Modulation of Neisseria gonorrhoeae susceptibility to vertebrate antibacterial peptides due to a member of the resistance/nodulation/division efflux pump family

Defensins: antimicrobial peptides of innate immunity

The production of natural antibiotic peptides has emerged as an important mechanism of innate immunity in plants and animals. Defensins are diverse members of a large family of antimicrobial peptides, contributing to the antimicrobial action of granulocytes, mucosal host defence in the small intestine and epithelial host defence in the skin and elsewhere. This review, inspired by a spate of recent studies of defensins in human diseases and animal models, focuses on the biological function of defensins.

Endogenous production of antimicrobial peptides in innate immunity and human disease

Antimicrobial peptides are diverse and evolutionarily ancient molecules produced by all living organisms. Peptides belonging to the cathelicidin and defensin gene families exhibit an immune strategy as they defend against infection by inhibiting microbial survival, and modify hosts through triggering tissue-specific defense and repair events. A variety of processes have evolved in microbes to evade the action of antimicrobial peptides, including the ability to degrade or inactivate antimicrobial peptides, or suppress host production of the peptide in response to infection. Animal models and clinical investigations have shown that an absence of cathelicidin or defensin antimicrobials can lead to disease. In this article, we review important recent advances in understanding the biology of antimicrobial peptides and their role in normal immunity and human disease.

Arming the enemy: the evolution of resistance to self-proteins

A remarkable range of novel antibiotics is attracting increasing interest as a major new weapon in the campaign against bacterial infection. They are based on the toxic peptides that provide the innate immune system of animals, and it is claimed that bacteria will be unable to evolve resistance to them because they attack the 'Achilles' heel' of bacterial membrane structure. Both experimental evidence and theoretical arguments suggest that this claim is doubtful. If so, the introduction of these substances into general use may provoke the evolution of resistance to our own defence proteins and thus compromise our natural defences against infection.

Antimicrobial peptides: current status and therapeutic potential

Antimicrobial peptides (AMPs) are effector molecules of the innate immune system. A variety of AMPs have been isolated from species of all kingdoms and are classified based on their structure and amino acid motifs. AMPs have a broad antimicrobial spectrum and lyse microbial cells by interaction with biomembranes. Besides their direct antimicrobial function, they have multiple roles as mediators of inflammation with impact on epithelial and inflammatory cells influencing diverse processes such as cell proliferation, immune induction, wound healing, cytokine release, chemotaxis and protease-antiprotease balance. AMPs qualify as prototypes of innovative drugs that may be used as antimicrobials, anti-lipopolysaccharide drugs or modifiers of inflammation. Several strategies have been followed to identify lead candidates for drug development, to modify the peptides' structures, and to produce sufficient amounts for pre-clinical and clinical studies. This review summarises the current knowledge about the basic and applied biology of AMPs.

Radiolabelled antimicrobial peptides for infection detection

It can be difficult to establish whether a febrile episode in a patient is suggestive of an infectious or non-infectious cause. Besides clinical history, physical examination, and laboratory assays, scintigraphic imaging of bacterial and fungal infections using antimicrobial peptides labelled with technetium-99m (99mTc) can be useful. Key to this latter approach is that some of these peptides accumulate at sites of infection but not in sterile inflammatory lesions, because of their preferential binding to bacteria and fungi over mammalian cells. Here we report on imaging of infections with these peptides in laboratory animals. On the basis of their favourable binding characteristics, fast and easy penetration into the infected area, and rapid clearance from the circulation (half-life approximately 30 min) via the kidneys, several 99mTc-antimicrobial peptides have been selected that distinguish infectious foci from sites of sterile inflammation. Accumulation of 99mTc-antimicrobial peptides at sites of experimental infection correlated well with the number of viable bacteria/yeasts present. This finding allowed us to monitor with 99mTc-antimicrobial peptides the efficacy of antimicrobial therapy in animals with experimental infections. In conclusion, non-microbicidal amounts of 99mTc-antimicrobial peptides are promising candidates for the scintigraphic imaging of bacterial/fungal infections and for monitoring the efficacy of antimicrobial therapy in patients.

Mechanisms of antimicrobial peptide action and resistance

Antimicrobial peptides have been isolated and characterized from tissues and organisms representing virtually every kingdom and phylum, ranging from prokaryotes to humans. Yet, recurrent structural and functional themes in mechanisms of action and resistance are observed among peptides of widely diverse source and composition. Biochemical distinctions among the peptides themselves, target versus host cells, and the microenvironments in which these counterparts convene, likely provide for varying degrees of selective toxicity among diverse antimicrobial peptide types. Moreover, many antimicrobial peptides employ sophisticated and dynamic mechanisms of action to effect rapid and potent activities consistent with their likely roles in antimicrobial host defense. In balance, successful microbial pathogens have evolved multifaceted and effective countermeasures to avoid exposure to and subvert mechanisms of antimicrobial peptides. A clearer recognition of these opposing themes will significantly advance our understanding of how antimicrobial peptides function in defense against infection. Furthermore, this understanding may provide new models and strategies for developing novel antimicrobial agents, that may also augment immunity, restore potency or amplify the mechanisms of conventional antibiotics, and minimize antimicrobial resistance mechanisms among pathogens. From these perspectives, the intention of this review is to illustrate the contemporary structural and functional themes among mechanisms of antimicrobial peptide action and resistance.

