Mode of action of human β-defensin 3 against Staphylococcus aureus and transcriptional analysis of responses to defensin challenge

Staphylococcus aureus response and adaptation to vancomycin

Antibiotic resistance is an increasing challenge for the human pathogen Staphylococcus aureus. Methicillin-resistant S. aureus (MRSA) clones have spread globally, and a growing number display decreased susceptibility to vancomycin, the favoured antibiotic for treatment of MRSA infections. These vancomycin-intermediate S. aureus (VISA) or heterogeneous vancomycin-intermediate S. aureus (hVISA) strains arise from accumulation of a variety of point mutations, leading to cell wall thickening and reduced vancomycin binding to the cell wall building block, Lipid II, at the septum. They display only minor changes in vancomycin susceptibility, with varying tolerance between cells in a population, and therefore, they can be difficult to detect. In this review, we summarize current knowledge of VISA and hVISA. We discuss the role of genetic strain background or epistasis for VISA development and the possibility of strains being 'transient' VISA with gene expression changes mediated by, for example, VraTSR, GraXSR, or WalRK signal transduction systems, leading to temporary vancomycin tolerance. Additionally, we address collateral susceptibility to other antibiotics than vancomycin. Specifically, we estimate how mutations in rpoB, encoding the β-subunit of the RNA polymerase, affect overall protein structure and compare changes with rifampicin resistance. Ultimately, such in-depth analysis of VISA and hVISA strains in terms of genetic and transcriptional changes, as well as changes in protein structures, may pave the way for improved detection and guide antibiotic therapy by revealing strains at risk of VISA development. Such tools will be valuable for keeping vancomycin an asset also in the future.

Identification of Genes Associated with Resistance to Persulcatusin, a Tick Defensin from Ixodes persulcatus

Antimicrobial peptides (AMPs) are present in a wide range of plants, animals, and microorganisms. Since AMPs are characterized by their effectiveness against emergent antibiotic-resistant bacteria, they are attracting attention as next-generation antimicrobial compounds that could solve the problem of drug-resistant bacteria. Persulcatusin (IP), an antibacterial peptide derived from the hard tick Ixodes persulcatus, shows high antibacterial activity against various Gram- positive bacteria as well as multidrug-resistant bacteria. However, reports on the antibacterial action and resistance mechanisms of IP are scarce. In this study, we spontaneously generated mutants showing increased a minimum inhibitory concentration (MIC) of IP and analyzed their cross-resistance to other AMPs and antibiotics. We also used fluorescent probes to investigate the target of IP activity by evaluating IP-induced damage to the bacterial cytoplasmic membrane. Our findings suggest that the antimicrobial activity of IP on bacterial cytoplasmic membranes occurs via a mechanism of action different from that of known AMPs. Furthermore, we screened for mutants with high susceptibility to IP using a transposon mutant library and identified 16 genes involved in IP resistance. Our results indicate that IP, like other AMPs, depolarizes the bacterial cytoplasmic membrane, but it may also alter membrane structure and inhibit cell-wall synthesis.

An Update on the Therapeutic Potential of Antimicrobial Peptides against Acinetobacter baumannii Infections

The rise in antibiotic-resistant strains of clinically important pathogens is a major threat to global health. The World Health Organization (WHO) has recognized the urgent need to develop alternative treatments to address the growing list of priority pathogens. Antimicrobial peptides (AMPs) rank among the suggested options with proven activity and high potential to be developed into effective drugs. Many AMPs are naturally produced by living organisms protecting the host against pathogens as a part of their innate immunity. Mechanisms associated with AMP actions include cell membrane disruption, cell wall weakening, protein synthesis inhibition, and interference in nucleic acid dynamics, inducing apoptosis and necrosis. Acinetobacter baumannii is a critical pathogen, as severe clinical implications have developed from isolates resistant to current antibiotic treatments and conventional control procedures, such as UV light, disinfectants, and drying. Here, we review the natural AMPs representing primary candidates for new anti-A. baumannii drugs in post-antibiotic-era and present computational tools to develop the next generation of AMPs with greater microbicidal activity and reduced toxicity.

Combined proteomic and transcriptomic analysis of the antimicrobial mechanism of tannic acid against Staphylococcus aureus

Staphylococcus aureus is a zoonotic opportunistic pathogen that represents a significant threat to public health. Previous studies have shown that tannic acid (TA) has an inhibitory effect on a variety of bacteria. In this study, the proteome and transcriptome of S. aureus were analyzed to comprehensively assess changes in genes and proteins induced by TA. Initial observations of morphological changes revealed that TA damaged the integrity of the cell membrane. Next, proteomic and genetic analyses showed that exposure to TA altered the expression levels of 651 differentially expressed proteins (DEPs, 283 upregulated and 368 downregulated) and 503 differentially expressed genes (DEGs, 191 upregulated and 312 downregulated). Analysis of the identified DEPs and DEGs suggested that TA damages the integrity of the cell envelope by decreasing the expression and protein abundance of enzymes involved in the synthesis of peptidoglycans, teichoic acids and fatty acids, such as murB, murQ, murG, fmhX and tagA. After treatment with TA, the assembly of ribosomes in S. aureus was severely impaired by significant reductions in available ribosome components, and thus protein synthesis was hindered. The levels of genes and proteins associated with amino acids and purine synthesis were remarkably decreased, which further reduced bacterial viability. In addition, ABC transporters, which are involved in amino acid and ion transport, were also badly affected. Our results reveal the molecular mechanisms underlying the effects of TA on S. aureus and provide a theoretical basis for the application of TA as an antibacterial chemotherapeutic agent.

Antibiotics: Precious Goods in Changing Times

Antibiotics represent a first line of defense of diverse microorganisms, which produce and use antibiotics to counteract natural enemies or competitors for nutritional resources in their nearby environment. For antimicrobial activity, nature has invented a great variety of antibiotic modes of action that involve the perturbation of essential bacterial structures or biosynthesis pathways of macromolecules such as the bacterial cell wall, DNA, RNA, or proteins, thereby threatening the specific microbial lifestyle and eventually even survival. However, along with highly inventive modes of antibiotic action, nature also developed a comparable set of resistance mechanisms that help the bacteria to circumvent antibiotic action. Microorganisms have evolved specific adaptive responses that allow to appropriately react to the presence of antimicrobial agents, thereby ensuring survival during antimicrobial stress. In times of rapid development and spread of antibiotic (multi-)resistance, new resistance-breaking strategies to counteract bacterial infections are desperately needed. This chapter is an update to Chapter 1 of the first edition of this book and intends to give an overview of common antibiotics and their target pathways. It will also present examples for new antibiotics with novel modes of action, illustrating that nature's repertoire of innovative new antimicrobial agents has not been fully exploited yet, and we still might find new drugs that help to evade established antimicrobial resistance strategies.

Recent insights into the role of defensins in diabetic wound healing

Diabetic wound, one of the most common serious complications of diabetic patients, is an important factor in disability and death. Much of the research on the pathophysiology of diabetic wound healing has long focused on mechanisms mediated by hyperglycemia, chronic inflammation, microcirculatory and macrocirculatory dysfunction. However, recent evidence suggests that defensins may play a crucial role in the development and perpetuation of diabetic wound healing. The available findings suggest that defensins exert a beneficial influence on diabetic wound healing through antimicrobial, immunomodulatory, angiogenic, tissue regenerator effects, and insulin resistance improvement. Therefore, summarizing the existing research progress on defensins in the diabetic wound may present a promising strategy for diabetic patients.

