Characterization of a higBA toxin-antitoxin locus in Vibrio cholerae

Insight into the environmental cues modulating the expression of bacterial toxin-antitoxin systems

Bacteria require sophisticated sensing mechanisms to adjust their metabolism in response to stressful conditions and survive in hostile environments. Among them, toxin-antitoxin (TA) systems play a crucial role in bacterial adaptation to environmental challenges. TA systems are considered as stress-responsive elements, consisting of both toxin and antitoxin genes, typically organized in operons or encoded on complementary DNA strands. A decrease in the antitoxin-toxin ratio, often triggered by specific stress conditions, leads to toxin excess, disrupting essential cellular processes and inhibiting bacterial growth. These systems are categorized into eight types based on the nature of the antitoxin (RNA or protein) and the mode of action of toxin inhibition. While the well-established biological roles of TA systems include phage inhibition and the maintenance of genetic elements, the environmental cues regulating their expression remain insufficiently documented. In this review, we highlight the diversity and complexity of the environmental cues influencing TA systems expression. A comprehensive understanding of how these genetic modules are regulated could provide deeper insights into their functions and support the development of innovative antimicrobial strategies.

Comparative Genomics Reveal Distinct Environment Preference and Functional Adaptation Among Lineages of Gemmatimonadota

Bacteria in the phylum Gemmatimonadota are globally distributed and abundant in microbial communities of various environments, playing an important role in driving biogeochemical cycling on Earth. Although high diversities in taxonomic composition and metabolic capabilities have been reported, little is known about the environmental preferences and associated functional features that facilitate adaptation among different Gemmatimonadota lineages. This study systematically analyzed the relationships between the environments, taxonomy, and functions of Gemmatimonadota lineages, by using a comparative genomics approach based on 1356 Gemmatimonadota genomes (213 high-quality and non-redundant genomes) available in a public database (NCBI). The taxonomic analysis showed that the 99.5% of the genomes belong to the class Gemmatimonadetes, and the rest of the genomes belong to the class Glassbacteria. Functional profiling revealed clear environmental preference among different lineages of Gemmatimonadota, and a marine group and two non-marine groups were identified and tested to be significantly different in functional composition. Further annotation and statistical comparison revealed a large number of functional genes (e.g., amiE, coxS, yfbK) that were significantly enriched in genomes from the marine group, supporting enhanced capabilities in energy acquisition, genetic information regulation (e.g., DNA repair), electrolyte homeostasis, and growth rate control. These genomic features are important for their survival in the marine environment, which is oligotrophic, variable, and with high salinity. The findings enhanced our understanding of the metabolic processes and environmental adaptation of Gemmatimonadota, and further advanced the understanding of the interactions of microorganisms and their habitats.

H. Shyu D, Chen YY, Zala D. Whole Genome Sequencing and Pan-Genomic Analysis of Multidrug-Resistant Vibrio cholerae VC01 Isolated from a Clinical Sample

Cholera, a disease caused by the Vibrio cholerae bacteria, threatens public health worldwide. The organism mentioned above has a significant historical record of being identified as a prominent aquatic environmental pollutant capable of adapting its phenotypic and genotypic traits to react to host patients effectively. This study aims to elucidate the heterogeneity of the sporadic clinical strain of V. cholerae VC01 among patients residing in Silvasa. The study involved conducting whole-genome sequencing of the isolate obtained from patients exhibiting symptoms, including those not commonly observed in clinical practice. The strain was initially identified through a combination of biochemical analysis, microscopy, and 16s rRNA-based identification, followed by type strain-based identification. The investigation demonstrated the existence of various genetic alterations and resistance profiles against multiple drugs, particularly chloramphenicol (catB9), florfenicol (floR), oxytetracycline (tet(34)), sulfonamide (sul2), and Trimethoprim (dfrA1). The pan-genomic analysis indicated that 1099 distinct clusters were detected within the genome sequences of recent isolates worldwide. The present study helps to establish a correlation between the mutation and the coexistence of antimicrobial resistance toward current treatment.

Systematic transcriptome analysis allows the identification of new type I and type II Toxin/Antitoxin systems located in the superintegron of Vibrio cholerae

Vibrio cholerae N16961 genome encodes 18 type II Toxin/Antitoxin (TA) systems, all but one located inside gene cassettes of its chromosomal superintegron (SI). This study aims to investigate additional TA systems in this genome. We screened for all two-genes operons of uncharacterized function by analyzing previous RNAseq data. Assays on nine candidates, revealed one additional functional type II TA encoded by the VCA0497-0498 operon, carried inside a SI cassette. We showed that VCA0498 antitoxin alone and in complex with VCA0497 represses its own operon promoter. VCA0497-0498 is the second element of the recently identified dhiT/dhiA superfamily uncharacterized type II TA system. RNAseq analysis revealed that another SI cassette encodes a novel type I TA system: VCA0495 gene and its two associated antisense non-coding RNAs, ncRNA495 and ncRNA496. Silencing of both antisense ncRNAs lead to cell death, demonstrating the type I TA function. Both VCA0497 and VCA0495 toxins do not show any homology to functionally characterized toxins, however our preliminary data suggest that their activity may end up in mRNA degradation, directly or indirectly. Our findings increase the TA systems number carried in this SI to 19, preferentially located in its distal end, confirming their importance in this large cassette array.

Do plasmids containing heavy metal resistance genes play a role in neonatal sepsis and invasive disease caused by Klebsiella pneumoniae and Klebsiella variicola?

Introduction. Klebsiella species are some of those most implicated in neonatal sepsis. However, many isolates from infections appear unremarkable; they are generally susceptible to antibiotics and often of sporadic types not associated with virulence.Hypothesis/Gap Statement. Investigation is needed to identify if such isolates have virulence characteristics.Aim. To sequence multiple isolates of a range of types from cases of neonatal invasive disease to identify elements that may explain their virulence, and to determine if such elements are more common among these isolates than generally.Methodology. In total, 14 isolates of K. pneumoniae/K. variicola belonging to 13 distinct types from blood or CSF from neonatal infections were sequenced using long-read nanopore technology. PCR assays were used to screen a general set of isolates for heavy metal resistance genes arsC, silS and merR.Results. Overall, 12/14 isolates carried one or more plasmids. Ten carried a large plasmid (186 to 310 kb) containing heavy metal resistance genes associated with hypervirulence plasmids, with most (nine) carrying genes for resistance to copper, silver and one other heavy metal (arsenic, tellurite or mercury), but lacking the genes encoding capsule-upregulation and siderophores. Most isolates (9/14) lacked any additional antibiotic resistance genes other than those intrinsic in the species. However, a representative of an outbreak strain carried a plasmid containing bla _CTX-M-15, qnrS1, aac3_IIa, dfrA17, sul1, mph(A), tet(A), bla _TEM1B and aadA5, but no heavy metal resistance genes. arsC, silS and merR were widely found among 100 further isolates screened, with most carbapenemase-gene-positive isolates (20/27) carrying at least one.Conclusion. Plasmids containing heavy metal resistance genes were a striking feature of isolates from neonatal sepsis but are widely found. They share elements in common with virulence and antibiotic resistance plasmids, perhaps providing a basis from which such plasmids evolve.