The NorM efflux pump of Neisseria gonorrhoeae and Neisseria meningitidis recognizes antimicrobial cationic compounds

In Neisseria gonorrhoeae and Neisseria meningitidis, we identified a gene that would encode a protein highly similar to NorM of Vibrio parahaemolyticus (Y. Morita et al., Antimicrob. Agents Chemother. 42:1778-1782, 1998). A nonpolar insertional mutation in either the gonococcal or meningococcal norM gene resulted in increased bacterial sensitivity to compounds harboring a quaternary ammonium on an aromatic ring (e.g., ethidium bromide, acriflavine hydrochloride, 2-N-methylellipticinium, and berberine). The presence of point mutations within the -35 region of a putative norM promoter or a likely ribosome binding site resulted in an increased resistance of gonococci and meningococci to the same compounds, as well as to norfloxacin and ciprofloxacin. Structure-activity relationship studies with putative NorM substrates have found that a cationic moiety is essential for NorM recognition.

The increased bactericidal activity of a fatty acid-modified synthetic antimicrobial peptide of human cathepsin G correlates with its enhanced capacity to interact with model membranes

The bactericidal potency of a synthetic peptide (CG 117-136) of human lysosomal cathepsin G (cat G) can be substantially increased by covalent attachment to its N- or C-termini, of saturated, linear fatty acids (FAs), namely those with C-8, C-10 and C-12 hydrocarbon chains. In order to understand better the mechanism by which FA moieties increase the bactericidal activity of CG 117-136, the interaction of N-terminally FA-modified peptides with artificial membranes was studied. First, the content of secondary structure motifs in the modified and unmodified peptides was determined by circular dichroism (CD). A marked increase in the propensity of FA-modified CG 117-136 to form an alpha-helix structure was observed for the C-8, C-10 and C-12 derivatives compared with unmodified/short-chain and long-chain (C-14, C-16, C-18) derivatives. These effects were observed both in the presence of large unilamellar liposomes or in trifuluoroethanol, a membrane-stimulating agent. Second, the capacity of peptides to insert into large unilamellar liposomes as a function of FA length was determined by their ability to release a trapped fluorescent dye. FA derivatives with the highest alpha-helical content were found to be the most effective in releasing a fluorescent dye, compared with an unmodified peptide and/or derivatives having a low alpha-helical content. The ability of the peptides to attain alpha-helical structure in the membrane-like environment and the ability to disrupt the liposomal membrane, therefore correlate remarkably well with their increased ability to kill bacteria. A plausible explanation for improved bactericidal action of the modified peptide is that the FA moiety facilitates formation of the peptide with an alpha-helical structure formation in membranes, which is essential for disrupting the integrity of the bacterial cytoplasmic membrane.

Regulation of bacterial drug export systems

The active transport of toxic compounds by membrane-bound efflux proteins is becoming an increasingly frequent mechanism by which cells exhibit resistance to therapeutic drugs. This review examines the regulation of bacterial drug efflux systems, which occurs primarily at the level of transcription. Investigations into these regulatory networks have yielded a substantial volume of information that has either not been forthcoming from or complements that obtained by analysis of the transport proteins themselves. Several local regulatory proteins, including the activator BmrR from Bacillus subtilis and the repressors QacR from Staphylococcus aureus and TetR and EmrR from Escherichia coli, have been shown to mediate increases in the expression of drug efflux genes by directly sensing the presence of the toxic substrates exported by their cognate pump. This ability to bind transporter substrates has permitted detailed structural information to be gathered on protein-antimicrobial agent-ligand interactions. In addition, bacterial multidrug efflux determinants are frequently controlled at a global level and may belong to stress response regulons such as E. coli mar, expression of which is controlled by the MarA and MarR proteins. However, many regulatory systems are ill-adapted for detecting the presence of toxic pump substrates and instead are likely to respond to alternative signals related to unidentified physiological roles of the transporter. Hence, in a number of important pathogens, regulatory mutations that result in drug transporter overexpression and concomitant elevated antimicrobial resistance are often observed.

