Deletion of mtrC in Haemophilus ducreyi increases sensitivity to human antimicrobial peptides and activates the CpxRA regulon

Cathelicidins: Opportunities and Challenges in Skin Therapeutics and Clinical Translation

Cathelicidins are a group of cationic, amphipathic peptides that play a vital role in the innate immune response of many vertebrates, including humans. Produced by immune and epithelial cells, they serve as natural defenses against a wide range of pathogens, including bacteria, viruses, and fungi. In humans, the cathelicidin LL-37 is essential for wound healing, maintaining skin barrier integrity, and combating infections. Cathelicidins of different origins have shown potential in treating various skin conditions, including melanoma, acne, and diabetic foot ulcers. Despite their promising therapeutic potential, cathelicidins face significant challenges in clinical application. Many peptide-based therapies have failed in clinical trials due to unclear efficacy and safety concerns. Additionally, the emergence of bacterial resistance, which contradicts initial claims of non-resistance, further complicates their development. To successfully translate cathelicidins into effective clinical treatments, therefore, several obstacles must be addressed, including a better understanding of their mechanisms of action, sustainable large-scale production, optimized formulations for drug delivery and stability, and strategies to overcome microbial resistance. This review examines the current knowledge of cathelicidins and their therapeutic applications and discusses the challenges that hinder their clinical use and must be overcome to fully exploit their potential in medicine.

The role of bacterial transport systems in the removal of host antimicrobial peptides in Gram-negative bacteria

Antibiotic resistance is a global issue that threatens our progress in healthcare and life expectancy. In recent years, antimicrobial peptides (AMPs) have been considered as promising alternatives to the classic antibiotics. AMPs are potentially superior due to their lower rate of resistance development, since they primarily target the bacterial membrane ('Achilles' heel' of the bacteria). However, bacteria have developed mechanisms of AMP resistance, including the removal of AMPs to the extracellular space by efflux pumps such as the MtrCDE or AcrAB-TolC systems, and the internalization of AMPs to the cytoplasm by the Sap transporter, followed by proteolytic digestion. In this review, we focus on AMP transport as a resistance mechanism compiling all the experimental evidence for the involvement of efflux in AMP resistance in Gram-negative bacteria and combine this information with the analysis of the structures of the efflux systems involved. Finally, we expose some open questions with the aim of arousing the interest of the scientific community towards the AMPs-efflux pumps interactions. All the collected information broadens our understanding of AMP removal by efflux pumps and gives some clues to assist the rational design of AMP-derivatives as inhibitors of the efflux pumps.

The Potential of Human Peptide LL-37 as an Antimicrobial and Anti-Biofilm Agent

The rise in antimicrobial resistant bacteria threatens the current methods utilized to treat bacterial infections. The development of novel therapeutic agents is crucial in avoiding a post-antibiotic era and the associated deaths from antibiotic resistant pathogens. The human antimicrobial peptide LL-37 has been considered as a potential alternative to conventional antibiotics as it displays broad spectrum antibacterial and anti-biofilm activities as well as immunomodulatory functions. While LL-37 has shown promising results, it has yet to receive regulatory approval as a peptide antibiotic. Despite the strong antimicrobial properties, LL-37 has several limitations including high cost, lower activity in physiological environments, susceptibility to proteolytic degradation, and high toxicity to human cells. This review will discuss the challenges associated with making LL-37 into a viable antibiotic treatment option, with a focus on antimicrobial resistance and cross-resistance as well as adaptive responses to sub-inhibitory concentrations of the peptide. The possible methods to overcome these challenges, including immobilization techniques, LL-37 delivery systems, the development of LL-37 derivatives, and synergistic combinations will also be considered. Herein, we describe how combination therapy and structural modifications to the sequence, helicity, hydrophobicity, charge, and configuration of LL-37 could optimize the antimicrobial and anti-biofilm activities of LL-37 for future clinical use.

Interactions of the Skin Pathogen Haemophilus ducreyi With the Human Host

The obligate human pathogen Haemophilus ducreyi causes both cutaneous ulcers in children and sexually transmitted genital ulcers (chancroid) in adults. Pathogenesis is dependent on avoiding phagocytosis and exploiting the suppurative granuloma-like niche, which contains a myriad of innate immune cells and memory T cells. Despite this immune infiltrate, long-lived immune protection does not develop against repeated H. ducreyi infections—even with the same strain. Most of what we know about infectious skin diseases comes from naturally occurring infections and/or animal models; however, for H. ducreyi, this information comes from an experimental model of infection in human volunteers that was developed nearly three decades ago. The model mirrors the progression of natural disease and serves as a valuable tool to determine the composition of the immune cell infiltrate early in disease and to identify host and bacterial factors that are required for the establishment of infection and disease progression. Most recently, holistic investigation of the experimentally infected skin microenvironment using multiple “omics” techniques has revealed that non-canonical bacterial virulence factors, such as genes involved in central metabolism, may be relevant to disease progression. Thus, the immune system not only defends the host against H. ducreyi, but also dictates the nutrient availability for the invading bacteria, which must adapt their gene expression to exploit the inflammatory metabolic niche. These findings have broadened our view of the host-pathogen interaction network from considering only classical, effector-based virulence paradigms to include adaptations to the metabolic environment. How both host and bacterial factors interact to determine infection outcome is a current focus in the field. Here, we review what we have learned from experimental H. ducreyi infection about host-pathogen interactions, make comparisons to what is known for other skin pathogens, and discuss how novel technologies will deepen our understanding of this infection.

