Peptidoglycan recognition proteins kill bacteria by inducing oxidative, thiol, and metal stress

Targeting the permeability barrier and peptidoglycan recycling pathways to disarm Pseudomonas aeruginosa against the innate immune system

Antimicrobial resistance is a continuously increasing threat that severely compromises our antibiotic arsenal and causes thousands of deaths due to hospital-acquired infections by pathogens such as Pseudomonas aeruginosa, situation further aggravated by the limited development of new antibiotics. Thus, alternative strategies such as those targeting bacterial resistance mechanisms, virulence or potentiating the activity of our immune system resources are urgently needed. We have recently shown that mutations simultaneously causing the peptidoglycan recycling blockage and the β-lactamase AmpC overexpression impair the virulence of P.aeruginosa. These findings suggested that peptidoglycan metabolism might be a good target not only for fighting antibiotic resistance, but also for the attenuation of virulence and/or potentiation of our innate immune weapons. Here we analyzed the activity of the innate immune elements peptidoglycan recognition proteins (PGRPs) and lysozyme against P. aeruginosa. We show that while lysozyme and PGRPs have a very modest basal effect over P. aeruginosa, their bactericidal activity is dramatically increased in the presence of subinhibitory concentrations of the permeabilizing agent colistin. We also show that the P. aeruginosa lysozyme inhibitors seem to play a very residual protective role even in permeabilizing conditions. In contrast, we demonstrate that, once the permeability barrier is overpassed, the activity of lysozyme and PGRPs is dramatically enhanced when inhibiting key peptidoglycan recycling components (such as the 3 AmpDs, AmpG or NagZ), indicating a decisive protective role for cell-wall recycling and that direct peptidoglycan-binding supports, at least partially, the activity of these enzymes. Finally, we show that recycling blockade when occurring simultaneously with AmpC overexpression determines a further decrease in the resistance against PGRP2 and lysozyme, linked to quantitative changes in the cell-wall. Thus, our results help to delineate new strategies against P. aeruginosa infections, simultaneously targeting β–lactam resistance, cell-wall metabolism and virulence, ultimately enhancing the activity of our innate immune weapons.

Bactericidal peptidoglycan recognition protein induces oxidative stress in Escherichia coli through a block in respiratory chain and increase in central carbon catabolism

Mammalian Peptidoglycan Recognition Proteins (PGRPs) kill both Gram-positive and Gram-negative bacteria through simultaneous induction of oxidative, thiol and metal stress responses in bacteria. However, metabolic pathways through which PGRPs induce these bactericidal stress responses are unknown. We screened Keio collection of Escherichia coli deletion mutants and revealed that deleting genes for respiratory chain flavoproteins or for tricarboxylic acid (TCA) cycle resulted in increased resistance of E. coli to PGRP killing. PGRP-induced killing depended on the production of hydrogen peroxide, which required increased supply of NADH for respiratory chain oxidoreductases from central carbon catabolism (glycolysis and TCA cycle), and was controlled by cAMP-Crp. Bactericidal PGRP induced a rapid decrease in respiration, which suggested that the main source of increased production of hydrogen peroxide was a block in respiratory chain and diversion of electrons from NADH oxidoreductases to oxygen. CpxRA two-component system was a negative regulator of PGRP-induced oxidative stress. By contrast, PGRP-induced thiol stress (depletion of thiols) and metal stress (increase in intracellular free Zn²⁺ through influx of extracellular Zn²⁺ ) were mostly independent of oxidative stress. Thus, manipulating pathways that induce oxidative, thiol and metal stress in bacteria could be a useful strategy to design new approaches to antibacterial therapy.