Antimicrobial peptides from freshwater invertebrate species: potential for future applications

Invertebrates are a significant source of antimicrobial peptides because they lack an adaptive immune system and must rely on their innate immunity to survive in a pathogen-infested environment. Various antimicrobial peptides that represent major components of invertebrate innate immunity have been described in a number of investigations over the last few decades. In freshwater invertebrates, antimicrobial peptides have been identified in arthropods, annelids, molluscs, crustaceans, and cnidarians. Freshwater invertebrate species contain antimicrobial peptides from the families astacidin, macin, defensin, and crustin, as well as other antimicrobial peptides that do not belong to these families. They show broad spectrum activities greatly directed against bacteria and to a less extent against fungi and viruses. This review focuses on antimicrobial peptides found in freshwater invertebrates, highlighting their features, structure-activity connections, antimicrobial processes, and possible applications in the food industry, animal husbandry, aquaculture, and medicine. The methods for their synthesis, purification, and characterization, as well as the obstacles and strategies for their development and application, are also discussed.

Human β-Defensin-3 is Associated With Platelet-Derived Extracellular Vesicles and is a Potential Contributor to Endothelial Dysfunction

While platelets are the essential mediators of hemostasis, they are being increasingly recognized for their potential of contributing to host defenses. Here, using immunofluorescent microscopy, western blot, and ELISA, we found that human β-defensin 3 (hBD-3), an important antimicrobial peptide produced by epithelial cells, can be detected in human platelets and megakaryocytes. Flow cytometry and immuno-electron microscopy revealed hBD-3 on the surface of thrombin activated platelets. Moreover, hBD-3 was also found in platelet derived extracellular vesicles (p-EVs), isolated from platelet poor plasma and from platelet supernatants following thrombin stimulation. Incubation of platelets with hBD-3 peptide resulted in modest platelet activation and pre-incubation of platelets with synthetic hBD-3 prior to exposure to thrombin appeared to increase hBD-3 content in platelet lysates as well as in p-EVs, suggesting that hBD-3 can be initially taken up by platelets, perhaps via their open canalicular system. Interestingly, in vitro exposure of primary human endothelial cells to either hBD-3 peptide or purified p-EVs, caused significant endothelial dysfunction as documented by diminished levels of phosphorylated endothelial nitric oxide synthase (eNOS), Krüppel like factor-2 (KLF-2), and elevated relative expression of von Willebrand Factor (vWF). Pre-incubation of platelets with hBD-3 appeared to augment endothelial dysfunction caused by p-EVs. Overall, the current study provides evidence that hBD-3 enriched EVs can be released by activated platelets and may play a role in positive feedback of platelet activation as well as in endothelial dysfunction. Theoretically, these effects could contribute to both cellular recruitment to the endothelium creating a pro-thrombotic vascular microenvironment which serve as a bridge between innate immunity and hemostasis.

The hypersusceptible antibiotic screening strain Staphylococcus aureus SG511-Berlin harbors multiple mutations in regulatory genes

The genetic plasticity of Staphylococcus aureus has facilitated the evolution of many virulent and drug-resistant strains. Here we present the sequence of the 2.74 Mbp genome of S. aureus SG511-Berlin, which is frequently used for antibiotic screening. Although S. aureus SG511 and the related methicillin-resistant S. aureus MRSA252 share a high similarity in their core genomes, indicated by an average nucleotide identity (ANI) of 99.83%, the accessory genomes of these strains differed, as nearly no mobile elements and resistance determinants were identified in the genome of S. aureus SG511. Susceptibility testing showed that S. aureus SG511 was susceptible to most of the tested antibiotics of different classes. Intriguingly, and in contrast to the standard laboratory strain S. aureus HG001, S. aureus SG511 was even hyper-susceptible towards cell wall and membrane targeting agents, with the exception of the MurA-inhibitor fosfomycin. In depth comparative genome analysis revealed that, in addition to the loss of function mutation in the antibiotic sensor histidine kinase gene graS, further mutations had occurred in the lysyltransferase gene mprF, the structural giant protein gene ebh, and the regulator genes codY and saeR, which might contribute to antibiotic susceptibility. In addition, an insertion element in agrC abolishes Agr-activity in S. aureus SG511, and the spa and sarS genes, which encode the surface protein SpA and its transcriptional regulator, were deleted. Thus, the lack of mobile resistance genes together with multiple mutations affecting cell envelope morphology may render S. aureus SG511 hyper-susceptible towards most cell wall targeting agents.

The Lactococcal dgkB (yecE) and dxsA Genes for Lipid Metabolism Are Involved in the Resistance to Cell Envelope-Acting Antimicrobials

The emergence of antibiotic-resistant bacteria led to an urgent need for next-generation antimicrobial agents with novel mechanisms of action. The use of positively charged antimicrobial peptides that target cytoplasmic membrane is an especially promising strategy since essential functions and the conserved structure of the membrane hinder the development of bacterial resistance. Aureocin A53- and enterocin L50-like bacteriocins are highly cationic, membrane-targeting antimicrobial peptides that have potential as next-generation antibiotics. However, the mechanisms of resistance to these bacteriocins and cross-resistance against antibiotics must be examined before application to ensure their safe use. Here, in the model bacterium Lactococcus lactis, we studied the development of resistance to selected aureocin A53- and enterocin L50-like bacteriocins and its correlation with antibiotics. First, to generate spontaneous resistant mutants, L.lactis was exposed to bacteriocin BHT-B. Sequencing of their genomes revealed single nucleotide polymorphisms (SNPs) in the dgkB (yecE) and dxsA genes encoding diacylglycerol kinase and 1-deoxy-D-xylulose 5-phosphate synthase, respectively. Then, selected mutants underwent susceptibility tests with a wide array of bacteriocins and antibiotics. The highest alterations in the sensitivity of studied mutants were seen in the presence of cytoplasmic membrane targeting bacteriocins (K411, Ent7, EntL50, WelM, SalC, nisin) and antibiotics (daptomycin and gramicidin) as well as lipid II cycle-blocking bacteriocins (nisin and Lcn972) and antibiotics (bacitracin). Interestingly, decreased via the SNPs accumulation sensitivity to membrane-active bacteriocins and antibiotics resulted in the concurrently increased vulnerability to bacitracin, carbenicillin, or chlortetracycline. It is suspected that SNPs may result in alterations to the efficiency of the nascent enzymes rather than a total loss of their function as neither deletion nor overexpression of dxsA restored the phenotype observed in spontaneous mutants.

Fabatin-loaded silica nanoparticle-induced apoptosis via mitochondrial dysfunction: targeting the PI3K/AKT molecular pathway as a therapeutic implication against triple negative breast cancer

Mechanism induced by F-SNP on inducing apoptosis in triple negative breast cancer.

Antimicrobial and Amyloidogenic Activity of Peptides. Can Antimicrobial Peptides Be Used against SARS-CoV-2?

At present, much attention is paid to the use of antimicrobial peptides (AMPs) of natural and artificial origin to combat pathogens. AMPs have several points that determine their biological activity. We analyzed the structural properties of AMPs, as well as described their mechanism of action and impact on pathogenic bacteria and viruses. Recently published data on the development of new AMP drugs based on a combination of molecular design and genetic engineering approaches are presented. In this article, we have focused on information on the amyloidogenic properties of AMP. This review examines AMP development strategies from the perspective of the current high prevalence of antibiotic-resistant bacteria, and the potential prospects and challenges of using AMPs against infection caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2).