Impaired Efflux of the Siderophore Enterobactin Induces Envelope Stress in Escherichia coli

The Cpx response is one of several envelope stress responses that monitor and maintain the integrity of the gram-negative bacterial envelope. While several conditions that are known or predicted to generate misfolded inner membrane proteins activate the Cpx response, the molecular nature of the Cpx inducing cue is not yet known. Studies have demonstrated that mutation of multidrug efflux pumps activates the Cpx response in many gram-negative bacteria. In Vibrio cholerae, pathway activation is due to accumulation of the catechol siderophore vibriobactin. However, the mechanism by which the Cpx response is activated by mutation of efflux pumps in Escherichia coli remains unknown. Here we show that inhibition of efflux by deletion of tolC, the outer membrane channel of several multidrug efflux pumps, activates the Cpx response in E. coli as a result of impaired efflux of the siderophore enterobactin. Enterobactin accumulation in the tolC mutant reduces activity of the nicotinamide adenine dinucleotide (NADH) oxidation arm of the aerobic respiratory chain. However, the Cpx pathway remains active in the tolC mutant when either NADH dehydrogenase I, NADH dehydrogenase II, or cytochrome bo₃ is absent. Finally, we show that the Cpx response down-regulates transcription of the enterobactin biosynthesis operon. These results suggest that the Cpx response promotes adaptation to envelope stress in enteric bacteria that are exposed to iron-limited environments, which are rich in envelope-damaging compounds and conditions.

Mechanisms of sexually transmitted infection-induced inflammation in women: implications for HIV risk

INTRODUCTION

Globally, sexually transmitted infections (STI) affect >300 million people annually, and are a major cause of sexual and reproductive health complications in women. In this commentary, we describe how STIs interact with the immune and non-immune cells, both within and below the cervicovaginal mucosal barrier, to cause inflammation, which in turn has been associated with increased HIV acquisition risk.

DISCUSSION

STIs have a major impact on the female genital mucosa, which is an important biological and physical barrier that forms the first line of defence against invading microorganisms such as HIV. Pattern recognition of STI pathogens, by receptors expressed either on the cell surface or inside the cell, typically triggers inflammation at the mucosal barrier. The types of mucosal responses vary by STI, and can be asymptomatic or culminate in the formation of discharge, ulcers and/or warts. While the aim of this response is to clear the invading microbes, in many cases these responses are either evaded or cause pathology that impairs barrier integrity and increases HIV access to target cells in the sub-mucosa. In addition, innate responses to STIs can result in an increased number of immune cells, including those that are the primary targets of HIV, and may contribute to the association between STIs and increased susceptibility to HIV acquisition. Many of these cells are mediators of adaptive immunity, including tissue-resident cells that may also display innate-like functions. Bacterial vaginosis (BV) is another common cause of inflammation, and evidence for multiple interactions between BV, STIs and HIV suggest that susceptibility to these conditions should be considered in concert.

CONCLUSIONS

STIs and other microbes can induce inflammation in the genital tract, perturbing the normal robust function of the mucosal barrier against HIV. While the impact of STIs on the mucosal immune system and HIV acquisition is often under-appreciated, understanding their interactions of the infections with the immune responses play an important role in improving treatment and reducing the risk of HIV acquisition. The frequent sub-clinical inflammation associated with STIs underscores the need for better STI diagnostics to reverse the immunological consequences of infection.

Both MisR (CpxR) and MisS (CpxA) Are Required for Neisseria gonorrhoeae Infection in a Murine Model of Lower Genital Tract Infection

