from the i6 genome will facilitate characterization and fine-tuning of the beneficial and detrimental traits of this species.

dbAPIS: a database of anti-prokaryotic immune system genes

Anti-prokaryotic immune system (APIS) proteins, typically encoded by phages, prophages, and plasmids, inhibit prokaryotic immune systems (e.g. restriction modification, toxin-antitoxin, CRISPR-Cas). A growing number of APIS genes have been characterized and dispersed in the literature. Here we developed dbAPIS (https://bcb.unl.edu/dbAPIS), as the first literature curated data repository for experimentally verified APIS genes and their associated protein families. The key features of dbAPIS include: (i) experimentally verified APIS genes with their protein sequences, functional annotation, PDB or AlphaFold predicted structures, genomic context, sequence and structural homologs from different microbiome/virome databases; (ii) classification of APIS proteins into sequence-based families and construction of hidden Markov models (HMMs); (iii) user-friendly web interface for data browsing by the inhibited immune system types or by the hosts, and functions for searching and batch downloading of pre-computed data; (iv) Inclusion of all types of APIS proteins (except for anti-CRISPRs) that inhibit a variety of prokaryotic defense systems (e.g. RM, TA, CBASS, Thoeris, Gabija). The current release of dbAPIS contains 41 verified APIS proteins and ∼4400 sequence homologs of 92 families and 38 clans. dbAPIS will facilitate the discovery of novel anti-defense genes and genomic islands in phages, by providing a user-friendly data repository and a web resource for an easy homology search against known APIS proteins.

ArdA genes from pKM101 and from B. bifidum chromosome have a different range of regulated genes

The ardA genes are present in a wide variety of conjugative plasmids and play an important role in overcoming the restriction barrier. To date, there is no information on the chromosomal ardA genes. It is still unclear whether they keep their antirestriction activity and why bacterial chromosomes contain these genes. In the present study, we confirmed the antirestriction function of the ardA gene from the Bifidobacterium bifidum chromosome. Transcriptome analysis in Escherichia coli showed that the range of regulated genes varies significantly for ardA from conjugative plasmid pKM101 and from the B. bifidum chromosome. Moreover, if the targets for both ardA genes match, they often show an opposite effect on regulated gene expression. The results obtained indicate two seemingly mutually exclusive conclusions. On the one hand, the pleiotropic effect of ardA genes was shown not only on restriction-modification system, but also on expression of a number of other genes. On the other hand, the range of affected genes varies significally for ardA genes from different sources, which indicates the specificity of ardA to inhibited targets. Author Summary. Conjugative plasmids, bacteriophages, as well as transposons, are capable to transfer various genes, including antibiotic resistance genes, among bacterial cells. However, many of those genes pose a threat to the bacterial cells, therefore bacterial cells have special restriction systems that limit such transfer. Antirestriction genes have previously been described as a part of conjugative plasmids, and bacteriophages and transposons. Those plasmids are able to overcome bacterial cell protection in the presence of antirestriction genes, which inhibit bacterial restriction systems. This work unveils the antirestriction mechanisms, which play an important role in the bacterial life cycle. Here, we clearly show that antirestriction genes, which are able to inhibit cell protection, exist not only in plasmids but also in the bacterial chromosomes themselves. Moreover, antirestrictases have not only an inhibitory function but also participate in the regulation of other bacterial genes. The regulatory function of plasmid antirestriction genes also helps them to overcome the bacterial cell protection against gene transfer, whereas the regulatory function of genomic antirestrictases has no such effect.

The bacterial genetic determinants of Escherichia coli capacity to cause bloodstream infections in humans

Escherichia coli is both a highly prevalent commensal and a major opportunistic pathogen causing bloodstream infections (BSI). A systematic analysis characterizing the genomic determinants of extra-intestinal pathogenic vs. commensal isolates in human populations, which could inform mechanisms of pathogenesis, diagnostic, prevention and treatment is still lacking. We used a collection of 912 BSI and 370 commensal E. coli isolates collected in France over a 17-year period (2000-2017). We compared their pangenomes, genetic backgrounds (phylogroups, STs, O groups), presence of virulence-associated genes (VAGs) and antimicrobial resistance genes, finding significant differences in all comparisons between commensal and BSI isolates. A machine learning linear model trained on all the genetic variants derived from the pangenome and controlling for population structure reveals similar differences in VAGs, discovers new variants associated with pathogenicity (capacity to cause BSI), and accurately classifies BSI vs. commensal strains. Pathogenicity is a highly heritable trait, with up to 69% of the variance explained by bacterial genetic variants. Lastly, complementing our commensal collection with an older collection from 1980, we predict that pathogenicity continuously increased through 1980, 2000, to 2010. Together our findings imply that E. coli exhibit substantial genetic variation contributing to the transition between commensalism and pathogenicity and that this species evolved towards higher pathogenicity.

Broadness and specificity: ArdB, ArdA, and Ocr against various restriction-modification systems

ArdB, ArdA, and Ocr proteins inhibit the endonuclease activity of the type I restriction-modification enzymes (RMI). In this study, we evaluated the ability of ArdB, ArdA, and Ocr to inhibit different subtypes of Escherichia coli RMI systems (IA, IB, and IC) as well as two Bacillus licheniformis RMI systems. Furthermore we explored, the antirestriction activity of ArdA, ArdB, and Ocr against a type III restriction-modification system (RMIII) EcoPI and BREX. We found that DNA-mimic proteins, ArdA and Ocr exhibit different inhibition activity, depending on which RM system tested. This effect might be linked to the DNA mimicry nature of these proteins. In theory, DNA-mimic might competitively inhibit any DNA-binding proteins; however, the efficiency of inhibition depend on the ability to imitate the recognition site in DNA or its preferred conformation. In contrast, ArdB protein with an undescribed mechanism of action, demonstrated greater versatility against various RMI systems and provided similar antirestriction efficiency regardless of the recognition site. However, ArdB protein could not affect restriction systems that are radically different from the RMI such as BREX or RMIII. Thus, we assume that the structure of DNA-mimic proteins allows for selective inhibition of any DNA-binding proteins depending on the recognition site. In contrast, ArdB-like proteins inhibit RMI systems independently of the DNA recognition site.

