The AraC Negative Regulator family modulates the activity of histone-like proteins in pathogenic bacteria

A Mini-Review of Enteroaggregative Escherichia coli with a Specific Target on the Virulence Factors Controlled by the AggR Master Regulator

Enteroaggregative Escherichia coli (EAEC) strains have been linked to several outbreaks of severe diarrhea around the world, and this bacterium is now commonly resistant to antibiotics. As part of the pathophysiology of EAEC, the characteristic pattern of adherence looks like stacked bricks on the intestinal epithelium. This phenotype depends on an aggregative adhesion plasmid (pAA), which codes for a regulatory protein named AggR. The AggR protein is a master regulator that transcriptionally actives the main virulence genes in this E. coli pathotype, such as those that encode the aggregative adhesion fimbriae, dispersin and its secretion apparatus, Aar regulatory protein, and type VI secretion system. Several reports have shown that AggR positively affects most EAEC virulence genes, functioning as a classic transcriptional activator in the promoter region of these genes, interacting with the RNA polymerase. This minireview article integrates the information about virulence determinants of EAEC controlled by the AggR regulator.

Highly-conserved regulatory activity of the ANR family in the virulence of diarrheagenic bacteria through interaction with master and global regulators

ANR (AraC negative regulators) are a novel class of small regulatory proteins commonly found in enteric pathogens. Aar (AggR-activated regulator), the best-characterized member of the ANR family, regulates the master transcriptional regulator of virulence AggR and the global regulator HNS in enteroaggregative Escherichia coli (EAEC) by protein-protein interactions. On the other hand, Rnr (RegA-negative regulator) is an ANR homolog identified in attaching and effacing (AE) pathogens, including Citrobacter rodentium and enteropathogenic Escherichia coli (EPEC), sharing only 25% identity with Aar. We previously found that C. rodentium lacking Rnr exhibits prolonged shedding and increased gut colonization in mice compared to the parental strain. To gain mechanistic insights into this phenomenon, we characterized the regulatory role of Rnr in the virulence of prototype EPEC strain E2348/69 by genetic, biochemical, and human organoid-based approaches. Accordingly, RNA-seq analysis revealed more than 500 genes differentially regulated by Rnr, including the type-3 secretion system (T3SS). The abundance of EspA and EspB in whole cells and bacterial supernatants confirmed the negative regulatory activity of Rnr on T3SS effectors. We found that besides HNS and Ler, twenty-six other transcriptional regulators were also under Rnr control. Most importantly, the deletion of aar in EAEC or rnr in EPEC increases the adherence of these pathogens to human intestinal organoids. In contrast, the overexpression of ANR drastically reduces bacterial adherence and the formation of AE lesions in the intestine. Our study suggests a conserved regulatory mechanism and a central role of ANR in modulating intestinal colonization by these enteropathogens despite the fact that EAEC and EPEC evolved with utterly different virulence programs.

Bacteroides faecium sp. nov. isolated from human faeces

A novel bacterial strain, CBA7301^T, was isolated from human faeces and was characterised using a polyphasic taxonomic approach. A phylogenetic analysis based on 16S rRNA gene sequences revealed that CBA7301^T represented a member of the genus Bacteroides, in the family Bacteroidaceae. The similarity between the 16S rRNA gene sequence of CBA7301^T and that of its most closely related species, Bacteroides faecichinchillae JCM 17102^T, was 96.2 %, and the average nucleotide identity between these two strains was 77.9 %. The genome size was 6 782 182 bp, and the DNA G+C content was 42.5 mol%. Cells of CBA7301^T were Gram-stain-negative, strictly anaerobic and rod-shaped. The optimal growth of this organism occurred at 30-35 °C, pH 7.0 and 0.5 % (w/v) NaCl. The respiratory quinone was menaquinone 10. The predominant polar lipids were phosphatidylethanolamine, phospholipids and aminophospholipids. The major cellular fatty acid was anteiso-C_15 : 0. According to the results of the polyphasic taxonomic analysis, CBA7301^T represents a novel species of the genus Bacteroides, which we named Bacteroides faecium sp. nov. The type strain is CBA7301^T (=KCCM 43355^T=ATCC TSD-227^T).