Use of Wild Type or Recombinant Lactic Acid Bacteria as an Alternative Treatment for Gastrointestinal Inflammatory Diseases: A Focus on Inflammatory Bowel Diseases and Mucositis

The human gastrointestinal tract (GIT) is highly colonized by bacterial communities, which live in a symbiotic relationship with the host in normal conditions. It has been shown that a dysfunctional interaction between the intestinal microbiota and the host immune system, known as dysbiosis, is a very important factor responsible for the development of different inflammatory conditions of the GIT, such as the idiopathic inflammatory bowel diseases (IBD), a complex and multifactorial disorder of the GIT. Dysbiosis has also been implicated in the pathogenesis of other GIT inflammatory diseases such as mucositis usually caused as an adverse effect of chemotherapy. As both diseases have become a great clinical problem, many research groups have been focusing on developing new strategies for the treatment of IBD and mucositis. In this review, we show that lactic acid bacteria (LAB) have been capable in preventing and treating both disorders in animal models, suggesting they may be ready for clinical trials. In addition, we present the most current studies on the use of wild type or genetically engineered LAB strains designed to express anti-inflammatory proteins as a promising strategy in the treatment of IBD and mucositis.

Recognition of microbial glycans by soluble human lectins

Human innate immune lectins that recognize microbial glycans can conduct microbial surveillance and thereby help prevent infection. Structural analysis of soluble lectins has provided invaluable insight into how these proteins recognize their cognate carbohydrate ligands and how this recognition gives rise to biological function. In this opinion, we cover the structural features of lectins that allow them to mediate microbial recognition, highlighting examples from the collectin, Reg protein, galectin, pentraxin, ficolin and intelectin families. These analyses reveal how some lectins (e.g., human intelectin-1) can recognize glycan epitopes that are remarkably diverse, yet still differentiate between mammalian and microbial glycans. We additionally discuss strategies to identify lectins that recognize microbial glycans and highlight tools that facilitate these discovery efforts.

Pathogenic Streptococcus strains employ novel escape strategy to inhibit bacteriostatic effect mediated by mammalian peptidoglycan recognition protein

Pathogenic streptococcal species are responsible for some of the most lethal and prevalent animal and human infections. Previous reports have identified a candidate pathogenicity island (PAI) in two highly virulent clinical isolates of Streptococcus suis type 2, a causative agent of high-mortality streptococcal toxic shock syndrome. This PAI contains a type-IVC secretion system C subgroup (type-IVC secretion system) that is involved in the secretion of unknown pathogenic effectors that are responsible for streptococcal toxic shock syndrome caused by highly virulent strains of S. suis. Both virulence protein B4 and virulence protein D4 were demonstrated to be key components of this type-IVC secretion system. In this study, we identify a new PAI family across 3 streptococcal species; Streptococcus genomic island contains type-IV secretion system, which contains a genomic island type-IVC secretion system and a novel PPIase molecule, SP1. SP1 is shown to interact with a component of innate immunity, peptidoglycan recognition protein (PGLYRP-1) and to perturb the PGLYRP-1-mediated bacteriostatic effect by interacting with protein PGLYRP-1. Our study elucidates a novel mechanism by which bacteria escape by components of the innate immune system by secretion of the SP1 protein in pathogenic Streptococci, which then interacts with PGLYRP-1 from the host. Our results provide potential targets for the development of new antimicrobial drugs against bacteria with resistance to innate host immunity.

Proteomic characterization of the interactions between fish serum proteins and waterborne bacteria reveals the suppression of anti-oxidative defense as a serum-mediated antimicrobial mechanism

