Resistance to antimicrobial peptides in Gram-negative bacteria

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

Design of Gram-negative selective antimicrobial peptides

Lipopolysaccharide (LPS), a major component of Gram-negative bacteria, signals bacterial invasion and triggers defensive host responses. However, excessive responses also lead to the serious pathophysiological consequence of septic shock. To develop Gram-negative selective compounds that can inhibit the effects of LPS-induced sepsis, we have designed constrained cyclic antimicrobial peptides based on a cystine-stabilized beta-stranded framework mimicking the putative LPS-binding sites of the LPS-binding protein family. Our prototype termed R4A, c(PACRCRAG-PARCRCAG), consists of an eight amino acid degenerated repeat constrained by a head-to-tail cyclic peptide backbone and two cross-bracing disulfides. NMR study of K4A, an R4A analogue with four Arg --> Lys replacements, confirmed the amphipathic design elements with four Lys on one face of the antiparallel beta-strand and two hydrophobic cystine pairs plus two Ala on the opposite face. K4A and R4A displayed moderate microbicidal potency and Gram-negative selectivity. However, R4A analogues with single or multiple replacements of Ala and Gly with Arg or bulky hydrophobic amino acids displayed increased potency and selectivity in both low- and high-salt conditions. Analogues R5L and R6Y containing additional cationic and bulky hydrophobic amino acids proved the best mimics of the amphipathic topology of the "active-site" beta-strands of LPS-binding proteins. They displayed potent activity against Gram-negative E. coli with a minimal inhibitory concentration of 20 nM and a >200-fold selectivity over Gram-positive S. aureus. Our results suggest that an LPS-targeted design may present an effective approach for preparing selective peptide antibiotics.     

Insect antimicrobial peptides show potentiating functional interactions against Gram-negative bacteria

Antimicrobial peptides (AMPs) and proteins are important components of innateimmunity against pathogens in insects. The production of AMPs is costly owing toresource-based trade-offs, and strategies maximizing the efficacy of AMPs at lowconcentrations are therefore likely to be advantageous. Here, we show thepotentiating functional interaction of co-occurring insect AMPs (the bumblebeelinear peptides hymenoptaecin and abaecin) resulting in more potentantimicrobial effects at low concentrations. Abaecin displayed no detectableactivity against Escherichia coli when tested alone atconcentrations of up to 200 μM, whereas hymenoptaecin affected bacterialcell growth and viability but only at concentrations greater than 2 μM.In combination, as little as 1.25 μM abaecin enhanced the bactericidaleffects of hymenoptaecin. To understand these potentiating functionalinteractions, we investigated their mechanisms of action using atomic forcemicroscopy and fluorescence resonance energy transfer-based quenching assays.Abaecin was found to reduce the minimal inhibitory concentration ofhymenoptaecin and to interact with the bacterial chaperone DnaK (anevolutionarily conserved central organizer of the bacterial chaperone network)when the membrane was compromised by hymenoptaecin. These naturally occurringpotentiating interactions suggest that combinations of AMPs could be usedtherapeutically against Gram-negative bacterial pathogens that have acquiredresistance to common antibiotics.     

The lipopolysaccharide transport system of Gram-negative bacteria

The cell envelope of Gram-negative bacteria consists of two distinct membranes, the inner (IM) and the outer membrane (OM) separated by the periplasm. The OM contains in the outer leaflet the lipopolysaccharide (LPS), a complex lipid with important biological activities. In the host it elicits the innate immune response whereas in the bacterium it is responsible for the peculiar permeability barrier properties exhibited by the OM. The chemical structure of LPS and its biosynthetic pathways have been fully elucidated. By contrast only recently details of the transport and assembly of LPS into the OM have emerged. LPS is synthesized in the cytoplasm and at the inner leaflet of the IM and needs to cross two different compartments, the IM and the periplasm, to reach its final destination at the OM. This review focuses on recent studies that led to our present understanding of the protein machine implicated in LPS transport and in assembly at the cell surface.     

Class II antimicrobial peptides from lactic acid bacteria

Strains of lactic acid bacteria (LAB) produce a wide variety of antibacterial peptides. More than fifty of these so-called peptide bacteriocins have been isolated in the last few years. They contain 20-60 amino acids, and are cationic and hydrophobic in nature. Several of these bacteriocins consist of two complementary peptides. The peptide bacteriocins of LAB are inhibitory at concentrations in the nanomolar range, and cause membrane permeabilization and leakage of intracellular components in sensitive cells. The inhibitory spectrum is limited to gram-positive bacteria, and in many cases to bacteria closely related to the producing strain. Among the target organisms are food spoilage bacteria and pathogens such as Listeria, so that many of these antimicrobial peptides could have a potential as food preservatives as well as in medical applications.     