Conjugation's Toolkit: the Roles of Nonstructural Proteins in Bacterial Sex

Bacterial conjugation, a form of horizontal gene transfer, relies on a type 4 secretion system (T4SS) and a set of nonstructural genes that are closely linked. These nonstructural genes aid in the mobile lifestyle of conjugative elements but are not part of the T4SS apparatus for conjugative transfer, such as the membrane pore and relaxosome, or the plasmid maintenance and replication machineries. While these nonstructural genes are not essential for conjugation, they assist in core conjugative functions and mitigate the cellular burden on the host. This review compiles and categorizes known functions of nonstructural genes by the stage of conjugation they modulate: dormancy, transfer, and new host establishment. Themes include establishing a commensalistic relationship with the host, manipulating the host for efficient T4SS assembly and function and assisting in conjugative evasion of recipient cell immune functions. These genes, taken in a broad ecological context, play important roles in ensuring proper propagation of the conjugation system in a natural environment.

Anti-restriction protein ArdA promotes clinical Klebsiella pneumoniae carbapenemase (KPC)-producing K. pneumoniae spread and its molecular mechanism

BACKGROUND

Klebsiella pneumoniae carbapenemase (KPC)-producing K. pneumoniae (KPC-KP) has spread worldwide and has become a major threat to public health. The restriction modification system provides an innate defence of bacteria against plasmids or transposons, while many different types of plasmid encoding the anti-restriction protein ArdA can specifically affect the restriction activity in bacteria.

OBJECTIVES

To detect the codistribution of ArdA and blaKPC-2 plasmids in KPC-KP and explore the molecular mechanism of ArdA promoting KPC-KP spread.

METHODS

We collected 65 clinical CRKP isolates from Ningbo, China, and 68 cases of plasmid complete sequences in GenBank to determine the prevalence of ArdA gene on the K. pneumoniae blaKPC-2 plasmid. The anti-restriction function of ArdA in promoting horizontal gene transfer (HGT) was verified by transformation, conjugation and transduction methods, and the pull-down experiment was used to investigate the molecular mechanism of ArdA protein in vitro.

RESULTS

We found that ArdA was widely distributed in KPC-KP in 100% of cases, which was detected in 0% of drug susceptible K. pneumoniae, and the plasmids containing the ArdA gene in 90% of the 30 cases randomly retrieved from the database. We also verified that ArdA has a good anti-restriction function (P < 0.05) through two aspects of HGT (transformation, transduction), and explored the non-occurrence interaction of ArdA and the hsdM subunit protein of EcoKI enzyme from the perspective of protein molecules.

CONCLUSIONS

These findings suggest that the coexistence advantage of ArdA with the blaKPC-2 plasmids may provide KPC-producing K. pneumoniae with a very efficient evasion of the restriction of type I systems, which not only favours ArdA-containing mobile genetic elements in the same species HGT between bacteria also facilitates HGT between other bacterial species.

Cells with stochastically increased methyltransferase to restriction endonuclease ratio provide an entry for bacteriophage into protected cell population

The action of Type II restriction-modification (RM) systems depends on restriction endonuclease (REase), which cleaves foreign DNA at specific sites, and methyltransferase (MTase), which protects host genome from restriction by methylating the same sites. We here show that protection from phage infection increases as the copy number of plasmids carrying the Type II RM Esp1396I system is increased. However, since increased plasmid copy number leads to both increased absolute intracellular RM enzyme levels and to a decreased MTase/REase ratio, it is impossible to determine which factor determines resistance/susceptibility to infection. By controlled expression of individual Esp1396I MTase or REase genes in cells carrying the Esp1396I system, we show that a shift in the MTase to REase ratio caused by overproduction of MTase or REase leads, respectively, to decreased or increased protection from infection. Consistently, due to stochastic variation of MTase and REase amount in individual cells, bacterial cells that are productively infected by bacteriophage have significantly higher MTase to REase ratios than cells that ward off the infection. Our results suggest that cells with transiently increased MTase to REase ratio at the time of infection serve as entry points for unmodified phage DNA into protected bacterial populations.

Combined comparative genomics and clinical modeling reveals plasmid-encoded genes are independently associated with Klebsiella infection

Members of the Klebsiella pneumoniae species complex frequently colonize the gut and colonization is associated with subsequent infection. To identify genes associated with progression from colonization to infection, we undertook a case-control comparative genomics study. Concordant cases (N = 85), where colonizing and invasive isolates were identical strain types, were matched to asymptomatically colonizing controls (N = 160). Thirty-seven genes are associated with infection, 27 of which remain significant following adjustment for patient variables and bacterial phylogeny. Infection-associated genes are not previously characterized virulence factors, but instead a diverse group of stress resistance, regulatory and antibiotic resistance genes, despite careful adjustment for antibiotic exposure. Many genes are plasmid borne, and for some, the relationship with infection is mediated by gut dominance. Five genes were validated in a geographically-independent cohort of colonized patients. This study identifies several genes reproducibly associated with progression to infection in patients colonized by diverse Klebsiella.