Bacterial strategies for overcoming host innate and adaptive immune responses

In higher organisms a variety of host defense mechanisms control the resident microflora and, in most cases, effectively prevent invasive microbial disease. However, it appears that microbial organisms have coevolved with their hosts to overcome protective host barriers and, in selected cases, actually take advantage of innate host responses. Many microbial pathogens avoid host recognition or dampen the subsequent immune activation through sophisticated interactions with host responses, but some pathogens benefit from the stimulation of inflammatory reactions. This review will describe the spectrum of strategies used by microbes to avoid or provoke activation of the host's immune response as well as our current understanding of the role this immunomodulatory interference plays during microbial pathogenesis.

Cloning and characterization of the Burkholderia vietnamiensis norM gene encoding a multi-drug efflux protein

Polymyxin B-sensitive mutants in Burkholderia vietnamiensis (Burkholderia cepacia genomovar V) were generated with a mini-Tn5 encoding tetracycline resistance. One of the transposon mutants had an insertion in the norM gene encoding a multi-drug efflux protein. Expression of B. vietnamiensis norM in an Escherichia coli acrAB deletion mutant complemented its norfloxacin hypersensitivity, indicating that the protein functions in drug efflux. However, no effect on antibiotic sensitivity other than sensitivity to polymyxin B was observed in the B. vietnamiensis norM mutant. We demonstrate that increased polymyxin sensitivity in B. vietnamiensis was associated with the presence of tetracycline in the growth medium, a phenotype that was partially suppressed by expression of the norM gene.

Proteinases of common pathogenic bacteria degrade and inactivate the antibacterial peptide LL-37

Effectors of the innate immune system, the anti-bacterial peptides, have pivotal roles in preventing infection at epithelial surfaces. Here we show that proteinases of the significant human pathogens Pseudomonas aeruginosa, Enterococcus faecalis, Proteus mirabilis and Streptococcus pyogenes, degrade the antibacterial peptide LL-37. Analysis by mass spectrometry of fragments generated by P. aeruginosa elastase in vitro revealed that the initial cleavages occurred at Asn-Leu and Asp-Phe, followed by two breaks at Arg-Ile, thus inactivating the peptide. Proteinases of the other pathogens also degraded LL-37 as determined by SDS-PAGE. Ex vivo, P. aeruginosa elastase induced LL-37 degradation in human wound fluid, leading to enhanced bacterial survival. The degradation was blocked by the metalloproteinase inhibitors GM6001 and 1, 10-phenantroline (both of which inhibited P. aeruginosa elastase, P. mirabilis proteinase, and E. faecalis gelatinase), or the inhibitor E64 (which inhibited S. pyogenes cysteine proteinase). Additional experiments demonstrated that dermatan sulphate and disaccharides of the structure [DeltaUA(2S)-GalNAc(4,6S)], or sucroseoctasulphate, inhibited the degradation of LL-37. The results indicate that proteolytic degradation of LL-37 is a common virulence mechanism and that molecules which block this degradation could have therapeutic potential.

How do bacteria resist human antimicrobial peptides?

Cationic antimicrobial peptides (CAMPs), such as defensins, cathelicidins and thrombocidins, are an important human defense mechanism, protecting skin and epithelia against invading microorganisms and assisting neutrophils and platelets. Staphylococcus aureus, Salmonella enterica and other bacterial pathogens have evolved countermeasures to limit the effectiveness of CAMPs, including the repulsion of CAMPs by reducing the net negative charge of the bacterial cell envelope through covalent modification of anionic molecules (e.g. teichoic acids, phospholipids and lipid A); expelling CAMPs through energy-dependent pumps; altering membrane fluidity; and cleaving CAMPs with proteases. Mutants susceptible to CAMPs are more efficiently inactivated by phagocytes and are virulence-attenuated, indicating that CAMP resistance plays a key role in bacterial infections.

Inducible, but not constitutive, resistance of gonococci to hydrophobic agents due to the MtrC-MtrD-MtrE efflux pump requires TonB-ExbB-ExbD proteins

The MtrC-MtrD-MtrE efflux pump possessed by Neisseria gonorrhoeae is very similar to the MexA-MexB-OprM efflux pump of Pseudomonas aeruginosa. Because the antimicrobial resistance property afforded by the MexA-MexB-OprM efflux pump also requires the TonB protein, we asked whether a similar requirement exists for the gonococcal efflux pump. Unlike earlier studies with P. aeruginosa, we found that constitutive levels of gonococcal resistance to hydrophobic antimicrobial agents (i.e., Triton X-100 [TX-100]) did not require the TonB, ExbB, or ExbD protein. However, inducible levels of TX-100 resistance in gonococci had an absolute requirement for the TonB-ExbB-ExbD system, suggesting that such resistance in gonococci has an energy requirement above and beyond that required for constitutive pump activity.