Modulation of AggR levels reveals features of virulence regulation in enteroaggregative E. coli

Enteroaggregative Escherichia coli (EAEC) strains are one of the diarrheagenic pathotypes. EAEC strains harbor a virulence plasmid (pAA2) that encodes, among other virulence determinants, the aggR gene. The expression of the AggR protein leads to the expression of several virulence determinants in both plasmids and chromosomes. In this work, we describe a novel mechanism that influences AggR expression. Because of the absence of a Rho-independent terminator in the 3'UTR, aggR transcripts extend far beyond the aggR ORF. These transcripts are prone to PNPase-mediated degradation. Structural alterations in the 3'UTR result in increased aggR transcript stability, leading to increased AggR levels. We therefore investigated the effect of increased AggR levels on EAEC virulence. Upon finding the previously described AggR-dependent virulence factors, we detected novel AggR-regulated genes that may play relevant roles in EAEC virulence. Mutants exhibiting high AggR levels because of structural alterations in the aggR 3'UTR show increased mobility and increased pAA2 conjugation frequency. Furthermore, among the genes exhibiting increased fold change values, we could identify those of metabolic pathways that promote increased degradation of arginine, fatty acids and gamma-aminobutyric acid (GABA), respectively. In this paper, we discuss how the AggR-dependent increase in specific metabolic pathways activity may contribute to EAEC virulence.

Responses of Escherichia coli and Listeria monocytogenes to ozone treatment on non-host tomato: Efficacy of intervention and evidence of induced acclimation

Because of the continuous rise of foodborne illnesses caused by the consumption of raw fruits and vegetables, effective post-harvest anti-microbial strategies are necessary. The aim of this study was to evaluate the anti-microbial efficacy of ozone (O₃) against two common causes of fresh produce contamination, the Gram-negative Escherichia coli O157:H7 and Gram-positive Listeria monocytogenes, and to relate its effects to potential mechanisms of xenobiosis by transcriptional network modeling. The study on non-host tomato environment correlated the dose × time aspects of xenobiosis by examining the correlation between bacterial survival in terms of log-reduction and defense responses at the level of gene expression. In E. coli, low (1 μg O₃/g of fruit) and moderate (2 μg O₃/g of fruit) doses caused insignificant reduction in survival, while high dose (3 μg/g of fruit) caused significant reduction in survival in a time-dependent manner. In L. monocytogenes, moderate dose caused significant reduction even with short-duration exposure. Distinct responses to O₃ xenobiosis between E. coli and L. monocytogenes are likely related to differences in membrane and cytoplasmic structure and components. Transcriptome profiling by RNA-Seq showed that primary defenses in E. coli were attenuated after exposure to a low dose, while the responses at moderate dose were characterized by massive upregulation of pathogenesis and stress-related genes, which implied the activation of defense responses. More genes were downregulated during the first hour at high dose, with a large number of such genes getting significantly upregulated after 2 hr and 3 hr. This trend suggests that prolonged exposure led to potential adaptation. In contrast, massive downregulation of genes was observed in L. monocytogenes regardless of dose and exposure duration, implying a mechanism of defense distinct from that of E. coli. The nature of bacterial responses revealed by this study should guide the selection of xenobiotic agents for eliminating bacterial contamination on fresh produce without overlooking the potential risks of adaptation.

An update of the unceasingly growing and diverse AraC/XylS family of transcriptional activators

Transcriptional factors play an important role in gene regulation in all organisms, especially in Bacteria. Here special emphasis is placed in the AraC/XylS family of transcriptional regulators. This is one of the most abundant as many predicted members have been identified and more members are added because more bacterial genomes are sequenced. Given the way more experimental evidence has mounded in the past decades, we decided to update the information about this captivating family of proteins. Using bioinformatics tools on all the data available for experimentally characterized members of this family, we found that many members that display a similar functional classification can be clustered together and in some cases they have a similar regulatory scheme. A proposal for grouping these proteins is also discussed. Additionally, an analysis of surveyed proteins in bacterial genomes is presented. Altogether, the current review presents a panoramic view into this family and we hope it helps to stimulate future research in the field.