Toxin-antitoxin systems in pathogenic Vibrio species: a mini review from a structure perspective

Toxin-antitoxin (TA) genetic modules have been found to widely exist in bacterial chromosomes and mobile genetic elements. They are composed of stable toxins and less stable antitoxins that can counteract the toxicity of toxins. The interactions between toxins and antitoxins could play critical roles in the virulence and persistence of pathogenic bacteria. There are at least eight types of TA systems which have been identified in a variety of bacteria. Vibrio, a genus of Gram-negative bacteria, is widespread in aquatic environments and can cause various human diseases, such as epidemic cholera. In this review, we mainly explore the structures and functions of TA modules found in common Vibrio pathogens, mainly V. cholerae, for better understanding of TA action mechanisms in pathogenic bacteria.

Functional characterization of HigBA toxin-antitoxin system in an Arctic bacterium, Bosea sp. PAMC 26642

Toxin-antitoxin (TA) systems are growth-controlling genetic elements consisting of an intracellular toxin protein and its cognate antitoxin. TA systems have been spread among microbial genomes through horizontal gene transfer and are now prevalent in most bacterial and archaeal genomes. Under normal growth conditions, antitoxins tightly counteract the activity of the toxins. Upon stresses, antitoxins are inactivated, releasing activated toxins, which induce growth arrest or cell death. In this study, among nine functional TA modules in Bosea sp. PAMC 26642 living in Arctic lichen, we investigated the functionality of BoHigBA2. BohigBA2 is located close to a genomic island and adjacent to flagellar gene clusters. The expression of BohigB2 induced the inhibition of E. coli growth at 37°C, which was more manifest at 18°C, and this growth defect was reversed when BohigA2 was co-expressed, suggesting that this BoHigBA2 module might be an active TA module in Bosea sp. PAMC 26642. Live/dead staining and viable count analyses revealed that the BoHigB2 toxin had a bactericidal effect, causing cell death. Furthermore, we demonstrated that BoHigB2 possessed mRNA-specific ribonuclease activity on various mRNAs and cleaved only mRNAs being translated, which might impede overall translation and consequently lead to cell death. Our study provides the insight to understand the cold adaptation of Bosea sp. PAMC 26642 living in the Arctic.

Rapid Identification of Secondary Structure and Binding Site Residues in an Intrinsically Disordered Protein Segment

Mycobacterium tuberculosis harbours nine toxin-antitoxin (TA) systems of the MazEF family. MazEF TA modules are of immense importance due to the perceived role of the MazF toxin in M. tuberculosis persistence and disease. The MazE antitoxin has a disordered C-terminal domain that binds the toxin, MazF and neutralizes its endoribonuclease activity. However, the structure of most MazEF TA complexes remains unsolved till date, obscuring structural and functional information about the antitoxins. We present a facile method to identify toxin binding residues on the disordered antitoxin. Charged residue scanning mutagenesis was used to screen a yeast surface displayed MazE6 antitoxin library against its purified cognate partner, the MazF6 toxin. Binding residues were deciphered by probing the relative reduction in binding to the ligand by flow cytometry. We have used this to identify putative antitoxin interface residues and local structure attained by the antitoxin upon interaction in the MazEF6 TA system and the same methodology is readily applicable to other intrinsically disordered protein regions.

Toxin-antitoxin systems and their medical applications: current status and future perspective

Almost all bacteria synthesize two types of toxins-one for its survival by regulating different cellular processes and another as a strategy to interact with host cells for pathogenesis. Usually, "bacterial toxins" are contemplated as virulence factors that harm the host organism. However, toxins produced by bacteria, as a survival strategy against the host, also hamper its cellular processes. To overcome this, the bacteria have evolved with the production of a molecule, referred to as antitoxin, to negate the deleterious effect of the toxin against itself. The toxin and antitoxins are encoded by a two-component toxin-antitoxin (TA) system. The antitoxin, a protein or RNA, sequesters the toxins of the TA system for neutralization within the bacterial cell. In this review, we have described different TA systems of bacteria and their potential medical and biotechnological applications. It is of interest to note that while bacterial toxin-antitoxin systems have been well studied, the TA system in unicellular eukaryotes, though predicted by the investigators, have never been paid the desired attention. In the present review, we have also touched upon the TA system of eukaryotes identified to date. KEY POINTS: Bacterial toxins harm the host and also affect the bacterial cellular processes. The antitoxin produced by bacteria protect it from the toxin's harmful effects. The toxin-antitoxin systems can be targeted for various medical applications.

High diversity of Vibrio spp. associated with different ecological niches in a marine aquaria system and description of Vibrio aquimaris sp. nov

The aim of the study was to characterise the diversity and niche-specific colonization of Vibrio spp. in a marine aquaria system by a cultivation-dependent approach. A total of 53 Vibrio spp. isolates were cultured from different ecological niches in a marine aquarium including microplastic (MP) and sandy sediment particles (12 weeks after added sterile to the system), detritus, and the surrounding aquarium water. Based on the 16S rRNA gene sequence phylogeny and multilocus sequence analysis (MLSA) the isolates were assigned to seven different phylotypes. Six phylotypes were identified by high probability to the species level. The highest phylotype diversity was cultured from detritus and water (six out of seven phylotypes), while only two phylotypes were cultured from MP and sediment particles. Genomic fingerprinting indicated an even higher genetic diversity of Vibrio spp. at the strain (genotype) level. Again, the highest diversity of genotypes was recovered from detritus and water while only few partially particle-type specific genotypes were cultured from MP and sediment particles. Phylotype V-2 formed an independent branch in the MLSA tree and could not be assigned to a described Vibrio species. Isolates of this phylotype showed highest 16S rRNA gene sequence similarity to type strains of Vibrio japonicus (98.5%) and Vibrio caribbeanicus (98.4%). A representative isolate, strain THAF100^T, was characterised by a polyphasic taxonomic approach and Vibrio aquimaris sp. nov., with strain THAF100^T (=DSM 109633^T=LMG 31434^T=CIP 111709^T) as type strain, is proposed as novel species.

Evaluating the Potential for Cross-Interactions of Antitoxins in Type II TA Systems

The diversity of Type-II toxin–antitoxin (TA) systems in bacterial genomes requires tightly controlled interaction specificity to ensure protection of the cell, and potentially to limit cross-talk between toxin–antitoxin pairs of the same family of TA systems. Further, there is a redundant use of toxin folds for different cellular targets and complexation with different classes of antitoxins, increasing the apparent requirement for the insulation of interactions. The presence of Type II TA systems has remained enigmatic with respect to potential benefits imparted to the host cells. In some cases, they play clear roles in survival associated with unfavorable growth conditions. More generally, they can also serve as a “cure” against acquisition of highly similar TA systems such as those found on plasmids or invading genetic elements that frequently carry virulence and resistance genes. The latter model is predicated on the ability of these highly specific cognate antitoxin–toxin interactions to form cross-reactions between chromosomal antitoxins and invading toxins. This review summarizes advances in the Type II TA system models with an emphasis on antitoxin cross-reactivity, including with invading genetic elements and cases where toxin proteins share a common fold yet interact with different families of antitoxins.