Fish blood is one of the crucial tissues of innate immune system, but the full repertoire of fish serum components involved in antibacterial defense is not fully identified. In this study, we demonstrated that turbot serum, but not the heat-inactivated control, significantly reduced the number of Edwardsiella tarda (E. tarda). By conjugating serum proteins with fluorescent dyes, we showed that E. tarda were coated with multiple fish proteins. In order to identify these proteins, we used E. tarda to capture turbot serum proteins and subjected the samples to shotgun proteomic analysis. A total of 76 fish proteins were identified in high confidence, including known antimicrobial proteins such as immunoglobins and complement components. 34 proteins with no previously known immunological functions were also identified. The expression of one of these proteins, IQ motif containing H (IQCH), was exclusively in fish brain and gonads and was induced during bacterial infection. This approach also allowed the study of the corresponding proteomic changes in E. tarda exposed to turbot serum, which is a general decrease of bacterial protein expression except for an upregulation of membrane components after serum treatment. Interestingly, while most other known stresses stimulate bacterial antioxidant enzymes, fish serum induced a rapid suppression of antioxidant proteins and led to an accumulation of reactive oxygen species. Heat treatment of fish serum eliminated this effect, suggesting that heat labile factors in the fish serum overrode bacterial antioxidant defenses. Taken together, this work offers a comprehensive view of the interactions between fish serum proteins and bacteria, and reveals previously unknown factors and mechanisms in fish innate immunity.

Involvement of PGRP-SC2 from Artemia sinica in the innate immune response against bacteria and expression pattern at different developmental stages

Peptidoglycan-recognition protein-SC2 precursor-like protein (PGRP-SC2) is a vital protein in innate immunity with a vita role in response to bacteria challenge in invertebrates. Here, a 678-bp full-length cDNA of pgrp-sc2 from A. sinica was obtained containing a 558-bp open reading frame encoding 185 amino acids with a calculated molecular mass of 19.6 kDa. The predicted protein contains a PGRP and an Amidase2 domain, indicating that PGRP-SC2 is a PGRP family member and has N-acetylmuramoyl-l-alanine amidase activity. The expression and localization of pgrp-sc2/PGRP-SC2 in A.sinica during embryonic development and bacterial challenge were determined by qPCR, WB and ISH. During different A. sinica embryonic development stages, the expression level of pgrp-sc2/PGRP-SC2 was most highly expressed at 0 and 5 h and after challenge by Gram-positive bacteria, it increased with increasing bacterial concentrations, indicating that it plays a vital role in A. sinica early embryonic development and innate immunity.

Functional and technological properties of camel milk proteins: a review

This review summarises current knowledge on camel milk proteins, with focus on significant peculiarities in protein composition and molecular properties. Camel milk is traditionally consumed as a fresh or naturally fermented product. Within the last couple of years, an increasing quantity is being processed in dairy plants, and a number of consumer products have been marketed. A better understanding of the technological and functional properties, as required for product improvement, has been gained in the past years. Absence of the whey protein β-LG and a low proportion of к-casein cause differences in relation to dairy processing. In addition to the technological properties, there are also implications for human nutrition and camel milk proteins are of interest for applications in infant foods, for food preservation and in functional foods. Proposed health benefits include inhibition of the angiotensin converting enzyme, antimicrobial and antioxidant properties as well as an antidiabetogenic effect. Detailed investigations on foaming, gelation and solubility as well as technological consequences of processing should be investigated further for the improvement of camel milk utilisation in the near future.

Mycobacteriophage SWU1 gp39 can potentiate multiple antibiotics against Mycobacterium via altering the cell wall permeability

M. tuberculosis is intrinsically tolerant to many antibiotics largely due to the imperviousness of its unusual mycolic acid-containing cell wall to most antimicrobials. The emergence and increasingly widespread of multidrug-resistant tuberculosis (MDR-TB) and extensively drug-resistant tuberculosis (XDR-TB) revitalized keen interest in phage-inspired therapy. SWU1gp39 is a novel gene from mycobacteriophage SWU1 with unknown function. SWU1gp39 expressed in M. smegmatis conferred the host cell increased susceptibility to multiple antibiotics, including isoniazid, erythromycin, norfloxacin, ampicillin, ciprofloxacin, ofloxacin, rifampicin and vancomycin, and multiple environment stresses such as H2O2, heat shock, low pH and SDS. By using EtBr/Nile red uptake assays, WT-pAL-gp39 strain showed higher cell wall permeability than control strain WT-pAL. Moreover, the WT-pAL-gp39 strain produced more reactive oxygen species and reduced NAD(+)/NADH ratio. RNA-Seq transcriptomes of the WT-pAL-gp39 and WT-pAL revealed that the transcription of 867 genes was differentially regulated, including genes associated with lipid metabolism. Taken together, our results implicated that SWU1gp39, a novel gene from mycobacteriophage, disrupted the lipid metabolism of host and increased cell wall permeability, ultimately potentiated the efficacy of multiple antibiotics and stresses against mycobacteria.