The Novel Streptococcal Transcriptional Regulator XtgS Negatively Regulates Bacterial Virulence and Directly Represses PseP Transcription

Streptococcus agalactiae (group B streptococcus [GBS]) has received continuous attention for its involvement in invasive infections and its broad host range. Transcriptional regulators have an important impact on bacterial adaptation to various environments. Research on transcriptional regulators will shed new light on GBS pathogenesis. In this study, we identified a novel XRE-family transcriptional regulator encoded on the GBS genome, designated XtgS. Our data demonstrate that XtgS inactivation significantly increases bacterial survival in host blood and animal challenge test, suggesting that it is a negative regulator of GBS pathogenicity. Further transcriptomic analysis and quantitative reverse transcription-PCR (qRT-PCR) mainly indicated that XtgS significantly repressed transcription of its upstream gene pseP Based on electrophoretic mobility shift and lacZ fusion assays, we found that an XtgS homodimer directly binds a palindromic sequence in the pseP promoter region. Meanwhile, the PseP and XtgS combination naturally coexists in diverse Streptococcus genomes and has a strong association with sequence type, serotype diversification and host adaptation of GBS. Therefore, this study reveals that XtgS functions as a transcriptional regulator that negatively affects GBS virulence and directly represses PseP expression, and it provides new insights into the relationships between transcriptional regulator and genome evolution.

Characterization of a novel AraC/XylS-regulated family of N-acyltransferases in pathogens of the order Enterobacterales

Enteroaggregative Escherichia coli (EAEC) is a diarrheagenic pathotype associated with traveler’s diarrhea, foodborne outbreaks and sporadic diarrhea in industrialized and developing countries. Regulation of virulence in EAEC is mediated by AggR and its negative regulator Aar. Together, they control the expression of at least 210 genes. On the other hand, we observed that about one third of Aar-regulated genes are related to metabolism and transport. In this study we show the AggR/Aar duo controls the metabolism of lipids. Accordingly, we show that AatD, encoded in the AggR-regulated aat operon (aatPABCD) is an N-acyltransferase structurally similar to the essential Apolipoprotein N-acyltransferase Lnt and is required for the acylation of Aap (anti-aggregation protein). Deletion of aatD impairs post-translational modification of Aap and causes its accumulation in the bacterial periplasm. trans-complementation of 042aatD mutant with the AatD homolog of ETEC or with the N-acyltransferase Lnt reestablished translocation of Aap. Site-directed mutagenesis of the E207 residue in the putative acyltransferase catalytic triad disrupted the activity of AatD and caused accumulation of Aap in the periplasm due to reduced translocation of Aap at the bacterial surface. Furthermore, Mass spectroscopy revealed that Aap is acylated in a putative lipobox at the N-terminal of the mature protein, implying that Aap is a lipoprotein. Lastly, deletion of aatD impairs bacterial colonization of the streptomycin-treated mouse model. Our findings unveiled a novel N-acyltransferase family associated with bacterial virulence, and that is tightly regulated by AraC/XylS regulators in the order Enterobacterales.

Although the regulatory scheme of AggR is well understood, the biological relevance of half of AggR-regulated proteins remains unknown. In this study we provide experimental evidence that the AggR-regulated AatD is a novel N-acyltransferase restricted to pathogens of the order Enterobacterales, including EAEC, ETEC, Yersinia sp., and C. rodentium. AatD is structurally similar to Lnt. However, unlike Lnt which is essential for cellular functions, AatD is a dedicated N-acyltransferase required for post-translational modification of virulence factors. Aap was identified as a lipoprotein acylated by AatD. Lipid modification in Aap provides an important post-translational mechanism to regulate the trafficking, stability and subcellular localization of Aap. In the absence of AatD, Aap is retained in the periplasmic space and cannot be translocated to the bacterial surface, presumably, restricting the biological function of the protein. Our data suggest that AggR and Aar virulence regulators, not only regulate the expression of Aap virulence factor at the transcriptional level, but also regulate translocation of Aap to the bacterial surface, which is required for full virulence of EAEC, unveiling an important mechanism of virulence regulation.

