Functional interrelationships between cell membrane and cell wall in antimicrobial peptide-mediated killing of Staphylococcus aureus

An energy coupling factor transporter of Streptococcus sanguinis impacts antibiotic susceptibility as well as metal and membrane homeostasis

Streptococcus sanguinis is a prevalent member of human microbiome capable of acting as a causative agent of oral and respiratory infections. S. sanguinis competitive success within the infection niche is dependent on acquisition of metal ions and vitamins. Among the systems that bacteria use for micronutrient uptake is the energy coupling factor (ECF) transporter system EcfAAT. Here we describe physiological changes arising from EcfAAT transporter disruption. We found that EcfAAT contributes to S. sanguinis antibiotic sensitivity as well as metal and membrane homeostasis. Specifically, our work found that disruption of EcfAAT results in increased polymyxin susceptibility. We performed assessment of cell-associated metal content and found depletion of iron, magnesium, and manganese. Furthermore, membrane composition analysis revealed significant enrichment in unsaturated fatty acid species resulting in increased membrane fluidity. Our results demonstrate how disruption of a single EcfAAT transporter can have broad consequences on bacterial cell homeostasis. ECF transporters are of interest within the context of infection biology in bacterial species other than streptococci, hence work described here will further the understanding of how micronutrient uptake systems contribute to bacterial pathogenesis.

Importance

Proficiency in micronutrient uptake is key for pathogen success in bacteria-bacteria and bacteria-host interactions within the infection context. Micronutrient uptake mechanisms are of interest in furthering the understanding of bacterial physiology within infection niche and as targets for design of antimicrobials. Here we describe how a deletion of a nutrient uptake transporter in S. sanguinis alters bacterial sensitivity to antibiotics. We also show that a defect in this candidate nutrient uptake system has consequences on the intracellular metal content, and also results in changes in membrane fatty acid composition and fluidity. This study demonstrates how disruption of a single nutrient uptake system disrupts bacterial physiology resulting in increased antibiotic sensitivity.

Gp05, a Prophage-Encoded Virulence Factor, Contributes to Persistent Methicillin-Resistant Staphylococcus aureus Endovascular Infection

Persistent methicillin-resistant Staphylococcus aureus (MRSA) endovascular infections represent a serious public health threat. We recently demonstrated that the presence of a novel prophage ϕSA169 was associated with vancomycin (VAN) treatment failure in experimental MRSA endocarditis. In this study, we assessed the role of a ϕSA169 gene, ϕ80α_gp05 (gp05), in VAN-persistent outcome using gp05 isogenic MRSA strain sets. Of note, Gp05 significantly influences the intersection of MRSA virulence factors, host immune responses, and antibiotic treatment efficacy, including the following: (i) activity of the significant energy-yielding metabolic pathway (e.g., tricarboxylic acid cycle); (ii) carotenoid pigment production; (iii) (p)ppGpp (guanosine tetra- and pentaphosphate) production, which activates the stringent response and subsequent downstream functional factors (e.g., phenol-soluble modulins and polymorphonuclear neutrophil bactericidal activity); and (iv) persistence to VAN treatment in an experimental infective endocarditis model. These data suggest that Gp05 is a significant virulence factor which contributes to the persistent outcomes in MRSA endovascular infection by multiple pathways. IMPORTANCE Persistent endovascular infections are often caused by MRSA strains that are susceptible to anti-MRSA antibiotics in vitro by CLSI breakpoints. Thus, the persistent outcome represents a unique variant of traditional antibiotic resistance mechanisms and a significant therapeutic challenge. Prophage, a critical mobile genetic element carried by most MRSA isolates, provides their bacterial host with metabolic advantages and resistance mechanisms. However, how prophage-encoded virulence factors interact with the host defense system and antibiotics, driving the persistent outcome, is not well known. In the current study, we demonstrated that a novel prophage gene, gp05, significantly impacts tricarboxylic acid cycle activity, stringent response, and pigmentation, as well as vancomycin treatment outcome in an experimental endocarditis model using isogenic gp05 overexpression and chromosomal deletion mutant MRSA strain sets. The findings significantly advance our understanding of the role of Gp05 in persistent MRSA endovascular infection and provide a potential target for development of novel drugs against these life-threatening infections.

Tailored anti-biofilm activity - Liposomal delivery for mimic of small antimicrobial peptide

The eradication of bacteria embedded in biofilms is among the most challenging obstacles in the management of chronic wounds. These biofilms are found in most chronic wounds; moreover, the biofilm-embedded bacteria are considerably less susceptible to conventional antimicrobial treatment than the planktonic bacteria. Antimicrobial peptides and their mimics are considered attractive candidates in the pursuit of novel therapeutic options for the treatment of chronic wounds and general bacterial eradication. However, some limitations linked to these membrane-active antimicrobials are making their clinical use challenging. Novel innovative delivery systems addressing these limitations represent a smart solution. We hypothesized that incorporation of a novel synthetic mimic of an antimicrobial peptide in liposomes could improve its anti-biofilm effect as well as the anti-inflammatory activity. The small synthetic mimic of an antimicrobial peptide, 7e-SMAMP, was incorporated into liposomes (~280 nm) tailored for skin wounds and evaluated for its potential activity against both biofilm formation and eradication of pre-formed biofilms. The 7e-SMAMP-liposomes significantly lowered inflammatory response in murine macrophages (~30 % reduction) without affecting the viability of macrophages or keratinocytes. Importantly, the 7e-SMAMP-liposomes completely eradicated biofilms produced by Staphylococcus aureus and Escherichia coli above concentrations of 6.25 μg/mL, whereas in Pseudomonas aeruginosa the eradication reached 75 % at the same concentration. Incorporation of 7e-SMAMP in liposomes improved both the inhibition of biofilm formation as well as biofilm eradication in vitro, as compared to non-formulated antimicrobial, therefore confirming its potential as a novel therapeutic option for bacteria-infected chronic wounds.

Computing Individual Area per Head Group Reveals Lipid Bilayer Dynamics

Lipid bilayers express a range of phases from solid-like to gel-like to liquid-like as a function of temperature and lipid surface concentration. The area occupied per lipid head group serves as one useful indicator of the bilayer phase, in conjunction with the two-dimensional radial distribution function (i.e., structure factor) within the bilayer. Typically, the area per head group is determined by dividing the bilayer area equally among all head groups. Such an approach is less satisfactory for a multicomponent set of diverse lipids. In this work, area determination is performed on a lipid-by-lipid basis by attributing to a lipid the volume that surrounds each atom. Voronoi tessellation provides this division of the interfacial region on a per-atom basis. The method is applied to a multicomponent system of water, NaCl, and 19 phospholipid types that was devised recently [Langmuir2022, 38, 9481-9499] as a computational representation of the Gram-positive Staphylococcus aureus phospholipid bilayer. Results demonstrate that lipids and water molecules occupy similar extents of area within the interfacial region; ascribing area only to head groups implicitly incorporates assumptions about head group hydration. Results further show that lipid tails provide non-negligible contributions to area on the membrane side of the bilayer-water interface. Results for minimum and maximum area of individual lipids reveal that spontaneous fluctuations displace head groups more than 10 Å from the interfacial region during an NPT simulation at 310 K, leading to a zero contribution to total area at some times. Total area fluctuations and fluctuations per individual lipid relax with a correlation time of ∼10 ns. The method complements density profile as an approach to quantify the structure and dynamics of computational lipid bilayers.