Molecular Characterization of SehB, a Type II Antitoxin of Salmonella enterica Serotype Typhimurium: Amino Acid Residues Involved in DNA-Binding, Homodimerization, Toxin Interaction, and Virulence

Salmonella enterica serotype Typhimurium is a bacterium that causes gastroenteritis and diarrhea in humans. The genome of S. Typhimurium codes for diverse virulence factors, among which are the toxin-antitoxin (TA) systems. SehAB is a type II TA, where SehA is the toxin and SehB is the antitoxin. It was previously reported that the absence of the SehB antitoxin affects the growth of S. Typhimurium. In addition, the SehB antitoxin can interact directly with the SehA toxin neutralizing its toxic effect as well as repressing its own expression. We identified conserved residues on SehB homologous proteins. Point mutations were introduced at both N- and C-terminal of SehB antitoxin to analyze the effect of these changes on its transcription repressor function, on its ability to form homodimers and on the virulence of S. Typhimurium. All changes in amino acid residues at both the N- and C-terminal affected the repressor function of SehB antitoxin and they were required for DNA-binding activity. Mutations in the amino acid residues at the N-terminal showed a lower capacity for homodimer formation of the SehB protein. However, none of the SehB point mutants were affected in the interaction with the SehA toxin. In terms of virulence, the eight single-amino acid mutations were attenuated for virulence in the mouse model. In agreement with our results, the eight amino acid residues of SehB antitoxin were required for its repressor activity, affecting both homodimerization and DNA-binding activity, supporting the notion that both activities of SehB antitoxin are required to confer virulence to Salmonella enterica.

Detection of a Single-Copy Plasmid, pXFSL21, in Xylella fastidiosa Strain Stag's Leap with Two Toxin-Antitoxin Systems Using Next-Generation Sequencing

Plasmids are important genetic elements contributing to bacterial evolution and environmental adaptation. Xylella fastidiosa is a nutritionally fastidious Gram-negative bacterium causing economically devastating diseases such as Pierce's disease (PD) of grapevine. In this study, the plasmid status of a highly virulent PD strain, Stag's Leap, originally isolated from Napa Valley, CA, was studied using sequencing and bioinformatics tools. DNA samples extracted from a pure culture in periwinkle wilt medium (in vitro DNA) and a PD-symptomatic grapevine artificially inoculated in the greenhouse (in planta DNA) were subject to next-generation sequencing (NGS) analyses (Illumina MiSeq or HiSeq). Sequence analyses and polymerase chain reaction experiments revealed the presence of a circular plasmid, pXFSL21, of 21,665 bp. This plasmid existed as a single copy per bacterial genome under both in vitro and in planta conditions. Two toxin-antitoxin (T-A) systems (ydcD-ydcE and higB-higA) were detected in pXFSL21, a possible mechanism for the long-term survival of this single-copy plasmid in the bacterial population. BLAST searches against the GenBank database (version 222) detected homologs of the two T-A systems in chromosomes or plasmids of some X. fastidiosa strains. However, double T-A systems were found only in pXFSL21. pXFSL21 was not found in other known PD strains and, therefore, could serve as a molecular marker for strain Stag's Leap monitoring and tracking. The NGS-based technique outlined in this article provides an effective tool for identifying single- or low-copy-number plasmids in fastidious prokaryotes.

Bioinformatics and Functional Assessment of Toxin-Antitoxin Systems in Staphylococcus aureus

Staphylococcus aureus is a nosocomial pathogen that can cause chronic to persistent infections. Among different mediators of pathogenesis, toxin-antitoxin (TA) systems are emerging as the most prominent. These systems are frequently studied in Escherichia coli and Mycobacterial species but rarely explored in S. aureus. In the present study, we thoroughly analyzed the S. aureus genome and screened all possible TA systems using the Rasta bacteria and toxin-antitoxin database. We further searched E. coli and Mycobacterial TA homologs and selected 67 TA loci as putative TA systems in S. aureus. The host inhibition of growth (HigBA) TA family was predominantly detected in S. aureus. In addition, we detected seven pathogenicity islands in the S. aureus genome that are enriched with virulence genes and contain 26 out of 67 TA systems. We ectopically expressed multiple TA genes in E. coli and S. aureus that exhibited bacteriostatic and bactericidal effects on cell growth. The type I Fst toxin created holes in the cell wall while the TxpA toxin reduced cell size and induced cell wall septation. Besides, we identified a new TA system whose antitoxin functions as a transcriptional autoregulator while the toxin functions as an inhibitor of autoregulation. Altogether, this study provides a plethora of new as well as previously known TA systems that will revitalize the research on S. aureus TA systems.

Antimicrobial resistance not related to 1,2,3 integrons and Superintegron in Vibrio spp. isolated from seawater sample of Lima (Peru)

Antimicrobial resistance (AMR) in microorganisms has been attributed to integrons, which have the ability to capture antimicrobial resistance gene cassettes and express them in their hosts. 170 strains of Vibrio spp. were isolated from Lima (Peru) seawater samples and identified by biochemical tests and PCR. AMR profiles were generated using 15 standard antibiotics. The presence of class 1, 2 and 3 integrons and Superintegron in these strains were also investigated by PCR. Ten species of Vibrio were identified with V. alginolyticus the most frequent. All strains were resistant to antibiotics, especially to penicillin group. No resistance to norfloxacin or tetracycline was observed. Class 1, 2 and 3 integrons were not found, only one Superintegron containing the mutT gene was identified in V. cholerae L22 strain. This indicated that AMR is not related to integrons as mentioned previously and that these strains can be reservoirs of resistance genes in marine environments.

Identification of four type II toxin-antitoxin systems in Actinobacillus pleuropneumoniae

Toxin-antitoxin (TA) systems are small genetic elements that are widely prevalent in the genomes of bacteria and archaea. These modules have been identified in various bacteria and proposed to play an important role in bacterial physiology and virulence. However, their presence in the genomes of Actinobacillus species has received no attention. In this study, we describe the identification of four type II TA systems in Actinobacillus pleuropneumoniae, the causative agent of porcine pleuropneumonia. Reverse transcription PCR analysis revealed that the genes encoding the toxin and antitoxin are co-transcribed. Overexpression of each toxin inhibited the growth of Escherichia coli, and the toxic effect could be counteracted by its cognate antitoxin. The pull-down experiments demonstrated that each toxin interacts with its cognate antitoxin in vivo. The promoter activity assays showed that each antitoxin could autoregulate either positively or negatively the TA operon transcription. In addition, the APJL_0660/0659 TA system is present in half of the detected serovars of A. pleuropneumoniae, while the others are present in all. Collectively, we identified four type II TA systems in A. pleuropneumoniae, and this study has laid the foundation for further functional study of these TA systems.

Novel toxins from type II toxin-antitoxin systems with acetyltransferase activity

Type II toxin-antitoxin (TA) systems are widespread in bacterial and archeal genomes. These modules are very dynamic and participate in bacterial genome evolution through horizontal gene transfer. TA systems are commonly composed of a labile antitoxin and a stable toxin. Toxins appear to preferentially inhibit the protein synthesis process. Toxins use a variety of molecular mechanisms and target nearly every step of translation to achieve their inhibitory function. This review focuses on a recently identified TA family that includes acetyltransferase toxins. The AtaT and TacT toxins are the best-characterized to date in this family. AtaT and TacT both inhibit translation by acetylating the amino acid charged on tRNAs. However, the specificities of these 2 toxins are different as AtaT inhibits translation initiation by acetylation of the initiator tRNA whereas TacT acetylates elongator tRNAs. The molecular mechanisms of these toxins are discussed, as well as the functions and possible evolutionary origins of this diverse toxin family.