Antimicrobial peptides: therapeutic potential for the treatment of Candida infections

The increasing frequency of fungal infections in immunocompromised patients together with the emergence of strains resistant to currently used antifungal drugs point to an increased need for a new class of antimycotics. Antimicrobial peptides are promising candidates for the treatment of fungal infections since they have both mechanisms of action distinct from available antifungal agents and the ability to regulate the host immune defence systems as well. This review focuses on Candida albicans as a large amount of work on the mechanisms of action of classical antifungals as well as antimicrobial peptides, such as defensins, protegrins, histatins and lactoferrin (LF)-derived peptides, has been performed in this yeast. Analogues of these antimicrobial peptides and combinations of antimicrobial peptides with classical antimycotics are under investigation for treatment of candidiasis.

Binding of protegrin-1 to Pseudomonas aeruginosa and Burkholderia cepacia

BACKGROUND

Pseudomonas aeruginosa and Burkholderia cepacia infections of cystic fibrosis patients' lungs are often resistant to conventional antibiotic therapy. Protegrins are antimicrobial peptides with potent activity against many bacteria, including P. aeruginosa. The present study evaluates the correlation between protegrin-1 (PG-1) sensitivity/resistance and protegrin binding in P. aeruginosa and B. cepacia.

METHODS

The PG-1 sensitivity/resistance and PG-1 binding properties of P. aeruginosa and B. cepacia were assessed using radial diffusion assays, radioiodinated PG-1, and surface plasmon resonance (BiaCore).

RESULTS

The six P. aeruginosa strains examined were very sensitive to PG-1, exhibiting minimal active concentrations from 0.0625-0.5 microg/ml in radial diffusion assays. In contrast, all five B. cepacia strains examined were greater than 10-fold to 100-fold more resistant, with minimal active concentrations ranging from 6-10 microg/ml. When incubated with a radioiodinated variant of PG-1, a sensitive P. aeruginosa strain bound considerably more protegrin molecules per cell than a resistant B. cepacia strain. Binding/diffusion and surface plasmon resonance assays revealed that isolated lipopolysaccharide (LPS) and lipid A from the sensitive P. aeruginosa strains bound PG-1 more effectively than LPS and lipid A from resistant B. cepacia strains.

CONCLUSION

These findings support the hypothesis that the relative resistance of B. cepacia to protegrin is due to a reduced number of PG-1 binding sites on the lipid A moiety of its LPS.

The antimicrobial peptides lactoferricin B and magainin 2 cross over the bacterial cytoplasmic membrane and reside in the cytoplasm

The localization of immunolabelled antimicrobial peptides was studied using transmission electron microscopy. Staphylococcus aureus and Escherichia coli were exposed to lactoferricin B (17-41), lactoferricin B (17-31) and D-lactoferricin B (17-31). E. coli was also exposed to cecropin P1 and magainin 2. The lactoferricins were found in the cytoplasm of both bacteria. In S. aureus the amount of cytoplasmic lactoferricin B (17-41) was time- and concentration-dependent, reaching a maximum within 30 min. Cecropin P1 was confined to the cell wall, while magainin 2 was found in the cytoplasm of E. coli. The finding of intracellularly localized magainin is not reported previously.

Identification of Proteus mirabilis mutants with increased sensitivity to antimicrobial peptides

Antimicrobial peptides (APs) are important components of the innate defenses of animals, plants, and microorganisms. However, some bacterial pathogens are resistant to the action of APs. For example, Proteus mirabilis is highly resistant to the action of APs, such as polymyxin B (PM), protegrin, and the synthetic protegrin analog IB-367. To better understand this resistance, a transposon mutagenesis approach was used to generate P. mirabilis mutants sensitive to APs. Four unique PM-sensitive mutants of P. mirabilis were identified (these mutants were >2 to >128 times more sensitive than the wild type). Two of these mutants were also sensitive to IB-367 (16 and 128 times more sensitive than the wild type). Lipopolysaccharide (LPS) profiles of the PM- and protegrin-sensitive mutants demonstrated marked differences in both the lipid A and O-antigen regions, while the PM-sensitive mutants appeared to have alterations of either lipid A or O antigen. Matrix-assisted laser desorption ionization-time of flight mass spectrometry analysis of the wild-type and PM-sensitive mutant lipid A showed species with one or two aminoarabinose groups, while lipid A from the PM- and protegrin-sensitive mutants was devoid of aminoarabinose. When the mutants were streaked on an agar-containing medium, the swarming motility of the PM- and protegrin-sensitive mutants was completely inhibited and the swarming motility of the mutants sensitive to only PM was markedly decreased. DNA sequence analysis of the mutagenized loci revealed similarities to an O-acetyltransferase (PM and protegrin sensitive) and ATP synthase and sap loci (PM sensitive). These data further support the role of LPS modifications as an elaborate mechanism in the resistance of certain bacterial species to APs and suggest that LPS surface charge alterations may play a role in P. mirabilis swarming motility.