Prediction of lipoprotein signal peptides in Gram-negative bacteria

A method to predict lipoprotein signal peptides in Gram-negative Eubacteria, LipoP, has been developed. The hidden Markov model (HMM) was able to distinguish between lipoproteins (SPaseII-cleaved proteins), SPaseI-cleaved proteins, cytoplasmic proteins, and transmembrane proteins. This predictor was able to predict 96.8% of the lipoproteins correctly with only 0.3% false positives in a set of SPaseI-cleaved, cytoplasmic, and transmembrane proteins. The results obtained were significantly better than those of previously developed methods. Even though Gram-positive lipoprotein signal peptides differ from Gram-negatives, the HMM was able to identify 92.9% of the lipoproteins included in a Gram-positive test set. A genome search was carried out for 12 Gram-negative genomes and one Gram-positive genome. The results for Escherichia coli K12 were compared with new experimental data, and the predictions by the HMM agree well with the experimentally verified lipoproteins. A neural network-based predictor was developed for comparison, and it gave very similar results. LipoP is available as a Web server at www.cbs.dtu.dk/services/LipoP/.     

Design of self-processing antimicrobial peptides for plant protection

Small antimicrobial peptides are excellent candidates for inclusion in self-processing proteins that could be used to confer pathogen resistance in transgenic plants. Antimicrobial peptides as small as 22 amino acids in length have been designed to incorporate the residual amino acids left from protein processing by the tobacco etch virus'(TEVs') NIa protease. Also, by minimizing the length of these peptides and the number of highly hydrophobic residues, haemolytic activity was reduced without affecting the peptide's antimicrobial activity.     

革兰氏阴性菌脂多糖运输系统的构成及作用机制

革兰氏阴性菌包含有两层组分不同的膜结构——内膜和外膜,对大多数革兰氏阴性菌而言,脂多糖(lipopolysaccharides,LPS)是其外膜上最主要的脂质成分,锚定在外膜小叶(the outer leaflet of the OM)上,是革兰氏阴性菌固有免疫的重要组成部分。脂多糖运输系统(lipopolysaccharide transport system,Lpt)将胞内装配完整的LPS正确装配到外膜,使得与脂多糖相关的阻渗、有机溶剂耐受性、疏水性抗生素耐受性、膜通透性等功能得以实现。该运输系统的正确作用主要依赖7个不同的脂多糖运输蛋白(Lpt ABCDEFG)协同完成,整个系统贯穿细菌内膜至外膜,由内膜上ABC转运体复合物Lpt B2FG、胞质内转运协同蛋白Lpt A/C及被许多学者称作脂多糖运输的"命门"的外膜蛋白复合物Lpt DE共同构成。本文就革兰氏阴性菌脂多糖的具体结构功能进行简介,进而综述脂多糖运输系统的7个蛋白的构成和作用机制,以期为进一步研究该系统中每个蛋白的功能提供理论基础及参考。     

Synthetic molecular evolution of antimicrobial peptides

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Stress responses as determinants of antimicrobial resistance in Gram-negative bacteria

Bacteria encounter a myriad of potentially growth-compromising conditions in nature and in hosts of pathogenic bacteria. These 'stresses' typically elicit protective and/or adaptive responses that serve to enhance bacterial survivability. Because they impact upon many of the same cellular components and processes that are targeted by antimicrobials, adaptive stress responses can influence antimicrobial susceptibility. In targeting and interfering with key cellular processes, antimicrobials themselves are 'stressors' to which protective stress responses have also evolved. Cellular responses to nutrient limitation (nutrient stress), oxidative and nitrosative stress, cell envelope damage (envelope stress), antimicrobial exposure and other growth-compromising stresses, have all been linked to the development of antimicrobial resistance in Gram-negative bacteria - resulting from the stimulation of protective changes to cell physiology, activation of resistance mechanisms, promotion of resistant lifestyles (biofilms), and induction of resistance mutations.     

The mechanism of secretion of hemolysin and other polypeptides from Gram-negative bacteria

In the secretion of polypeptides from Gram-negative bacteria, the outer membrane constitutes a specific barrier which has to be circumvented. In the majority of systems, secretion is two-step process, with initial export to the periplasm involving an N-terminal signal sequence. Transport across the outer membrane then involves a variable number of ancillary polypeptides including both periplasmic and outer membrane. While such ancillary proteins are probably specific for each secreted protein, the mechanism of movement across the outer membrane is unknown. In contrast to these systems, secretion of theE. coli hemolysin (HlyA) has several distinctive features. These include a novel targeting signal located within the last 50 or so C-terminal amino acids, the absence of any periplasmic intermediates in transfer, and a specific membrane-bound translocator, HlyB, with important mammalian homologues such as P-glycoprotein (Mdr) and the cystic fibrosis protein. In this review we discuss the nature of the HlyA targeting signal, the structure and function of HlyB, and the probability that HlyA is secreted directly to the medium through a trans-envelope complex composed of HlyB and HlyD.     