Functional comparison of anti-restriction and anti-methylation activities of ArdA, KlcA, and KlcAHS from Klebsiella pneumoniae

Anti-restriction proteins are typically encoded by plasmids, conjugative transposons, or phages to improve their chances of entering a new bacterial host with a type I DNA restriction and modification (RM) system. The invading DNA is normally destroyed by the RM system. The anti-restriction proteins ArdA, KlcA, and their homologues are usually encoded on plasmid of carbapenemase-resistant Klebsiella pneumoniae. We found that the plasmid sequence and restriction proteins affected horizontal gene transfer, and confirmed the anti-restriction and anti-methylation activities of ArdA and KlcA during transformation and transduction. Among the three anti-restriction proteins, ArdA shows stronger anti-restriction and anti-methylation effects, and KlcAHS was weaker. KlcA shows anti-methylation only during transformation. Understanding the molecular mechanism underlying the clinical dissemination of K. pneumoniae and other clinically resistant strains from the perspective of restrictive and anti-restrictive systems will provide basic theoretical support for the prevention and control of multidrug-resistant bacteria, and new strategies for delaying or even controlling the clinical dissemination of resistant strains in the future.

The Spread of Antibiotic Resistance Genes In Vivo Model

Infections caused by antibiotic-resistant bacteria are a major public health threat. The emergence and spread of antibiotic resistance genes (ARGs) in the environment or clinical setting pose a serious threat to human and animal health worldwide. Horizontal gene transfer (HGT) of ARGs is one of the main reasons for the dissemination of antibiotic resistance in vitro and in vivo environments. There is a consensus on the role of mobile genetic elements (MGEs) in the spread of bacterial resistance. Most drug resistance genes are located on plasmids, and the spread of drug resistance genes among microorganisms through plasmid-mediated conjugation transfer is the most common and effective way for the spread of multidrug resistance. Experimental studies of the processes driving the spread of antibiotic resistance have focused on simple in vitro model systems, but the current in vitro protocols might not correctly reflect the HGT of antibiotic resistance genes in realistic conditions. This calls for better models of how resistance genes transfer and disseminate in vivo. The in vivo model can better mimic the situation that occurs in patients, helping study the situation in more detail. This is crucial to develop innovative strategies to curtail the spread of antibiotic resistance genes in the future. This review aims to give an overview of the mechanisms of the spread of antibiotic resistance genes and then demonstrate the spread of antibiotic resistance genes in the in vivo model. Finally, we discuss the challenges in controlling the spread of antibiotic resistance genes and their potential solutions.

Towards a better understanding of antimicrobial resistance dissemination: what can be learnt from studying model conjugative plasmids?

Bacteria can evolve rapidly by acquiring new traits such as virulence, metabolic properties, and most importantly, antimicrobial resistance, through horizontal gene transfer (HGT). Multidrug resistance in bacteria, especially in Gram-negative organisms, has become a global public health threat often through the spread of mobile genetic elements. Conjugation represents a major form of HGT and involves the transfer of DNA from a donor bacterium to a recipient by direct contact. Conjugative plasmids, a major vehicle for the dissemination of antimicrobial resistance, are selfish elements capable of mediating their own transmission through conjugation. To spread to and survive in a new bacterial host, conjugative plasmids have evolved mechanisms to circumvent both host defense systems and compete with co-resident plasmids. Such mechanisms have mostly been studied in model plasmids such as the F plasmid, rather than in conjugative plasmids that confer antimicrobial resistance (AMR) in important human pathogens. A better understanding of these mechanisms is crucial for predicting the flow of antimicrobial resistance-conferring conjugative plasmids among bacterial populations and guiding the rational design of strategies to halt the spread of antimicrobial resistance. Here, we review mechanisms employed by conjugative plasmids that promote their transmission and establishment in Gram-negative bacteria, by following the life cycle of conjugative plasmids.

Genome archaeology of two laboratory Salmonella enterica enterica sv Typhimurium

The Salmonella research community has used strains and bacteriophages over decades, exchanging useful new isolates among laboratories for the study of cell surface antigens, metabolic pathways and restriction-modification (RM) studies. Here we present the sequences of two laboratory Salmonella strains (STK005, an isolate of LB5000; and its descendant ER3625). In the ancestry of LB5000, segments of ∼15 and ∼42 kb were introduced from Salmonella enterica sv Abony 803 into S. enterica sv Typhimurium LT2, forming strain SD14; this strain is thus a hybrid of S. enterica isolates. Strains in the SD14 lineage were used to define flagellar antigens from the 1950s to the 1970s, and to define three RM systems from the 1960s to the 1980s. LB5000 was also used as a host in phage typing systems used by epidemiologists. In the age of cheaper and easier sequencing, this resource will provide access to the sequence that underlies the extensive literature.

Comparative Genomics Suggests Mechanisms of Genetic Adaptation toward the Catabolism of the Phenylurea Herbicide Linuron in Variovorax

Biodegradation of the phenylurea herbicide linuron appears a specialization within a specific clade of the Variovorax genus. The linuron catabolic ability is likely acquired by horizontal gene transfer but the mechanisms involved are not known. The full-genome sequences of six linuron-degrading Variovorax strains isolated from geographically distant locations were analyzed to acquire insight into the mechanisms of genetic adaptation toward linuron metabolism. Whole-genome sequence analysis confirmed the phylogenetic position of the linuron degraders in a separate clade within Variovorax and indicated that they unlikely originate from a common ancestral linuron degrader. The linuron degraders differentiated from Variovorax strains that do not degrade linuron by the presence of multiple plasmids of 20–839 kb, including plasmids of unknown plasmid groups. The linuron catabolic gene clusters showed 1) high conservation and synteny and 2) strain-dependent distribution among the different plasmids. Most of them were bordered by IS1071 elements forming composite transposon structures, often in a multimeric array configuration, appointing IS1071 as a key element in the recruitment of linuron catabolic genes in Variovorax. Most of the strains carried at least one (catabolic) broad host range plasmid that might have been a second instrument for catabolic gene acquisition. We conclude that clade 1 Variovorax strains, despite their different geographical origin, made use of a limited genetic repertoire regarding both catabolic functions and vehicles to acquire linuron biodegradation.