Antimicrobial Mechanism of pBD2 against Staphylococcus aureus

Antimicrobial peptides (AMPs) show high antibacterial activity against pathogens, which makes them potential new therapeutics to prevent and cure diseases. Porcine beta defensin 2 (pBD2) is a newly discovered AMP and has shown antibacterial activity against different bacterial species including multi-resistant bacteria. In this study, the functional mechanism of pBD2 antibacterial activity against Staphylococcus aureus was investigated. After S. aureus cells were incubated with different concentrations of pBD2, the morphological changes in S. aureus and locations of pBD2 were detected by electron microscopy. The differentially expressed genes (DEGs) were also analyzed. The results showed that the bacterial membranes were broken, bulging, and perforated after treatment with pBD2; pBD2 was mainly located on the membranes, and some entered the cytoplasm. Furthermore, 31 DEGs were detected and confirmed by quantitative real-time PCR (qRT-PCR). The known functional DEGs were associated with transmembrane transport, transport of inheritable information, and other metabolic processes. Our data suggest that pBD2 might have multiple modes of action, and the main mechanism by which pBD2 kills S. aureus is the destruction of the membrane and interaction with DNA. The results imply that pBD2 is an effective bactericide for S. aureus, and deserves further study as a new therapeutic substance against S. aureus.

Plant defensin antibacterial mode of action against Pseudomonas species

BACKGROUND

Though many plant defensins exhibit antibacterial activity, little is known about their antibacterial mode of action (MOA). Antimicrobial peptides with a characterized MOA induce the expression of multiple bacterial outer membrane modifications, which are required for resistance to these membrane-targeting peptides. Mini-Tn5-lux mutant strains of Pseudomonas aeruginosa with Tn insertions disrupting outer membrane protective modifications were assessed for sensitivity against plant defensin peptides. These transcriptional lux reporter strains were also evaluated for lux gene expression in response to sublethal plant defensin exposure. Also, a plant pathogen, Pseudomonas syringae pv. syringae was modified through transposon mutagenesis to create mutants that are resistant to in vitro MtDef4 treatments.

RESULTS

Plant defensins displayed specific and potent antibacterial activity against strains of P. aeruginosa. A defensin from Medicago truncatula, MtDef4, induced dose-dependent gene expression of the aminoarabinose modification of LPS and surface polycation spermidine production operons. The ability for MtDef4 to damage bacterial outer membranes was also verified visually through fluorescent microscopy. Another defensin from M. truncatula, MtDef5, failed to induce lux gene expression and limited outer membrane damage was detected with fluorescent microscopy. The transposon insertion site on MtDef4 resistant P. syringae pv. syringae mutants was sequenced, and modifications of ribosomal genes were identified to contribute to enhanced resistance to plant defensin treatments.

CONCLUSIONS

MtDef4 damages the outer membrane similar to polymyxin B, which stimulates antimicrobial peptide resistance mechanisms to plant defensins. MtDef5, appears to have a different antibacterial MOA. Additionally, the MtDef4 antibacterial mode of action may also involve inhibition of translation.

Clinical Applications of Antimicrobial Peptides (AMPs): Where do we Stand Now?

In this era of multi-drug resistance (MDR), antimicrobial peptides (AMPs) are one of the most promising classes of potential drug candidates to combat communicable as well as noncommunicable diseases such as cancers and diabetes. AMPs show a wide spectrum of biological activities which include antiviral, antifungal, anti-mitogenic, anticancer, and anti-inflammatory properties. Apart from these prospective therapeutic potentials, the AMPs can act as food preservatives and immune modulators. Therefore, AMPs have the potential to replace conventional drugs and may gain a significant global drug market share. Although several AMPs have shown therapeutic potential in vitro or in vivo, in most cases they have failed the clinical trial owing to various issues. In this review, we discuss in brief (i) molecular mechanisms of AMPs in various diseases, (ii) importance of AMPs in pharmaceutical industries, (iii) the challenges in using AMPs as therapeutics and how to overcome, (iv) available AMP therapeutics in market, and (v) AMPs under clinical trials. Here, we specifically focus on the therapeutic AMPs in the areas of dermatology, surgery, oncology and metabolic diseases.

Interactions of Tea-Derived Catechin Gallates with Bacterial Pathogens

Green tea-derived galloylated catechins have weak direct antibacterial activity against both Gram-positive and Gram-negative bacterial pathogens and are able to phenotypically transform, at moderate concentrations, methicillin-resistant Staphylococcus aureus (MRSA) clonal pathogens from full β-lactam resistance (minimum inhibitory concentration 256-512 mg/L) to complete susceptibility (~1 mg/L). Reversible conversion to susceptibility follows intercalation of these compounds into the bacterial cytoplasmic membrane, eliciting dispersal of the proteins associated with continued cell wall peptidoglycan synthesis in the presence of β-lactam antibiotics. The molecules penetrate deep within the hydrophobic core of the lipid palisade to force a reconfiguration of cytoplasmic membrane architecture. The catechin gallate-induced staphylococcal phenotype is complex, reflecting perturbation of an essential bacterial organelle, and includes prevention and inhibition of biofilm formation, disruption of secretion of virulence-related proteins, dissipation of halotolerance, cell wall thickening and cell aggregation and poor separation of daughter cells during cell division. These features are associated with the reduction of capacity of potential pathogens to cause lethal, difficult-to-treat infections and could, in combination with β-lactam agents that have lost therapeutic efficacy due to the emergence of antibiotic resistance, form the basis of a new approach to the treatment of staphylococcal infections.

Human Defensins: A Novel Approach in the Fight against Skin Colonizing Staphylococcus aureus

Staphylococcus aureus is a microorganism capable of causing numerous diseases of the human skin. The incidence of S. aureus skin infections reflects the conflict between the host skin′s immune defenses and the S. aureus’ virulence elements. Antimicrobial peptides (AMPs) are small protein molecules involved in numerous biological activities, playing a very important role in the innate immunity. They constitute the defense of the host′s skin, which prevents harmful microorganisms from entering the epithelial barrier, including S. aureus. However, S. aureus uses ambiguous mechanisms against host defenses by promoting colonization and skin infections. Our review aims to provide a reference collection on host-pathogen interactions in skin disorders, including S. aureus infections and its resistance to methicillin (MRSA). In addition to these, we discuss the involvement of defensins and other innate immunity mediators (i.e., toll receptors, interleukin-1, and interleukin-17), involved in the defense of the host against the skin disorders caused by S. aureus, and then focus on the evasion mechanisms developed by the pathogenic microorganism under analysis. This review provides the “state of the art” on molecular mechanisms underlying S. aureus skin infection and the pharmacological potential of AMPs as a new therapeutic strategy, in order to define alternative directions in the fight against cutaneous disease.