Generation and Computational Characterization of a Complex Staphylococcus aureus Lipid Bilayer

Studies indicate a crucial cell membrane role in the antibiotic resistance of Staphylococcus aureus. To simulate its membrane structure and dynamics, a complex molecular-scale computational representation of the S. aureus lipid bilayer was developed. Phospholipid types and their amounts were optimized by reverse Monte Carlo to represent characterization data from the literature, leading to 19 different phospholipid types that combine three headgroups [phosphatidylglycerol, lysyl-phosphatidylglycerol (LPG), and cardiolipin] and 10 tails, including iso- and anteiso-branched saturated chains. The averaged lipid bilayer thickness was 36.7 Å, and area per headgroup was 67.8 Ų. Phosphorus and nitrogen density profiles showed that LPG headgroups tended to be bent and oriented more parallel to the bilayer plane. The water density profile showed that small amounts reached the membrane center. Carbon density profiles indicated hydrophobic interactions for all lipids in the middle of the bilayer. Bond vector order parameters along each tail demonstrated different C-H ordering even within distinct lipids of the same type; however, all tails followed similar trends in average order parameter. These complex simulations further revealed bilayer insights beyond those attainable with monodisperse, unbranched lipids. Longer tails often extended into the opposite leaflet. Carbon at and beyond a branch showed significantly decreased ordering compared to carbon in unbranched tails; this feature arose in every branched lipid. Diverse tail lengths distributed these disordered methyl groups throughout the middle third of the bilayer. Distributions in mobility and ordering reveal diverse properties that cannot be obtained with monodisperse lipids.

Multilocus sequence type-dependent activity of human and animal cathelicidins against community-, hospital-, and livestock-associated methicillin-resistant Staphylococcus aureus isolates

Sequence type (ST) 5 methicillin-resistant Staphylococcus aureus (MRSA) with staphylococcal cassette chromosome mec (SCCmec) type II (ST5-MRSA-II) and ST72-MRSA-IV represent the most significant genotypes for healthcare- (HA) and community-associated (CA) MRSA in Korea, respectively. In addition to the human-type MRSA strains, the prevalence of livestock-associated (LA) MRSA clonal lineages, such as ST541 and ST398 LA-MRSA-V in pigs and ST692 LA-MRSA-V and ST188 LA-MRSA-IV in chickens, has recently been found. In this study, clonotype-specific resistance profiles to cathelicidins derived from humans (LL-37), pigs (PMAP-36), and chickens (CATH-2) were examined using six different ST groups of MRSA strains: ST5 HA-MRSA-II, ST72 CA-MRSA-IV, ST398 LA-MRSA-V, ST541 LA-MRSA-V, ST188 LA-MRSA-IV, and ST692 LA-MRSA-V. Phenotypic characteristics often involved in cathelicidin resistance, such as net surface positive charge, carotenoid production, and hydrogen peroxide susceptibility were also determined in the MRSA strains. Human- and animal-type MRSA strains exhibited clonotype-specific resistance profiles to LL-37, PMAP-36, or CATH-2, indicating the potential role of cathelicidin resistance in the adaptation and colonization of human and animal hosts. The ST5 HA-MRSA isolates showed enhanced resistance to all three cathelicidins and hydrogen peroxide than ST72 CA-MRSA isolates by implementing increased surface positive charge and carotenoid production. In contrast, LA-MRSA strains employed mechanisms independent of surface charge regulation and carotenoid production for cathelicidin resistance. These results suggest that human- and livestock-derived MRSA strains use different strategies to counteract the bactericidal action of cathelicidins during the colonization of their respective host species.

Gamma-aminobutyric acid fermentation in MRS-based medium by the fructophilic Lactiplantibacillus plantarum Y7

Among the key metabolites produced by probiotic lactic acid bacteria (LAB), the use of gamma-aminobutyric acid (GABA), which alleviates hypertension, depression, and sleepiness in humans, is gaining popularity. Thus, GABA-producing LAB are sought after. GABA-producing LAB were preliminarily screened in acidified-MRS broth and quantified via GABase assays. The one-factor-at-a-time strategy was applied to determine the optimal conditions for GABA production. GABA production in reconstituted skim milk medium (RSM) and antibiotic susceptibility testing were performed to evaluate the potential of the strain as a yogurt starter. L. plantarum Y7 produced 4,856.86 ± 82.47 μg/mL of GABA at optimal culture conditions. Co-cultivation of Y7 and commercial Lactobacillus bulgaricus affected the amount of GABA production (6.85 ± 0.20 μg/mL) in RSM. Y7 was susceptible to ampicillin, erythromycin, and tetracycline. Therefore, L. plantarum Y7 represents a promising strain for GABA production in the food industry.

Peptide-Membrane Interactions Monitored by Fluorescence Lifetime Imaging: A Study Case of Transportan 10

The interest on detailed analysis of peptide-membrane interactions is of great interest in both fundamental and applied sciences as these may relate to both functional and pathogenic events. Such interactions are highly dynamic and spatially heterogeneous, making the investigation of the associated phenomena highly complex. The specific properties of membranes and peptide structural details, together with environmental conditions, may determine different events at the membrane interface, which will drive the fate of the peptide-membrane system. Here, we use an experimental approach based on the combination of spectroscopy and fluorescence microscopy methods to characterize the interactions of the multifunctional amphiphilic peptide transportan 10 with model membranes. Our approach, based on the use of suitable fluorescence reporters, exploits the advantages of phasor plot analysis of fluorescence lifetime imaging microscopy measurements to highlight the molecular details of occurring membrane alterations in terms of rigidity and hydration. Simultaneously, it allows following dynamic events in real time without sample manipulation distinguishing, with high spatial resolution, whether the peptide is adsorbed to or inserted in the membrane.

Boosting Synergistic Effects of Short Antimicrobial Peptides With Conventional Antibiotics Against Resistant Bacteria

The global spread of antibiotic-resistant infections has meant that there is an urgent need to develop new antimicrobial alternatives. In this study, we developed a strategy to boost and/or synergize the activity of conventional antibiotics by combination with antimicrobial peptides tagged with the bulky non-natural amino acid β-naphthylalanine (Nal) to their N- or C-terminus. A checkerboard method was used to evaluate synergistic effects of the parent peptide and the Nal-tagged peptides. Moreover, boron-dipyrro-methene labeled vancomycin was used to characterize the synergistic mechanism of action between the peptides and vancomycin on the bacterial strains. These Nal-tagged antimicrobial peptides also reduced the antibiotic-induced release of lipopolysaccharide from Gram-negative bacteria by more than 99.95%. Our results demonstrate that Nal-tagged peptides could help in developing antimicrobial peptides that not only have enhanced antibacterial activities but also increase the synergistic effects with conventional antibiotics against antibiotic-resistant bacteria.

Role of the Staphylococcus aureus Extracellular Loop of GraS in Resistance to Distinct Human Defense Peptides in PMN and Invasive Cardiovascular infections

GraS is a membrane sensor in Staphylococcus aureus that induces mprF and dltABCD expression to alter the surface positive charge upon exposure to cationic human defense peptides (HDPs). The sensing domain of GraS likely resides in the 9-residue extracellular loop (EL). In this study, we assessed a hospital-acquired methicillin-resistant S. aureus (HA-MRSA) strain (COL) for the specific role of two distinct EL mutations: F38G (bulk) and D/35/37/41K (charged inversion). Activation of mprF by polymyxin B (PMB) was reduced in the D35/37/41K mutant versus the D35/37/41G mutant, correlating with reduced surface positive charge; in contrast, these effects were less prominent in the F38G mutant but still lower than those in the parent. These data indicated that both electrostatic charge and steric bulk of the EL of GraS influence induction of genes impacting HDP resistance. Using mprF expression as a readout, we confirmed GraS signaling was pH dependent, increasing as pH was lowered (from pH 7.5 down to pH 5.5). In contrast to PMB activation, reduction of mprF was comparable at pH 5.5 between the P38G and D35/37/41K point mutants, indicating a mechanistic divergence between GraS activation by acidic pH versus cationic peptides. Survival assays in human blood and purified polymorphonuclear leukocytes (PMNs) revealed lower survival of the D35/37/41K mutant versus the F38G mutant, with both being lower than that of the parent. Virulence studies in the rabbit endocarditis model mirrored whole blood and PMN killing assay data described above. Collectively, these data confirmed the importance of specific residues within the EL of GraS in conferring essential bacterial responses for MRSA survival in infections.