In vitro activity of PR-39, a proline-arginine-rich peptide, against susceptible and multi-drug-resistant Mycobacterium tuberculosis

We have investigated the in vitro activity of antimicrobial peptides against Mycobacterium tuberculosis using a radiometric method and cfu determinations. PR-39, a proline-arginine-rich antibacterial peptide from porcine leucocytes, was found to be active against drug-susceptible as well as multi-drug-resistant (MDR) clinical isolates of M. tuberculosis. The activity of PR-39 was concentration dependent, with 80% growth inhibition of M. tuberculosis H37Rv at 50 mg/L. The MDR M. tuberculosis strains E1380/94 and P34/95 were less susceptible to PR-39, with 39 and 49% growth inhibition at 50 mg/L peptide, respectively, suggesting a lower susceptibility than strain H37Rv and drug-susceptible clinical isolates. Reduction of counts of M. tuberculosis H37Rv and the MDR M. tuberculosis strain E1380/94 by PR-39 indicated that the growth inhibition seen in the radiometric assay is due to a mycobactericidal effect of the peptide. These observations suggest that antimicrobial peptides may play an important role in host defence against MDR M. tuberculosis.

Downregulation of bactericidal peptides in enteric infections: a novel immune escape mechanism with bacterial DNA as a potential regulator

Antibacterial peptides are active defense components of innate immunity. Several studies confirm their importance at epithelial surfaces as immediate barrier effectors in preventing infection. Here we report that early in Shigella spp. infections, expression of the antibacterial peptides LL-37 and human beta-defensin-1 is reduced or turned off. The downregulation is detected in biopsies from patients with bacillary dysenteries and in Shigella- infected cell cultures of epithelial and monocyte origin. This downregulation of immediate defense effectors might promote bacterial adherence and invasion into host epithelium and could be an important virulence parameter. Analyses of bacterial molecules causing the downregulation indicate Shigella plasmid DNA as one mediator.

Structural features and biological activities of the cathelicidin-derived antimicrobial peptides

Cathelicidins are a numerous group of mammalian proteins that carry diverse antimicrobial peptides at the C-terminus of a highly conserved preproregion. These peptides, which become active when released from the proregion, display a remarkable variety of sizes, sequences, and structures, and in fact comprise representatives of all the structural groups in which the known antimicrobial peptides have been classified. Most of the cathelicidin-derived peptides exert a broad spectrum and potent antimicrobial activity and also bind to lipopolysaccharide and neutralize its effects. In addition, some of them have recently been shown to exert other activities and might participate in host defense also by virtue of their ability to induce expression of molecules involved in a variety of biological processes. This review is aimed at providing a general overview of the cathelicidins and of the peptides derived therefrom, with emphasis on aspects such as structure, biological activities in vitro and in vivo, and structure/activity relationship studies.

Antisense PNA effects in Escherichia coli are limited by the outer-membrane LPS layer

Antisense peptide nucleic acids (PNAs) can inhibit Escherichia coli gene expression and cell growth through sequence-specific RNA binding, and this opens possibilities for novel anti-infective agents and tools for microbial functional genomics. However, the cellular effects of PNAs are limited relative to effects in cell extracts, presumably because of cell barrier components such as the outer-membrane lipopolysaccharide (LPS) layer or drug efflux pumps, both of which function to exclude antibiotics and other foreign molecules. To evaluate the importance of such cellular factors on PNA effects, the authors developed a positive assay for antisense inhibition by targeting the lac operon repressor and compared PNA susceptibilities in mutant and wild-type E. coli by assessing lacZ induction. Strains with defective LPS (AS19 and D22) were more permeable to the antibiotic nitrocefin and more susceptible to PNA than the wild-type. Also, PNA potency was improved in wild-type cells grown in the presence of certain cell-wall-permeabilizing agents. In contrast, the activities of the Acr and Emr drug efflux pumps were not found to affect PNA susceptibility. The results show that the LPS layer is a major barrier against cell entry, but PNAs that can enter E. coli are likely to remain active inside cells.

Mechanisms of bacterial resistance and response to bile

Enteric bacteria are resistant to the bactericidal effects of intestinal bile, but these resistance mechanisms are not completely understood. It is becoming increasingly apparent that enteric bacteria have evolved to utilize bile as a signal for the temporal production of virulence factors and other adaptive mechanisms. A greater understanding of the resistance and response of bacteria to bile may assist the development of novel therapeutic, prevention, and diagnostic strategies to treat enteric and extraintestinal infections.