Exploring Pleiotropic Functions of Canine β-Defensin 103: Nasal Cavity Expression, Antimicrobial Activity, and Melanocortin Receptor Activity

Canine β-defensin 103 (cBD103) and its common variant cBD103ΔG23 are multitasking polypeptides. As a β-defensin, cBD103 is one of many antimicrobial agents used by the innate immunity to thwart pathogenic colonization. In this study, we showed that cBD103 was expressed throughout the nasal cavity, with primary expression in the nares as well as respiratory and olfactory epithelia. In the rostral nasal concha, cBD103 was expressed in the epithelium, and to a lesser degree in the lamina propria, but was absent in goblet cells. In the main olfactory epithelium, virtually all cells in the epithelial layer and select cells associated with Bowman's glands expressed cBD103. We also showed that the ΔG23 mutation did not appreciably alter the antimicrobial activity of the peptide against several species of microorganisms tested in nutrient-rich or minimal media or minimal media with salt added. Moreover, we showed antimicrobial activity in minimal media did not necessarily predict the inhibitory action of the peptide in nutrient-rich media. Both forms of cBD103 caused ultrastructural changes (membrane blebbing, condensation of intracellular contents and cell wall lysis) in Escherichia coli and Staphylococcus aureus. As a ligand of the melanocortin receptors, we showed that cBD103ΔG23 increased ERK1/2 activation and cAMP accumulation when bound to the human or canine melanocortin-4 receptor, acting as a weak allosteric agonist.

Antibacterial Activity of Plant Defensins

Plant defensins are antimicrobial host defense peptides expressed in all higher plants. Performing a significant role in plant innate immunity, plant defensins display potent activity against a wide range of pathogens. Vertebrate and invertebrate defensins have well-characterized antibacterial activity, but plant defensins are commonly considered to display antimicrobial activity against only fungi. In this review, we highlight the often-overlooked antibacterial activity of plant defensins. Also, we illustrate methods to evaluate defensins for antibacterial activity and describe the current advances in uncovering their antibacterial modes of action.

β-Defensins: Farming the Microbiome for Homeostasis and Health

Diverse commensal populations are now regarded as key to physiological homeostasis and protection against disease. Although bacteria are the most abundant component of microbiomes, and the most intensively studied, the microbiome also consists of viral, fungal, archael, and protozoan communities, about which comparatively little is known. Host-defense peptides (HDPs), originally described as antimicrobial, now have renewed significance as curators of the pervasive microbial loads required to maintain homeostasis and manage microbiome diversity. Harnessing HDP biology to transition away from non-selective, antibiotic-mediated treatments for clearance of microbes is a new paradigm, particularly in veterinary medicine. One family of evolutionarily conserved HDPs, β-defensins which are produced in diverse combinations by epithelial and immune cell populations, are multifunctional cationic peptides which manage the cross-talk between host and microbes and maintain a healthy yet dynamic equilibrium across mucosal systems. They are therefore key gatekeepers to the oral, respiratory, reproductive and enteric tissues, preventing pathogen-associated inflammation and disease and maintaining physiological normality. Expansions in the number of genes encoding these natural antibiotics have been described in the genomes of some species, the functional significance of which has only recently being appreciated. β-defensin expression has been documented pre-birth and disruptions in their regulation may play a role in maladaptive neonatal immune programming, thereby contributing to subsequent disease susceptibility. Here we review recent evidence supporting a critical role for β-defensins as farmers of the pervasive and complex prokaryotic ecosystems that occupy all body surfaces and cavities. We also share some new perspectives on the role of β-defensins as sensors of homeostasis and the immune vanguard particularly at sites of immunological privilege where inflammation is attenuated.

Role of Two-Component System Response Regulator bceR in the Antimicrobial Resistance, Virulence, Biofilm Formation, and Stress Response of Group B Streptococcus

Group B Streptococcus (GBS; Streptococcus agalactiae) is a leading cause of sepsis in neonates and pregnant mothers worldwide. Whereas the hyper-virulent serogroup III clonal cluster 17 has been associated with neonatal disease and meningitis, serogroup III ST283 was recently implicated in invasive disease among non-pregnant adults in Asia. Here, through comparative genome analyses of invasive and non-invasive ST283 strains, we identified a truncated DNA-binding regulator of a two-component system in a non-invasive strain that was homologous to Bacillus subtilis bceR, encoding the bceRSAB response regulator, which was conserved among GBS strains. Using isogenic knockout and complementation mutants of the ST283 strain, we demonstrated that resistance to bacitracin and the human antimicrobial peptide cathelicidin LL-37 was reduced in the ΔbceR strain with MICs changing from 64 and 256 μg/ml to 0.25 and 64 μg/ml, respectively. Further, the ATP-binding cassette transporter was upregulated by sub-inhibitory concentrations of bacitracin in the wild-type strain. Upregulation of dltA in the wild-type strain was also observed and thought to explain the increased resistance to antimicrobial peptides. DltA, an enzyme involved in D-alanylation during the synthesis of wall teichoic acids, which mediates reduced antimicrobial susceptibility, was previously shown to be regulated by the bceR-type regulator in Staphylococcus aureus. In a murine infection model, we found that the ΔbceR mutation significantly reduced the mortality rate compared to that with the wild-type strain (p < 0.01). Moreover, this mutant was more susceptible to oxidative stress compared to the wild-type strain (p < 0.001) and was associated with reduced biofilm formation (p < 0.0001). Based on 2-DGE and mass spectrometry, we showed that downregulation of alkyl hydroperoxide reductase (AhpC), a Gls24 family stress protein, and alcohol dehydrogenase (Adh) in the ΔbceR strain might explain the attenuated virulence and compromised stress response. Together, we showed for the first time that the bceR regulator in GBS plays an important role in bacitracin and antimicrobial peptide resistance, virulence, survival under oxidative stress, and biofilm formation.

Self-Assembling Myristoylated Human α-Defensin 5 as a Next-Generation Nanobiotics Potentiates Therapeutic Efficacy in Bacterial Infection

The increasing prevalence of antibacterial resistance globally underscores the urgent need to the update of antibiotics. Here, we describe a strategy for inducing the self-assembly of a host-defense antimicrobial peptide (AMP) into nanoparticle antibiotics (termed nanobiotics) with significantly improved pharmacological properties. Our strategy involves the myristoylation of human α-defensin 5 (HD5) as a therapeutic target and subsequent self-assembly in aqueous media in the absence of exogenous excipients. Compared with its parent HD5, the C-terminally myristoylated HD5 (HD5-myr)-assembled nanobiotic exhibited significantly enhanced broad-spectrum bactericidal activity in vitro. Mechanistically, it selectively killed Escherichia coli ( E. coli) and methicillin-resistant Staphylococcus aureus (MRSA) through disruption of the cell wall and/or membrane structure. The in vivo results further demonstrated that the HD5-myr nanobiotic protected against skin infection by MRSA and rescued mice from E. coli-induced sepsis by lowering the systemic bacterial burden and alleviating organ damage. The self-assembled HD5-myr nanobiotic also showed negligible hemolytic activity and substantially low toxicity in animals. Our findings validate this design rationale as a simple yet versatile strategy for generating AMP-derived nanobiotics with excellent in vivo tolerability. This advancement will likely have a broad impact on antibiotic discovery and development efforts aimed at combating antibacterial resistance.

Targeting a cell wall biosynthesis hot spot

Covering: up to 2017History points to the bacterial cell wall biosynthetic network as a very effective target for antibiotic intervention, and numerous natural product inhibitors have been discovered. In addition to the inhibition of enzymes involved in the multistep synthesis of the macromolecular layer, in particular, interference with membrane-bound substrates and intermediates essential for the biosynthetic reactions has proven a valuable antibacterial strategy. A prominent target within the peptidoglycan biosynthetic pathway is lipid II, which represents a particular "Achilles' heel" for antibiotic attack, as it is readily accessible on the outside of the cytoplasmic membrane. Lipid II is a unique non-protein target that is one of the structurally most conserved molecules in bacterial cells. Notably, lipid II is more than just a target molecule, since sequestration of the cell wall precursor may be combined with additional antibiotic activities, such as the disruption of membrane integrity or disintegration of membrane-bound multi-enzyme machineries. Within the membrane bilayer lipid II is likely organized in specific anionic phospholipid patches that form a particular "landing platform" for antibiotics. Nature has invented a variety of different "lipid II binders" of at least 5 chemical classes, and their antibiotic activities can vary substantially depending on the compounds' physicochemical properties, such as amphiphilicity and charge, and thus trigger diverse cellular effects that are decisive for antibiotic activity.