Mycobacteriophage SWU1 gp39 can potentiate multiple antibiotics against Mycobacterium via altering the cell wall permeability

M. tuberculosis is intrinsically tolerant to many antibiotics largely due to the imperviousness of its unusual mycolic acid-containing cell wall to most antimicrobials. The emergence and increasingly widespread of multidrug-resistant tuberculosis (MDR-TB) and extensively drug-resistant tuberculosis (XDR-TB) revitalized keen interest in phage-inspired therapy. SWU1gp39 is a novel gene from mycobacteriophage SWU1 with unknown function. SWU1gp39 expressed in M. smegmatis conferred the host cell increased susceptibility to multiple antibiotics, including isoniazid, erythromycin, norfloxacin, ampicillin, ciprofloxacin, ofloxacin, rifampicin and vancomycin, and multiple environment stresses such as H2O2, heat shock, low pH and SDS. By using EtBr/Nile red uptake assays, WT-pAL-gp39 strain showed higher cell wall permeability than control strain WT-pAL. Moreover, the WT-pAL-gp39 strain produced more reactive oxygen species and reduced NAD(+)/NADH ratio. RNA-Seq transcriptomes of the WT-pAL-gp39 and WT-pAL revealed that the transcription of 867 genes was differentially regulated, including genes associated with lipid metabolism. Taken together, our results implicated that SWU1gp39, a novel gene from mycobacteriophage, disrupted the lipid metabolism of host and increased cell wall permeability, ultimately potentiated the efficacy of multiple antibiotics and stresses against mycobacteria.

Effect of cooperation of chaperones and gene dosage on the expression of porcine PGLYRP-1 in Pichia pastoris

Mammalian peptidoglycan recognition proteins (PGLYRPs) are highly conserved pattern-recognition molecules of the innate immune system with considerable bactericidal activity, which manifest their potential values for the application to food and pharmaceutical industry. However, the effective expression of porcine PGLYRP-1 in Pichia pastoris has not been reported so far. In this study, expression in P. pastoris was explored as an efficient way to produce functional porcine PGLYRP-1. Cooperation of chaperones co-expression and gene dosage (including protein disulfide isomerase (PDI)/binding protein (BiP) and pglyrp-1) were used to enhance functional expression of antimicrobial protein in P. pastoris. Overexpression of PDI was certainly able to increase secretion level of PGLYRP-1 protein because the increase in secreted PGLYRP-1 secretion was correlated with the copy numbers of PDI in high copy pglyrp-1 clones. However, co-expression of BiP was proved to be detrimental to PGLYRP-1 secretion. In addition, we also found that excessive expression of PDI and/or BiP could decrease the mRNA expression of pglyrp-1 gene. This showed that PDI and BiP as the target genes of unfolded protein response (UPR) might regulate the transcription of the target protein. These data demonstrated for the first time that the combination of chaperones and gene dosages could improve the yield of PGLYRP-1, which could facilitate the application to food and pharmaceutical industry.