Evaluation of Short-Chain Antimicrobial Peptides With Combined Antimicrobial and Anti-inflammatory Bioactivities for the Treatment of Zoonotic Skin Pathogens From Canines

The incidence of zoonotic Staphylococcus pseudintermedius and Microsporum canis infections is rapidly growing worldwide in the context of an increasing frequency of close contact between animals and humans, presenting challenges in both human and veterinary medicine. Moreover, the development of microbial resistance and emergence of recalcitrant biofilms, accompanied by the insufficiency of new antimicrobial agents, have become major obstacles in treating superficial skin infections caused by various microbes including S. pseudintermedius and M. canis. Over recent years, the prospects of antimicrobial peptides as emerging antimicrobials to combat microbial infections have been demonstrated. In our study, two novel short-chain peptides, namely, allomyrinasin and andricin B, produced by Allomyrina dichotoma and Andrias davidianus, were revealed to exhibit potent antimicrobial efficacy against clinical isolates of S. pseudintermedius and M. canis with remarkable and rapid fungicidal and bactericidal effects, while allomyrinasin exhibited inhibition of biofilm formation and eradication of mature biofilm. These peptides displayed synergistic activity when combined with amoxicillin and terbinafine against S. pseudintermedius and M. canis. Cytoplasmic leakage via cytomembrane permeabilization serves as a mechanism of action. Extremely low hemolytic activity and serum stability in vitro, as well as superior anti-infective efficacy in reducing bacterial counts and relieving the inflammatory response in vivo, were detected. The potent antibacterial, antifungal, and anti-inflammatory activities of allomyrinasin and andricin B might indicate promising anti-infective alternatives for the treatment of S. pseudintermedius and M. canis infections in the context of human and veterinary medicine.

Removal of peptidoglycan and inhibition of active cellular processes leads to daptomycin tolerance in Enterococcus faecalis

Daptomycin is a cyclic lipopeptide antibiotic used in the clinic for treatment of severe enterococcal infections. Recent reports indicate that daptomycin targets active cellular processes, specifically, peptidoglycan biosynthesis. Within, we examined the efficacy of daptomycin against Enterococcus faecalis under a range of environmental growth conditions including inhibitors that target active cellular processes. Daptomycin was far less effective against cells in late stationary phase compared to cells in exponential phase, and this was independent of cellular ATP levels. Further, the addition of either the de novo protein synthesis inhibitor chloramphenicol or the fatty acid biosynthesis inhibitor cerulenin induced survival against daptomycin far better than controls. Alterations in metabolites associated with peptidoglycan synthesis correlated with protection against daptomycin. This was further supported as removal of peptidoglycan induced physiological daptomycin tolerance, a synergistic relation between daptomycin and fosfomycin, an inhibitor of the fist committed step peptidoglycan synthesis, was observed, as well as an additive effect when daptomycin was combined with ampicillin, which targets crosslinking of peptidoglycan strands. Removal of the peptidoglycan of Enterococcus faecium, Staphylococcus aureus, and Bacillus subtilis also resulted in significant protection against daptomycin in comparison to whole cells with intact cell walls. Based on these observations, we conclude that bacterial growth phase and metabolic activity, as well as the presence/absence of peptidoglycan are major contributors to the efficacy of daptomycin.

The Stringent Response Contributes to Persistent Methicillin-Resistant Staphylococcus aureus Endovascular Infection Through the Purine Biosynthetic Pathway

Persistent methicillin-resistant Staphylococcus aureus (MRSA) endovascular infections represent a significant clinical-therapeutic challenge. Of particular concern is antibiotic treatment failure in infections caused by MRSA that are "susceptible" to antibiotic in vitro. In the current study, we investigate specific purine biosynthetic pathways and stringent response mechanism(s) related to this life-threatening syndrome using genetic matched persistent and resolving MRSA clinical bacteremia isolates (PB and RB, respectively), and isogenic MRSA strain sets. We demonstrate that PB isolates (vs RB isolates) have significantly higher (p)ppGpp production, phenol-soluble-modulin expression, polymorphonuclear leukocyte lysis and survival, fibronectin/endothelial cell (EC) adherence, and EC damage. Importantly, an isogenic strain set, including JE2 parental, relP-mutant and relP-complemented strains, translated the above findings into significant outcome differences in an experimental endocarditis model. These observations indicate a significant regulation of purine biosynthesis on stringent response, and suggest the existence of a previously unknown adaptive genetic mechanism in persistent MRSA infection.

Lipidated Short Analogue of α-Melanocyte Stimulating Hormone Exerts Bactericidal Activity against the Stationary Phase of Methicillin-Resistant Staphylococcus aureus and Inhibits Biofilm Formation

Stationary phase Staphylococcus aureus, especially methicillin-resistant S. aureus (MRSA), has been widely associated with many persistent infections as well as biofilm-associated infections, which are challenging due to their increasing antibiotic resistance. α-Melanocyte stimulating hormone (α-MSH) is an antimicrobial peptide (AMP) with well-established potent activity against S. aureus , but little is known about its antimicrobial efficacy against the stationary phase of the bacteria. We investigated the in vitro activities of two palmitoylated analogues, Pal-α-MSH(6-13) and Pal-α-MSH(11-13), of the C-terminal fragments of α-MSH against biofilm-producing strains of methicillin-sensitive S. aureus (MSSA) and MRSA. While both the peptides demonstrated anti-staphylococcal efficacy, Pal-α-MSH(11-13) emerged as the most effective AMP as palmitoylation led to a remarkable enhancement in its activity against stationary phase bacteria. Similar to α-MSH, both the designed analogues were membrane-active and exhibited improved bacterial membrane depolarization and permeabilization, as further confirmed via electron microscopy studies. Of the two peptides, Pal-α-MSH(11-13) was able to retain its activity in the presence of standard microbiological media, which otherwise is a major limiting factor toward the therapeutic use of α-MSH-based peptides. More importantly, Pal-α-MSH(11-13) was also highly effective in inhibiting the formation of biofilms. Furthermore, it did not lead to resistance development in MRSA cells even upon 18 serial passages at sub-MIC concentrations. These observations support the potential use of Pal-α-MSH(11-13) in the treatment of planktonic as well as sessile S. aureus infections.

Targeted and Intracellular Antibacterial Activity against S. agalactiae of the Chimeric Peptides Based on Pheromone and Cell-Penetrating Peptides

The significance of the complex bacterial ecosystem in the human body and the impediment of the mammalian membrane against many antibiotics together emphasize the necessity to develop antimicrobial agents with precise antimicrobial and cell-penetrating activities. A simple and feasible method for generating dual-function antimicrobial peptides inspired by highly hydrophobic peptide pheromone and cationic cell-penetrating peptides is presented. Furthermore, the extension of the peptide candidate library is achieved by modifying the charged domain. The bacteria-selective peptides L1, L2, L10, and L11 kill Streptococcus agalactiae by disrupting the membrane structure, and the targeted mechanism is suggested where the peptides offset the entrapment of S. agalactiae rather than of other bacteria. Moreover, L2 and L10 possess intracellular antibacterial activity and carrier property, which is mainly dependent on endocytosis. Given their suitable biocompatibility, high tolerance, no drug resistance, and effective antimicrobial capacity in a mouse mastitis model, L2 and L10 can be powerful weapons against S. agalactiae pathogen infection.

MgrA Governs Adherence, Host Cell Interaction, and Virulence in a Murine Model of Bacteremia Due to Staphylococcus aureus

BACKGROUND

MgrA is an important global virulence gene regulator in Staphylococcus aureus. In the present study, the role of mgrA in host-pathogen interactions related to virulence was explored in both methicillin-resistant S. aureus (MRSA) and methicillin-susceptible S. aureus (MSSA) strains.

METHODS

In vitro susceptibilities to human defense peptides (HDPs), adherence to fibronectin (Fn) and endothelial cells (ECs), EC damage, α-toxin production, expression of global regulator (eg, agr RNAIII) and its downstream effectors (eg, α-toxin [hla] and Fn binding protein A [fnbA]), MgrA binding to fnbA promoter, and the effect on HDP-induced mprF and dltA expression were analyzed. The impact of mgrA on virulence was evaluated using a mouse bacteremia model.

RESULTS

mgrA mutants displayed significantly higher susceptibility to HDPs, which might be related to the decreased HDP-induced mprF and dltA expression but decreased Fn and EC adherence, EC damage, α-toxin production, agr RNAIII, hla and fnbA expression, and attenuated virulence in the bacteremia model as compared to their respective parental and mgrA-complemented strains. Importantly, direct binding of MgrA to the fnbA promoter was observed.