The Ethanolamine-Sensing Transcription Factor EutR Promotes Virulence and Transmission during Citrobacter rodentium Intestinal Infection

Enteric pathogens exploit chemical and nutrient signaling to gauge their location within a host and control expression of traits important for infection. Ethanolamine-containing molecules are essential in host physiology and play important roles in intestinal processes. The transcription factor EutR is conserved in the Enterobacteriaceae and is required for ethanolamine sensing and metabolism. In enterohemorrhagic Escherichia coli (EHEC) O157:H7, EutR responds to ethanolamine to activate expression of traits required for host colonization and disease; however, the importance of EutR to EHEC intestinal infection has not been examined. Because EHEC does not naturally colonize or cause disease in mice, we employed the natural murine pathogen Citrobacter rodentium as a model of EHEC virulence to investigate the importance of EutR in vivo EHEC and C. rodentium possess the locus of enterocyte effacement (LEE), which is the canonical virulence trait of attaching and effacing pathogens. Our findings demonstrate that ethanolamine sensing and EutR-dependent regulation of the LEE are conserved in C. rodentium Moreover, during infection, EutR is required for maximal LEE expression, colonization, and transmission efficiency. These findings reveal that EutR not only is important for persistence during the primary host infection cycle but also is required for maintenance in a host population.

Xenobiotic Effects of Chlorine Dioxide to Escherichia coli O157:H7 on Non-host Tomato Environment Revealed by Transcriptional Network Modeling: Implications to Adaptation and Selection

Escherichia coli serotype O157:H7 is one of the major agents of pathogen outbreaks associated with fresh fruits and vegetables. Gaseous chlorine dioxide (ClO₂) has been reported to be an effective intervention to eliminate bacterial contamination on fresh produce. Although remarkable positive effects of low doses of ClO₂ have been reported, the genetic regulatory machinery coordinating the mechanisms of xenobiotic effects and the potential bacterial adaptation remained unclear. This study examined the temporal transcriptome profiles of E. coli O157:H7 during exposure to different doses of ClO₂ in order to elucidate the genetic mechanisms underlying bacterial survival under such harsh conditions. Dosages of 1 μg, 5 μg, and 10 μg ClO₂ per gram of tomato fruits cause different effects with dose-by-time dynamics. The first hour of exposure to 1 μg and 5 μg ClO₂ caused only partial killing with significant growth reduction starting at the second hour, and without further significant reduction at the third hour. However, 10 μg ClO₂ exposure led to massive bacterial cell death at 1 h with further increase in cell death at 2 and 3 h. The first hour exposure to 1 μg ClO₂ caused activation of primary defense and survival mechanisms. However, the defense response was attenuated during the second and third hours. Upon treatment with 5 μg ClO₂, the transcriptional networks showed massive downregulation of pathogenesis and stress response genes at the first hour of exposure, with decreasing number of differentially expressed genes at the second and third hours. In contrast, more genes were further downregulated with exposure to 10 μg ClO₂ at the first hour, with the number of both upregulated and downregulated genes significantly decreasing at the second hour. A total of 810 genes were uniquely upregulated at the third hour at 10 μg ClO₂, suggesting that the potency of xenobiotic effects had led to potential adaptation. This study provides important knowledge on the possible selection of target molecules for eliminating bacterial contamination on fresh produce without overlooking potential risks of adaptation.

Dual Function of Aar, a Member of the New AraC Negative Regulator Family, in Escherichia coli Gene Expression

Enteroaggregative Escherichia coli (EAEC) is an E. coli pathotype associated with diarrhea and growth faltering. EAEC virulence gene expression is controlled by the autoactivated AraC family transcriptional regulator, AggR. AggR activates transcription of a large number of virulence genes, including Aar, which in turn acts as a negative regulator of AggR itself. Aar has also been shown to affect expression of E. coli housekeeping genes, including H-NS, a global regulator that acts at multiple promoters and silences AT-rich genes (such as those in the AggR regulon). Although Aar has been shown to bind both AggR and H-NS in vitro, functional significance of these interactions has not been shown in vivo In order to dissect this regulatory network, we removed the complex interdependence of aggR and aar by placing the genes under the control of titratable promoters. We measured phenotypic and genotypic changes on downstream genes in EAEC strain 042 and E. coli K-12 strain DH5α, which lacks the AggR regulon. In EAEC, we found that low expression of aar increases aafA fimbrial gene expression via H-NS; however, when aar is more highly expressed, it acts as a negative regulator via AggR. In DH5α, aar affected expression of E. coli genes in some cases via H-NS and in some cases independent of H-NS. Our data support the model that Aar interacts in concert with AggR, H-NS, and possibly other regulators and that these interactions are likely to be functionally significant in vivo.