Pglyrp-Regulated Gut Microflora Prevotella falsenii, Parabacteroides distasonis and Bacteroides eggerthii Enhance and Alistipes finegoldii Attenuates Colitis in Mice

Dysbiosis is a hallmark of inflammatory bowel disease (IBD), but it is unclear which specific intestinal bacteria predispose to and which protect from IBD and how they are regulated. Peptidoglycan recognition proteins (Pglyrps) are antibacterial, participate in maintaining intestinal microflora, and modulate inflammatory responses. Mice deficient in any one of the four Pglyrp genes are more sensitive to dextran sulfate sodium (DSS)-induced colitis, and stools from Pglyrp-deficient mice transferred to wild type (WT) germ-free mice predispose them to much more severe colitis than stools from WT mice. However, the identities of these Pglyrp-regulated bacteria that predispose Pglyrp-deficient mice to colitis or protect WT mice from colitis are not known. Here we identified significant changes in β-diversity of stool bacteria in Pglyrp-deficient mice compared with WT mice. The most consistent changes in microbiome in all Pglyrp-deficient mice were in Bacteroidales, from which we selected four species, two with increased abundance (Prevotella falsenii and Parabacteroides distasonis) and two with decreased abundance (Bacteroides eggerthii and Alistipes finegoldii). We then gavaged WT mice with stock type strains of these species to test the hypothesis that they predispose to or protect from DSS-induced colitis. P. falsenii, P. distasonis, and B. eggerthii all enhanced DSS-induced colitis in both WT mice with otherwise undisturbed intestinal microflora and in WT mice with antibiotic-depleted intestinal microflora. By contrast, A. finegoldii (which is the most abundant species in WT mice) attenuated DSS-induced colitis both in WT mice with otherwise undisturbed intestinal microflora and in WT mice with antibiotic-depleted intestinal microflora, similar to the colitis protective effect of the entire normal microflora. These results identify P. falsenii, P. distasonis, and B. eggerthii as colitis-promoting species and A. finegoldii as colitis-protective species.

Biophysical and physiological characterization of ZraP from Escherichia coli, the periplasmic accessory protein of the atypical ZraSR two-component system

ZraP is an octamer containing four interfacial metal-binding sites contributing to dimer stability. Zinc binding enhances its chaperone properties and zinc-bound ZraP represses the expression of the zraPSR operon. None of the Zra proteins are involved in zinc resistance.

Bacillithiol: a key protective thiol in Staphylococcus aureus

Bacillithiol is a low-molecular-weight thiol analogous to glutathione and is found in several Firmicutes, including Staphylococcus aureus. Since its discovery in 2009, bacillithiol has been a topic of interest because it has been found to contribute to resistance during oxidative stress and detoxification of electrophiles, such as the antibiotic fosfomycin, in S. aureus. The rapid increase in resistance of methicillin-resistant Staphylococcus aureus (MRSA) to available therapeutic agents is a great health concern, and many research efforts are focused on identifying new drugs and targets to combat this organism. This review describes the discovery of bacillithiol, studies that have elucidated the physiological roles of this molecule in S. aureus and other Bacilli, and the contribution of bacillithiol to S. aureus fitness during pathogenesis. Additionally, the bacillithiol biosynthesis pathway is evaluated as a novel drug target that can be utilized in combination with existing therapies to treat S. aureus infections.

Survival in amoeba--a major selection pressure on the presence of bacterial copper and zinc resistance determinants? Identification of a "copper pathogenicity island"

The presence of metal resistance determinants in bacteria usually is attributed to geological or anthropogenic metal contamination in different environments or associated with the use of antimicrobial metals in human healthcare or in agriculture. While this is certainly true, we hypothesize that protozoan predation and macrophage killing are also responsible for selection of copper/zinc resistance genes in bacteria. In this review, we outline evidence supporting this hypothesis, as well as highlight the correlation between metal resistance and pathogenicity in bacteria. In addition, we introduce and characterize the "copper pathogenicity island" identified in Escherichia coli and Salmonella strains isolated from copper- and zinc-fed Danish pigs.