CONCLUSIONS

These results suggest that mgrA mediates host-pathogen interactions and virulence and may provide a novel therapeutic target for invasive S. aureus infections.

Occurrence and Characteristics of Methicillin-Resistant and -Susceptible Staphylococcus aureus Isolated from the Beef Production Chain in Korea

The emergence and persistence of methicillin-susceptible Staphylococcus aureus (MSSA) and methicillin-resistant S. aureus (MRSA) in livestock animals have been reported as a potential risk factor for transmission to humans. In this study, we investigated the nationwide prevalence and characteristics of MRSA and MSSA in the Korean beef production system, including retail markets, slaughterhouses, and cattle farms. From a total of 1,285 samples, only 5 MRSA strains were isolated: from a farmer (1 ST72 MRSA), a carcass sample from a slaughterhouse (1 ST72 MRSA), and beef cattle (3 ST5 MRSA). In addition, 11 MSSA strains were isolated from beef cattle (n=3), humans (1 farmer, 1 slaughterhouse worker, and 4 retail market workers), and carcass samples (n=1) and slaughterhouse environment (n=1). Although the prevalence of MRSA and MSSA in beef cattle was much lower than that reported in pigs, 5/5 MRSA and 2/11 MSSA strains displayed multiple drug resistance (MDR) phenotypes. Unlike the swine-associated MRSA, no correlation was found between tetracycline/zinc resistance and MDR phenotype. However, MRSA strains had an identical set of staphylococcal enterotoxins and exhibited enhanced levels of resistance to antimicrobial peptides (PMAP-36 and LL-37) compared to the MSSA strains. In conclusion, continued and systemic surveillance of livestock, meat products, and humans in close contact with livestock/meat products is necessary to prevent the transmission of MRSA and MSSA to humans.

Mechanisms of bactericidal action and resistance of polymyxins for Gram-positive bacteria

Polymyxins are cationic antimicrobial peptides used as the last-line therapy to treat multidrug-resistant Gram-negative bacterial infections. The bactericidal activity of polymyxins against Gram-negative bacteria relies on the electrostatic interaction between the positively charged polymyxins and the negatively charged lipid A of lipopolysaccharide (LPS). Given that Gram-positive bacteria lack an LPS-containing outer membrane, it is generally acknowledged that polymyxins are less active against Gram-positive bacteria. However, Gram-positive bacteria produce negatively charged teichoic acids, which may act as the target of polymyxins. More and more studies suggest that polymyxins have potential as a treatment for Gram-positive bacterial infection. This mini-review discusses recent advances in the mechanism of the antibacterial activity and resistance of polymyxins in Gram-positive bacteria.Key Points• Teichoic acids play a key role in the action of polymyxins on Gram-positive bacteria.• Polymyxin kills Gram-positive bacteria by disrupting cell surface and oxidative damage.• Modification of teichoic acids and phospholipids contributes to polymyxin resistance in Gram-positive bacteria.• Polymyxins have potential as a treatment for Gram-positive bacterial infection.

Pseudomonas aeruginosa Increases the Sensitivity of Biofilm-Grown Staphylococcus aureus to Membrane-Targeting Antiseptics and Antibiotics

The thick mucus in the airways of cystic fibrosis (CF) patients predisposes them to frequent, polymicrobial respiratory infections. Pseudomonas aeruginosa and Staphylococcus aureus are frequently coisolated from the airways of individuals with CF, as well as from diabetic foot ulcers and other wounds. Both organisms form biofilms, which are notoriously difficult to eradicate and promote chronic infection. In this study, we have shown that P. aeruginosa-secreted factors can increase the efficacy of compounds that alone have little or no bactericidal activity against S. aureus biofilms. In particular, we discovered that P. aeruginosa exoproducts can potentiate the antistaphylococcal activity of phenol-based antiseptics and other membrane-active drugs. Our findings illustrate that polymicrobial interactions can dramatically increase antibacterial efficacy in vitro and suggest that altering membrane physiology promotes the ability of certain drugs to kill bacterial biofilms—knowledge that may provide a path for the discovery of new biofilm-targeting antimicrobial strategies.

Pseudomonas aeruginosa and Staphylococcus aureus often cause chronic, recalcitrant infections in large part due to their ability to form biofilms. The biofilm mode of growth enables these organisms to withstand antibacterial insults that would effectively eliminate their planktonic counterparts. We found that P. aeruginosa supernatant increased the sensitivity of S. aureus biofilms to multiple antimicrobial compounds, including fluoroquinolones and membrane-targeting antibacterial agents, including the antiseptic chloroxylenol. Treatment of S. aureus with the antiseptic chloroxylenol alone did not decrease biofilm cell viability; however, the combination of chloroxylenol and P. aeruginosa supernatant led to a 4-log reduction in S. aureus biofilm viability compared to exposure to chloroxylenol alone. We found that the P. aeruginosa-produced small molecule 2-n-heptyl-4-hydroxyquinoline N-oxide (HQNO) is responsible for the observed heightened sensitivity of S. aureus to chloroxylenol. Similarly, HQNO increased the susceptibility of S. aureus biofilms to other compounds, including both traditional and nontraditional antibiotics, which permeabilize bacterial membranes. Genetic and phenotypic studies support a model whereby HQNO causes an increase in S. aureus membrane fluidity, thereby improving the efficacy of membrane-targeting antiseptics and antibiotics. Importantly, our data show that P. aeruginosa exoproducts can enhance the ability of various antimicrobial agents to kill biofilm populations of S. aureus that are typically difficult to eradicate. Finally, our discovery that altering membrane fluidity shifts antimicrobial sensitivity profiles of bacterial biofilms may guide new approaches to target persistent infections, such as those commonly found in respiratory tract infections and in chronic wounds.

Escherichia coli as a Model for the Description of the Antimicrobial Mechanism of a Cationic Polymer Surface: Cellular Target and Bacterial Contrast Response

In this study, we use Escherichia coli as a model to investigate the antimicrobial mechanism of a film made of a copolymer based on monomethylether poly(ethylene glycol), methyl methacrylate, and 2-dimethyl(aminoethyl) methacrylate, whose surface is active towards Gram-negative and Gram-positive bacteria. The polymer contains not quaternized amino groups that can generate a charged surface by protonation when in contact with water. For this purpose, we adopted a dual strategy based on the analysis of cell damage caused by contact with the polymer surface and on the evaluation of the cell response to the surface toxic action. The lithic effect on the protoplasts of E. coli showed that the polymer surface can affect the structure of cytoplasmic membranes, while assays of calcein leakage from large unilamellar vesicles at different phospholipid compositions indicated that action on membranes does not need a functionally active cell. On the other hand, the significant increase in sensitivity to actinomycin D demonstrates that the polymer interferes also with the structure of the outer membrane, modifying its permeability. The study on gene expression, based on the analysis of the transcripts in a temporal window where the contact with the polymer is not lethal and the damage is reversible, showed that some key genes of the synthesis and maintenance of the outer membrane structure ( fabR, fadR, fabA, waaA, waaC, kdsA, pldA, and pagP), as well as regulators of cellular response to oxidative stress ( soxS), are more expressed when bacteria are exposed to the polymer surface. All together these results identified the outer membrane as the main cellular target of the antimicrobial surface and indicated a specific cellular response to damage, providing more information on the antimicrobial mechanism. In this perspective, data reported here could play a pivotal role in a microbial growth control strategy based not only on the structural improvements of the materials but also on the possibility of intervening on the cellular pathways involved in the contrast reaction to these and other polymers with similar mechanisms.