Ribosome-dependent Vibrio cholerae mRNAse HigB2 is regulated by a β-strand sliding mechanism

Toxin–antitoxin (TA) modules are small operons involved in bacterial stress response and persistence. higBA operons form a family of TA modules with an inverted gene organization and a toxin belonging to the RelE/ParE superfamily. Here, we present the crystal structures of chromosomally encoded Vibrio cholerae antitoxin (VcHigA2), toxin (VcHigB2) and their complex, which show significant differences in structure and mechanisms of function compared to the higBA module from plasmid Rts1, the defining member of the family. The VcHigB2 is more closely related to Escherichia coli RelE both in terms of overall structure and the organization of its active site. VcHigB2 is neutralized by VcHigA2, a modular protein with an N-terminal intrinsically disordered toxin-neutralizing segment followed by a C-terminal helix-turn-helix dimerization and DNA binding domain. VcHigA2 binds VcHigB2 with picomolar affinity, which is mainly a consequence of entropically favorable de-solvation of a large hydrophobic binding interface and enthalpically favorable folding of the N-terminal domain into an α-helix followed by a β-strand. This interaction displaces helix α3 of VcHigB2 and at the same time induces a one-residue shift in the register of β-strand β3, thereby flipping the catalytically important Arg64 out of the active site.

Translation-dependent mRNA cleavage by YhaV in Escherichia coli

Many bacteria have toxin-antitoxin (TA) systems, where toxin gene expression inhibits their own cell growth. mRNA is one of the well-known targets of the toxins in the type II toxin-antitoxin systems. Here, we examined the ribosome dependency of the endoribonuclease activity of YhaV, one of the toxins in type II TA systems, on mRNA in vitro and in vivo. A polysome profiling assay revealed that YhaV is bound to the 70S ribosomes and 50S ribosomal subunits. Moreover, we found that while YhaV cleaves ompF and lpp mRNAs in a translation-dependent manner, they did not cleave the 5' untranslated region in primer extension experiments. From these results, we conclude that YhaV is a ribosome-dependent toxin that cleaves mRNA in a translation-dependent manner.

Characterization and Comparative Overview of Complete Sequences of the First Plasmids of Pandoraea across Clinical and Non-clinical Strains

To date, information on plasmid analysis in Pandoraea spp. is scarce. To address the gap of knowledge on this, the complete sequences of eight plasmids from Pandoraea spp. namely Pandoraea faecigallinarum DSM 23572^T (pPF72-1, pPF72-2), Pandoraea oxalativorans DSM 23570^T (pPO70-1, pPO70-2, pPO70-3, pPO70-4), Pandoraea vervacti NS15 (pPV15) and Pandoraea apista DSM 16535^T (pPA35) were studied for the first time in this study. The information on plasmid sequences in Pandoraea spp. is useful as the sequences did not match any known plasmid sequence deposited in public databases. Replication genes were not identified in some plasmids, a situation that has led to the possibility of host interaction involvement. Some plasmids were also void of par genes and intriguingly, repA gene was also not discovered in these plasmids. This further leads to the hypothesis of host-plasmid interaction. Plasmid stabilization/stability protein-encoding genes were observed in some plasmids but were not established for participating in plasmid segregation. Toxin-antitoxin systems MazEF, VapBC, RelBE, YgiT-MqsR, HigBA, and ParDE were identified across the plasmids and their presence would improve plasmid maintenance. Conjugation genes were identified portraying the conjugation ability amongst Pandoraea plasmids. Additionally, we found a shared region amongst some of the plasmids that consists of conjugation genes. The identification of genes involved in replication, segregation, toxin-antitoxin systems and conjugation, would aid the design of drugs to prevent the survival or transmission of plasmids carrying pathogenic properties. Additionally, genes conferring virulence and antibiotic resistance were identified amongst the plasmids. The observed features in the plasmids shed light on the Pandoraea spp. as opportunistic pathogens.

Structure, Biology, and Therapeutic Application of Toxin-Antitoxin Systems in Pathogenic Bacteria

Bacterial toxin–antitoxin (TA) systems have received increasing attention for their diverse identities, structures, and functional implications in cell cycle arrest and survival against environmental stresses such as nutrient deficiency, antibiotic treatments, and immune system attacks. In this review, we describe the biological functions and the auto-regulatory mechanisms of six different types of TA systems, among which the type II TA system has been most extensively studied. The functions of type II toxins include mRNA/tRNA cleavage, gyrase/ribosome poison, and protein phosphorylation, which can be neutralized by their cognate antitoxins. We mainly explore the similar but divergent structures of type II TA proteins from 12 important pathogenic bacteria, including various aspects of protein–protein interactions. Accumulating knowledge about the structure–function correlation of TA systems from pathogenic bacteria has facilitated a novel strategy to develop antibiotic drugs that target specific pathogens. These molecules could increase the intrinsic activity of the toxin by artificially interfering with the intermolecular network of the TA systems.

Toxin-Antitoxin Systems in Clinical Pathogens

Toxin-antitoxin (TA) systems are prevalent in bacteria and archaea. Although not essential for normal cell growth, TA systems are implicated in multiple cellular functions associated with survival under stress conditions. Clinical strains of bacteria are currently causing major human health problems as a result of their multidrug resistance, persistence and strong pathogenicity. Here, we present a review of the TA systems described to date and their biological role in human pathogens belonging to the ESKAPE group (Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa and Enterobacter spp.) and others of clinical relevance (Escherichia coli, Burkholderia spp., Streptococcus spp. and Mycobacterium tuberculosis). Better understanding of the mechanisms of action of TA systems will enable the development of new lines of treatment for infections caused by the above-mentioned pathogens.

Cryptic prophages as targets for drug development

Bacterial chromosomes may contain up to 20% phage DNA that encodes diverse proteins ranging from those for photosynthesis to those for autoimmunity; hence, phages contribute greatly to the metabolic potential of pathogens. Active prophages carrying genes encoding virulence factors and antibiotic resistance can be excised from the host chromosome to form active phages and are transmissible among different bacterial hosts upon SOS responses. Cryptic prophages are artifacts of mutagenesis in which lysogenic phage are captured in the bacterial chromosome: they may excise but they do not form active phage particles or lyse their captors. Hence, cryptic prophages are relatively permanent reservoirs of genes, many of which benefit pathogens, in ways we are just beginning to discern. Here we explore the role of active prophage- and cryptic prophage-derived proteins in terms of (i) virulence, (ii) antibiotic resistance, and (iii) antibiotic tolerance; antibiotic tolerance occurs as a result of the non-heritable phenotype of dormancy which is a result of activation of toxins of toxin/antitoxin loci that are frequently encoded in cryptic prophages. Therefore, cryptic prophages are promising targets for drug development.

The HigB/HigA toxin/antitoxin system of Pseudomonas aeruginosa influences the virulence factors pyochelin, pyocyanin, and biofilm formation

Toxin/antitoxin (TA) systems are prevalent in most bacterial and archaeal genomes, and one of the emerging physiological roles of TA systems is to help regulate pathogenicity. Although TA systems have been studied in several model organisms, few studies have investigated the role of TA systems in pseudomonads. Here, we demonstrate that the previously uncharacterized proteins HigB (unannotated) and HigA (PA4674) of Pseudomonas aeruginosa PA14 form a type II TA system in which antitoxin HigA masks the RNase activity of toxin HigB through direct binding. Furthermore, toxin HigB reduces production of the virulence factors pyochelin, pyocyanin, swarming, and biofilm formation; hence, this system affects the pathogencity of this strain in a manner that has not been demonstrated previously for TA systems.