Analysis of the Virulence Profile and Phenotypic Features of Typical and Atypical Enteroaggregative Escherichia coli (EAEC) Isolated From Diarrheal Patients in Brazil

Enteroaggregative Escherichia coli (EAEC) is an important agent of acute and persistent diarrhea in children and adults worldwide. Here we report a characterization of 220 EAEC isolates, 88.2% (194/220) of which were typical and 11.8% (26/220) were atypical, obtained from diarrheal patients during seven years (2010-2016) of epidemiological surveillance in Brazil. The majority of the isolates were assigned to phylogroups A (44.1%, 97/220) or B1 (21.4%, 47/220). The aggregative adherence (AA) pattern was detected in 92.7% (204/220) of the isolates, with six of them exhibiting AA concomitantly with a chain-like adherence pattern; and agg5A and agg4A were the most common adhesin-encoding genes, which were equally detected in 14.5% (32/220) of the isolates. Each of 12 virulence factor-encoding genes (agg4A, agg5A, pic, aap, aaiA, aaiC, aaiG, orf3, aar, air, capU, and shf) were statistically associated with typical EAEC (P < 0.05). The genes encoding the newly described aggregate-forming pili (AFP) searched (afpB, afpD, afpP, and afpA2), and/or its regulator (afpR), were exclusively detected in atypical EAEC (57.7%, 15/26), and showed a significant association with this subgroup of EAEC (P < 0.001). In conclusion, we presented an extensive characterization of the EAEC circulating in the Brazilian settings and identified the afp genes as putative markers for increasing the efficiency of atypical EAEC diagnosis.

Plasmid-chromosome cross-talks

Plasmids can be acquired by recipient bacteria at a significant cost while conferring them advantageous traits. To counterbalance the costs of plasmid carriage, both plasmids and host bacteria have developed a tight regulatory network that may involve a cross-talk between the chromosome and the plasmids. Although plasmid regulation by chromosomal regulators is generally well known, chromosome regulation by plasmid has been far less investigated. Yet, a growing number of studies have highlighted an impact of plasmids on their host bacteria. Here, we describe the plasmid-chromosome cross-talk from the plasmid point of view. We summarize data about the chromosomal adaptive mutations generated by plasmid carriage; the impact of the loss of a domesticated plasmid or the gain of a new plasmid. Then, we present the control of plasmid-encoded regulators on chromosomal gene expression. The involvement of regulators homologous to chromosome-encoded proteins is illustrated by the H-NS-like proteins, and by the Rap-Phr system. Finally, plasmid-specific regulators of chromosomal gene expression are presented, which highlight the involvement of transcription factors and sRNAs. A comprehensive analysis of the mechanisms that allow a given plasmid to impact the chromosome of bacterium will help to understand the tight cross-talk between plasmids and the chromosome.

Transcription of Bacterial Chromatin

Decades of research have probed the interplay between chromatin (genomic DNA associated with proteins and RNAs) and transcription by RNA polymerase (RNAP) in all domains of life. In bacteria, chromatin is compacted into a membrane-free region known as the nucleoid that changes shape and composition depending on the bacterial state. Transcription plays a key role in both shaping the nucleoid and organizing it into domains. At the same time, chromatin impacts transcription by at least five distinct mechanisms: (i) occlusion of RNAP binding; (ii) roadblocking RNAP progression; (iii) constraining DNA topology; (iv) RNA-mediated interactions; and (v) macromolecular demixing and heterogeneity, which may generate phase-separated condensates. These mechanisms are not mutually exclusive and, in combination, mediate gene regulation. Here, we review the current understanding of these mechanisms with a focus on gene silencing by H-NS, transcription coordination by HU, and potential phase separation by Dps. The myriad questions about transcription of bacterial chromatin are increasingly answerable due to methodological advances, enabling a needed paradigm shift in the field of bacterial transcription to focus on regulation of genes in their native state. We can anticipate answers that will define how bacterial chromatin helps coordinate and dynamically regulate gene expression in changing environments.