Temperature-regulated efflux pump/potassium antiporter system mediates resistance to cationic antimicrobial peptides in Yersinia

Most bacterial pathogens are resistant to cationic antimicrobial peptides (CAMPs) that are key components of the innate immunity of both vertebrates and invertebrates. In Gram-negative bacteria, the known CAMPs resistance mechanisms involve outer membrane (OM) modifications and specifically those in the lipopolysaccharide (LPS) molecule. Here we report, the characterization of a novel CAMPs resistance mechanism present in Yersinia that is dependent on an efflux pump/potassium antiporter system formed by the RosA and RosB proteins. The RosA/RosB system is activated by a temperature shift to 37 degrees C, but is also induced by the presence of the CAMPs, such as polymyxin B. This is the first report of a CAMPs resistance system that is induced by the presence of CAMPs. It is proposed that the RosA/RosB system protects the bacteria by both acidifying the cytoplasm to prevent the CAMPs action and pumping the CAMPs out of the cell.

Animal antimicrobial peptides: an overview

Antibiotic peptides are a key component of the innate immune systems of most multicellular organisms. Despite broad divergences in sequence and taxonomy, most antibiotic peptides share a common mechanism of action, i.e., membrane permeabilization of the pathogen. This review provides a general introduction to the subject, with emphasis on aspects such as structural types, post-translational modifications, mode of action or mechanisms of resistance. Some of these questions are treated in depth in other reviews in this issue. The review also discusses the role of antimicrobial peptides in nature, including several pathological conditions, as well as recent accounts of their application at the preclinical level.

In vitro resistance of Staphylococcus aureus to thrombin-induced platelet microbicidal protein is associated with alterations in cytoplasmic membrane fluidity

Platelet microbicidal proteins (PMPs) are small, cationic peptides which possess potent microbicidal activities against common bloodstream pathogens, such as Staphylococcus aureus. We previously showed that S. aureus strains exhibiting resistance to thrombin-induced PMP (tPMP-1) in vitro have an enhanced capacity to cause human and experimental endocarditis (T. Wu, M. R. Yeaman, and A. S. Bayer, Antimicrob. Agents Chemother. 38:729-732, 1994; A. S. Bayer et al., Antimicrob. Agents Chemother. 42:3169-3172, 1998; V. K. Dhawan et al., Infect. Immun. 65:3293-3299, 1997). However, the mechanisms mediating tPMP-1 resistance in S. aureus are not fully delineated. The S. aureus cell membrane appears to be a principal target for the action of tPMP-1. To gain insight into the basis of tPMP-1 resistance, we compared several parameters of membrane structure and function in three tPMP-1-resistant (tPMP-1(r)) strains and their genetically related, tPMP-1-susceptible (tPMP-1(s)) counterpart strains. The tPMP-1(r) strains were derived by three distinct methods: transposon mutagenesis, serial passage in the presence of tPMP-1 in vitro, or carriage of a naturally occurring multiresistance plasmid (pSK1). All tPMP-1(r) strains were found to possess elevated levels of longer-chain, unsaturated membrane lipids, in comparison to their tPMP-1(s) counterparts. This was reflected in corresponding differences in cell membrane fluidity in the strain pairs, with tPMP-1(r) strains exhibiting significantly higher degrees of fluidity as assessed by fluorescence polarization. These data provide further support for the concept that specific alterations in the cytoplasmic membrane of S. aureus strains are associated with tPMP-1 resistance in vitro.

Epithelial antimicrobial peptides in host defense against infection

One component of host defense at mucosal surfaces seems to be epithelium-derived antimicrobial peptides. Antimicrobial peptides are classified on the basis of their structure and amino acid motifs. Peptides of the defensin, cathelicidin, and histatin classes are found in humans. In the airways, alpha-defensins and the cathelicidin LL-37/hCAP-18 originate from neutrophils. beta-Defensins and LL-37/hCAP-18 are produced by the respiratory epithelium and the alveolar macrophage and secreted into the airway surface fluid. Beside their direct antimicrobial function, antimicrobial peptides have multiple roles as mediators of inflammation with effects on epithelial and inflammatory cells, influencing such diverse processes as proliferation, immune induction, wound healing, cytokine release, chemotaxis, protease-antiprotease balance, and redox homeostasis. Further, antimicrobial peptides qualify as prototypes of innovative drugs that might be used as antibiotics, anti-lipopolysaccharide drugs, or modifiers of inflammation.

Plasmid-mediated resistance to thrombin-induced platelet microbicidal protein in staphylococci: role of the qacA locus

Thrombin-induced platelet microbicidal protein 1 (tPMP-1) is a small, cationic peptide released from rabbit platelets following thrombin stimulation. In vitro resistance to this peptide among strains of Staphylococcus aureus correlates with the survival advantage of such strains at sites of endothelial damage in humans as well as in experimental endovascular infections. The mechanisms involved in the phenotypic resistance of S. aureus to tPMP-1 are not fully delineated. The plasmid-encoded staphylococcal gene qacA mediates multidrug resistance to multiple organic cations via a proton motive force-dependent efflux pump. We studied whether the qacA gene might also confer resistance to cationic tPMP-1. Staphylococcal plasmids encoding qacA were found to confer resistance to tPMP-1 in an otherwise susceptible parental strain. Deletions which removed the region containing the qacA gene in the S. aureus multiresistance plasmid pSK1 abolished tPMP-1 resistance. Resistance to tPMP-1 in the qacA-bearing strains was inoculum independent but peptide concentration dependent, with the level of resistance decreasing at higher peptide concentrations for a given inoculum. There was no apparent cross-resistance in qacA-bearing strains to other endogenous cationic antimicrobial peptides which are structurally distinct from tPMP-1, including human neutrophil defensin 1, protamine, or the staphylococcal lantibiotics pep5 and nisin. These data demonstrate that the staphylococcal multidrug resistance gene qacA also mediates in vitro resistance to cationic tPMP-1.