Plasmid- and strain-specific factors drive variation in ESBL-plasmid spread in vitro and in vivo

Horizontal gene transfer, mediated by conjugative plasmids, is a major driver of the global rise of antibiotic resistance. However, the relative contributions of factors that underlie the spread of plasmids and their roles in conjugation in vivo are unclear. To address this, we investigated the spread of clinical Extended Spectrum Beta-Lactamase (ESBL)-producing plasmids in the absence of antibiotics in vitro and in the mouse intestine. We hypothesised that plasmid properties would be the primary determinants of plasmid spread and that bacterial strain identity would also contribute. We found clinical Escherichia coli strains natively associated with ESBL-plasmids conjugated to three distinct E. coli strains and one Salmonella enterica serovar Typhimurium strain. Final transconjugant frequencies varied across plasmid, donor, and recipient combinations, with qualitative consistency when comparing transfer in vitro and in vivo in mice. In both environments, transconjugant frequencies for these natural strains and plasmids covaried with the presence/absence of transfer genes on ESBL-plasmids and were affected by plasmid incompatibility. By moving ESBL-plasmids out of their native hosts, we showed that donor and recipient strains also modulated transconjugant frequencies. This suggests that plasmid spread in the complex gut environment of animals and humans can be predicted based on in vitro testing and genetic data.

Discovery of multiple anti-CRISPRs highlights anti-defense gene clustering in mobile genetic elements

Many prokaryotes employ CRISPR-Cas systems to combat invading mobile genetic elements (MGEs). In response, some MGEs have developed strategies to bypass immunity, including anti-CRISPR (Acr) proteins; yet the diversity, distribution and spectrum of activity of this immune evasion strategy remain largely unknown. Here, we report the discovery of new Acrs by assaying candidate genes adjacent to a conserved Acr-associated (Aca) gene, aca5, against a panel of six type I systems: I-F (Pseudomonas, Pectobacterium, and Serratia), I-E (Pseudomonas and Serratia), and I-C (Pseudomonas). We uncover 11 type I-F and/or I-E anti-CRISPR genes encoded on chromosomal and extrachromosomal MGEs within Enterobacteriaceae and Pseudomonas, and an additional Aca (aca9). The acr genes not only associate with other acr genes, but also with genes encoding inhibitors of distinct bacterial defense systems. Thus, our findings highlight the potential exploitation of acr loci neighborhoods for the identification of previously undescribed anti-defense systems.

Conquering CRISPR: how phages overcome bacterial adaptive immunity

The rise of antibiotic-resistant bacteria has led to renewed interest in the use of their natural enemies, phages, for the prevention and treatment of infections. However, phage therapy requires detailed knowledge of the interactions between these entities. Bacteria defend themselves against phage predation with a large repertoire of defences. Among these, CRISPR-Cas systems stand out due to their adaptive character, mechanistic complexity and diversity, and present a significant hurdle for phage infection. Here, we provide an overview of how phages can circumvent CRISPR-Cas defence, ranging from target sequence mutations and DNA modifications to anti-CRISPR proteins and nucleus-like protective structures. An in-depth understanding of these phage evasion strategies is crucial for the successful development of phage therapy applications.

A Review of the Functional Annotations of Important Genes in the AHPND-Causing pVA1 Plasmid

Acute hepatopancreatic necrosis disease (AHPND) is a lethal shrimp disease. The pathogenic agent of this disease is a special Vibrio parahaemolyticus strain that contains a pVA1 plasmid. The protein products of two toxin genes in pVA1, pirAvp and pirBvp, targeted the shrimp's hepatopancreatic cells and were identified as the major virulence factors. However, in addition to pirAvp and pirBvp, pVA1 also contains about ~90 other open-reading frames (ORFs), which may encode functional proteins. NCBI BLASTp annotations of the functional roles of 40 pVA1 genes reveal transposases, conjugation factors, and antirestriction proteins that are involved in horizontal gene transfer, plasmid transmission, and maintenance, as well as components of type II and III secretion systems that may facilitate the toxic effects of pVA1-containing Vibrio spp. There is also evidence of a post-segregational killing (PSK) system that would ensure that only pVA1 plasmid-containing bacteria could survive after segregation. Here, in this review, we assess the functional importance of these pVA1 genes and consider those which might be worthy of further study.

Antirestriction Protein ArdB (R64) Interacts with DNA

The antirestriction ArdB protein inhibits the endonuclease activity of type I restriction/modification (RM) systems in vivo; however, the mechanism of inhibition remains unknown. In this study, we showed that recombinant ArdB from Escherichia coli cells co-purified with DNA. When overexpressed in E. coli cells, a portion of ArdB protein formed insoluble DNA-free aggregates. Only native ArdB, but not the ArdBΔD141 mutant lacking the antirestriction activity, co-purified with DNA upon anion-exchange and affinity chromatography or total DNA isolation from formaldehyde-treated cells. These observations confirm the hypothesis that ArdB blocks DNA translocation via the R subunits of the R₂M₂S complex of type I RM enzymes.