Carriage of Staphylococcus schleiferi from canine otitis externa: antimicrobial resistance profiles and virulence factors associated with skin infection

The recent emergence of Staphylococcus schleiferi in dogs with otitis externa or skin and soft tissue infections has become a significant zoonotic issues. In the current study, we investigated 1) the carriage rates of S. schleiferi among major staphylococci in healthy dogs and dogs with otitis externa, 2) antibiotic susceptibility profiles of S. schleiferi, particularly methicillin resistance (MR), and 3) virulence factors associated with skin and soft tissue infections such as ability to form biofilm, resistance to cationic antimicrobial peptides (CAMPs), and carriage of staphylococcal enterotoxin genes. Among the 21 S. schleiferi isolates, 5 isolates (24%) were determined to be methicillin-resistant (MRSS). Staphylococcal cassette chromosome mec (SCCmec) typing revealed the presence of SCCmec type V in 4 MRSS isolates and type VII in one MRSS. Higher levels of antibiotic resistance, especially multidrug resistance, were observed in MRSS isolates compared to the methicillin-susceptible S. schleiferi (MSSS) isolates. In addition, MRSS isolates exhibited enhanced ability to form biofilm under static condition and all the 5 MRSS isolates carried three or more enterotoxin genes. However, there were no significant differences in resistance to CAMPs between MRSS and MSSS isolates. These findings suggest that coagulase-negative S. schleiferi is becoming more prevalent in canine otitis externa cases. Our results also highlight the presence of multidrug-resistant MRSS isolates with enhanced biofilm production and carriage of multiple enterotoxins.

Potential role of host defense antimicrobial peptide resistance in increased virulence of health care-associated MRSA strains of sequence type (ST) 5 versus livestock-associated and community-associated MRSA strains of ST72

The most significant community-associated methicillin-resistant Staphylococcus aureus (CA-MRSA) in Korea is sequence type (ST) 72 with staphylococcal cassette chromosome mec (SCCmec) type IV (ST72-MRSA-IV). Although the impact of CA-MRSA on the clinical outcomes versus healthcare-associated (HA)-MRSA remains unclear, it has recently been revealed that ST5 HA-MRSA-II is associated with higher mortality compared with ST72 CA-MRSA-IV, suggesting higher virulence in ST5 HA-MRSA-II strains. In this investigation, human-/animal-originated ST72-MRSA-IV strains were examined for virulence phenotypes and compared with those of ST5-MRSA-II strains, the established HA-MRSA in Korea. Overall, ST5 HA-MRSA-II strains demonstrated higher levels of resistance to host defense-cationic antimicrobial peptides of human (LL-37), bovine (BMAP-28), and bacterial (polymyxin B) origins versus ST72-MRSA-IV strains via enhanced surface positive charge. Hemolysis profiles, gelatinase activity, and staphylococcal superantigen gene profiles were not different between ST72 CA-MRSA and ST5 HA-MRSA strains. However, ST5 HA-MRSA strains were able to downregulate initial cytokine response in murine macrophages.

The activity and action mechanism of novel short selective LL-37-derived anticancer peptides against clinical isolates of Escherichia coli

Human cathelicidin LL-37 has recently attracted interest as a potential therapeutic agent, mostly because of its ability to kill a wide variety of pathogens and cancer cells. In this study, we aimed to investigate the antibacterial activity and cytotoxicity of previously designed LL-37 anticancer derivatives (i.e., P7, P22, and P38). Calcein release assay and field emission-scanning electron microscopy (FE-SEM) were performed to elucidate the possible mechanism of action of P38, the peptide with the highest bactericidal activity. In silico analysis demonstrated the amphipathic alpha-helical structure for three peptides. Antibacterial activity of P38 against multidrug-resistant (MDR) clinical isolates of Escherichia coli was higher than that of P7 and P22. P38 caused no hemolysis or cytotoxicity. Treating calcein-loaded E. coli with 4× MIC of P38 resulted in more than 96% leakage of calcein. Noticeably, FE-SEM revealed that P38 killed E. coli by disrupting the bacterial membrane. Molecular docking studies showed that P38 had a much higher affinity for the outer membrane of Gram-negative bacteria compared with both P22 and P7. Owing to the bactericidal activity of P38 against MDR E. coli isolates and its negligible cytotoxicity, P38 has the potential for further studies in a mouse model of infectious disease.

Molecular mechanisms of thioridazine resistance in Staphylococcus aureus

Staphylococcus aureus has developed resistance towards the most commonly used anti-staphylococcal antibiotics. Therefore, there is an urgent need to find new treatment opportunities. A new approach relies on the use of helper compounds, which are able to potentiate the effect of antibiotics. A well-studied helper compound is thioridazine, which potentiates the effect of the β-lactam antibiotic dicloxacillin against Methicillin-resistant Staphylococcus aureus (MRSA). In order to identify thioridazine's mechanism of action and how it potentiates the effect of dicloxacillin, we generated thioridazine resistant strains of MRSA USA300 by serial passage experiments. Selected strains were whole-genome sequenced to find mutations causing thioridazine resistance. Genes observed to be mutated were attempted deleted in MRSA USA300. The cls gene encoding a cardiolipin synthase important for synthesis of the membrane lipid cardiolipin was found to be mutated in thioridazine resistant strains. Deletion of this gene resulted in a two-fold increased Minimum inhibitory concentrations (MIC) value for thioridazine compared to the wild type and decreased susceptibility similar to the thioridazine resistant strains. Since cardiolipin likely plays a role in resistance towards thioridazine, it might also be important for the mechanism of action behind the potentiating effect of thioridazine. TDZ is known to intercalate into the membrane and we show here that TDZ can depolarize the plasma membrane. However, our results indicate that the membrane potential reducing effect of TDZ is independent of the resistance mechanism.

Adaptations of Vancomycin-Intermediate Sequence Type 72 Methicillin-Resistant Staphylococcus aureus for Daptomycin Nonsusceptibility

In Korea, the major clonal type of community-associated methicillin-resistant Staphylococcus aureus (CA-MRSA) is sequence type 72 (ST72) with staphylococcal cassette chromosome mec (SCCmec) type IV (ST72-MRSA-IV). In this study, we used a previously well-characterized isogenic pair of ST72 vancomycin (VAN) susceptible-and VAN intermediate-MRSA strains (VSSA303 and VISA072) and several VSSA strains complemented with plasmids expressing single-point mutated genes (dprAG196C, femAF92C, vraRE127K, and vraSRE127K) identified in the VISA strain. Using the strain set, we assessed the (1) susceptibilities to daptomycin (DAP) and cationic antimicrobial peptides (CAMPs), (2) alterations in cell envelope phenotypes, such as cell wall autolysis, surface positive charge, and membrane potential (ΔΨ), (3) transcriptional expression profiles of genes involved in surface charge regulation and changes of ΔΨ, and (4) cytokine stimulation profiles in murine macrophages. The vraRE127K mutation could enhance surface positive charge through mprF- and dltABCD-independent mechanisms with thickened cell wall. However, none of the single-point mutated genes increased DAP resistance. The DAP nonsusceptible (DAP-NS) phenotype observed in VISA072 strain likely resulted from the combined effects of low ΔΨ and increased positive surface charge. These results suggest that physicochemical alterations in cell envelope are involved in the survival response of DAP-NS VISA072 in sites of infections.

Impact of Multiple Single-Nucleotide Polymorphisms Within mprF on Daptomycin Resistance in Staphylococcus aureus

A number of single nucleotide polymorphisms (SNPs) within the mprF open reading frame (ORF) have been associated with daptomycin-resistance (DAP-R) in Staphylococcus aureus. Such SNPs have been found throughout the mprF ORF, although there are clearly preferred "hot spots" within this gene frequently linked to DAP-R phenotype. These mprF SNPs are often correlated with a gain-in-function phenotype, either in terms of increased production (synthase activity) and/or enhanced translocation (translocase activity) of lysyl-phosphatidylglycerol (L-PG) within its cell membrane. However, it is unclear if multiple hot spot mprF SNPs can accumulate within mprF ORFs and cause additive elevations of DAP minimum inhibitory concentrations (MICs). In this study, we used a previously well-characterized plasmid complementation system in S. aureus Newman ΔmprF mutant to express: (1) single point-mutated forms of mprF ORFs cloned from two DAP-R S. aureus strains (mprF_S295L or mprF_T345A) and (2) dual point-mutated forms of mprF ORFs simultaneously harboring SNPs in the central bifunctional domain and synthase domain in MprF, respectively (mprF_S295L+L826F or mprF_T345A+L826F). The current study revealed that, although individual hot spot point mutations within mprF ORF can recapitulate signature DAP-R-associated phenotypes (i.e., increased DAP MICs, enhanced surface positive charge, and increased L-PG synthesis), accumulation of such hot spot point mutations paradoxically caused reduction in these latter three metrics.