Antibiotics: Precious Goods in Changing Times

Antibiotics represent a first line of defense of diverse microorganisms, which produce and use antibiotics to counteract natural enemies or competitors for nutritional resources in their nearby environment. For antimicrobial activity, nature has invented a great variety of mechanisms of antibiotic action that involve the perturbation of essential bacterial structures or biosynthesis pathways of macromolecules such as the bacterial cell wall, DNA, RNA, or proteins, thereby threatening the specific microbial lifestyle and eventually even survival. However, along with highly inventive modes of antibiotic action, nature also developed a comparable set of resistance mechanisms that help the bacteria to circumvent antibiotic action. Microorganisms have evolved specific adaptive responses that allow appropriately reacting to the presence of antimicrobial agents, ensuring survival during antimicrobial stress. In times of rapid development and spread of antibiotic (multi-)resistance, we need to explore new, resistance-breaking strategies to counteract bacterial infections. This chapter intends to give an overview of common antibiotics and their target pathways. It will also discuss recent advances in finding new antibiotics with novel modes of action, illustrating that nature's repertoire of innovative new antimicrobial agents has not been fully exploited yet, and we still might find new drugs that help to evade established antimicrobial resistance strategies.

Development of in vitro resistance to chitosan is related to changes in cell envelope structure of Staphylococcus aureus

The bacterial cell envelope is believed to be a principal target for initiating the staphylocidal pathway of chitosan. The present study was therefore designed to investigate possible changes in cell surface phenotypes related to the in vitro chitosan resistance development in the laboratory strain S. aureus SG511-Berlin. Following a serial passage experiment, a stable chitosan-resistant variant (CRV) was identified, exhibiting >50-fold reduction in its sensitivity towards chitosan. Our analyses of the CRV identified phenotypic and genotypic features that readily distinguished it from its chitosan-susceptible parental strain, including: (i) a lower overall negative cell surface charge; (ii) cross-resistance to a number of antimicrobial agents; (iii) major alterations in cell envelope structure, cellular bioenergetics and metabolism (based on transcriptional profiling); and (iv) a repaired sensor histidine kinase GraS. Our data therefore suggest a close nexus between changes in cell envelope properties with the in vitro chitosan-resistant phenotype in S. aureus SG511-Berlin.

Targeting bactoprenol-coupled cell envelope precursors

Targeting the bactoprenol-coupled cell wall precursor lipid II is a validated antibacterial strategy. In this review, selected prototype lipid II-binding antibiotics of different chemical classes are discussed. Although these compounds attack the same molecular target, they trigger nuanced and diverse cellular effects. Consequently, the mechanisms of antibacterial resistance and the likelihood of resistance development may vary substantially.

Comparative Analysis of the Antimicrobial Activities of Plant Defensin-Like and Ultrashort Peptides against Food-Spoiling Bacteria

Antimicrobial peptides offer potential as novel therapeutics to combat food spoilage and poisoning caused by pathogenic and nonpathogenic bacteria. Our previous studies identified the peptide human beta-defensin 3 (HBD3) as a potent antimicrobial agent against a wide range of beer-spoiling bacteria. Thus, HBD3 is an excellent candidate for development as an additive to prevent food and beverage spoilage. To expand the repertoire of peptides with antimicrobial activity against bacteria associated with food spoilage and/or food poisoning, we carried out an in silico discovery pipeline to identify peptides with structure and activity similar to those of HBD3, focusing on peptides of plant origin. Using a standardized assay, we compared the antimicrobial activities of nine defensin-like plant peptides to the activity of HBD3. Only two of the peptides, fabatin-2 and Cp-thionin-2, displayed antimicrobial activity; however, the peptides differed from HBD3 in being sensitive to salt and were thermostable. We also compared the activities of several ultrashort peptides to that of HBD3. One of the peptides, the synthetic tetrapeptide O3TR, displayed biphasic antimicrobial activity but had a narrower host range than HBD3. Finally, to determine if the peptides might act in concert to improve antimicrobial activity, we compared the activities of the peptides in pairwise combinations. The plant defensin-like peptides fabatin-2 and Cp-thionin-2 displayed a synergistic effect with HBD3, while O3TR was antagonistic. Thus, some plant defensin-like peptides are effective antimicrobials and may act in concert with HBD3 to control bacteria associated with food spoilage and food poisoning.

IMPORTANCE

Food spoilage and food poisoning caused by bacteria can have major health and economic implications for human society. With the rise in resistance to conventional antibiotics, there is a need to identify new antimicrobials to combat these outbreaks in our food supply. Here we screened plant peptide databases to identify peptides that share structural similarity with the human defensin peptide HBD3, which has known antimicrobial activity against food-spoiling bacteria. We show that two of the plant peptides display antimicrobial activity against bacteria associated with food spoilage. When combined with HBD3, the peptides are highly effective. We also analyzed the activity of an easily made ultrashort synthetic peptide, O3TR. We show that this small peptide also displays antimicrobial activity against food-spoiling bacteria but is not as effective as HBD3 or the plant peptides. The plant peptides identified are good candidates for development as natural additives to prevent food spoilage.

Killing of staphylococci by θ-defensins involves membrane impairment and activation of autolytic enzymes

θ-Defensins are cyclic antimicrobial peptides expressed in leukocytes of Old world monkeys. To get insight into their antibacterial mode of action, we studied the activity of RTDs (rhesus macaque θ-defensins) against staphylococci. We found that in contrast to other defensins, RTDs do not interfere with peptidoglycan biosynthesis, but rather induce bacterial lysis in staphylococci by interaction with the bacterial membrane and/or release of cell wall lytic enzymes. Potassium efflux experiments and membrane potential measurements revealed that the membrane impairment by RTDs strongly depends on the energization of the membrane. In addition, RTD treatment caused the release of Atl-derived cell wall lytic enzymes probably by interaction with membrane-bound lipoteichoic acid. Thus, the premature and uncontrolled activity of these enzymes contributes strongly to the overall killing by θ-defensins. Interestingly, a similar mode of action has been described for Pep5, an antimicrobial peptide of bacterial origin.

VraH Is the Third Component of the Staphylococcus aureus VraDEH System Involved in Gallidermin and Daptomycin Resistance and Pathogenicity

In bacteria, extracellular signals are transduced into the cell predominantly by two-component systems (TCSs) comprising a regulatory unit triggered by a specific signal. Some of the TCSs control executing units such as ABC transporters involved in antibiotic resistance. For instance, inStaphylococcus aureus, activation of BraSR leads to the upregulation ofvraDEexpression that encodes an ABC transporter playing a role in bacitracin and nisin resistance. In this study, we show that the small staphylococcal transmembrane protein VraH forms, together with VraDE, a three-component system. Although the expression ofvraHin the absence ofvraDEwas sufficient to mediate low-level resistance, only this VraDEH entity conferred high-level resistance against daptomycin and gallidermin. In most staphylococcal genomes,vraHis located immediately downstream ofvraDE, forming an operon, whereas in some species it is localized differently. In an invertebrate infection model, VraDEH significantly enhancedS. aureuspathogenicity. In analogy to the TCS connectors, VraH can be regarded as an ABC connector that modulates the activity of ABC transporters involved in antibiotic resistance.