Copper Tolerance and Characterization of a Copper-Responsive Operon, copYAZ, in an M1T1 Clinical Strain of Streptococcus pyogenes

Infection with Streptococcus pyogenes is associated with a breadth of clinical manifestations ranging from mild pharyngitis to severe necrotizing fasciitis. Elevated levels of intracellular copper are highly toxic to this bacterium, and thus, the microbe must tightly regulate the level of this metal ion by one or more mechanisms, which have, to date, not been clearly defined. In this study, we have identified two virulence mechanisms by which S. pyogenes protects itself against copper toxicity. We defined a set of putative genes, copY (for a regulator), copA (for a P1-type ATPase), and copZ (for a copper chaperone), whose expression is regulated by copper. Our results indicate that these genes are highly conserved among a range of clinical S. pyogenes isolates. The copY, copA, and copZ genes are induced by copper and are transcribed as a single unit. Heterologous expression assays revealed that S. pyogenes CopA can confer copper tolerance in a copper-sensitive Escherichia coli mutant by preventing the accumulation of toxic levels of copper, a finding that is consistent with a role for CopA in copper export. Evaluation of the effect of copper stress on S. pyogenes in a planktonic or biofilm state revealed that biofilms may aid in protection during initial exposure to copper. However, copper stress appears to prevent the shift from the planktonic to the biofilm state. Therefore, our results indicate that S. pyogenes may use several virulence mechanisms, including altered gene expression and a transition to and from planktonic and biofilm states, to promote survival during copper stress.

IMPORTANCE

Bacterial pathogens encounter multiple stressors at the host-pathogen interface. This study evaluates a virulence mechanism(s) utilized by S. pyogenes to combat copper at sites of infection. A better understanding of pathogen tolerance to stressors such as copper is necessary to determine how host-pathogen interactions impact bacterial survival during infections. These insights may lead to the identification of novel therapeutic targets that can be used to address antibiotic resistance.

Cyclic di-GMP modulates gene expression in Lyme disease spirochetes at the tick-mammal interface to promote spirochete survival during the blood meal and tick-to-mammal transmission

Borrelia burgdorferi, the Lyme disease spirochete, couples environmental sensing and gene regulation primarily via the Hk1/Rrp1 two-component system (TCS) and Rrp2/RpoN/RpoS pathways. Beginning with acquisition, we reevaluated the contribution of these pathways to spirochete survival and gene regulation throughout the enzootic cycle. Live imaging of B. burgdorferi caught in the act of being acquired revealed that the absence of RpoS and the consequent derepression of tick-phase genes impart a Stay signal required for midgut colonization. In addition to the behavioral changes brought on by the RpoS-off state, acquisition requires activation of cyclic di-GMP (c-di-GMP) synthesis by the Hk1/Rrp1 TCS; B. burgdorferi lacking either component is destroyed during the blood meal. Prior studies attributed this dramatic phenotype to a metabolic lesion stemming from reduced glycerol uptake and utilization. In a head-to-head comparison, however, the B. burgdorferi Δglp mutant had a markedly greater capacity to survive tick feeding than B. burgdorferi Δhk1 or Δrrp1 mutants, establishing unequivocally that glycerol metabolism is only one component of the protection afforded by c-di-GMP. Data presented herein suggest that the protective response mediated by c-di-GMP is multifactorial, involving chemotactic responses, utilization of alternate substrates for energy generation and intermediary metabolism, and remodeling of the cell envelope as a means of defending spirochetes against threats engendered during the blood meal. Expression profiling of c-di-GMP-regulated genes through the enzootic cycle supports our contention that the Hk1/Rrp1 TCS functions primarily, if not exclusively, in ticks. These data also raise the possibility that c-di-GMP enhances the expression of a subset of RpoS-dependent genes during nymphal transmission.