Novel Miniature Membrane Active Lipopeptidomimetics against Planktonic and Biofilm Embedded Methicillin-Resistant Staphylococcus aureus

Escalating multidrug resistance and highly evolved virulence mechanisms have aggravated the clinical menace of methicillin-resistant Staphylococcus aureus (MRSA) infections. Towards development of economically viable staphylocidal agents here we report eight structurally novel tryptophan-arginine template based peptidomimetics. Out of the designed molecules, three lipopeptidomimetics (S-6, S-7 and S-8) containing 12-amino dodecanoic acid exhibited cell selectivity and good to potent activity against clinically relevant pathogens MRSA, methicillin-resistant Staphylococcus epidermidis and vancomycin-resistant Enterococcus faecium (MIC: 1.4–22.7 μg/mL). Mechanistically, the active peptidomimetics dissipated membrane potential and caused massive permeabilization on MRSA concomitant with loss of viability. Against stationary phase MRSA under nutrient-depleted conditions, active peptidomimetics S-7 and S-8 achieved > 6 log reduction in viability upon 24 h incubation while both S-7 (at 226 μg/mL) and S-8 (at 28 μg/mL) also destroyed 48 h mature MRSA biofilm causing significant decrease in viability (p < 0.05). Encouragingly, most active peptidomimetic S-8 maintained efficacy against MRSA in presence of serum/plasma while exhibiting no increase in MIC over 17 serial passages at sub-MIC concentrations implying resistance development to be less likely. Therefore, we envisage that the current template warrants further optimization towards the development of cell selective peptidomimetics for the treatment of device associated MRSA infections.

Targeting biofilms and persisters of ESKAPE pathogens with P14KanS, a kanamycin peptide conjugate

BACKGROUND

The worldwide emergence of antibiotic resistance represents a serious medical threat. The ability of these resistant pathogens to form biofilms that are highly tolerant to antibiotics further aggravates the situation and leads to recurring infections. Thus, new therapeutic approaches that adopt novel mechanisms of action are urgently needed. To address this significant problem, we conjugated the antibiotic kanamycin with a novel antimicrobial peptide (P14LRR) to develop a kanamycin peptide conjugate (P14KanS).

METHODS

Antibacterial activities were evaluated in vitro and in vivo using a Caenorhabditis elegans model. Additionally, the mechanism of action, antibiofilm activity and anti-inflammatory effect of P14KanS were investigated.

RESULTS

P14KanS exhibited potent antimicrobial activity against ESKAPE pathogens. P14KanS demonstrated a ≥128-fold improvement in MIC relative to kanamycin against kanamycin-resistant strains. Mechanistic studies confirmed that P14KanS exerts its antibacterial effect by selectively disrupting the bacterial cell membrane. Unlike many antibiotics, P14KanS demonstrated rapid bactericidal activity against stationary phases of both Gram-positive and Gram-negative pathogens. Moreover, P14KanS was superior in disrupting adherent bacterial biofilms and in killing intracellular pathogens as compared to conventional antibiotics. Furthermore, P14KanS demonstrated potent anti-inflammatory activity via the suppression of LPS-induced proinflammatory cytokines. Finally, P14KanS protected C. elegans from lethal infections of both Gram-positive and Gram-negative pathogens.

CONCLUSIONS

The potent in vitro and in vivo activity of P14KanS warrants further investigation as a potential therapeutic agent for bacterial infections.

GENERAL SIGNIFICANCE

This study demonstrates that equipping kanamycin with an antimicrobial peptide is a promising method to tackle bacterial biofilms and address bacterial resistance to aminoglycosides.

Phenotypic and genotypic correlates of daptomycin-resistant methicillin-susceptible Staphylococcus aureus clinical isolates

Daptomycin (DAP) has potent activity in vitro and in vivo against both methicillin-susceptible Staphylococcus aureus (MSSA) and methicillin-resistant S. aureus (MRSA) strains. DAP-resistance (DAP-R) in S. aureus has been mainly observed in MRSA strains, and has been linked to single nucleotide polymorphisms (SNPs) within the mprF gene leading to altered cell membrane (CM) phospholipid (PL) profiles, enhanced positive surface charge, and changes in CM fluidity. The current study was designed to delineate whether these same genotypic and phenotypic perturbations are demonstrated in clinically-derived DAP-R MSSA strains. We used three isogenic DAP-susceptible (DAP-S)/DAP-R strainpairs and compared: (i) presence of mprF SNPs, (ii) temporal expression profiles of the two key determinants (mprF and dltABCD) of net positive surface charge, (iii) increased production of mprF-dependent lysinylated-phosphatidylglycerol (L-PG), (iv) positive surface charge assays, and (v) susceptibility to cationic host defense peptides (HDPs) of neutrophil and platelet origins. Similar to prior data in MRSA, DAP-R (vs DAP-S) MSSA strains exhibited hallmark hot-spot SNPs in mprF, enhanced and dysregulated expression of both mprF and dltA, L-PG overproduction, HDP resistance and enhanced positive surface charge profiles. However, in contrast to most DAP-R MRSA strains, there were no changes in CM fluidity seen. Thus, charge repulsion via mprF-and dlt-mediated enhancement of positive surface charge may be the main mechanism to explain DAP-R in MSSA strains.

Antimicrobial Activity of Cationic Antimicrobial Peptides against Gram-Positives: Current Progress Made in Understanding the Mode of Action and the Response of Bacteria

Antimicrobial peptides (AMPs) have been proposed as a novel class of antimicrobials that could aid the fight against antibiotic resistant bacteria. The mode of action of AMPs as acting on the bacterial cytoplasmic membrane has often been presented as an enigma and there are doubts whether the membrane is the sole target of AMPs. Progress has been made in clarifying the possible targets of these peptides, which is reported in this review with as focus gram-positive vegetative cells and spores. Numerical estimates are discussed to evaluate the possibility that targets, other than the membrane, could play a role in susceptibility to AMPs. Concerns about possible resistance that bacteria might develop to AMPs are addressed. Proteomics, transcriptomics, and other molecular techniques are reviewed in the context of explaining the response of bacteria to the presence of AMPs and to predict what resistance strategies might be. Emergent mechanisms are cell envelope stress responses as well as enzymes able to degrade and/or specifically bind (and thus inactivate) AMPs. Further studies are needed to address the broadness of the AMP resistance and stress responses observed.

Polymyxin: Alternative Mechanisms of Action and Resistance

Antibiotic resistance among pathogenic bacteria is an ever-increasing issue worldwide. Unfortunately, very little has been achieved in the pharmaceutical industry to combat this problem. This has led researchers and the medical field to revisit past drugs that were deemed too toxic for clinical use. In particular, the cyclic cationic peptides polymyxin B and colistin, which are specific for Gram-negative bacteria, have been used as "last resort" antimicrobials. Before the 1980s, these drugs were known for their renal and neural toxicities; however, new clinical practices and possibly improved manufacturing have made them safer to use. Previously suggested to primarily attack the membranes of Gram-negative bacteria and to not easily select for resistant mutants, recent research exploring resistance and mechanisms of action has provided new perspectives. This review focuses primarily on the proposed alternative mechanisms of action, known resistance mechanisms, and how these support the alternative mechanisms of action.

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.