The Role of the AggR Regulon in the Virulence of the Shiga Toxin-Producing Enteroaggregative Escherichia coli Epidemic O104:H4 Strain in Mice

An O104:H4 Shiga toxin (Stx)-producing enteroaggregative Escherichia coli (EAEC) strain caused a large outbreak of bloody diarrhea and the hemolytic uremic syndrome in 2011. We previously developed an ampicillin (Amp)-treated C57BL/6 mouse model to measure morbidity (weight loss) and mortality of mice orally infected with the prototype Stx-EAEC strain C227-11. Here, we hypothesized that mice fed C227-11 cured of the pAA plasmid or deleted for individual genes on that plasmid would display reduced virulence compared to animals given the wild-type (wt) strain. C227-11 cured of the pAA plasmid or deleted for the known pAA-encoded virulence genes aggR, aggA, sepA, or aar were fed to Amp-treated C57BL/6 mice at doses of 10¹⁰–10¹¹CFU. Infected animals were then either monitored for morbidity and lethality for 28 days or euthanized to determine intestinal pathology and colonization levels at selected times. The pAA-cured, aggR, and aggA mutants of strain C227-11 all showed reduced colonization at various intestinal sites. However, the aggR mutant was the only mutant attenuated for virulence as it showed both reduced morbidity and mortality. The aar mutant showed increased expression of the aggregative adherence fimbriae (AAF) and caused greater systemic effects in infected mice when compared to the C227-11 wt strain. However, unexpectedly, both the aggA and aar mutants displayed increased weight loss compared to wt. The sepA mutant did not exhibit altered morbidity or mortality in the Amp-treated mouse model compared to wt. Our data suggest that the increased morbidity due to the aar mutant could possibly be via an effect on expression of an as yet unknown virulence-associated factor under AggR control.

Enteroaggregative Escherichia coli subclinical and clinical infections

PURPOSE OF REVIEW

The current review is to update the results on epidemiology, pathobiology, and genes related to virulence, clinical presentation, molecular diagnosis, antimicrobial resistance, and extraintestinal infection of enteroaggregative Escherichia coli (EAEC).

RECENT FINDINGS

EAEC subclinical infection was significantly associated with reduced length at 2 years of age and EAEC and coinfections were associated with reduced delta weight-for-length and weight-for-age z-scores in the first 6 months of age in the MAL-ED birth cohort study. EAEC was associated with malnutrition in children 6-24 months of age in prospective case-control studies in Bangladesh and Brazil. Virulence gene-based studies have suggested aggregative fimbriae II may be a major contributor to disease, whereas AggR-activated regulator a marker of less severe disease. The high ability of EAEC colonization likely exacerbates effects of other microbial virulence strategies. Molecular diagnosis has been useful for understanding EAEC burden, although different criteria may relate to different pathogenic outcomes.

SUMMARY

EAEC gained special interest in the past few years, especially due to association with growth decrements in children with subclinical infections and its important role as a copathogen. Understanding of EAEC pathogenesis advanced but further research is needed for elucidating both microbial and host factors influencing infection outcomes.

Evolution of Bacterial Global Modulators: Role of a Novel H-NS Paralogue in the Enteroaggregative Escherichia coli Strain 042

Bacterial genomes sometimes contain genes that code for homologues of global regulators, the function of which is unclear. In members of the family Enterobacteriaceae, cells express the global regulator H-NS and its paralogue StpA. In Escherichia coli, out of providing a molecular backup for H-NS, the role of StpA is poorly characterized. The enteroaggregative E. coli strain 042 carries, in addition to the hns and stpA genes, a third gene encoding an hns paralogue (hns2). We present in this paper information about its biological function. Transcriptomic analysis has shown that the H-NS2 protein targets a subset of the genes targeted by H-NS. Genes targeted by H-NS2 correspond mainly with horizontally transferred (HGT) genes and are also targeted by the Hha protein, a fine-tuner of H-NS activity. Compared with H-NS, H-NS2 expression levels are lower. In addition, H-NS2 expression exhibits specific features: it is sensitive to the growth temperature and to the nature of the culture medium. This novel H-NS paralogue is widespread within the Enterobacteriaceae. IMPORTANCE Global regulators such as H-NS play key relevant roles enabling bacterial cells to adapt to a changing environment. H-NS modulates both core and horizontally transferred (HGT) genes, but the mechanism by which H-NS can differentially regulate these genes remains to be elucidated. There are several instances of bacterial cells carrying genes that encode homologues of the global regulators. The question is what the roles of these proteins are. We noticed that the enteroaggregative E. coli strain 042 carries a new hitherto uncharacterized copy of the hns gene. We decided to investigate why this pathogenic E. coli strain requires an extra H-NS paralogue, termed H-NS2. In our work, we show that H-NS2 displays specific expression and regulatory properties. H-NS2 targets a subset of H-NS-specific genes and may help to differentially modulate core and HGT genes by the H-NS cellular pool.