Hit 'em where it hurts: The growing and structurally diverse family of peptides that target lipid-II

Understanding the mode of action of antibiotics is becoming more and more important in the time that microorganisms start to develop resistance. One very well validated target of several classes of antibiotics is the peptidoglycan precursor lipid II. In this review different classes of lipid II targeting antibiotics will be discussed in detail, including the lantibiotics, human invertebrate defensins and the recently discovered teixobactin. By hitting bacteria where it hurts, at the level of lipid II, we expect to be able to develop efficient antibacterial agents in the future. This article is part of a Special Issue entitled: Antimicrobial peptides edited by Karl Lohner and Kai Hilpert.

Peptides and Peptidomimetics for Antimicrobial Drug Design

The purpose of this paper is to introduce and highlight a few classes of traditional antimicrobial peptides with a focus on structure-activity relationship studies. After first dissecting the important physiochemical properties that influence the antimicrobial and toxic properties of antimicrobial peptides, the contributions of individual amino acids with respect to the peptides antibacterial properties are presented. A brief discussion of the mechanisms of action of different antimicrobials as well as the development of bacterial resistance towards antimicrobial peptides follows. Finally, current efforts on novel design strategies and peptidomimetics are introduced to illustrate the importance of antimicrobial peptide research in the development of future antibiotics.

A helix-PXXP-helix peptide with antibacterial activity without cytotoxicity against MDRPA-infected mice

In response to the growing problem of multidrug-resistant pathogenic microbes, much attention is being paid to naturally occurring and synthetic antimicrobial peptides (AMPs) and the effects of their structural modification. Among these modifications, amino acid substitution is a simple approach to enhancing biological activity and reducing cytotoxicity. An earlier study indicated that HPA3, an analog of HP (2-20) derived from the N-terminus of Helicobacter pylori ribosomal protein L1, forms large pores and shows considerable cytotoxicity. However, HPA3P, in which a proline (Pro) is substituted for glutamic acid (Glu) at position 9 of HPA3, shows markedly less cytotoxicity. This may be attributable to the presence of a Pro-kink into middle of the HPA3P structure within the membrane environment. Unfortunately, HPA3P is not an effective antibacterial agent in vivo. We therefore designed a helix-PXXP-helix structure (HPA3P2), in which Pro was substituted for the Glu and phenylalanine (Phe) at positions 9 and 12 of HPA3, yielding a molecule with a flexible central hinge. As compared to HPA3P, HPA3P3 exhibited dramatically increased antibacterial activity in vivo. ICR mice infected with clinically isolated multidrug-resistant Pseudomonas aeruginosa showed 100% survival when administered one 0.5-mg/kg dose of HPA3P2 or three 0.1-mg/kg doses of HPA3P2. Moreover, in a mouse model of septic shock induced by P. aeruginosa LPS, HPA3P2 reduced production of pro-inflammatory mediators and correspondingly reduced lung (alveolar) and liver tissue damage. The changes in HPA3 behavior with the introduction of Pro likely reflects alterations of the mechanism of action: i) HPA3 forms pores in the bacterial cell membranes, ii) HPA3P permeates the cell membranes and binds to intracellular RNA and DNA, and iii) HPA3P2 acts on the outer cellular membrane component LPS. Collectively, these results suggest HPA3P2 has the potential to be an effective antibiotic for use against multidrug-resistant bacterial strains.

Defensin-based anti-infective strategies

Cationic and amphiphilic peptides are widely distributed in eukaryotic organisms and constitute a first line of host defense against invading pathogens. Some of these host defense peptides (HDPs) combine specific antibiotic activities with modulation of immune responses. Moreover, they are active against bacteria resistant to conventional antibiotics and show only modest resistance development under in vitro selection pressure. Based on these features, HDPs and particularly defensins are considered a promising source of novel anti-infective agents. This review summarizes the current knowledge about defensins from different kingdoms and discusses their potential for therapeutic application.

Peptide antimicrobials: cell wall as a bacterial target

Endogenous host defense peptides (HDPs) are among the most ancient immune mediators, constituting a first line of defense against invading pathogens across the evolutionary continuum. Generally, HDPs are small (<10 kDa), cationic, and amphipathic polypeptides, often broadly classified based on structure. In eukaryotes, major HDP classes include disulfide-stabilized (e.g., defensins), and α-helical or extended (e.g., cathelicidins) peptides. Prokaryote HDPs are generally referred to as bacteriocins, colicins, or lantibiotics, many of which undergo extensive posttranslational modifications. One target for prokaryotic and eukaryotic HDPs is the bacterial cell wall, an essential structural feature conserved among broad classes of bacteria. A primary building block of the cell wall is peptidoglycan, a macromolecular complex that arises through a series of reactions including membrane translocation, extracellular anchoring, and side chain cross-linking. Each of these steps represents a potential target for HDP inhibition, leading to bacteriostatic or bactericidal outcomes. Thus, understanding the relationships between HDPs and cell wall targets may shed light on new peptide antimicrobial agents and strategies to meet the daunting challenge of antibiotic resistance.

Lipid composition is a determinant for human defensin HNP1 selectivity

Human neutrophilpeptide 1 (HNP1) is a human defensin with antimicrobial activity against different bacteria (both Gram-positive and negative), fungi, and viruses. HNP1 is stored in the cytoplasmic azurophilic granules of neutrophils. To elucidate the mode of action of this antimicrobial peptide, studies based on its lipid selectivity were carried out. Large unilamellar vesicles with different lipid compositions were used as biomembranes model systems (mammal, fungal, and bacterial models). Changes on the intrinsic fluorescence of HNP1 upon membrane binding/insertion show that HNP1 has quite distinct preferences for mammalian and fungal membrane model systems. HNP1 showed low interaction with glucosylceramide rich membranes, but high sterol selectivity: it has a higher partition for ergosterol-containing membranes (as fungal membranes) and lower interaction with cholesterol-containing membranes (as in mammalian cells). These results reveal that lipid selectivity is a determinant step for HNP1 action. Fluorescence quenching data obtained using acrylamide indicate that HNP1 interacts with membranes without a full insertion in the lipid bilayer. Generalized polarization of laurdan indicates a change in membrane fluidity in the presence of HNP1 for POPC membranes but not for ergosterol-enriched membranes.

Mechanisms of daptomycin resistance in Staphylococcus aureus: role of the cell membrane and cell wall

The bactericidal, cell membrane-targeting lipopeptide antibiotic daptomycin (DAP) is an important agent in treating invasive Staphylococcus aureus infections. However, there have been numerous recent reports of development of daptomycin resistance (DAP-R) during therapy with this agent. The mechanisms of DAP-R in S. aureus appear to be quite diverse. DAP-R strains often exhibit progressive accumulation of single nucleotide polymorphisms in the multipeptide resistance factor gene (mprF) and the yycFG components of the yycFGHI operon. Both loci are involved in key cell membrane (CM) events, with mprF being responsible for the synthesis and outer CM translocation of the positively charged phospholipid, lysyl-phosphotidylglycerol (L-PG), while the yyc operon is involved in the generalized response to stressors such as antimicrobials. In addition, other perturbations of the CM have been identified in DAP-R strains, including extremes in CM order, resistance to CM depolarization and permeabilization, and reduced surface binding of DAP. Moreover, modifications of the cell wall (CW) appear to also contribute to DAP-R, including enhanced expression of the dlt operon (involved in d-alanylation of CW teichoic acids) and progressive CW thickening.