Temporins and their synergism against Gram-negative bacteria and in lipopolysaccharide detoxification

Ribosomally synthesized antimicrobial peptides (AMPs) represent an essential component of the ancient and non-specific innate immune system in all forms of life, with the primary role of killing infectious microorganisms. Amphibian skin is one of the richest storehouses for them. Each frog species produces its own set of peptides with up to 10 isoforms, as in the case of the species Rana temporaria. Nowadays, human health is facing two major threats: (i) the increasing emergence of resistant pathogens to one or more available drugs, and (ii) the onset of septic shock, which is associated with the release of lipopolysaccharide (LPS) from the cell walls of Gram-negative bacteria, particularly upon antibiotic treatment. AMPs are considered as potential new anti-infective compounds with a novel mode of action, because many of them can kill bacteria and, at the same time, neutralize the toxic effects of LPS. Recent studies have suggested that the production of large number of structurally similar AMPs within the same animal is a strategy used by nature to increase the spectrum of antimicrobial activities, by using combinations of the peptide's isoforms. The biological rationale for their coexistence within the same organism is discussed. In addition, the distinctive and attractive synergistic effects of temporins in both antimicrobial and anti-endotoxin activities are reviewed, along with their plausible underlying molecular mechanism.     

Molecular characterization of antimicrobial resistance in Gram-negative bacteria isolated from bovine mastitis in Egypt

The aim of this study was to characterize the genetic basis of multidrug resistance in Gram-negative bacteria isolated from bovine mastitis cases in Egypt. Multidrug resistance phenotypes were found in 34 of 112 (30.4%) Gram-negative bacterial isolates, which harbored at least one antimicrobial resistance gene. The most prevalent multidrug-resistant (MDR) species were Enterobacter cloacae (8 isolates, 7.1%), Klebsiella pneumoniae (7 isolates, 6.3%), Klebsiella oxytoca (7 isolates, 6.3%), Escherichia coli (5 isolates, 4.5%), and Citrobacter freundii (3 isolates, 2.7%). The most commonly observed resistance phenotypes were against ampicillin (97.0%), streptomycin (94.1%), tetracycline (91.2%), trimethoprim-sulfamethoxazole (88.2%), nalidixic acid (85.3%), and chloramphenicol (76.5%). Class 1 integrons were detected in 28 (25.0%) isolates. The gene cassettes within class 1 integrons included those encoding resistance to trimethoprim (dfrA1, dfrA5, dfrA7, dfrA12, dfrA15, dfrA17, and dfrA25), aminoglycosides (aadA1, aadA2, aadA5, aadA7, aadA12, aadA22, and aac(3)-Id), chloramphenicol (cmlA), erythromycin (ereA2), and rifampicin (arr-3). Class 2 integrons were identified in 6 isolates (5.4%) with three different profiles. Furthermore, the β-lactamase encoding genes, bla(TEM), bla(SHV), bla(CTX-M), and bla(OXA), the plasmid-mediated quinolone resistance genes, qnr and aac(6)-Ib-cr, and the florfenicol resistance gene, floR, were also identified. To the best of our knowledge, the results identified class 2 integrons, qnr and aac(6)-Ib-cr from cases of mastitis for the first time. This is the first report of molecular characterization for antimicrobial resistance in Gram-negative bacteria isolated from bovine mastitis in Africa.     

A synergism between temporins toward Gram-negative bacteria overcomes resistance imposed by the lipopolysaccharide protective layer

Temporins are short and homologous antimicrobial peptides (AMPs) isolated from the frog skin of Rana genus. To date, very little is known about the biological significance of the presence of closely related AMPs in single living organisms. Here we addressed this question using temporins A, B, and L isolated from Rana temporaria. We found that temporins A and B are only weakly active toward Gram-negative bacteria. However, a marked synergism occurs when each is mixed with temporin L. To shed light on the underlying mechanisms involved in these activities, we used various experimental strategies to investigate: (i) the effect of the peptides' interaction on both the viability and membrane permeability of intact bacteria and spheroplasts; (ii) their interaction with lipopolysaccharides (LPS) and the effect of LPS on the oligomeric state of temporins, alone or combining one with another; (iii) their structure in solution and when bound to LPS, by using circular dichroism and ATR-FTIR spectroscopies. Our data reveal that temporin L synergizes with A and B by preventing their oligomerization in LPS. This should promote their translocation across the outer membrane into the cytoplasmic membrane. To the best of our knowledge, this is the first study that explains how a combination of native AMPs from the same species can overcome bacterial resistance imposed by the LPS leaflet.     