During infection, Neisseria gonorrhoeae senses and responds to stress; such responses may be modulated by MisRS (NGO0177 and NGO0176), a two-component system that is a homolog of CpxRA. In Escherichia coli, CpxRA senses and responds to envelope stress; CpxA is a sensor kinase/phosphatase for CpxR, a response regulator. When a cpxA mutant is grown in medium containing glucose, CpxR is phosphorylated by acetyl phosphate but cannot be dephosphorylated, resulting in constitutive activation. Kandler and coworkers (J. L. Kandler, C. L. Holley, J. L. Reimche, V. Dhulipala, J. T. Balthazar, A. Muszyński, R. W. Carlson, and W. M. Shafer, Antimicrob Agents Chemother 60:4690-4700, 2016, https://doi.org/10.1128/AAC.00823-16) showed that MisR (CpxR) is required for the maintenance of membrane integrity and resistance to antimicrobial peptides, suggesting a role in gonococcal survival in vivo Here, we evaluated the contributions of MisR and MisS (CpxA) to gonococcal infection in a murine model of cervicovaginal colonization and identified MisR-regulated genes using RNA sequencing (RNA-Seq). The deletion of misR or misS severely reduced the capacity of N. gonorrhoeae to colonize mice or maintain infection over a 7-day period and reduced microbial fitness after exposure to heat shock. Compared to the wild type (WT), the inactivation of misR identified 157 differentially regulated genes, most of which encoded putative envelope proteins. The inactivation of misS identified 17 differentially regulated genes compared to the WT and 139 differentially regulated genes compared to the misR mutant, 111 of which overlapped those differentially expressed in the comparison of the WT versus the misR mutant. These data indicate that an intact MisRS system is required for gonococcal infection of mice. Provided the MisR is constitutively phosphorylated in the misS mutant, the data suggest that controlled but not constitutive activation is required for gonococcal infection in mice.

The Vibrio cholerae VexGH RND Efflux System Maintains Cellular Homeostasis by Effluxing Vibriobactin

Resistance-nodulation-division (RND) superfamily efflux systems have been widely studied for their role in antibiotic resistance, but their native biological functions remain poorly understood. We previously showed that loss of RND-mediated efflux in Vibrio cholerae resulted in activation of the Cpx two-component regulatory system, which mediates adaptation to stress resulting from misfolded membrane proteins. Here, we investigated the mechanism linking RND-mediated efflux to the Cpx response. We performed transposon mutagenesis screening of RND-deficient V. cholerae to identify Cpx suppressors. Suppressor mutations mapped to genes involved in the biosynthesis of the catechol siderophore vibriobactin. We subsequently demonstrated that vibriobactin secretion is impaired in mutants lacking the VexGH RND efflux system and that impaired vibriobactin secretion is responsible for Cpx system activation, suggesting that VexGH secretes vibriobactin. This conclusion was bolstered by results showing that vexGH expression is induced by iron limitation and that vexH-deficient cells exhibit reduced fitness during growth under iron-limiting conditions. Our results support a model where VexGH contributes to cellular homeostasis by effluxing vibriobactin. In the absence of vexGH, retained vibriobactin appears to chelate iron from iron-rich components of the respiratory chain, with the deferrated proteins functioning to activate the Cpx response. Our collective results demonstrate that a native function of the V. cholerae VexGH RND efflux system is in vibriobactin secretion and that vibriobactin efflux is critical for maintenance of cellular homeostasis.IMPORTANCE RND efflux systems are ubiquitous Gram-negative transporters that play critical roles in antimicrobial resistance. In addition to antimicrobial resistance, RND transporters also affect the expression of diverse phenotypes, including virulence, cell metabolism, and stress responses. The latter observations suggest that RND transporters fulfill unknown physiological functions in the cell independently of their role in antimicrobial resistance. Vibrio cholerae is representative of many Gram-negative bacteria in encoding multiple RND transporters that are redundant in antimicrobial resistance and affect multiple phenotypes. Here we describe a novel function of the V. cholerae VexGH RND transporter in vibriobactin secretion. We show that vibriobactin production in VexGH-deficient cells impacts cell homeostasis, leading to activation of the Cpx stress response and reduced fitness under iron-limiting conditions. Our results highlight a native physiological function of an RND transporter and provide insight into the selective forces that maintain what was thought to be a redundant multidrug transporter.

The SapA Protein Is Involved in Resistance to Antimicrobial Peptide PR-39 and Virulence of Actinobacillus pleuropneumoniae

Antimicrobial peptides are essential to the innate immune defense of the mammal against bacterial infection. However, pathogenic bacteria have evolved multiple strategies to resist and evade antimicrobial peptides, which is vital to bacterial survival and colonization in hosts. PR-39 is a linear porcine antimicrobial peptide containing 39 amino acid residues with a high proline content. Resistance to antimicrobial peptide PR-39 has been observed in Actinobacillus pleuropneumoniae. However, little is known about the factors required for this resistance. In the present study, PR-39 exposure increased the expression of the sapA gene in A. pleuropneumoniae. The sapA gene, which encodes a putative peptide transport periplasmic protein, was deleted from this bacterium. The ΔsapA mutant showed increased sensitivity to PR-39 compared to the wild-type MD12 and complemented PΔsapA strains. However, the ΔsapA mutant did not exhibit any alterations in outer membrane integrity. Scanning electron microscopy showed that the ΔsapA mutant displayed morphological defects, as indicated by a deformed and sunken shape after PR-39 treatment. In addition, disruption of the SapA protein led to reduced colonization and attenuated virulence of A. pleuropneumoniae in the BALB/c mouse model. Collectively, these data suggest that SapA acts as one mechanism for A. pleuropneumoniae to counteract PR-39-mediated killing. To the best of our knowledge, this is the first study to show a mechanism underlying antimicrobial peptide resistance in A. pleuropneumoniae.