Phage T7 DNA mimic protein Ocr is a potent inhibitor of BREX defence

BREX (for BacteRiophage EXclusion) is a superfamily of common bacterial and archaeal defence systems active against diverse bacteriophages. While the mechanism of BREX defence is currently unknown, self versus non-self differentiation requires methylation of specific asymmetric sites in host DNA by BrxX (PglX) methyltransferase. Here, we report that T7 bacteriophage Ocr, a DNA mimic protein that protects the phage from the defensive action of type I restriction-modification systems, is also active against BREX. In contrast to the wild-type phage, which is resistant to BREX defence, T7 lacking Ocr is strongly inhibited by BREX, and its ability to overcome the defence could be complemented by Ocr provided in trans. We further show that Ocr physically associates with BrxX methyltransferase. Although BREX+ cells overproducing Ocr have partially methylated BREX sites, their viability is unaffected. The result suggests that, similar to its action against type I R-M systems, Ocr associates with as yet unidentified BREX system complexes containing BrxX and neutralizes their ability to both methylate and exclude incoming phage DNA.

ArdC, a ssDNA-binding protein with a metalloprotease domain, overpasses the recipient hsdRMS restriction system broadening conjugation host range

Plasmids, when transferred by conjugation in natural environments, must overpass restriction-modification systems of the recipient cell. We demonstrate that protein ArdC, encoded by broad host range plasmid R388, was required for conjugation from Escherichia coli to Pseudomonas putida. Expression of ardC was required in the recipient cells, but not in the donor cells. Besides, ardC was not required for conjugation if the hsdRMS system was deleted in P. putida recipient cells. ardC was also required if the hsdRMS system was present in E. coli recipient cells. Thus, ArdC has antirestriction activity against the HsdRMS system and consequently broadens R388 plasmid host range. The crystal structure of ArdC was solved both in the absence and presence of Mn²⁺. ArdC is composed of a non-specific ssDNA binding N-terminal domain and a C-terminal metalloprotease domain, although the metalloprotease activity was not needed for the antirestriction function. We also observed by RNA-seq that ArdC-dependent conjugation triggered an SOS response in the P. putida recipient cells. Our findings give new insights, and open new questions, into the antirestriction strategies developed by plasmids to counteract bacterial restriction strategies and settle into new hosts.

Horizontal gene transfer is the main mechanism by which bacteria acquire and disseminate new traits, such as antibiotic resistance genes, that allow adaptation and evolution. Here we identified a gene, ardC, that enables a plasmid to increase its conjugative host range, and thus positively contributes to plasmid fitness. The crystal structure of the antirestriction protein ArdC revealed a fold different from other antirestriction proteins. Our results have wide implications for understanding how a gene enlarges the environments a plasmid can colonize and point to new targets to harness the bacterial DNA uptake control.

Anti‑restriction protein KlcAHS enhances carbapenem resistance

The KlcAHS gene was previously identified as coexisting with the blaKPC‑2 gene in the backbone region of a series of blaKPC‑2‑harboring plasmids. The purpose of the present study was to determine the association between the KlcAHS and blaKPC‑2 genes. KlcAHS deletion and complementation experiments were used to evaluate the association between KlcAHS and carbapenem minimal inhibition concentrations (MICs). Reverse transcription‑quantitative polymerase chain reaction (RT‑qPCR) analysis was used to detect changes in the expression levels of blaKPC‑2 upon knocking out the KlcAHS gene in a blaKPC‑2‑harboring plasmid. The imipenem MIC of the transformants harboring ΔKlcAHSpHS10842 was lower (16 µg/ml) than that of the transformants harboring wild‑type pHS10842 (32 µg/ml), whereas the kanamycin MIC of the transformants harboring pET24a was lower (1,024 µg/ml) than that of the transformants harboring pET24a‑KlcAHS (2,048 µg/ml). The imipenem MICs of the two NM1049 Escherichia coli strains carrying plasmids pHS092839 or ΔKlcAHSpHS092839 exceeded 16 µg/ml, whereas the ertapenem MIC of the host strains harboring ΔKlcAHSpHS092839 was 4 µg/ml compared with ≥8 µg/ml observed in the host strains carrying pHS092839. The RT‑qPCR results demonstrated that the messenger RNA expression levels of blaKPC‑2 in the transformants carrying ΔKlcAHSpHS092839 were significantly downregulated (P=0.007) compared with those in the transformants carrying pHS092839. These findings revealed that KlcAHS elevated the MIC values of various antibiotics by upregulating the expression levels of blaKPC‑2. Therefore, KlcAHS can confer increased resistance to carbapenems in host strains. The survival probability of clinical pathogens may be enhanced by the presence of the KlcAHS gene in antibiotics used on a large scale.

ArdB Protective Activity for Unmodified λ Phage Against EcoKI Restriction Decreases in UV-Treated Escherichia coli

Anti-restriction proteins ArdB/KlcA specifically inhibit restriction (endonuclease) activity of restriction-modification (RM) type I systems. Molecular mechanisms of ArdB/KlcA-based anti-restriction remain unknown. In this study, we quantitate effects of ArdB on protection of unmodified λ phage DNA from EcoKI restriction. After UV irradiations, which produce significant amounts of unmodified chromosomal DNA in Escherichia coli K12 cells, the protective activity of ArdB decreases. Unlike ArdB, DNA-mimicking protein Ocr retains its ability to protect the unmodified λ phage regardless of UV dose. We hypothesize that the observed decrease in ArdB protective activity in UV-treated cells is due to its binding to unmodified chromosomal DNA, which decreases effective concentrations of free ArdB molecules available for λ phage protection against type I restriction enzymes.