Modulating the activity of short arginine-tryptophan containing antibacterial peptides with N-terminal metallocenoyl groups

A series of small synthetic arginine and tryptophan containing peptides was prepared and analyzed for their antibacterial activity. The effect of N-terminal substitution with metallocenoyl groups such as ferrocene (FcCO) and ruthenocene (RcCO) was investigated. Antibacterial activity in different media, growth inhibition, and killing kinetics of the most active peptides were determined. The toxicity of selected derivatives was determined against erythrocytes and three human cancer cell lines. It was shown that the replacement of an N-terminal arginine residue with a metallocenoyl moiety modulates the activity of WRWRW-peptides against Gram-positive and Gram-negative bacteria. MIC values of 2–6 µM for RcCO-W(RW)₂ and 1–11 µM for (RW)₃ were determined. Interestingly, W(RW)₂-peptides derivatized with ferrocene were significantly less active than those derivatized with ruthenocene which have similar structural but different electronic properties, suggesting a major influence of the latter. The high activities observed for the RcCO-W(RW)₂- and (RW)₃-peptides led to an investigation of the origin of activity of these peptides using several important activity-related parameters. Firstly, killing kinetics of the RcCO-W(RW)₂-peptide versus killing kinetics of the (RW)₃ derivative showed faster reduction of the colony forming units for the RcCO-W(RW)₂-peptide, although MIC values indicated higher activity for the (RW)₃-peptide. This was confirmed by growth inhibition studies. Secondly, hemolysis studies revealed that both peptides did not lead to significant destruction of erythrocytes, even up to 500 µg/mL for (RW)₃ and 250 µg/mL for RcCO-W(RW)₂. In addition, toxicity against three human cancer cell lines (HepG2, HT29, MCF7) showed that the (RW)₃-peptide had an IC₅₀ value of ~140 µM and the RcW(RW)₂ one of ~90 µM, indicating a potentially interesting therapeutic window. Both the killing kinetics and growth inhibition studies presented in this work point to a membrane-based mode of action for these two peptides, each having different kinetic parameters.

Transcriptome analysis of responses to rhodomyrtone in methicillin-resistant Staphylococcus aureus

Rhodomyrtone, purified from Rhodomyrtus tomentosa (Aiton) Hassk, exhibits a high degree of potency against methicillin-resistant Staphylococcus aureus (MRSA). We recently demonstrated that exposure of MRSA to a subinhibitory concentration (0.174 µg/ml) of rhodomyrtone resulted in the alteration of expression of several functional classes of bacterial proteins. To provide further insight into the antibacterial mode of action of this compound, we determined the impact of exposure to rhodomyrtone on the gene transcriptional profile of MRSA using microarray analysis. Exposure of MRSA to subinhibitory concentrations (0.5MIC; 0.5 µg/ml) of rhodomyrtone revealed significant modulation of gene expression, with induction of 64 genes and repression of 35 genes. Prominent changes in response to exposure to rhodomyrtone involved genes encoding proteins essential to metabolic pathways and processes such as amino acid metabolism, membrane function, ATP-binding cassette (ABC) transportation and lipoprotein and nucleotide metabolism. Genes involved in the synthesis of the aspartate family of amino acids, in particular proteins encoded by the dap operon were prominent. The diaminopimelate (DAP) biosynthetic pathway is the precursor of lysine synthesis and is essential for peptidoglycan biosynthesis. However, phenotypic analysis of the peptidoglycan amino acid content of rhodomyrtone-treated MRSA did not differ significantly from that extracted from control cells. Genes involved in the biosynthesis of amino acids and peptidoglycan, and a high affinity ATP-driven K ((+)) transport system, were investigated by quantitative reverse transcription-PCR (qRT-PCR) using EMRSA-16 1, 4, or 18 h after exposure to rhodomyrtone and in general the data concurred with that obtained by microarray, highlighting the relevance of the DAP biosynthetic pathway to the mode of action of rhodomyrtone.

Progressive structuring of a branched antimicrobial peptide on the path to the inner membrane target

In recent years, interest has grown in the antimicrobial properties of certain natural and non-natural peptides. The strategy of inserting a covalent branch point in a peptide can improve its antimicrobial properties while retaining host biocompatibility. However, little is known regarding possible structural transitions as the peptide moves on the access path to the presumed target, the inner membrane. Establishing the nature of the interactions with the complex bacterial outer and inner membranes is important for effective peptide design. Structure-activity relationships of an amphiphilic, branched antimicrobial peptide (B2088) are examined using environment-sensitive fluorescent probes, electron microscopy, molecular dynamics simulations, and high resolution NMR in solution and in condensed states. The peptide is reconstituted in bacterial outer membrane lipopolysaccharide extract as well as in a variety of lipid media mimicking the inner membrane of Gram-negative pathogens. Progressive structure accretion is observed for the peptide in water, LPS, and lipid environments. Despite inducing rapid aggregation of bacteria-derived lipopolysaccharides, the peptide remains highly mobile in the aggregated lattice. At the inner membranes, the peptide undergoes further structural compaction mediated by interactions with negatively charged lipids, probably causing redistribution of membrane lipids, which in turn results in increased membrane permeability and bacterial lysis. These findings suggest that peptides possessing both enhanced mobility in the bacterial outer membrane and spatial structure facilitating its interactions with the membrane-water interface may provide excellent structural motifs to develop new antimicrobials that can overcome antibiotic-resistant Gram-negative pathogens.

Further insights into the mode of action of the lipoglycopeptide telavancin through global gene expression studies

Telavancin is a novel semisynthetic lipoglycopeptide derivative of vancomycin with a decylaminoethyl side chain that is active against Gram-positive bacteria, including Staphylococcus aureus strains resistant to methicillin or vancomycin. A dual mechanism of action has been proposed for telavancin involving inhibition of peptidoglycan biosynthesis and membrane depolarization. Here we report the results of genome-wide transcriptional profiling of the response of S. aureus to telavancin using microarrays. Short (15-min) challenge of S. aureus with telavancin revealed strong expression of the cell wall stress stimulon, a characteristic response to inhibition of cell wall biosynthesis. In the transcriptome obtained after 60-min telavancin challenge, in addition to induction of the cell wall stress stimulon, there was induction of various genes, including lrgA and lrgB, lysine biosynthesis operon (dap) genes, vraD and vraE, and hlgC, that have been reported to be induced by known membrane-depolarizing and active agents, including carbonyl cyanide m-chlorophenylhydrazone, daptomycin, bacitracin, and other antimicrobial peptides These genes were either not induced or only weakly induced by the parent molecule vancomycin. We suggest that expression of these genes is a response of the cell to mitigate and detoxify such molecules and is diagnostic of a membrane-depolarizing or membrane-active molecule. The results indicate that telavancin causes early and significant induction of the cell wall stress stimulon due to strong inhibition of peptidoglycan biosynthesis, with evidence in support of membrane depolarization and membrane activity that is expressed after a longer duration of drug treatment.