Cross-resistance to human cationic antimicrobial peptides and to polymyxins mediated by the plasmid-encoded MCR-1?

OBJECTIVES

To evaluate whether acquired resistance to cationic antimicrobial peptide (CAMP) group molecules, being normal components of the human immune system, may select co-resistance to antibiotic peptides such as polymyxins, considering they share the same mechanism of action. We aimed to evaluate strains producing the recently identified plasmid-encoded polymyxin resistance determinant MCR-1, which is a phosphoethanolamine transferase that modifies the lipopolysaccharide structure of Gram-negative bacteria.

METHODS

In vitro susceptibility studies were performed using human CAMPs, namely cathelicidin LL-37, α-defensin 5 (HD5), and β-defensin 3 (HDB3), towards MCR-1-producing and colistin-resistant Escherichia coli or Klebsiella pneumoniae.

RESULTS

Cross-resistance to CAMPs and colistin mediated by MCR-1 or chromosomal mechanisms was neither observed in E. coli nor in K. pneumoniae.

CONCLUSION

Future therapeutic development of human CAMPs is not likely to be impeded by the spread of MCR-1 plasmid-mediated resistance to polymyxins, at least in E. coli.

Bacterial Evasion of Host Antimicrobial Peptide Defenses

Antimicrobial peptides (AMPs), also known as host defense peptides, are small naturally occurring microbicidal molecules produced by the host innate immune response that function as a first line of defense to kill pathogenic microorganisms by inducing deleterious cell membrane damage. AMPs also possess signaling and chemoattractant activities and can modulate the innate immune response to enhance protective immunity or suppress inflammation. Human pathogens have evolved defense molecules and strategies to counter and survive the AMPs released by host immune cells such as neutrophils and macrophages. Here, we review the various mechanisms used by human bacterial pathogens to resist AMP-mediated killing, including surface charge modification, active efflux, alteration of membrane fluidity, inactivation by proteolytic digestion, and entrapment by surface proteins and polysaccharides. Enhanced understanding of AMP resistance at the molecular level may offer insight into the mechanisms of bacterial pathogenesis and augment the discovery of novel therapeutic targets and drug design for the treatment of recalcitrant multidrug-resistant bacterial infections.

Role of the Gram-Negative Envelope Stress Response in the Presence of Antimicrobial Agents

Bacterial survival necessitates endurance of many types of antimicrobial compound. Many Gram-negative envelope stress responses, which must contend with an outer membrane and a dense periplasm containing the cell wall, have been associated with the status of protein folding, membrane homeostasis, and physiological functions such as efflux and the proton motive force (PMF). In this review, we discuss evidence that indicates an emerging role for Gram-negative envelope stress responses in enduring exposure to diverse antimicrobial substances, focusing on recent studies of the γ-proteobacterial Cpx envelope stress response.

Mucosal physical and chemical innate barriers: Lessons from microbial evasion strategies

The innate immune system has evolved since millions of years under a selective pressure. Among the different host mechanisms selected and conserved as a first line of defense, the gastrointestinal mucus layer constitutes an efficient physical and chemical barrier against invading microbes. Mucin glycoproteins and antimicrobial peptides are the major components of the mucus barrier, and evidences prove that they form an effective protection against most microbes. However, successful pathogens have evolved evasion strategies to circumvent this defense barrier. Here, we discuss the interactions between pathogens, mucins, and antimicrobial peptides, and the mechanisms that pathogens have developed to evade the innate defense systems of the intestinal mucosal barrier.

Phosphoethanolamine Transferase LptA in Haemophilus ducreyi Modifies Lipid A and Contributes to Human Defensin Resistance In Vitro

Haemophilus ducreyi resists the cytotoxic effects of human antimicrobial peptides (APs), including α-defensins, β-defensins, and the cathelicidin LL-37. Resistance to LL-37, mediated by the sensitive to antimicrobial peptide (Sap) transporter, is required for H. ducreyi virulence in humans. Cationic APs are attracted to the negatively charged bacterial cell surface. In other gram-negative bacteria, modification of lipopolysaccharide or lipooligosaccharide (LOS) by the addition of positively charged moieties, such as phosphoethanolamine (PEA), confers AP resistance by means of electrostatic repulsion. H. ducreyi LOS has PEA modifications at two sites, and we identified three genes (lptA, ptdA, and ptdB) in H. ducreyi with homology to a family of bacterial PEA transferases. We generated non-polar, unmarked mutants with deletions in one, two, or all three putative PEA transferase genes. The triple mutant was significantly more susceptible to both α- and β-defensins; complementation of all three genes restored parental levels of AP resistance. Deletion of all three PEA transferase genes also resulted in a significant increase in the negativity of the mutant cell surface. Mass spectrometric analysis revealed that LptA was required for PEA modification of lipid A; PtdA and PtdB did not affect PEA modification of LOS. In human inoculation experiments, the triple mutant was as virulent as its parent strain. While this is the first identified mechanism of resistance to α-defensins in H. ducreyi, our in vivo data suggest that resistance to cathelicidin LL-37 may be more important than defensin resistance to H. ducreyi pathogenesis.