Staphylococcus aureus infective endocarditis versus bacteremia strains: Subtle genetic differences at stake

Infective endocarditis (IE)((1)) is a severe condition complicating 10-25% of Staphylococcus aureus bacteremia. Although host-related IE risk factors have been identified, the involvement of bacterial features in IE complication is still unclear. We characterized strictly defined IE and bacteremia isolates and searched for discriminant features. S. aureus isolates causing community-acquired, definite native-valve IE (n=72) and bacteremia (n=54) were collected prospectively as part of a French multicenter cohort. Phenotypic traits previously reported or hypothesized to be involved in staphylococcal IE pathogenesis were tested. In parallel, the genotypic profiles of all isolates, obtained by microarray, were analyzed by discriminant analysis of principal components (DAPC)((2)). No significant difference was observed between IE and bacteremia strains, regarding either phenotypic or genotypic univariate analyses. However, the multivariate statistical tool DAPC, applied on microarray data, segregated IE and bacteremia isolates: IE isolates were correctly reassigned as such in 80.6% of the cases (C-statistic 0.83, P<0.001). The performance of this model was confirmed with an independent French collection IE and bacteremia isolates (78.8% reassignment, C-statistic 0.65, P<0.01). Finally, a simple linear discriminant function based on a subset of 8 genetic markers retained valuable performance both in study collection (86.1%, P<0.001) and in the independent validation collection (81.8%, P<0.01). We here show that community-acquired IE and bacteremia S. aureus isolates are genetically distinct based on subtle combinations of genetic markers. This finding provides the proof of concept that bacterial characteristics may contribute to the occurrence of IE in patients with S. aureus bacteremia.

The Potential Use of Natural and Structural Analogues of Antimicrobial Peptides in the Fight against Neglected Tropical Diseases

Recently, research into the development of new antimicrobial agents has been driven by the increase in resistance to traditional antibiotics and Emerging Infectious Diseases. Antimicrobial peptides (AMPs) are promising candidates as alternatives to current antibiotics in the treatment and prevention of microbial infections. AMPs are produced by all known living species, displaying direct antimicrobial killing activity and playing an important role in innate immunity. To date, more than 2000 AMPs have been discovered and many of these exhibit broad-spectrum antibacterial, antiviral and anti-parasitic activity. Neglected tropical diseases (NTDs) are caused by a variety of pathogens and are particularly wide-spread in low-income and developing regions of the world. Alternative, cost effective treatments are desperately needed to effectively battle these medically diverse diseases. AMPs have been shown to be effective against a variety of NTDs, including African trypanosomes, leishmaniosis and Chagas disease, trachoma and leprosy. In this review, the potential of selected AMPs to successfully treat a variety of NTD infections will be critically evaluated.

Frequency and Distribution of Single-Nucleotide Polymorphisms within mprF in Methicillin-Resistant Staphylococcus aureus Clinical Isolates and Their Role in Cross-Resistance to Daptomycin and Host Defense Antimicrobial Peptides

MprF is responsible for the lysinylation of phosphatidylglycerol (PG) to synthesize the positively charged phospholipid (PL) species, lysyl-PG (L-PG). It has been proposed that the single-nucleotide polymorphisms (SNPs) within the mprF open reading frame (ORF) are associated with a gain-in-function phenotype in terms of daptomycin resistance in Staphylococcus aureus. (Note that although the official term is daptomycin nonsusceptibility, we use the term daptomycin resistance in this paper for ease of presentation.) Using 22 daptomycin-susceptible (DAP(s))/daptomycin-resistant (DAP(r)) clinical methicillin-resistant S. aureus (MRSA) strain pairs, we assessed (i) the frequencies and distribution of putative mprF gain-in-function SNPs, (ii) the relationships of the SNPs to both daptomycin resistance and cross-resistance to the prototypical endovascular host defense peptide (HDP) thrombin-induced platelet microbicidal protein (tPMP), and (iii) the impact of mprF SNPs on positive surface charge phenotype and modifications of membrane PL profiles. Most of the mprF SNPs identified in our DAP(r) strains were clustered within the two MprF loci, (i) the central bifunctional domain and (ii) the C-terminal synthase domain. Moreover, we were able to correlate the presence and location of mprF SNPs in DAP(r) strains with HDP cross-resistance, positive surface charge, and L-PG profiles. Although DAP(r) strains with mprF SNPs in the bifunctional domain showed higher resistance to tPMPs than DAP(r) strains with SNPs in the synthase domain, this relationship was not observed in positive surface charge assays. These results demonstrated that both charge-mediated and -unrelated mechanisms are involved in DAP resistance and HDP cross-resistance in S. aureus.

Bactericidal activity of curcumin I is associated with damaging of bacterial membrane

Curcumin, an important constituent of turmeric, is known for various biological activities, primarily due to its antioxidant mechanism. The present study focused on the antibacterial activity of curcumin I, a significant component of commercial curcumin, against four genera of bacteria, including those that are Gram-positive (Staphylococcus aureus and Enterococcus faecalis) and Gram-negative (Escherichia coli and Pseudomonas aeruginosa). These represent prominent human pathogens, particularly in hospital settings. Our study shows the strong antibacterial potential of curcumin I against all the tested bacteria from Gram-positive as well as Gram-negative groups. The integrity of the bacterial membrane was checked using two differential permeabilization indicating fluorescent probes, namely, propidium iodide and calcein. Both the membrane permeabilization assays confirmed membrane leakage in Gram-negative and Gram-positive bacteria on exposure to curcumin I. In addition, scanning electron microscopy and fluorescence microscopy were employed to confirm the membrane damages in bacterial cells on exposure to curcumin I. The present study confirms the broad-spectrum antibacterial nature of curcumin I, and its membrane damaging property. Findings from this study could provide impetus for further research on curcumin I regarding its antibiotic potential against rapidly emerging bacterial pathogens.

The antimicrobial activity of gramicidin A is associated with hydroxyl radical formation

Gramicidin A is an antimicrobial peptide that destroys gram-positive bacteria. The bactericidal mechanism of antimicrobial peptides has been linked to membrane permeation and metabolism disruption as well as interruption of DNA and protein functions. However, the exact bacterial killing mechanism of gramicidin A is not clearly understood. In the present study, we examined the antimicrobial activity of gramicidin A on Staphylococcus aureus using biochemical and biophysical methods, including hydroxyl radical and NAD+/NADH cycling assays, atomic force microscopy, and Fourier transform infrared spectroscopy. Gramicidin A induced membrane permeabilization and changed the composition of the membrane. The morphology of Staphylococcus aureus during gramicidin A destruction was divided into four stages: pore formation, water permeability, bacterial flattening, and lysis. Changes in membrane composition included the destruction of membrane lipids, proteins, and carbohydrates. Most interestingly, we demonstrated that gramicidin A not only caused membrane permeabilization but also induced the formation of hydroxyl radicals, which are a possible end product of the transient depletion of NADH from the tricarboxylic acid cycle. The latter may be the main cause of complete Staphylococcus aureus killing. This new finding may provide insight into the underlying bactericidal mechanism of gA.

Characterization of a novel small molecule that potentiates β-lactam activity against gram-positive and gram-negative pathogens

In a loss-of-viability screen using small molecules against methicillin-resistant Staphylococcus aureus (MRSA) strain USA300 with a sub-MIC of a β-lactam, we found a small molecule, designated DNAC-1, which potentiated the effect of oxacillin (i.e., the MIC of oxacillin decreased from 64 to 0.25 μg/ml). Fluorescence microscopy indicated a disruption in the membrane structures within 15 min of exposure to DNAC-1 at 2× MIC. This permeabilization was accompanied by a rapid loss of membrane potential, as monitored by use of the DiOC2 (3,3'-diethyloxacarbocyanine iodide) dye. Macromolecular analysis showed the inhibition of staphylococcal cell wall synthesis by DNAC-1. Transmission electron microscopy of treated MRSA USA300 cells revealed a slightly thicker cell wall, together with mesosome-like projections into the cytosol. The exposure of USA300 cells to DNAC-1 was associated with the mislocalization of FtsZ accompanied by the localization of penicillin-binding protein 2 (PBP2) and PBP4 away from the septum, as well as mild activation of the vraRS-mediated cell wall stress response. However, DNAC-1 does not have any generalized toxicity toward mammalian host cells. DNAC-1 in combination with ceftriaxone is also effective against an assortment of Gram-negative pathogens. Using a murine subcutaneous coinjection model with 10(8) CFU of USA300 as a challenge inoculum, DNAC-1 alone or DNAC-1 with a sub-MIC of oxacillin resulted in a 6-log reduction in bacterial load and decreased abscess formation compared to the untreated control. We propose that DNAC-1, by exerting a bimodal effect on the cell membrane and cell wall, is a viable candidate in the development of combination therapy against many common bacterial pathogens.