KlcAHS genes are ubiquitous in clinical, blaKPC-2-positive, Klebsiella pneumoniae isolates

Carbapenemase-producing Klebsiella pneumoniae has emerged and spread widely throughout the world. The mechanisms involved remain unclear. To provide insight, five plasmids were obtained from carbapenemase-producing K. pneumoniae clinical isolates. The five sequences were acquired, aligned and analyzed. In addition to the bla_KPC-2 gene, which encodes beta lactamase, essentially all the plasmids contained a putative anti-restriction protein-encoding gene, KlcA_HS. The KlcA_HS gene was found in 98.2% of the bla_KPC-2-positive, imipenem-resistant K. pneumoniae clinical isolates and in <1% of the bla_KPC-2-negative control group. A searched of the GenBank database indicated that KlcA_HS was mainly submitted by Chinese investigators beginning in 2010. Seventeen different KlcA amino acid sequences were found in the database using the restricting words: KlcA and Klebsiella pneumoniae. These sequences were used to generate a phylogenetic tree via MEGA6 software, revealing a distant evolutionary relationship between KlcA_HS and other KlcAs. The secondary structure of KlcA_HS, predicted with PROMALS3D software, exhibited highly conserved α-helices and β-strands. KlcA_HS expressed anti-restriction activity in vivo. In summary, KlcA_HS genes are ubiquitous in bla_KPC-2-positive Klebsiella pneumoniae clinical isolates collected at Huashan Hospital, China. The KlcA_HS protein possesses a secondary structure similar to that exhibited by anti-restriction proteins and displays anti-restriction activity. As such, KlcA_HS is a probable factor in the accelerated spread of bla_KPC-2 and carbapenem-resistance among clinical, K. pneumoniae isolates.

Reconsidering plasmid maintenance factors for computational plasmid design

Plasmids are genetic parasites of microorganisms. The genomes of naturally occurring plasmids are expected to be polished via natural selection to achieve long-term persistence in the microbial cell population. However, plasmid genomes are extremely diverse, and the rules governing plasmid genomes are not fully understood. Therefore, computationally designing plasmid genomes optimized for model and nonmodel organisms remains challenging. Here, we summarize current knowledge of the plasmid genome organization and the factors that can affect plasmid persistence, with the aim of constructing synthetic plasmids for use in gram-negative bacteria. Then, we introduce publicly available resources, plasmid data, and bioinformatics tools that are useful for computational plasmid design.

Determination of virulence and fitness genes associated with the pheU, pheV and selC integration sites of LEE-negative food-borne Shiga toxin-producing Escherichia coli strains

Background

In the current study, nine foodborne “Locus of Enterocyte Effacement” (LEE)-negative Shiga toxin-producing Escherichia coli (STEC) strains were selected for whole genome sequencing and analysis for yet unknown genetic elements within the already known LEE integration sites selC, pheU and pheV. Foreign DNA ranging in size from 3.4 to 57 kbp was detected and further analyzed. Five STEC strains contained an insertion of foreign DNA adjacent to the selC tRNA gene and five and seven strains contained foreign DNA adjacent to the pheU and pheV tRNA genes, respectively. We characterized the foreign DNA insertion associated with selC (STEC O91:H21 strain 17584/1), pheU (STEC O8:H4 strain RF1a and O55:Hnt strain K30) and pheV (STEC O91:H21 strain 17584/1 and O113:H21 strain TS18/08) as examples.

Results

In total, 293 open reading frames partially encoding putative virulence factors such as TonB-dependent receptors, DNA helicases, a hemolysin activator protein precursor, antigen 43, anti-restriction protein KlcA, ShiA, and phosphoethanolamine transferases were detected. A virulence type IV toxin-antitoxin system was detected in three strains. Additionally, the ato system was found in one strain. In strain 17584/1 we were able to define a new genomic island which we designated GIselC_17584/1. The island contained integrases and mobile elements in addition to genes for increased fitness and those playing a putative role in pathogenicity.

Conclusion

The data presented highlight the important role of the three tRNAs selC, pheU, and pheV for the genomic flexibility of E. coli.

Electronic supplementary material

The online version of this article (10.1186/s13099-018-0271-8) contains supplementary material, which is available to authorized users.

Spread and Persistence of Virulence and Antibiotic Resistance Genes: A Ride on the F Plasmid Conjugation Module

The F plasmid or F-factor is a large, 100-kbp, circular conjugative plasmid of Escherichia coli and was originally described as a vector for horizontal gene transfer and gene recombination in the late 1940s. Since then, F and related F-like plasmids have served as role models for bacterial conjugation. At present, more than 200 different F-like plasmids with highly related DNA transfer genes, including those for the assembly of a type IV secretion apparatus, are completely sequenced. They belong to the phylogenetically related MOBF12A group. F-like plasmids are present in enterobacterial hosts isolated from clinical as well as environmental samples all over the world. As conjugative plasmids, F-like plasmids carry genetic modules enabling plasmid replication, stable maintenance, and DNA transfer. In this plasmid backbone of approximately 60 kbp, the DNA transfer genes occupy the largest and mostly conserved part. Subgroups of MOBF12A plasmids can be defined based on the similarity of TraJ, a protein required for DNA transfer gene expression. In addition, F-like plasmids harbor accessory cargo genes, frequently embedded within transposons and/or integrons, which harness their host bacteria with antibiotic resistance and virulence genes, causing increasingly severe problems for the treatment of infectious diseases. Here, I focus on key genetic elements and their encoded proteins present on the F-factor and other typical F-like plasmids belonging to the MOBF12A group of conjugative plasmids.