Development and validation of a high-throughput cell-based screen to identify activators of a bacterial two-component signal transduction system

CpxRA is a two-component signal transduction system (2CSTS) found in many drug-resistant Gram-negative bacteria. In response to periplasmic stress, CpxA autophosphorylates and donates a phosphoryl group to its cognate response regulator, CpxR. Phosphorylated CpxR (CpxR-P) upregulates genes involved in membrane repair and downregulates multiple genes that encode virulence factors, which are trafficked across the cell membrane. Mutants that constitutively activate CpxRA in Salmonella enterica serovar Typhimurium and Haemophilus ducreyi are avirulent in mice and humans, respectively. Thus, the activation of CpxRA has high potential as a novel antimicrobial/antivirulence strategy. Using a series of Escherichia coli strains containing a CpxR-P-responsive lacZ reporter and deletions in genes encoding CpxRA system components, we developed and validated a novel cell-based high-throughput screen (HTS) for CpxRA activators. A screen of 36,000 compounds yielded one hit compound that increased reporter activity in wild-type cells. This is the first report of a compound that activates, rather than inhibits, a 2CSTS. The activity profile of the compound against CpxRA pathway mutants in the presence of glucose suggested that the compound inhibits CpxA phosphatase activity. We confirmed that the compound induced the accumulation of CpxR-P in treated cells. Although the hit compound contained a nitro group, a derivative lacking this group retained activity in serum and had lower cytotoxicity than that of the initial hit. This HTS is amenable for the screening of larger libraries to find compounds that activate CpxRA by other mechanisms, and it could be adapted to find activators of other two-component systems.

On the in vivo significance of bacterial resistance to antimicrobial peptides

Antimicrobial peptides (AMPs) are at the front-line of host defense during infection and play critical roles both in reducing the microbial load early during infection and in linking innate to adaptive immunity. However, successful pathogens have developed mechanisms to resist AMPs. Although considerable progress has been made in elucidating AMP-resistance mechanisms of pathogenic bacteria in vitro, less is known regarding the in vivo significance of such resistance. Nevertheless, progress has been made in this area, largely by using murine models and, in two instances, human models of infection. Herein, we review progress on the use of in vivo infection models in AMP research and discuss the AMP resistance mechanisms that have been established by in vivo studies to contribute to microbial infection. We posit that in vivo infection models are essential tools for investigators to understand the significance to pathogenesis of genetic changes that impact levels of bacterial susceptibility to AMPs. This article is part of a Special Issue entitled: Bacterial Resistance to Antimicrobial Peptides.

Host Antimicrobial Peptides in Bacterial Homeostasis and Pathogenesis of Disease

Innate immune responses function as a first line of host defense against the development of bacterial infection, and in some cases to preserve the sterility of privileged sites in the human host. Bacteria that enter these sites must counter host responses for colonization. From the host's perspective, the innate immune system works expeditiously to minimize the bacterial threat before colonization and subsequent dysbiosis. The multifactorial nature of disease further challenges predictions of how each independent variable influences bacterial pathogenesis. From bacterial colonization to infection and through disease, the microenvironments of the host are in constant flux as bacterial and host factors contribute to changes at the host-pathogen interface, with the host attempting to eradicate bacteria and the bacteria fighting to maintain residency. A key component of this innate host response towards bacterial infection is the production of antimicrobial peptides (AMPs). As an early component of the host response, AMPs modulate bacterial load and prevent establishment of infection. Under quiescent conditions, some AMPs are constitutively expressed by the epithelium. Bacterial infection can subsequently induce production of other AMPs in an effort to maintain sterility, or to restrict colonization. As demonstrated in various studies, the absence of a single AMP can influence pathogenesis, highlighting the importance of AMP concentration in maintaining homeostasis. Yet, AMPs can increase bacterial virulence through the co-opting of the peptides or alteration of bacterial virulence gene expression. Further, bacterial factors used to subvert AMPs can modify host microenvironments and alter colonization of the residential flora that principally maintain homeostasis. Thus, the dynamic interplay between host defense peptides and bacterial factors produced to quell peptide activity play a critical role in the progression and outcome of disease.