The toxin GraT inhibits ribosome biogenesis

Most bacteria encode numerous chromosomal toxin-antitoxin (TA) systems that are proposed to contribute to stress tolerance, as they are able to shift the cells to a dormant state. Toxins act on a variety of targets with the majority attacking the translational apparatus. Intriguingly, the toxicity mechanisms of even closely related toxins may differ essentially. Here, we report on a new type of TA toxin that inhibits ribosome biogenesis. GraT of the GraTA system has previously been described in Pseudomonas putida as an unusually moderate toxin at optimal growth temperatures. However, GraT causes a severe growth defect at lower temperatures. Here, we demonstrate that GraT causes the accumulation of free ribosomal subunits. Mapping the rRNA 5' ends reveals incomplete processing of the free subunits and quantification of modified nucleosides shows an underrepresentation of late subunit assembly specific modifications. This indicates that GraT inhibits ribosome subunit assembly. Interestingly, GraT effects can be alleviated by modification of the chaperone DnaK, a known facilitator of late stages in ribosome biogenesis. We show that GraT directly interacts with DnaK and suggest two possible models for the role of this interaction in GraT toxicity.

Developing Universal Genetic Tools for Rapid and Efficient Deletion Mutation in Vibrio Species Based on Suicide T-Vectors Carrying a Novel Counterselectable Marker, vmi480

Despite that Vibrio spp. have a significant impact on the health of humans and aquatic animals, the molecular basis of their pathogenesis is little known, mainly due to the limited genetic tools for the functional research of genes in Vibrio. In some cases, deletion of target DNAs in Vibrio can be achieved through the use of suicide vectors. However, these strategies are time-consuming and lack universality, and the widely used counterselectable gene sacB does not work well in Vibrio cells. In this study, we developed universal genetic tools for rapid and efficient deletion mutations in Vibrio species based on suicide T-Vectors carrying a novel counterselectable marker, vmi480. We explored two uncharacterized genes, vmi480 and vmi470, in a genomic island from Vibrio mimicus VM573 and confirmed that vmi480 and vmi470 constitute a two-component toxin-antitoxin system through deletion and expression of vmi480 and vmi470. The product of vmi480 exhibited strong toxicity to Escherichia coli cells. Based on vmi480 and the P_BAD or P_TAC promoter system, we constructed two suicide T-vectors, pLP11 and pLP12, and each of these vectors contained a multiple cloning region with two AhdI sites. Both vectors linearized by AhdI digestion could be stored and directly ligated with purified PCR products without a digestion step. By using pLP11 and pLP12 coupled with a highly efficient conjugation system provided by E. coli β2163, six genes from four representative Vibrio species were easily deleted. By using the counterselective marker vmi480, we obtained 3–12 positive colonies (deletion mutants) among no more than 20 colonies randomly selected on counterselection plates. The strategy does not require the digestion of PCR products and suicide vectors every time, and it avoids large-scale screening colonies on counterselective plates. These results demonstrate that we successfully developed universal genetic tools for rapid and efficient gene deletion in Vibrio species.

Comprehensive Functional Analysis of the 18 Vibrio cholerae N16961 Toxin-Antitoxin Systems Substantiates Their Role in Stabilizing the Superintegron

The role of chromosomal toxin-antitoxin (TA) systems, which are ubiquitous within the genomes of free-living bacteria, is still debated. We have scanned the Vibrio cholerae N16961 genome for class 2 TA genes and identified 18 gene pair candidates. Interestingly, all but one are located in the chromosome 2 superintegron (SI). The single TA found outside the SI is located on chromosome 1 and is related to the well-characterized HipAB family, which is known to play a role in antibiotic persistence. We investigated this clustering within the SI and its possible biological consequences by performing a comprehensive functional analysis on all of the putative TA systems. We demonstrate that the 18 TAs identified encode functional toxins and that their cognate antitoxins are able to neutralize their deleterious effects when expressed in Escherichia coli. In addition, we reveal that the 17 predicted TA systems of the SI are transcribed and expressed in their native context from their own promoters, a situation rarely found in integron cassettes. We tested the possibility of interactions between noncognate pairs of all toxins and antitoxins and found no cross-interaction between any of the different TAs. Although these observations do not exclude other roles, they clearly strengthen the role of TA systems in stabilizing the massive SI cassette array of V. cholerae.

IMPORTANCE

The chromosomal toxin-antitoxin systems have been shown to play various, sometimes contradictory roles, ranging from genomic stabilization to bacterial survival via persistence. Determining the interactions between TA systems hosted within the same bacteria is essential to understand the hierarchy between these different roles. We identify here the full set of class 2 TAs carried in the Vibrio cholerae N16961 genome and found they are all, with a single exception, located in the chromosome 2 superintegron. Their characterization, in terms of functionality, expression, and possible cross-interactions, supports their main role as being the stabilization of the 176-cassette-long array of the superintegron but does not exclude dual roles, such as stress response elements, persistence, and bacteriophage defense through abortive infection mechanisms.

Toxin-antitoxin systems in the mobile genome of Acidithiobacillus ferrooxidans

Toxin-antitoxin (TA) systems are genetic modules composed of a pair of genes encoding a stable toxin and an unstable antitoxin that inhibits toxin activity. They are widespread among plasmids and chromosomes of bacteria and archaea. TA systems are known to be involved in the stabilization of plasmids but there is no consensus about the function of chromosomal TA systems. To shed light on the role of chromosomally encoded TA systems we analyzed the distribution and functionality of type II TA systems in the chromosome of two strains from Acidithiobacillus ferrooxidans (ATCC 23270 and 53993), a Gram-negative, acidophilic, environmental bacterium that participates in the bioleaching of minerals. As in other environmental microorganisms, A. ferrooxidans has a high content of TA systems (28-29) and in twenty of them the toxin is a putative ribonuclease. According to the genetic context, some of these systems are encoded near or within mobile genetic elements. Although most TA systems are shared by both strains, four of them, which are encoded in the active mobile element ICEAfe1, are exclusive to the type strain ATCC 23270. We demostrated that two TA systems from ICEAfe1 are functional in E. coli cells, since the toxins inhibit growth and the antitoxins counteract the effect of their cognate toxins. All the toxins from ICEAfe1, including a novel toxin, are RNases with different ion requirements. The data indicate that some of the chromosomally encoded TA systems are actually part of the A. ferrooxidans mobile genome and we propose that could be involved in the maintenance of these integrated mobile genetic elements.

Genomic overview of the phytopathogen Pectobacterium wasabiae strain RNS 08.42.1A suggests horizontal acquisition of quorum-sensing genes

The blackleg and soft-rot diseases caused by pectinolytic enterobacteria such as Pectobacterium and Dickeya are major causes of losses affecting potato crop in the field and upon storage. In this work, we report the isolation, characterization and genome analysis of the Pectobacterium wasabiae (formerly identified as Pectobacterium carotovorum subsp. carotovorum) strain RNS 08.42.1A, that has been isolated from a Solanum tuberosum host plant in France. Comparative genomics with 3 other P. wasabiae strains isolated from potato plants in different areas in North America and Europe, highlighted both a strong similarity at the whole genome level (ANI > 99 %) and a conserved synteny of the virulence genes. In addition, our analyses evidenced a robust separation between these four P. wasabiae strains and the type strain P. wasabiae CFBP 3304(T), isolated from horseradish in Japan. In P. wasabiae RNS 08.42.1A, the expI and expR nucleotidic sequences are more related to those of some Pectobacterium atrosepticum and P. carotovorum strains (90 % of identity) than to those of the other potato P. wasabiae strains (70 to 74 % of identity). This could suggest a recruitment of these genes in the P. wasabiae strain RNS 08.42.1A by an horizontal transfer between pathogens infecting the same potato host plant.