Peptidoglycan recognition protein-peptidoglycan complexes increase monocyte/macrophage activation and enhance the inflammatory response

Peptidoglycan recognition proteins (PGRP) are pattern recognition receptors that can bind or hydrolyse peptidoglycan (PGN). Four human PGRP have been described: PGRP-S, PGRP-L, PGRP-Iα and PGRP-Iβ. Mammalian PGRP-S has been implicated in intracellular destruction of bacteria by polymorphonuclear cells, PGRP-Iα and PGRP-Iβ have been found in keratinocytes and epithelial cells, and PGRP-L is a serum protein that hydrolyses PGN. We have expressed recombinant human PGRP and observed that PGRP-S and PGRP-Iα exist as monomer and disulphide dimer proteins. The PGRP dimers maintain their biological functions. We detected the PGRP-S dimer in human serum and polymorphonuclear cells, from where it is secreted after degranulation; these cells being a possible source of serum PGRP-S. Recombinant PGRP do not act as bactericidal or bacteriostatic agents in the assayed conditions; however, PGRP-S and PGRP-Iα cause slight damage in the bacterial membrane. Monocytes/macrophages increase Staphylococcus aureus phagocytosis in the presence of PGRP-S, PGRP-Iα and PGRP-Iβ. All PGRP bind to monocyte/macrophage membranes and are endocytosed by them. In addition, all PGRP protect cells from PGN-induced apoptosis. PGRP increase THP-1 cell proliferation and enhance activation by PGN. PGRP-S-PGN complexes increase the membrane expression of CD14, CD80 and CD86, and enhance secretion of interleukin-8, interleukin-12 and tumour necrosis factor-α, but reduce interleukin-10, clearly inducing an inflammatory profile.

Dissecting the formation, structure and barrier function of the stratum corneum

The skin is the largest organ of the mammalian body. The outermost layer of mammalian skin, the stratum corneum (SC) of the epidermis, consists of piles of dead corneocytes that are the end-products of terminal differentiation of epidermal keratinocytes. The SC performs a crucial barrier function of epidermis. Langerhans cells, when activated, extend their dendrites through tight junctions just beneath the SC to capture external antigens. Recently, knowledge of the biology of corneocytes ('corneobiology') has progressed rapidly and many key factors that modulate its barrier function have been identified and characterized. In this review article on the SC, we summarize its evolution, formation, structure and function. Cornification is an important step of SC formation at the conversion of living epithelial cells to dead corneocytes, and consists of three major steps: formation of the intracellular keratin network, cornified envelopes and intercellular lipids. After cornification, the SC undergoes chemical reactions to form the mature SC with different functional layers. Finally, the SC is shed off at the surface ('desquamation'), mediated by a cascade of several proteases. This review will be helpful to understand our expanding knowledge of the biology of the SC, where immunity meets external antigens.

Diagnosing oxidative stress in bacteria: not as easy as you might think

Microorganisms are vulnerable to elevated levels of intracellular reactive oxygen species (ROS). This situation has led to proposals that many natural stresses might be toxic specifically because they accelerate endogenous ROS formation. Such a mechanism has been convincingly demonstrated for redox-cycling compounds. However, the evidence is much weaker for most other stressors. The hypothesis that clinical antibiotics generate lethal ROS stress has attracted much attention, and the author discusses some aspects of evidence that support or oppose this idea. Importantly, even if all cellular electron flow were somehow diverted to ROS formation, the resultant doses of H2O2 and O2(-) would more likely be bacteriostatic than bacteriocidal unless key defense mechanisms were simultaneously blocked.

Generation of reactive oxygen species by lethal attacks from competing microbes

Whether antibiotics induce the production of reactive oxygen species (ROS) that contribute to cell death is an important yet controversial topic. Here, we report that lethal attacks from bacterial and viral species also result in ROS production in target cells. Using soxS as an ROS reporter, we found soxS was highly induced in Escherichia coli exposed to various forms of attacks mediated by the type VI secretion system (T6SS), P1vir phage, and polymyxin B. Using a fluorescence ROS probe, we found enhanced ROS levels correlate with induced soxS in E. coli expressing a toxic T6SS antibacterial effector and in E. coli treated with P1vir phage or polymyxin B. We conclude that both contact-dependent and contact-independent interactions with aggressive competing bacterial species and viruses can induce production of ROS in E. coli target cells.