Reciprocal regulation of resistance-nodulation-division efflux systems and the Cpx two-component system in Vibrio cholerae

The Cpx two-component regulatory system has been shown in Escherichia coli to alleviate stress caused by misfolded cell envelope proteins. The Vibrio cholerae Cpx system was previously found to respond to cues distinct from those in the E. coli system, suggesting that this system fulfills a different physiological role in the cholera pathogen. Here, we used microarrays to identify genes that were regulated by the V. cholerae Cpx system. Our observations suggest that the activation of the V. cholerae Cpx system does not induce expression of genes involved in the mitigation of stress generated by misfolded cell envelope proteins but promotes expression of genes involved in antimicrobial resistance. In particular, activation of the Cpx system induced expression of the genes encoding the VexAB and VexGH resistance-nodulation-division (RND) efflux systems and their cognate outer membrane pore protein TolC. The promoters for these loci contained putative CpxR consensus binding sites, and ectopic cpxR expression activated transcription from the promoters for the RND efflux systems. CpxR was not required for intrinsic antimicrobial resistance, but CpxR activation enhanced resistance to antimicrobial substrates of VexAB and VexGH. Mutations that inactivated VexAB or VexGH efflux activity resulted in the activation of the Cpx response, suggesting that vexAB and vexGH and the cpxP-cpxRA system are reciprocally regulated. We speculate that the reciprocal regulation of the V. cholerae RND efflux systems and the Cpx two-component system is mediated by the intracellular accumulation of an endogenously produced metabolic by-product that is normally extruded from the cell by the RND efflux systems.

Everything old is new again: an update on current research on the Cpx envelope stress response

The Cpx envelope stress response (ESR) has been linked to proteins that are integrated into and secreted across the inner membrane for several decades. Initial studies of the cpx locus linked it to alterations in the protein content of both the inner and outer membrane, together with changes in proton motive driven transport and conjugation. Since the mid 1990s, the predominant view of the Cpx envelope stress response has been that it serves to detect and respond to secreted, misfolded proteins in the periplasm. Recent studies in Escherichia coli and other Gram negative organisms highlight a role for the Cpx ESR in specifically responding to perturbations that occur at the inner membrane (IM). It is clear that Cpx adaptation involves a broad suite of changes that encompass many functions in addition to protein folding. Interestingly, recent studies have refocused attention on Cpx-regulated phenotypes that were initially published over 30years ago, including antibiotic resistance and transport across the IM. In this review I will focus on the insights and models that have arisen from recent studies and that may help explain some of the originally published Cpx phenotypes. Although the molecular nature of the inducing signal for the Cpx ESR remains enigmatic, recently solved structures of signaling proteins are yielding testable models concerning the molecular mechanisms behind signaling. The identification of connections between the Cpx ESR and other stress responses in the cell reveals a complex web of interactions that involves Cpx-regulated expression of other regulators as well as small proteins and sRNAs. This article is part of a Special Issue entitled: Protein trafficking and secretion in bacteria. Guest Editors: Anastassios Economou and Ross Dalbey.

The Haemophilus ducreyi Fis protein is involved in controlling expression of the lspB-lspA2 operon and other virulence factors

Expression of the lspB-lspA2 operon encoding a virulence-related two-partner secretion system in Haemophilus ducreyi 35000HP is directly regulated by the CpxRA regulatory system (M. Labandeira-Rey, J. R. Mock, and E. J. Hansen, Infect. Immun. 77:3402-3411, 2009). In the present study, we show that this secretion system is also regulated by the small nucleoid-associated protein Fis. Inactivation of the H. ducreyi fis gene resulted in a reduction in expression of both the H. ducreyi LspB and LspA2 proteins. DNA microarray experiments showed that a H. ducreyi fis deletion mutant exhibited altered expression levels of genes encoding other important H. ducreyi virulence factors, including DsrA and Flp1, suggesting a possible global role for Fis in the control of virulence in this obligate human pathogen. While the H. ducreyi Fis protein has a high degree of sequence and structural similarity to the Fis proteins of other bacteria, its temporal pattern of expression was very different from that of enterobacterial Fis proteins. The use of a lacZ-based transcriptional reporter provided evidence which indicated that the H. ducreyi Fis homolog is a positive regulator of gyrB, a gene that is negatively regulated by Fis in enteric bacteria. Taken together, the Fis protein expression data and the observed regulatory effects of Fis in H. ducreyi suggest that this small DNA binding protein has a regulatory role in H. ducreyi which may differ in substantial ways from that of other Fis proteins.