Involvement of toxin-antitoxin modules in Burkholderia cenocepacia biofilm persistence

Biofilms are involved in the recalcitrance of infections due to the presence of persister cells. Although the molecular basis of persistence is still largely unknown, toxin-antitoxin modules (TA) are thought to play a role in this process. In this study, we investigated whether TA modules contribute to persistence toward antibiotics in Burkholderia cenocepacia J2315. Sixteen pairs of genes were identified based on their apparent similarity to TA modules. Overexpression of the putative toxins had various effects on growth, persistence, and biofilm formation. Toxins, whose overexpression resulted in growth inhibition, often increased the number of surviving persisters; in contrast, overexpression of putative toxins showing no effects on growth had no positive influence on the number of surviving persisters. Furthermore, the expression of the TA modules was compared between treated and untreated sessile and planktonic wild-type cultures. For 10 toxin-encoding genes, the expression was higher in untreated sessile cells than in untreated planktonic cells. Nine toxin-encoding genes were upregulated after treatment with tobramycin, but none after treatment with ciprofloxacin. These results indicate that most, but not all TA modules contribute to persistence in B. cenocepacia J2315 and that this contribution depends on the mode of growth and the antibiotic used.

Characterization of putative toxin/antitoxin systems in Vibrio parahaemolyticus

AIM

To obtain more information about the toxin/antitoxin (TA) systems in the Vibrio genus and also to examine their involvement in the induction of a viable but nonculturable (VBNC) state, we searched homologues of the Escherichia coli TA systems in the Vibrio parahaemolyticus genome.

METHODS AND RESULTS

We found that a gene cluster, vp1842/vp1843, in the V. parahaemolyticus genome database has homology to that encoding the E. coli TA proteins, DinJ/YafQ. Expression of the putative toxin gene vp1843 in E. coli cells strongly inhibited the cell growth, while coexpression with the putative antitoxin gene vp1842 neutralized this effect. Mutational analysis identified Lys37 and Pro45 in the gene product VP1843 of vp1843 as crucial residues for the growth retardation of E. coli cells. VP1843, unlike the E. coli toxin YafQ, has no protein synthesis inhibitory activity, and that instead the expression of vp1843 in E. coli caused morphological change of the cells.

CONCLUSIONS

The gene cluster vp1842/vp1843 encodes the V. parahaemolyticus TA system; VP1843 inhibits cell growth, whereas VP1842 serves as an antitoxin by forming a stable complex with VP1843.

SIGNIFICANCE AND IMPACT OF THE STUDY

The putative toxin, VP1843, may be involved in the induction of the VBNC state in V. parahaemolyticus by inhibiting cell division.

The ToxAvapA toxin-antitoxin locus contributes to the survival of nontypeable Haemophilus influenzae during infection

Nontypeable Haemophilus influenzae (NTHi) is an opportunistic pathogen that is a common cause of acute and recurrent mucosal infections. One uncharacterized NTHi toxin-antitoxin (TA) module, NTHI1912-1913, is a host inhibition of growth (higBA) homologue. We hypothesized that this locus, which we designated toxAvapA, contributed to NTHi survival during infection. We deleted toxAvapA and determined that growth of the mutant in defined media was not different from the parent strain. We tested the mutant for persistence during long-term in vitro co-culture with primary human respiratory tissues, which revealed that the ΔtoxAvapA mutant was attenuated for survival. We then performed challenge studies using the chinchilla model of otitis media and determined that mutant survival was also reduced in vivo. Following purification, the toxin exhibited ribonuclease activity on RNA in vitro, while the antitoxin did not. A microarray comparison of the transcriptome revealed that the tryptophan biosynthetic regulon was significantly repressed in the mutant compared to the parent strain. HPLC studies of conditioned medium confirmed that there was no significant difference in the concentration of tryptophan remaining in the supernatant, indicating that the uptake of tryptophan by the mutant was not affected. We conclude that the role of the NTHi toxAvapA TA module in persistence following stress is multifactorial and includes effects on essential metabolic pathways.

A new type of intrabacterial nanotransportation system for VacA in Helicobacter pylori

Helicobacter pylori possesses intrabacterial nanotransportation systems (ibNoTSs) for CagA and urease. Both systems are UreI-dependent and urea-independent, and activated by extrabacterial acid. The activation occurs/appears within 15 min after exposure to extrabacterial acid stimulation. Although it has been clarified that VacA is secreted via the type-V secretion machinery, it remains unclear how this toxin is transported toward the machinery. To clarify the intrabacterial nanotransportation system for H. pylori VacA, immunoelectron microscopic analysis was performed in this study. VacA shifted to the periphery of the bacterial cytoplasm at 30 min after the extracellular pH change, whereas CagA and urease did so within 15 min. Studies using an ureI-deletion mutant revealed that unlike CagA and urease transport, VacA transport was not UreI-dependent. VacA secretion was accelerated without an increase in the production of VacA 30 min after the exposure to extrabacterial acid. These findings indicated that H. pylori possesses a novel type of ibNoTS for VacA, which is different from that for CagA or urease, in response time and dependency of UreI.

A toxin-antitoxin module of Salmonella promotes virulence in mice

Toxin-antitoxin (TA) modules are widely prevalent in both bacteria and archaea. Originally described as stabilizing elements of plasmids, TA modules are also widespread on bacterial chromosomes. These modules promote bacterial persistence in response to specific environmental stresses. So far, the possibility that TA modules could be involved in bacterial virulence has been largely neglected, but recent comparative genomic studies have shown that the presence of TA modules is significantly associated with the pathogenicity of bacteria. Using Salmonella as a model, we investigated whether TA modules help bacteria to overcome the stress conditions encountered during colonization, thereby supporting virulence in the host. By bioinformatics analyses, we found that the genome of the pathogenic bacterium Salmonella Typhimurium encodes at least 11 type II TA modules. Several of these are conserved in other pathogenic strains but absent from non-pathogenic species indicating that certain TA modules might play a role in Salmonella pathogenicity. We show that one TA module, hereafter referred to as sehAB, plays a transient role in virulence in perorally inoculated mice. The use of a transcriptional reporter demonstrated that bacteria in which sehAB is strongly activated are predominantly localized in the mesenteric lymph nodes. In addition, sehAB was shown to be important for the survival of Salmonella in these peripheral lymphoid organs. These data indicate that the transient activation of a type II TA module can bring a selective advantage favouring virulence and demonstrate that TA modules are engaged in Salmonella pathogenesis.

A moderate toxin, GraT, modulates growth rate and stress tolerance of Pseudomonas putida

Chromosomal toxin-antitoxin (TA) systems are widespread among free-living bacteria and are supposedly involved in stress tolerance. Here, we report the first TA system identified in the soil bacterium Pseudomonas putida. The system, encoded by the loci PP1586-PP1585, is conserved in pseudomonads and belongs to the HigBA family. The new TA pair was named GraTA for the growth rate-affecting ability of GraT and the antidote activity of GraA. The GraTA system shares many features common to previously described type II TA systems. The overexpression of GraT is toxic to the antitoxin deletion mutants, since the toxin's neutralization is achieved by binding of the antitoxin. Also, the graTA operon structure and autoregulation by antitoxin resemble those of other TA loci. However, we were able to delete the antitoxin gene from the chromosome, which shows the unusually mild toxicity of innate GraT compared to previously described toxins. Furthermore, GraT is a temperature-dependent toxin, as its growth-regulating effect becomes more evident at lower temperatures. Besides affecting the growth rate, GraT also increases membrane permeability, resulting in higher sensitivity to some chemicals, e.g., NaCl and paraquat. Nevertheless, the active toxin helps the bacteria survive under different stressful conditions and increases their tolerance to several antibiotics, including streptomycin, kanamycin, and ciprofloxacin. Therefore, our data suggest that GraT may represent a new class of mild chromosomal regulatory toxins that have evolved to be less harmful to their host bacterium. Their moderate toxicity might allow finer growth and metabolism regulation than is possible with strong growth-arresting or bactericidal toxins.