Initial insights into structure-activity relationships of avian defensins

Numerous β-defensins have been identified in birds, and the potential use of these peptides as alternatives to antibiotics has been proposed, in particular to fight antibiotic-resistant and zoonotic bacterial species. Little is known about the mechanism of antibacterial activity of avian β-defensins, and this study was carried out to obtain initial insights into the involvement of structural features or specific residues in the antimicrobial activity of chicken AvBD2. Chicken AvBD2 and its enantiomeric counterpart were chemically synthesized. Peptide elongation and oxidative folding were both optimized. The similar antimicrobial activity measured for both L- and D-proteins clearly indicates that there is no chiral partner. Therefore, the bacterial membrane is in all likelihood the primary target. Moreover, this work indicates that the three-dimensional fold is required for an optimal antimicrobial activity, in particular for gram-positive bacterial strains. The three-dimensional NMR structure of chicken AvBD2 defensin displays the structural three-stranded antiparallel β-sheet characteristic of β-defensins. The surface of the molecule does not display any amphipathic character. In light of this new structure and of the king penguin AvBD103b defensin structure, the consensus sequence of the avian β-defensin family was analyzed. Well conserved residues were highlighted, and the potential strategic role of the lysine 31 residue of AvBD2 was emphasized. The synthetic AvBD2-K31A variant displayed substantial N-terminal structural modifications and a dramatic decrease in activity. Taken together, these results demonstrate the structural as well as the functional role of the critical lysine 31 residue in antimicrobial activity.

Exometabolome analysis identifies pyruvate dehydrogenase as a target for the antibiotic triphenylbismuthdichloride in multiresistant bacterial pathogens

The desperate need for new therapeutics against notoriously antibiotic-resistant bacteria has led to a quest for novel antibacterial target structures and compounds. Moreover, defining targets and modes of action of new antimicrobial compounds remains a major challenge with standard technologies. Here we characterize the antibacterial properties of triphenylbismuthdichloride (TPBC), which has recently been successfully used against device-associated infections. We demonstrate that TPBC has potent antimicrobial activity against many bacterial pathogens. Using an exometabolome profiling approach, a unique TPBC-mediated change in the metabolites of Staphylococcus aureus was identified, indicating that TPBC blocks bacterial pyruvate catabolism. Enzymatic studies showed that TPBC is a highly efficient, uncompetitive inhibitor of the bacterial pyruvate dehydrogenase complex. Our study demonstrates that metabolomics approaches can offer new avenues for studying the modes of action of antimicrobial compounds, and it indicates that inhibition of the bacterial pyruvate dehydrogenase complex may represent a promising strategy for combating multidrug-resistant bacteria.

Antibiotic activities of host defense peptides: more to it than lipid bilayer perturbation

Defensins are small basic amphiphilic peptides (up to 5 kDa) that have been shown to be important effector molecules of the innate immune system of animals, plants and fungi. In addition to immune modulatory functions, they have potent direct antimicrobial activity against a broad spectrum of bacteria, fungi and/or viruses, which makes them promising lead compounds for the development of next-generation antiinfectives. The mode of antibiotic action of defensins was long thought to result from electrostatic interaction between the positively charged defensins and negatively charged microbial membranes, followed by unspecific membrane permeabilization or pore-formation. Microbial membranes are more negatively charged than human membranes, which may explain to some extent the specificity of defensin action against microbes and associated low toxicity for the host. However, research during the past decade has demonstrated that defensin activities can be much more targeted and that microbe-specific lipid receptors are involved in the killing activity of various defensins. In this respect, human, fungal and invertebrate defensins have been shown to bind to and sequester the bacterial cell wall building block lipid II, thereby specifically inhibiting cell wall biosynthesis. Moreover, plant and insect defensins were found to interact with fungal sphingolipid receptors, resulting in fungal cell death. This review summarizes the current knowledge on the mode of action and structure of defensins from different kingdoms, with specific emphasis on their interaction with microbial lipid receptors.

Insertion of epicatechin gallate into the cytoplasmic membrane of methicillin-resistant Staphylococcus aureus disrupts penicillin-binding protein (PBP) 2a-mediated beta-lactam resistance by delocalizing PBP2

Epicatechin gallate (ECg) sensitizes methicillin-resistant Staphylococcus aureus (MRSA) to oxacillin and other beta-lactam agents; it also reduces the secretion of virulence-associated proteins, prevents biofilm formation, and induces gross morphological changes in MRSA cells without compromising the growth rate. MRSA is resistant to oxacillin because of the presence of penicillin-binding protein 2a (PBP2a), which allows peptidoglycan synthesis to continue after oxacillin-mediated acylation of native PBPs. We show that ECg binds predominantly to the cytoplasmic membrane (CM), initially decreasing the fluidity of the bilayer, and induces changes in gene expression indicative of an attempt to preserve and repair a compromised cell wall. On further incubation, the CM is reorganized; the amount of lysylphosphatidylglycerol is markedly reduced, with a concomitant increase in phosphatidylglycerol, and the proportion of branched chain fatty acids increases, resulting in a more fluid structure. We found no evidence that ECg modulates the enzymatic activity of PBP2a through direct binding to the protein but determined that PBP2 is delocalized from the FtsZ-anchored cell wall biosynthetic machinery at the septal division site following intercalation into the CM. We argue that many features of the ECg-induced phenotype can be explained by changes in the fluid dynamics of the CM.

In vitro antimicrobial efficacy of β-defensin 3 against Staphylococcus pseudintermedius isolates from healthy and atopic canine skin

β-Defensins (BDs) are highly conserved antimicrobial peptides important in innate defence against bacteria. β-Defensin 3 has a specific role in protecting the skin. This study quantified the minimal inhibitory concentration (MIC) of human (h)BD3 against Staphylococcus pseudintermedius isolates from atopic and healthy dogs. Single colony isolates (1 × 10(5) colony-forming units/mL log phase) were cultured with doubling dilutions of hBD3 in sodium phosphate buffer from 0.8 to 50 μg/mL at 37 °C for 2 h, before adding 100 μL of tryptone soy broth and incubating for a further 20 h. Bacterial growth was assessed as the mean optical density at 540 nm corrected for background. The median MIC was 12.5 μg hBD3/mL (range 3.125-25 μg/mL; n=22). Forty-five percent of the isolates were inhibited at ≤ 6.25 μg hBD3/mL, and 90% were inhibited at ≤ 12.5 μg hBD3/mL. Bacterial growth was not inhibited at ≤ 1.6 μg hBD3/mL. There were no significant differences in the inhibition by hBD3 of isolates from atopic (median MIC 12.5 μg/mL, range 6.25-25 μg/mL, n=14) and healthy dogs (median MIC 9.4 μg/mL, range 3.125-12.5 μg/mL, n = 8); from noninfected colonized sites (median MIC 12.5 μg/mL, range 3.125-25 μg/mL, n=16) and infected lesions (median MIC 9.4 μg/mL, range 6.25-12.5 μg/mL, n=6); or between sample sites (nose median MIC 12.5 μg/mL, range 6.25-25 μg/mL, n=5; perineum median MIC 12.5 μg/mL, range 3.125-25 μg/mL, n=7; ear median MIC 6.25 μg/mL, range 6.25-12.5 μg/mL, n=4; lesions median MIC 9.4 μg/mL, range 6.25-12.5 μg/mL, n=6). In conclusion, hBD3 inhibited the growth of canine S. pseudintermedius isolates in vitro irrespective of origin.