Activation of CpxRA in Haemophilus ducreyi primarily inhibits the expression of its targets, including major virulence determinants

Haemophilus ducreyi causes chancroid, a genital ulcer disease that facilitates the transmission of human immunodeficiency virus type 1. In humans, H. ducreyi is surrounded by phagocytes and must adapt to a hostile environment to survive. To sense and respond to environmental cues, bacteria frequently use two-component signal transduction (2CST) systems. The only obvious 2CST system in H. ducreyi is CpxRA; CpxR is a response regulator, and CpxA is a sensor kinase. Previous studies by Hansen and coworkers showed that CpxR directly represses the expression of dsrA, the lspB-lspA2 operon, and the flp operon, which are required for virulence in humans. They further showed that CpxA functions predominantly as a phosphatase in vitro to maintain the expression of virulence determinants. Since a cpxA mutant is avirulent while a cpxR mutant is fully virulent in humans, CpxA also likely functions predominantly as a phosphatase in vivo. To better understand the role of H. ducreyi CpxRA in controlling virulence determinants, here we defined genes potentially regulated by CpxRA by using RNA-Seq. Activation of CpxR by deletion of cpxA repressed nearly 70% of its targets, including seven established virulence determinants. Inactivation of CpxR by deletion of cpxR differentially regulated few genes and increased the expression of one virulence determinant. We identified a CpxR binding motif that was enriched in downregulated but not upregulated targets. These data reinforce the hypothesis that CpxA phosphatase activity plays a critical role in controlling H. ducreyi virulence in vivo. Characterization of the downregulated genes may offer new insights into pathogenesis.

Characterization of an antimicrobial peptide produced by Bacillus subtilis subsp. spizezinii showing inhibitory activity towards Haemophilus parasuis

Haemophilus parasuis is the pathogen that causes Glässer's disease, a major illness affecting young pigs. The aim of this work was to investigate the antagonistic activity of antimicrobial substances produced by Bacillus species against H. parasuis. Among the tested strains, only Bacillus subtilis ATCC 6633 inhibited H. parasuis growth. The antibacterial substance was purified by ammonium sulfate precipitation, gel filtration chromatography on Sephadex G-50 and ion-exchange chromatography on DEAE-cellulose. The purification was about 100-fold with a yield of 0.33 %. The purified substance was resistant up to 80 °C and pH ranging 3-7, but the substance lost its activity when it was treated with proteases. The peptide had a molecular mass of 1083 Da and its sequence was determined by MS as NRWCFAGDD, which showed no homology with other known antimicrobial peptides. The complete inhibition of H. parasuis growth was observed at 20 µg peptide ml(-1) after 20 min of exposure. The peptide obtained by chemical synthesis also showed antimicrobial activity on H. parasuis. The identification of antimicrobial substances that can be effective against H. parasuis is very relevant to combat this pathogen that causes important losses in swine production.

Permeases of the sap transporter are required for cathelicidin resistance and virulence of Haemophilus ducreyi in humans

BACKGROUND

Haemophilus ducreyi encounters several classes of antimicrobial peptides (APs) in vivo and utilizes the sensitive-to-antimicrobial-peptides (Sap) transporter as one mechanism of AP resistance. A mutant lacking the periplasmic solute-binding component, SapA, was somewhat more sensitive to the cathelicidin LL-37 than the parent strain and was partially attenuated for virulence. The partial attenuation led us to question whether the transporter is fully abrogated in the sapA mutant.

METHODS

We generated a nonpolar sapBC mutant, which lacks both inner membrane permeases of the Sap transporter, and tested the mutant for virulence in human volunteers. In vitro, we compared LL-37 resistance phenotypes of the sapBC and sapA mutants.

RESULTS

Unlike the sapA mutant, the sapBC mutant was fully attenuated for virulence in human volunteers. In vitro, the sapBC mutant exhibited significantly greater sensitivity than the sapA mutant to killing by LL-37. Similar to the sapA mutant, the sapBC mutant did not affect H. ducreyi's resistance to human defensins.

CONCLUSIONS

Compared with the sapA mutant, the sapBC mutant exhibited greater attenuation in vivo, which directly correlated with increased sensitivity to LL-37 in vitro. These results strongly suggest that the SapBC channel retains activity when SapA is removed.

Resistance to antimicrobial peptides in Gram-negative bacteria

Antimicrobial peptides (AMPs) are present in virtually all organisms and are an ancient and critical component of innate immunity. In mammals, AMPs are present in phagocytic cells, on body surfaces such as skin and mucosa, and in secretions and body fluids such as sweat, saliva, urine, and breast milk, consistent with their role as part of the first line of defense against a wide range of pathogenic microorganisms including bacteria, viruses, and fungi. AMPs are microbicidal and have also been shown to act as immunomodulators with chemoattractant and signaling activities. During the co-evolution of hosts and bacterial pathogens, bacteria have developed the ability to sense and initiate an adaptive response to AMPs to resist their bactericidal activity. Here, we review the various mechanisms used by Gram-negative bacteria to sense and resist AMP-mediated killing. These mechanisms play an important role in bacterial resistance to host-derived AMPs that are encountered during the course of infection. Bacterial resistance to AMPs should also be taken into consideration in the development and use of AMPs as anti-infective agents, for which there is currently a great deal of academic and commercial interest.