Crystallization of the HigBA2 toxin-antitoxin complex from Vibrio cholerae

The genome of Vibrio cholerae encodes two higBA toxin-antitoxin (TA) modules that are activated by amino-acid starvation. Here, the TA complex of the second module, higBA2, as well as the C-terminal domain of the corresponding HigA2 antitoxin, have been purified and crystallized. The HigBA2 complex crystallized in two crystal forms. Crystals of form I belonged to space group P2(1)2(1)2, with unit-cell parameters a = 129.0, b = 119.8, c = 33.4 Å, and diffracted to 3.0 Å resolution. The asymmetric unit is likely to contain a single complex consisting of two toxin monomers and one antitoxin dimer. The second crystal form crystallized in space group P3(2)21, with unit-cell parameters a = 134.5, c = 55.4 Å. These crystals diffracted to 2.2 Å resolution and probably contain a complex with a different stoichiometry. Crystals of the C-terminal domain of HigA2 belonged to space group C2, with unit-cell parameters a = 115.4, b = 61.2, c = 73.8 Å, β = 106.7°, and diffracted to 1.8 Å resolution.

Induced ectopic expression of HigB toxin in Mycobacterium tuberculosis results in growth inhibition, reduced abundance of a subset of mRNAs and cleavage of tmRNA

In Mycobacterium tuberculosis, the genes Rv1954A-Rv1957 form an operon that includes Rv1955 and Rv1956 which encode the HigB toxin and the HigA antitoxin respectively. We are interested in the role and regulation of this operon, since toxin-antitoxin systems have been suggested to play a part in the formation of persister cells in mycobacteria. To investigate the function of the higBA locus, effects of toxin expression on mycobacterial growth and transcript levels were assessed in M. tuberculosis H37Rv wild type and in an operon deletion background. We show that expression of HigB toxin in the absence of HigA antitoxin arrests growth and causes cell death in M. tuberculosis. We demonstrate HigB expression to reduce the abundance of IdeR and Zur regulated mRNAs and to cleave tmRNA in M. tuberculosis, Escherichia coli and Mycobacterium smegmatis. This study provides the first identification of possible target transcripts of HigB in M. tuberculosis.

Characterization of the phd-doc and ccd toxin-antitoxin cassettes from Vibrio superintegrons

Toxin-antitoxin (TA) systems have been reported in the genomes of most bacterial species, and their role when located on the chromosome is still debated. TA systems are particularly abundant in the massive cassette arrays associated with chromosomal superintegrons (SI). Here, we describe the characterization of two superintegron cassettes encoding putative TA systems. The first is the phd-doc(SI) system identified in Vibrio cholerae N16961. We determined its distribution in 36 V. cholerae strains and among five V. metschnikovii strains. We show that this cassette, which is in position 72 of the V. cholerae N16961 cassette array, is functional, carries its own promoter, and is expressed from this location. Interestingly, the phd-doc(SI) system is unable to control its own expression, most likely due to the absence of any DNA-binding domain on the antitoxin. In addition, this SI system is able to cross talk with the canonical P1 phage system. The second cassette that we characterized is the ccd(Vfi) cassette found in the V. fischeri superintegron. We demonstrate that CcdB(Vfi) targets DNA-gyrase, as the canonical CcB(F) toxin, and that ccd(Vfi) regulates its expression in a fashion similar to the ccd(F) operon of the conjugative plasmid F. We also establish that this cassette is functional and expressed in its chromosomal context in V. fischeri CIP 103206T. We tested its functional interactions with the ccdAB(F) system and found that CcdA(Vfi) is specific for its associated CcdB(Vfi) and cannot prevent CcdB(F) toxicity. Based on these results, we discuss the possible biological functions of these TA systems in superintegrons.

Structural overview of toxin-antitoxin systems in infectious bacteria: a target for developing antimicrobial agents

The bacterial toxin-antitoxin (TA) system is a module that may play a role in cell survival under stress conditions. Generally, toxin molecules act as negative regulators in cell survival and antitoxin molecules as positive regulators. Thus, the expression levels and interactions between toxins and antitoxins should be systematically harmonized so that bacteria can escape such harmful conditions. Since TA systems are able to control the fate of bacteria, they are considered potent targets for the development of new antimicrobial agents. TA systems are widely prevalent with a variety of systems existing in bacteria: there are three types of bacterial TA systems depending on the property of the antitoxin which binds either the protein toxin or mRNA coding the toxin protein. Moreover, the multiplicity of TA genes has been observed even in species of bacteria. Therefore, knowledge on TA systems such as the individual characteristics of TA systems, integrative working mechanisms of various TA systems in bacteria, interactions between toxin molecules and cellular targets, and so on is currently limited due to their complexity. In this regard, it would be helpful to know the structural characteristics of TA modules for understanding TA systems in bacteria. Until now, 85 out of the total structures deposited in PDB have been bacterial TA system proteins including TA complexes or isolated toxins/antitoxins. Here, we summarized the structural information of TA systems and analyzed the structural characteristics of known TA modules from several bacteria, especially focusing on the TA modules of several infectious bacteria.

Identification of type II toxin-antitoxin modules in Burkholderia pseudomallei

Type II toxin-antitoxin (TA) systems are believed to be widely distributed amongst bacteria although their biological functions are not clear. We have identified eight candidate TA systems in the genome of the human pathogen Burkholderia pseudomallei. Five of these were located in genome islands. Of the candidate toxins, BPSL0175 (RelE1) or BPSS1060 (RelE2) caused growth to cease when expressed in Escherichia coli, whereas expression of BPSS0390 (HicA) or BPSS1584 (HipA) (in an E. coli ΔhipBA background) caused a reduction in the number of culturable bacteria. The cognate antitoxins could restore growth and culturability of cells.

Chromosomal bacterial type II toxin-antitoxin systems

Most prokaryotic chromosomes contain a number of toxin-antitoxin (TA) modules consisting of a pair of genes that encode 2 components, a stable toxin and its cognate labile antitoxin. TA systems are also known as addiction modules, since the cells become "addicted" to the short-lived antitoxin product (the unstable antitoxin is degraded faster than the more stable toxin) because its de novo synthesis is essential for their survival. While toxins are always proteins, antitoxins are either RNAs (type I, type III) or proteins (type II). Type II TA systems are widely distributed throughout the chromosomes of almost all free-living bacteria and archaea. The vast majority of type II toxins are mRNA-specific endonucleases arresting cell growth through the mechanism of RNA cleavage, thus preventing the translation process. The physiological role of chromosomal type II TA systems still remains the subject of debate. This review describes the currently known type II toxins and their characteristics. The different hypotheses that have been proposed to explain their role in bacterial physiology are also discussed.