Induction of the mtrCDE-encoded efflux pump system of Neisseria gonorrhoeae requires MtrA, an AraC-like protein

The mtr (multiple transferable resistance) gene complex in Neisseria gonorrhoeae encodes an energy-dependent efflux pump composed of the MtrC-MtrD-MtrE cell envelope proteins that serves to export structurally diverse antimicrobial, hydrophobic agents (HAs). Many of these agents have membrane-acting detergent activity. Using Triton X-100 (TX-100) as a representative HA, we found that the mtrCDE efflux pump operon could be induced to higher levels of expression when an HA-sensitive strain was exposed to sublethal concentrations of this non-ionic detergent and the structurally related spermicide, nonoxynol-9. This induction was at the level of mtrCDE gene transcription and was independent of the MtrR repressor, which normally decreases mtrCDE gene expression. However, the enhanced resistance of gonococci to TX-100 was dependent on the expression of a previously undescribed gonococcal protein that belonged to the AraC/XylS family of transcriptional activators. We have termed this protein MtrA to signify its likely role in the activation of mtrCDE gene expression. Taken together with previous studies dealing with the genetic control of mtrCDE gene expression, we propose that gonococci can modulate their resistance to HAs through both positive and negative transcriptional control processes. The action of these regulatory processes is probably of importance in determining the survival capacity of gonococci at mucosal surfaces that contain detergent-like HAs.

The farAB-encoded efflux pump mediates resistance of gonococci to long-chained antibacterial fatty acids

Gonococci often infect mucosal surfaces bathed in antibacterial fatty acids (FAs). Resistance of gonococci to FAs and other antibacterial hydrophobic agents has been attributed to the mtrCDE-encoded efflux pump system and a heretofore undefined mechanism. This alternative resistance mechanism has been suggested to mediate gonococcal resistance to long-chained FAs independently of the mtr efflux pump. We have now identified this alternative FA resistance system in gonococci and report that it bears significant similarity to the emrAB-encoded efflux pump possessed by Escherichia coli and the vceAB-encoded pump of Vibrio cholerae. We termed the gonococcal version of this efflux pump farAB (fatty acid resistance) to signify its involvement in FA resistance expressed by gonococci and to distinguish it from the emrAB- or vceAB-encoded pumps that modulate bacterial susceptibility to uncoupling agents and certain antibiotics. Although the farAB system in gonococci was found to provide resistance to FAs independently of the mtrCDE-encoded efflux pump, its function was dependent on the MtrE outer membrane protein. Moreover, expression of the tandemly linked farA and farB genes was positively associated with the presence of the MtrR transcriptional regulatory protein that normally downregulates the expression of mtrCDE. Thus, the data presented herein suggest that, while the mtrCDE- and farAB-encoded systems act independently to mediate resistance of gonococci to host-derived, hydrophobic antimicrobial agents, their capacity to export these agents is dependent on the same outer membrane protein (MtrE), and their expression may be differentially controlled by the same transcriptional regulatory protein (MtrR).

Salt-resistant alpha-helical cationic antimicrobial peptides

Analogues based on the insect cecropin-bee melittin hybrid peptide (CEME) were studied and analyzed for activity and salt resistance. The new variants were designed to have an increase in amphipathic alpha-helical content (CP29 and CP26) and in overall positive charge (CP26). The alpha-helicity of these peptides was demonstrated by circular dichroism spectroscopy in the presence of liposomes. CP29 was shown to have activity against gram-negative bacteria that was similar to or better than those of the parent peptides, and CP26 had similar activity. CP29 had cytoplasmic membrane permeabilization activity, as assessed by the unmasking of cytoplasmic beta-galactosidase, similar to that of CEME and its more positively charged derivative named CEMA, whereas CP26 was substantially less effective. The activity of the peptides was not greatly attenuated by an uncoupler of membrane potential, carbonyl cyanide-m-chlorophenylhydrazone. The tryptophan residue in position 2 was shown to be necessary for interaction with cell membranes, as demonstrated by a complete lack of activity in the peptide CP208. Peptides CP29, CEME, and CEMA were resistant to antagonism by 0.1 to 0.3 M NaCl; however, CP26 was resistant to antagonism only by up to 160 mM NaCl. The peptides were generally more antagonized by 3 and 5 mM Mg2+ and by the polyanion alginate. It appeared that the positively charged C terminus in CP26 altered its ability to permeabilize the cytoplasmic membrane of Escherichia coli, although CP26 maintained its ability to kill gram-negative bacteria. These peptides are potential candidates for future therapeutic drugs.