A liaR deletion restores susceptibility to daptomycin and antimicrobial peptides in multidrug-resistant Enterococcus faecalis

Daptomycin is a lipopeptide antibiotic that is used clinically against many gram-positive bacterial pathogens and is considered a key frontline bactericidal antibiotic to treat multidrug-resistant enterococci. Emergence of daptomycin resistance during therapy of serious enterococcal infections is a major clinical issue. In this work, we show that deletion of the gene encoding the response regulator, LiaR (a member of the LiaFSR system that controls cell envelope homeostasis), from daptomycin-resistant Enterococcus faecalis not only reversed resistance to 2 clinically available cell membrane-targeting antimicrobials (daptomycin and telavancin), but also resulted in hypersusceptibility to these antibiotics and to a variety of antimicrobial peptides of diverse origin and with different mechanisms of action. The changes in susceptibility to these antibiotics and antimicrobial peptides correlated with in vivo attenuation in a Caenorhabditis elegans model. Mechanistically, deletion of liaR altered the localization of cardiolipin microdomains in the cell membrane. Our findings suggest that LiaR is a master regulator of the enterococcal cell membrane response to diverse antimicrobial agents and peptides; as such, LiaR represents a novel target to restore the activity of clinically useful antimicrobials against these organisms and, potentially, increase susceptibility to endogenous antimicrobial peptides.

Heterogeneity of mprF sequences in methicillin-resistant Staphylococcus aureus clinical isolates: role in cross-resistance between daptomycin and host defense antimicrobial peptides

Over the past several years, single-nucleotide polymorphisms (SNPs) within the mprF open reading frame (ORF) have been proposed to be associated with a gain-of-function phenotype in terms of daptomycin (DAP) nonsusceptibility (referred to as daptomycin resistance [DAP-R] herein for ease of presentation) in Staphylococcus aureus. We investigated the frequencies of SNPs within the mprF ORF and the relationships of such SNPs to cross-resistance between DAP and cationic host defense peptides (HDPs). Thirty-five well-characterized, unique DAP-susceptible (DAP-S) and DAP-R methicillin-resistant S. aureus (MRSA) isolates of the clonal complex 5 genotype were used. In addition to mprF SNPs and DAP-HDP cross-resistance, several other key genotypic and phenotypic metrics often associated with DAP-R were delineated, as follows: (i) mprF expression, (ii) membrane phospholipid content, (iii) positive surface charge, (iv) DAP binding, and (v) cell wall thickness profiles. A number of DAP-S strains (MICs of ≤ 1 μg/ml) exhibited mprF SNPs, occasionally with high-level mprF sequence variation from the genotype reference strain. However, none of these SNPs were localized to well-chronicled mprF hot spot locations associated with DAP-R in S. aureus. In contrast, all 8 DAP-R isolates demonstrated SNPs within such known mprF hot spots. Moreover, only the DAP-R strains showed MprF gain-of-function phenotypes, enhanced mprF expression, higher survival against two prototypical HDPs, and reduced DAP binding. Although a heterogenous array of mprF SNPs were often found in DAP-S strains, only selected hot spot SNPs, combined with concurrent mprF dysregulation, were associated with the DAP-R phenotype.

Site-specific mutation of the sensor kinase GraS in Staphylococcus aureus alters the adaptive response to distinct cationic antimicrobial peptides

The Staphylococcus aureus two-component regulatory system, GraRS, is involved in resistance to killing by distinct host defense cationic antimicrobial peptides (HD-CAPs). It is believed to regulate downstream target genes such as mprF and dltABCD to modify the S. aureus surface charge. However, the detailed mechanism(s) by which the histidine kinase, GraS, senses specific HD-CAPs is not well defined. Here, we studied a well-characterized clinical methicillin-resistant S. aureus (MRSA) strain (MW2), its isogenic graS deletion mutant (ΔgraS strain), a nonameric extracellular loop mutant (ΔEL strain), and four residue-specific ΔEL mutants (D37A, P39A, P39S, and D35G D37G D41G strains). The ΔgraS and ΔEL strains were unable to induce mprF and dltA expression and, in turn, demonstrated significantly increased susceptibilities to daptomycin, polymyxin B, and two prototypical HD-CAPs (hNP-1 and RP-1). Further, P39A, P39S, and D35G-D37G-D41G ΔEL mutations correlated with moderate increases in HD-CAP susceptibility. Reductions of mprF and dltA induction by PMB were also found in the ΔEL mutants, suggesting these residues are pivotal to appropriate activation of the GraS sensor kinase. Importantly, a synthetic exogenous soluble EL mimic of GraS protected the parental MW2 strain against hNP-1- and RP-1-mediated killing, suggesting a direct interaction of the EL with HD-CAPs in GraS activation. In vivo, the ΔgraS and ΔEL strains displayed dramatic reductions in achieved target tissue MRSA counts in an endocarditis model. Taken together, our results provide new insights into potential roles of GraS in S. aureus sensing of HD-CAPs to induce adaptive survival responses to these molecules.

Targeting methicillin-resistant Staphylococcus aureus with short salt-resistant synthetic peptides

The seriousness of microbial resistance combined with the lack of new antimicrobials has increased interest in the development of antimicrobial peptides (AMPs) as novel therapeutics. In this study, we evaluated the antimicrobial activities of two short synthetic peptides, namely, RRIKA and RR. These peptides exhibited potent antimicrobial activity against Staphylococcus aureus, and their antimicrobial effects were significantly enhanced by addition of three amino acids in the C terminus, which consequently increased the amphipathicity, hydrophobicity, and net charge. Moreover, RRIKA and RR demonstrated a significant and rapid bactericidal effect against clinical and drug-resistant Staphylococcus isolates, including methicillin-resistant Staphylococcus aureus (MRSA), vancomycin-intermediate S. aureus (VISA), vancomycin-resistant S. aureus (VRSA), linezolid-resistant S. aureus, and methicillin-resistant Staphylococcus epidermidis. In contrast to many natural AMPs, RRIKA and RR retained their activity in the presence of physiological concentrations of NaCl and MgCl2. Both RRIKA and RR enhanced the killing of lysostaphin more than 1,000-fold and eradicated MRSA and VRSA isolates within 20 min. Furthermore, the peptides presented were superior in reducing adherent biofilms of S. aureus and S. epidermidis compared to results with conventional antibiotics. Our findings indicate that the staphylocidal effects of our peptides were through permeabilization of the bacterial membrane, leading to leakage of cytoplasmic contents and cell death. Furthermore, peptides were not toxic to HeLa cells at 4- to 8-fold their antimicrobial concentrations. The potent and salt-insensitive antimicrobial activities of these peptides present an attractive therapeutic candidate for treatment of multidrug-resistant S. aureus infections.

The high resolution structure of tyrocidine A reveals an amphipathic dimer

Tyrocidine A, one of the first antibiotics ever to be discovered, is a cyclic decapeptide that binds to membranes of target bacteria, disrupting their integrity. It is active against a broad spectrum of Gram-positive organisms, and has recently engendered interest as a potential scaffold for the development of new drugs to combat antibiotic-resistant pathogens. We present here the X-ray crystal structure of tyrocidine A at a resolution of 0.95Å. The structure reveals that tyrocidine forms an intimate and highly amphipathic homodimer made up of four beta strands that associate into a single, highly curved antiparallel beta sheet. We used surface plasmon resonance and potassium efflux assays to demonstrate that tyrocidine binds tightly to mimetics of bacterial membranes with an apparent dissociation constant (K(D)) of 10 μM, and efficiently permeabilizes bacterial cells at concentrations equal to and below the K(D). Using variant forms of tyrocidine in which the fluorescent probe p-cyano-phenylalanine had been inserted on either the polar or apolar face of the molecule, we performed fluorescence quenching experiments, using both water-soluble and membrane-embedded quenchers. The quenching results, together with the structure, strongly support a membrane association model in which the convex, apolar face of tyrocidine's beta sheet is oriented toward the membrane interior, while the concave, polar face is presented to the aqueous phase.