Clinically Relevant Plasmid-Host Interactions Indicate that Transcriptional and Not Genomic Modifications Ameliorate Fitness Costs of Klebsiella pneumoniae Carbapenemase-Carrying Plasmids

The rapid dissemination of antimicrobial resistance (AMR) around the globe is largely due to mobile genetic elements, such as plasmids. They confer resistance to critically important drugs, including extended-spectrum beta-lactams, carbapenems, and colistin. Large, complex resistance plasmids have evolved alongside their host bacteria. However, much of the research on plasmid-host evolution has focused on small, simple laboratory plasmids in laboratory-adapted bacterial hosts. These and other studies have documented mutations in both host and plasmid genes which occur after plasmid introduction to ameliorate fitness costs of plasmid carriage. We describe here the impact of two naturally occurring variants of a large AMR plasmid (pKpQIL) on a globally successful pathogen. In our study, after pKpQIL plasmid introduction, no changes in coding domain sequences were observed in their natural host, Klebsiella pneumoniae However, significant changes in chromosomal and plasmid gene expression may have allowed the bacterium to adapt to the acquisition of the AMR plasmid. We hypothesize that this was sufficient to ameliorate the associated fitness costs of plasmid carriage, as pKpQIL plasmids were maintained without selection pressure. The dogma that removal of selection pressure (e.g., antimicrobial exposure) results in plasmid loss due to bacterial fitness costs is not true for all plasmid/host combinations. We also show that pKpQIL impacted the ability of K. pneumoniae to form a biofilm, an important aspect of virulence. This study used highly relevant models to study the interaction between AMR plasmids and pathogens and revealed striking differences from results of studies done on laboratory-adapted plasmids and strains.IMPORTANCE Antimicrobial resistance is a serious problem facing society. Many of the genes that confer resistance can be shared between bacteria through mobile genetic elements, such as plasmids. Our work shows that when two clinically relevant AMR plasmids enter their natural host bacteria, there are changes in gene expression, rather than changes to gene coding sequences. These changes in gene expression ameliorate the potential fitness costs of carriage of these AMR plasmids. In line with this, the plasmids were stable within their natural host and were not lost in the absence of selective pressure. We also show that better understanding of the impact of resistance plasmids on fundamental pathogen biology, including biofilm formation, is crucial for fighting drug-resistant infections.

Anti-Restriction Protein, KlcAHS, Promotes Dissemination of Carbapenem Resistance

Carbapenemase-producing Klebsiella pneumoniae (KPC) has emerged and spread throughout the world. A retrospective analysis was performed on carbapenem-resistant K. pneumoniae isolated at our teaching hospital during the period 2009–2010, when the initial outbreak occurred. To determine the mechanism(s) that underlies the increased infectivity exhibited by KPC, Multilocus Sequence Typing (MLST) was conducted. A series of plasmids was also extracted, sequenced and analyzed. Concurrently, the complete sequences of bla_KPC−2-harboring plasmids deposited in GenBank were summarized and aligned. The bla_KPC−2 and KlcA_HS genes in the carbapenem-resistant K. pneumoniae isolates were examined. E. coli strains, carrying different Type I Restriction and Modification (RM) systems, were selected to study the interaction between RM systems, anti-RM systems and horizontal gene transfer (HGT). The ST11 clone predominated among 102 carbapenem-resistant K. pneumoniae isolates, all harbored the bla_KPC−2 gene; 98% contained the KlcA_HS gene. KlcA_HS was one of the core genes in the backbone region of most bla_KPC−2 carrying plasmids. Type I RM systems in the host bacteria reduced the rate of pHS10842 plasmid transformation by 30- to 40-fold. Presence of the anti-restriction protein, KlcA_HS, on the other hand, increased transformation efficiency by 3- to 6-fold. These results indicate that RM systems can significantly restrict HGT. In contrast, KlcA_HS can disrupt the RM systems and promote HGT by transformation. These findings suggest that the anti-restriction protein, KlcA_HS, represents a novel mechanism that facilitates the increased transfer of bla_KPC-2 and KlcA_HS-carrying plasmids among K. pneumoniae strains.

Biosynthesis and Function of Modified Bases in Bacteria and Their Viruses

Naturally occurring modification of the canonical A, G, C, and T bases can be found in the DNA of cellular organisms and viruses from all domains of life. Bacterial viruses (bacteriophages) are a particularly rich but still underexploited source of such modified variant nucleotides. The modifications conserve the coding and base-pairing functions of DNA, but add regulatory and protective functions. In prokaryotes, modified bases appear primarily to be part of an arms race between bacteriophages (and other genomic parasites) and their hosts, although, as in eukaryotes, some modifications have been adapted to convey epigenetic information. The first half of this review catalogs the identification and diversity of DNA modifications found in bacteria and bacteriophages. What is known about the biogenesis, context, and function of these modifications are also described. The second part of the review places these DNA modifications in the context of the arms race between bacteria and bacteriophages. It focuses particularly on the defense and counter-defense strategies that turn on direct recognition of the presence of a modified base. Where modification has been shown to affect other DNA transactions, such as expression and chromosome segregation, that is summarized, with reference to recent reviews.

Type I restriction enzymes and their relatives

Type I restriction enzymes (REases) are large pentameric proteins with separate restriction (R), methylation (M) and DNA sequence-recognition (S) subunits. They were the first REases to be discovered and purified, but unlike the enormously useful Type II REases, they have yet to find a place in the enzymatic toolbox of molecular biologists. Type I enzymes have been difficult to characterize, but this is changing as genome analysis reveals their genes, and methylome analysis reveals their recognition sequences. Several Type I REases have been studied in detail and what has been learned about them invites greater attention. In this article, we discuss aspects of the biochemistry, biology and regulation of Type I REases, and of the mechanisms that bacteriophages and plasmids have evolved to evade them. Type I REases have a remarkable ability to change sequence specificity by domain shuffling and rearrangements. We summarize the classic experiments and observations that led to this discovery, and we discuss how this ability depends on the modular organizations of the enzymes and of their S subunits. Finally, we describe examples of Type II restriction-modification systems that have features in common with Type I enzymes, with emphasis on the varied Type IIG enzymes.