Biological properties of structurally related alpha-helical cationic antimicrobial peptides

A series of alpha-helical cationic antimicrobial peptide variants with small amino acid changes was designed. Alterations in the charge, hydrophobicity, or length of the variant peptides did not improve the antimicrobial activity, and there was no statistically significant correlation between any of these factors and the MIC for Pseudomonas aeruginosa, Escherichia coli, or Salmonella typhimurium. Individual peptides demonstrated synergy with conventional antibiotics against antibiotic-resistant strains of P. aeruginosa. The peptides varied considerably in the ability to bind E. coli O111:B4 lipopolysaccharide (LPS), and this correlated significantly with their antimicrobial activity and ability to block LPS-stimulated tumor necrosis factor and interleukin-6 production. In general, the peptides studied here demonstrated a broad range of activities, including antimicrobial, antiendotoxin, and enhancer activities.

Antimicrobial peptides in mammalian and insect host defence

During the past year, additional insights into systems that regulate antimicrobial peptide production in Drosophila were reported. Granulysin, a peptide stored in the cytoplasmic granules of human natural killer cells and cytolytic T cells, was shown to kill Mycobacterium tuberculosis. More data implicating antimicrobial peptides in the pathogenesis of bronchopulmonary infections in cystic fibrosis appeared. Studies that examined the potential contributions of antimicrobial peptides to regional innate immunity gained in prominence. Efforts to design peptide analogues to prevent or treat infections continued.

The lipopolysaccharide of Bordetella bronchiseptica acts as a protective shield against antimicrobial peptides

Resistance profiles of the two Bordetella species B. bronchiseptica and B. pertussis against various antimicrobial peptides were determined in liquid survival and agar diffusion assays. B. bronchiseptica exhibited significantly higher resistance against all tested peptides than B. pertussis. The most powerful agents acting on B. bronchiseptica were, in the order of their killing efficiencies, cecropin P > cecropin B > magainin-II-amide > protamine > melittin. Interestingly, for B. bronchiseptica, the resistance level was significantly affected by phase variation, as a bvgS deletion derivative showed an increased sensitivity to these peptides. Tn5-induced protamine-sensitive B. bronchiseptica mutants, which were found to be very susceptible to most of the cationic peptides, were isolated. In two of these mutants, the genetic loci inactivated by transposon insertion were identified as containing genes highly homologous to the wlbA and wlbL genes of B. pertussis that are involved in the biosynthesis of lipopolysaccharide (LPS). In agreement with this finding, the two peptide-sensitive mutants revealed structural changes in the LPS, resulting in the loss of the O-specific side chains and the prevalence of the LPS core structure. This demonstrates that LPS plays a major role in the resistance of B. bronchiseptica against the action of antimicrobial peptides and suggests that B. pertussis is much more susceptible to these peptides due to the lack of the highly charged O-specific sugar side chains.

Antibacterials - mechanisms of action

Recent studies on antibacterials have focused on the development of antimycobacterial agents and antibacterial peptides, and on furthering the understanding of agents that have been available for several decades, including imidazoles, beta-lactams and quinolones. New areas of research include antisense oligonucleotides, antibacterial peptides and a new class of agents, oxazolidinones.

The identification of exported proteins with gene fusions to invasin

Exported proteins are integral to understanding the biology of bacterial organisms. They have special significance in pathogenesis research because they can mediate critical interactions between pathogens and eukaryotic cell surfaces. Further, they frequently serve as targets for vaccines and diagnostic tests. The commonly used genetic assays for identifying exported proteins use fusions to alkaline phosphatase or beta-lactamase. These systems are not ideal for identifying outer membrane proteins because they identify a large number of inner membrane proteins as well. We addressed this problem by developing a gene fusion system that preferentially identifies proteins that contain cleavable signal sequences and are released from the inner membrane. This system selects fusions that restore outer membrane localization to an amino terminal-truncated Yersinia pseudotuberculosis invasin derivative. In the present study, a variety of Salmonella typhimurium proteins that localize beyond the inner membrane were identified with gene fusions to this invasin derivative. Previously undescribed proteins identified include ones that share homology with components of fimbrial operons, multiple drug resistance efflux pumps and a haemolysin. All of the positive clones analysed contain cleavable signal sequences. Moreover, over 40% of the genes identified encode putative outer membrane proteins. This system has several features that may make it especially useful in the study of genetically intractable organisms.