Single-cell, real-time detection of oxidative stress induced in Escherichia coli by the antimicrobial peptide CM15

Antibiotics target specific biochemical mechanisms in bacteria. In response to new drugs, pathogenic bacteria rapidly develop resistance. In contrast, antimicrobial peptides (AMPs) have retained broad spectrum antibacterial potency over millions of years. We present single-cell fluorescence assays that detect reactive oxygen species (ROS) in the Escherichia coli cytoplasm in real time. Within 30 s of permeabilization of the cytoplasmic membrane by the cationic AMP CM15 [combining residues 1-7 of cecropin A (from moth) with residues 2-9 of melittin (bee venom)], three fluorescence signals report oxidative stress in the cytoplasm, apparently involving O2 (-), H2O2, and •OH. Mechanistic studies indicate that active respiration is a prerequisite to the CM15-induced oxidative damage. In anaerobic conditions, signals from ROS are greatly diminished and the minimum inhibitory concentration increases 20-fold. Evidently the natural human AMP LL-37 also induces a burst of ROS. Oxidative stress may prove a significant bacteriostatic mechanism for a variety of cationic AMPs. If so, host organisms may use the local oxygen level to modulate AMP potency.

Copper is an endogenous modulator of neural circuit spontaneous activity

For reasons that remain insufficiently understood, the brain requires among the highest levels of metals in the body for normal function. The traditional paradigm for this organ and others is that fluxes of alkali and alkaline earth metals are required for signaling, but transition metals are maintained in static, tightly bound reservoirs for metabolism and protection against oxidative stress. Here we show that copper is an endogenous modulator of spontaneous activity, a property of functional neural circuitry. Using Copper Fluor-3 (CF3), a new fluorescent Cu(+) sensor for one- and two-photon imaging, we show that neurons and neural tissue maintain basal stores of loosely bound copper that can be attenuated by chelation, which define a labile copper pool. Targeted disruption of these labile copper stores by acute chelation or genetic knockdown of the CTR1 (copper transporter 1) copper channel alters the spatiotemporal properties of spontaneous activity in developing hippocampal and retinal circuits. The data identify an essential role for copper neuronal function and suggest broader contributions of this transition metal to cell signaling.

Peptidoglycan recognition protein 3 and Nod2 synergistically protect mice from dextran sodium sulfate-induced colitis

Aberrant immune response and changes in the gut microflora are the main causes of inflammatory bowel disease (IBD). Peptidoglycan recognition proteins (Pglyrp1, Pglyrp2, Pglyrp3, and Pglyrp4) are bactericidal innate immunity proteins that maintain normal gut microbiome, protect against experimental colitis, and are associated with IBD in humans. Nucleotide-binding oligomerization domain 2 (Nod2) is an intracellular bacterial sensor and may be required for maintaining normal gut microbiome. Mutations in Nod2 are strongly associated with Crohn's disease, but the causative mechanism is not understood, and the role of Nod2 in ulcerative colitis is not known. Because IBD is likely caused by variable multiple mutations in different individuals, in this study, we examined the combined role of Pglyrp3 and Nod2 in the development of experimental colitis in mice. We demonstrate that a combined deficiency of Pglyrp3 and Nod2 results in higher sensitivity to dextran sodium sulfate-induced colitis compared with a single deficiency. Pglyrp3(-/-)Nod2(-/-) mice had decreased survival and higher loss of body weight, increased intestinal bleeding, higher apoptosis of colonic mucosa, elevated expression of cytokines and chemokines, altered gut microbiome, and increased levels of ATP in the colon. Increased sensitivity to dextran sodium sulfate-induced colitis in Pglyrp3(-/-)Nod2(-/-) mice depended on increased apoptosis of intestinal epithelium, changed gut microflora, and elevated ATP. Pglyrp3 deficiency contributed colitis-predisposing intestinal microflora and increased intestinal ATP, whereas Nod2 deficiency contributed higher apoptosis and responsiveness to increased level of ATP. In summary, Pglyrp3 and Nod2 are both required for maintaining gut homeostasis and protection against colitis, but their protective mechanisms differ.