On the evolution of the sexually transmitted bacteria Haemophilus ducreyi and Klebsiella granulomatis

Haemophilus ducreyi and Klebsiella (Calymmatobacterium) granulomatis are sexually transmitted bacteria that cause characteristic, persisting ulceration on external genitals called chancroid and granuloma inguinale, respectively. Those ulcers are endemic in developing countries or exist, as does granuloma inguinale, only in some geographic "hot spots."H. ducreyi is placed in the genus Haemophilus (family Pasteurellacae); however, this phylogenetic position is not obvious. The multiple ways in which the bacterium may be adapted to its econiche through specialized nutrient acquisitions; defenses against the immune system; and virulence factors that increase attachment, fitness, and persistence within genital tissue are discussed below. The analysis of K. granulomatis phylogeny demonstrated a high degree of identity with other Klebsiella species, and the name K. granulomatis comb. nov. was proposed. Because of the difficulty in growing this bacterium on artificial media, its characteristics have not been sufficiently defined. More studies are needed to understand bacterial genetics related to the pathogenesis and evolution of K. granulomatis.

Sap transporter mediated import and subsequent degradation of antimicrobial peptides in Haemophilus

Antimicrobial peptides (AMPs) contribute to host innate immune defense and are a critical component to control bacterial infection. Nontypeable Haemophilus influenzae (NTHI) is a commensal inhabitant of the human nasopharyngeal mucosa, yet is commonly associated with opportunistic infections of the upper and lower respiratory tracts. An important aspect of NTHI virulence is the ability to avert bactericidal effects of host-derived antimicrobial peptides (AMPs). The Sap (sensitivity to antimicrobial peptides) ABC transporter equips NTHI to resist AMPs, although the mechanism of this resistance has remained undefined. We previously determined that the periplasmic binding protein SapA bound AMPs and was required for NTHI virulence in vivo. We now demonstrate, by antibody-mediated neutralization of AMP in vivo, that SapA functions to directly counter AMP lethality during NTHI infection. We hypothesized that SapA would deliver AMPs to the Sap inner membrane complex for transport into the bacterial cytoplasm. We observed that AMPs localize to the bacterial cytoplasm of the parental NTHI strain and were susceptible to cytoplasmic peptidase activity. In striking contrast, AMPs accumulated in the periplasm of bacteria lacking a functional Sap permease complex. These data support a mechanism of Sap mediated import of AMPs, a novel strategy to reduce periplasmic and inner membrane accumulation of these host defense peptides.

The opportunistic pathogen Haemophilus influenzae is a normal inhabitant of the human nasopharynx, and is commonly implicated in respiratory tract infections, particularly of the middle ear (otitis media), sinuses, and lung (pneumonia, chronic obstructive pulmonary disease and cystic fibrosis). We have identified a multifunctional bacterial uptake system that is required for critical mechanisms of bacterial survival in the host. This Sap transporter system recognizes and transports host immune defense molecules and is involved in uptake of an iron-containing nutrient (heme) that is host-limited, yet required for bacterial growth and survival. We propose that bacteria utilize this, and likely other similar transport systems, for numerous functions that are important for bacterial survival in the host, including host immune evasion and metabolism. Our findings significantly advance our understanding of how single bacterial protein systems co-operate and coordinate multiple functions to equip bacteria to survive and cause disease in the hostile host environment. Our long-range goal is to block this uptake system thereby starving the bacterium of essential nutrients and also promoting clearance by the host immune response. Removal of this important bacterial survival mechanism will thwart the ability for Haemophilus to survive as a pathogen and thus decrease the incidence of disease development.

Rapid divergence of two classes of Haemophilus ducreyi

Haemophilus ducreyi, the etiologic agent of chancroid, expresses variants of several key virulence factors. While previous reports suggested that H. ducreyi strains formed two clonal populations, the differences between, and diversity within, these populations were unclear. To assess their variability, we examined sequence diversity at 11 H. ducreyi loci, including virulence and housekeeping genes, augmenting published data sets with PCR-amplified genes to acquire data for at least 10 strains at each locus. While sequences from all 11 loci place strains into two distinct groups, there was very little variation within each group. The difference between alleles of the two groups was variable and large at 3 loci encoding surface-exposed proteins (0.4 < K(S) < 1.3, where K(S) is divergence at synonymous sites) but consistently small at genes encoding cytoplasmic or periplasmic proteins (K(S) < 0.09). The data suggest that the two classes have recently diverged, that recombination has introduced variant alleles into at least 3 distinct loci, and that these alleles have been confined to one of the two classes. In addition, recombination is evident among alleles within, but not between, classes. Rather than clones of the same species, these properties indicate that the two classes may form distinct species.