Oxidized phospholipids as potential molecular targets for antimicrobial peptides

The effects of oxidatively modified phospholipids on the association with model biomembranes of four antimicrobial peptides (AMPs), temporin B and L, indolicidin, and LL-37(F27W) were studied by Langmuir balance and fluorescence spectroscopy. In keeping with previous reports the negatively charged phospholipid phosphatidylglycerol (PG) enhanced the intercalation of all four peptides into lipid monolayers and liposomal bilayers under low ionic strength conditions. Interestingly, similar effect was observed for 1-palmitoyl-2-(9′-oxo-nonanoyl)-sn-glycero-3-phosphocholine (PoxnoPC), a zwitterionic oxidized phospholipid bearing an aldehyde function at the end of its truncated sn-2 acyl chain. Instead, the structurally similar 1-palmitoyl-2-azelaoyl-sn-glycero-3-phosphocholine (PazePC) containing a carboxylic moiety was less efficient in promoting the membrane association of these peptides. Physiological saline reduced the binding of the above peptides to membranes containing PG, whereas interactions with PoxnoPC were found to be insensitive to ionic strength. Notably, membrane intercalation of temporin L, the most surface active of the above peptides could be into PoxnoPC containing monolayers was strongly attenuated by methoxyamine, suggesting the importance of Schiff base formation between peptide amino groups and the lipid aldehyde function. PoxnoPC and similar aldehyde bearing oxidatively modified phospholipids could represent novel molecular targets for AMPs.     

Isolation and molecular characterization of multidrug-resistant Gram-negative bacteria from imported flamingos in Japan

Imported animals, especially those from developing countries, may constitute a potential hazard to native animals and to public health. In this study, a new flock of lesser flamingos imported from Tanzania to Hiroshima Zoological Park were screened for multidrug-resistant Gram-negative bacteria, integrons and antimicrobial resistance genes. Thirty-seven Gram-negative bacterial isolates were obtained from the flamingos. Seven isolates (18.9%) showed multidrug resistance phenotypes, the most common being against: ampicillin, streptomycin, tetracycline, trimethoprim/sulfamethoxazole and nalidixic acid. Molecular analyses identified class 1 and class 2 integrons, β-lactamase-encoding genes, bla _TEM-1 and bla _CTX-M-2 and the plasmid-mediated quinolone resistance genes, qnrS and qnrB. This study highlights the role of animal importation in the dissemination of multidrug-resistant bacteria, integrons and antimicrobial resistance genes from one country to another.     

Outer membrane lipoprotein Lpp is Gram-negative bacterial cell surface receptor for cationic antimicrobial peptides

Antimicrobial peptides/proteins (AMPs) are important components of the host innate defense mechanisms. Here we demonstrate that the outer membrane lipoprotein, Lpp, of Enterobacteriaceae interacts with and promotes susceptibility to the bactericidal activities of AMPs. The oligomeric Lpp was specifically recognized by several cationic α-helical AMPs, including SMAP-29, CAP-18, and LL-37; AMP-mediated bactericidal activities were blocked by anti-Lpp antibody blocking. Blebbing of the outer membrane and increase in membrane permeability occurred in association with the coordinate internalization of Lpp and AMP. Interestingly, the specific binding of AMP to Lpp was resistant to divalent cations and salts, which were able to inhibit the bactericidal activities of some AMPs. Furthermore, using His-tagged Lpp as a ligand, we retrieved several characterized AMPs, including SMAP-29 and hRNase 7, from a peptide library containing crude mammalian cell lysates. Overall, this study explores a new mechanism and target of antimicrobial activity and provides a novel method for screening of antimicrobials for use against drug-resistant bacteria.     

抗菌肽作用机制的研究进展

抗菌肽是一类来源于多种生物、能有效杀灭病原体的小分子多肽,具有活性谱广、作用强且迅速、不易产生耐药等众多优点.作为新一代抗感染候选药物,抗菌肽的作用机制还未完全清楚,但目前有两种观点已得到公认,即胞膜渗透作用破坏胞膜结构完整性和作用于胞内不同靶点干扰细菌生长及代谢平衡.本文主要就抗菌肽理化性质、二级结构、作用机制以及后两者间的关系做一总结,以便更好的理解抗菌肽的构效关系,为合理设计抗菌肽提供理论基础.