Complete genome sequence and phenotype microarray analysis of Cronobacter sakazakii SP291: a persistent isolate cultured from a powdered infant formula production facility

Outbreaks of human infection linked to the powdered infant formula (PIF) food chain and associated with the bacterium Cronobacter, are of concern to public health. These bacteria are regarded as opportunistic pathogens linked to life-threatening infections predominantly in neonates, with an under developed immune system. Monitoring the microbiological ecology of PIF production sites is an important step in attempting to limit the risk of contamination in the finished food product. Cronobacter species, like other microorganisms can adapt to the production environment. These organisms are known for their desiccation tolerance, a phenotype that can aid their survival in the production site and PIF itself. In evaluating the genome data currently available for Cronobacter species, no sequence information has been published describing a Cronobacter sakazakii isolate found to persist in a PIF production facility. Here we report on the complete genome sequence of one such isolate, Cronobacter sakazakii SP291 along with its phenotypic characteristics. The genome of C. sakazakii SP291 consists of a 4.3-Mb chromosome (56.9% GC) and three plasmids, denoted as pSP291-1, [118.1-kb (57.2% GC)], pSP291-2, [52.1-kb (49.2% GC)], and pSP291-3, [4.4-kb (54.0% GC)]. When C. sakazakii SP291 was compared to the reference C. sakazakii ATCC BAA-894, which is also of PIF origin, the annotated genome data identified two interesting functional categories, comprising of genes related to the bacterial stress response and resistance to antimicrobial and toxic compounds. Using a phenotypic microarray (PM), we provided a full metabolic profile comparing C. sakazakii SP291 and the previously sequenced C. sakazakii ATCC BAA-894. These data extend our understanding of the genome of this important neonatal pathogen and provides further insights into the genotypes associated with features that can contribute to its persistence in the PIF environment.

Protein structure validation and identification from unassigned residual dipolar coupling data using 2D-PDPA

More than 90% of protein structures submitted to the PDB each year are homologous to some previously characterized protein structure. The extensive resources that are required for structural characterization of proteins can be justified for the 10% of the novel structures, but not for the remaining 90%. This report presents the 2D-PDPA method, which utilizes unassigned residual dipolar coupling in order to address the economics of structure determination of routine proteins by reducing the data acquisition and processing time. 2D-PDPA has been demonstrated to successfully identify the correct structure of an array of proteins that range from 46 to 445 residues in size from a library of 619 decoy structures by using unassigned simulated RDC data. When using experimental data, 2D-PDPA successfully identified the correct NMR structures from the same library of decoy structures. In addition, the most homologous X-ray structure was also identified as the second best structural candidate. Finally, success of 2D-PDPA in identifying and evaluating the most appropriate structure from a set of computationally predicted structures in the case of a previously uncharacterized protein Pf2048.1 has been demonstrated. This protein exhibits less than 20% sequence identity to any protein with known structure and therefore presents a compelling and practical application of our proposed work.

Global transcriptional regulator KorC coordinates expression of three backbone modules of the broad-host-range RA3 plasmid from IncU incompatibility group

The broad-host-range conjugative RA3 plasmid from IncU incompatibility group has been isolated from the fish pathogen Aeromonas hydrophila. DNA sequencing has revealed a mosaic modular structure of RA3 with the stabilization module showing some similarity to IncP-1 genes and the conjugative transfer module highly similar to that from PromA plasmids. The integrity of the mosaic plasmid genome seems to be specified by its regulatory network. In this paper the transcriptional regulator KorC was analyzed. KorCRA3 (98 amino acids) is encoded in the stabilization region and represses four strong promoters by binding to a conserved palindrome sequence, designated OC on the basis of homology to the KorC operator sequences in IncP-1 plasmids. Two of the KorCRA3-regulated promoters precede the first two cistrons in the stabilization module, one fires towards replication module, remaining one controls a tricistronic operon, whose products are involved in the conjugative transfer process. Despite the similarity between the binding sites in IncU and IncP-1 plasmids, no cross-reactivity between their KorC proteins has been detected. KorC emerges as a global regulator of RA3, coordinating all its backbone functions: replication, stable maintenance and conjugative transfer.

A novel gene, ardD, determines antirestriction activity of the non-conjugative transposon Tn5053 and is located antisense within the tniA gene

The mercury-resistance transposon Tn5053 inhibits restriction activity of the type I restriction-modification endonuclease EcoKI in Escherichia coli K12 cells. This is the first report of antirestriction activity of a non-conjugative transposon. The gene (ardD) coding for the antirestriction protein has been cloned. The ardD gene is located within the tniA gene, coding for transposase, on the complementary strand. The direction of transcription is opposite to transcription of the tniA gene.

Identification of bacterial plasmids based on mobility and plasmid population biology

Plasmids contain a backbone of core genes that remains relatively stable for long evolutionary periods, making sense to speak about plasmid species. The identification and characterization of the core genes of a plasmid species has a special relevance in the study of its epidemiology and modes of transmission. Besides, this knowledge will help to unveil the main routes that genes, for example antibiotic resistance (AbR) genes, use to travel from environmental reservoirs to human pathogens. Global dissemination of multiple antibiotic resistances and virulence traits by plasmids is an increasing threat for the treatment of many bacterial infectious diseases. To follow the dissemination of virulence and AbR genes, we need to identify the causative plasmids and follow their path from reservoirs to pathogens. In this review, we discuss how the existing diversity in plasmid genetic structures gives rise to a large diversity in propagation strategies. We would like to propose that, using an identification methodology based on plasmid mobility types, we can follow the propagation routes of most plasmids in Gammaproteobacteria, as well as their cargo genes, in complex ecosystems. Once the dissemination routes are known, designing antidissemination drugs and testing their efficacy will become feasible. We discuss in this review how the existing diversity in plasmid genetic structures gives rise to a large diversity in propagation strategies. We would like to propose that, by using an identification methodology based on plasmid mobility types, we can follow the propagation routes of most plasmids in ?-proteobacteria, as well as their cargo genes, in complex ecosystems.