Growth and translation inhibition through sequence-specific RNA binding by Mycobacterium tuberculosis VapC toxin

The Mycobacterium tuberculosis genome harbors an unusually large number of toxin-antitoxin (TA) modules. Curiously, over half of these are VapBC (virulence-associated protein) family members. Nonetheless, the cellular target, precise mode of action, and physiological role of the VapC toxins in this important pathogen remain unclear. To better understand the function of this toxin family, we studied the features and biochemical properties of a prototype M. tuberculosis VapBC TA system, vapBC-mt4 (Rv0596c-Rv0595c). VapC-mt4 expression resulted in growth arrest, a hallmark of all TA toxins, in Escherichia coli, Mycobacterium smegmatis, and M. tuberculosis. Its expression led to translation inhibition accompanied by a gradual decrease in the steady-state levels of several mRNAs. VapC-mt4 exhibited sequence-specific endoribonuclease activity on mRNA templates at ACGC and AC(A/U)GC sequences. However, the cleavage activity of VapC-mt4 was comparatively weak relative to the TA toxin MazF-mt1 (Rv2801c). Unlike other TA toxins, translation inhibition and growth arrest preceded mRNA cleavage, suggesting that the RNA binding property of VapC-mt4, not RNA cleavage, initiates toxicity. In support of this hypothesis, expression of VapC-mt4 led to an increase in the recovery of total RNA with time in contrast to TA toxins that inhibit translation via direct mRNA cleavage. Additionally, VapC-mt4 exhibited stable, sequence-specific RNA binding in an electrophoretic mobility shift assay. Finally, VapC-mt4 inhibited protein synthesis in a cell-free system without cleaving the corresponding mRNA. Therefore, the activity of VapC-mt4 is mechanistically distinct from other TA toxins because it appears to primarily inhibit translation through selective, stable binding to RNA.

Toxins-antitoxins: diversity, evolution and function

Genes for toxin-antitoxin (TA) complexes are widespread in prokaryote genomes, and species frequently possess tens of plasmid and chromosomal TA loci. The complexes are categorized into three types based on genetic organization and mode of action. The toxins universally are proteins directed against specific intracellular targets, whereas the antitoxins are either proteins or small RNAs that neutralize the toxin or inhibit toxin synthesis. Within the three types of complex, there has been extensive evolutionary shuffling of toxin and antitoxin genes leading to considerable diversity in TA combinations. The intracellular targets of the protein toxins similarly are varied. Numerous toxins, many of which are sequence-specific endoribonucleases, dampen protein synthesis levels in response to a range of stress and nutritional stimuli. Key resources are conserved as a result ensuring the survival of individual cells and therefore the bacterial population. The toxin effects generally are transient and reversible permitting a set of dynamic, tunable responses that reflect environmental conditions. Moreover, by harboring multiple toxins that intercede in protein synthesis in response to different physiological cues, bacteria potentially sense an assortment of metabolic perturbations that are channeled through different TA complexes. Other toxins interfere with the action of topoisomersases, cell wall assembly, or cytoskeletal structures. TAs also play important roles in bacterial persistence, biofilm formation and multidrug tolerance, and have considerable potential both as new components of the genetic toolbox and as targets for novel antibacterial drugs.

Toxin-antitoxin (TA) systems are prevalent and transcribed in clinical isolates of Pseudomonas aeruginosa and methicillin-resistant Staphylococcus aureus

The percentage of bacterial infections refractory to standard antibiotic treatments is steadily increasing. Among the most problematic hospital and community-acquired pathogens are methicillin-resistant Staphylococcus aureus (MRSA) and Pseudomonas aeruginosa (PA). One novel strategy proposed for treating infections of multidrug-resistant bacteria is the activation of latent toxins of toxin-antitoxin (TA) protein complexes residing within bacteria; however, the prevalence and identity of TA systems in clinical isolates of MRSA and PA has not been defined. We isolated DNA from 78 MRSA and 42 PA clinical isolates and used PCR to probe for the presence of various TA loci. Our results showed that the genes for homologs of the mazEF TA system in MRSA and the relBE and higBA TA systems in PA were present in 100% of the respective strains. Additionally, reverse transcriptase PCR analysis revealed that these transcripts are produced in the clinical isolates. These results indicate that TA genes are prevalent and transcribed within MRSA and PA and suggest that activation of the toxin proteins could be an effective antibacterial strategy for these pathogens.

Characterization of a functional toxin-antitoxin module in the genome of the fish pathogen Piscirickettsia salmonis

This is the first report of a functional toxin-antitoxin (TA) locus in Piscirickettsia salmonis. The P. salmonis TA operon (ps-Tox-Antox) is an autonomous genetic unit containing two genes, a regulatory promoter site and an overlapping putative operator region. The ORFs consist of a toxic ps-Tox gene (P. salmonis toxin) and its upstream partner ps-Antox (P. salmonis antitoxin). The regulatory promoter site contains two inverted repeat motifs between the -10 and -35 regions, which may represent an overlapping operator site, known to mediate transcriptional auto-repression in most TA complexes. The Ps-Tox protein contains a PIN domain, normally found in prokaryote TA operons, especially those of the VapBC and ChpK families. The expression in Escherichia coli of the ps-Tox gene results in growth inhibition of the bacterial host confirming its toxicity, which is neutralized by coexpression of the ps-Antox gene. Additionally, ps-Tox is an endoribonuclease whose activity is inhibited by the antitoxin. The bioinformatic modelling of the two putative novel proteins from P. salmonis matches with their predicted functional activity and confirms that the active site of the Ps-Tox PIN domain is conserved.

Evidence for the role of horizontal transfer in generating pVT1, a large mosaic conjugative plasmid from the clam pathogen, Vibrio tapetis

The marine bacterium Vibrio tapetis is the causative agent of the brown ring disease, which affects the clam Ruditapes philippinarum and causes heavy economic losses in North of Europe and in Eastern Asia. Further characterization of V. tapetis isolates showed that all the investigated strains harbored at least one large plasmid. We determined the sequence of the 82,266 bp plasmid pVT1 from the CECT4600(T) reference strain and analyzed its genetic content. pVT1 is a mosaic plasmid closely related to several conjugative plasmids isolated from Vibrio vulnificus strains and was shown to be itself conjugative in Vibrios. In addition, it contains DNA regions that have similarity with several other plasmids from marine bacteria (Vibrio sp., Shewanella sp., Listonella anguillarum and Photobacterium profundum). pVT1 contains a number of mobile elements, including twelve Insertion Sequences or inactivated IS genes and an RS1 phage element related to the CTXphi phage of V. cholerae. The genetic organization of pVT1 underscores an important role of horizontal gene transfer through conjugative plasmid shuffling and transposition events in the acquisition of new genetic resources and in generating the pVT1 modular organization. In addition, pVT1 presents a copy number of 9, relatively high for a conjugative plasmid, and appears to belong to a new type of replicon, which may be specific to Vibrionaceae and Shewanelleacae.

Recent advancements in toxin and antitoxin systems involved in bacterial programmed cell death

Programmed cell death (PCD) systems have been extensively studied for their significant role in a variety of biological processes in eukaryotic organisms. Recently, more and more researches have revealed the existence of similar systems employed by bacteria in response to various environmental stresses. This paper summarized the recent researching advancements in toxin/antitoxin systems located on plasmids or chromosomes and their regulatory roles in bacterial PCD. The most studied yet disputed mazEF system was discussed in depth, and possible roles and status of such a special bacterial death and TA systems were also reviewed from the ecological and evolutionary perspectives.