Mode of action of the antimicrobial peptide Mel4 is independent of Staphylococcus aureus cell membrane permeability

Antibacterial efficacy and membrane mechanism of action of the Serratia-derived non-ionic lipopeptide, serrawettin W2-FL10

The study aimed to investigate the antibacterial activity, cytotoxicity, and mechanism of action of the non-ionic, cyclic lipopeptide, serrawettin W2-FL10 against Staphylococcus aureus. W2-FL10 exhibited potent activity against the Gram-positive bacteria S. aureus, Enterococcus faecalis, Enterococcus faecium, Listeria monocytogenes, and Bacillus subtilis, with minimum inhibitory concentration (MIC) values ranging from 6.3 to 31.3 μg/mL, while no activity was observed against Gram-negative bacteria. Broth microdilution assays showed that W2-FL10 interacted with key cell membrane components, such as lipid phosphatidyl glycerol and lipoteichoic acid of S. aureus. Upon membrane interaction, W2-FL10 dissipated membrane potential within 12 min and increased S. aureus membrane permeability within 28-40 min, albeit at slower rates and higher concentrations than the lytic peptide melittin. The observed membrane permeability, as detected with propidium iodide (PI), may be attributed to transmembrane pores/lesions, possibly dependent on dimer-driven lipopeptide oligomerization in the membrane. Scanning electron microscopy (SEM) imaging also visually confirmed the formation of lesions in the cell wall of one of the S. aureus strains, and cell damage within 1 h of exposure to W2-FL10, corroborating the rapid time-kill kinetics of the S. aureus strains. This bactericidal action against the S. aureus strains corresponded to membrane permeabilization by W2-FL10, indicating that self-promoted uptake into the cytosol may be part of the mode of action. Finally, this lipopeptide exhibited low to moderate cytotoxicity to the Chinese hamster ovarian (CHO) cell line in comparison to the control (emetine) with an optimal lipophilicity range (log D value of 2.5), signifying its potential as an antibiotic candidate.

IMPORTANCE

Antimicrobial resistance is a major public health concern, urgently requiring antibacterial compounds exhibiting low adverse health effects. In this study, a novel antibacterial lipopeptide analog is described, serrawettin W2-FL10 (derived from Serratia marcescens), with potent activity displayed against Staphylococcus aureus. Mechanistic studies revealed that W2-FL10 targets the cell membrane of S. aureus, causing depolarization and permeabilization because of transmembrane lesions/pores, resulting in the leakage of intracellular components, possible cytosolic uptake of W2-FL10, and ultimately cell death. This study provides the first insight into the mode of action of a non-ionic lipopeptide. The low to moderate cytotoxicity of W2-FL10 also highlights its application as a promising therapeutic agent for the treatment of bacterial infections.

Membrane-targeting, ultrashort lipopeptide acts as an antibiotic adjuvant and sensitizes MDR gram-negative pathogens toward narrow-spectrum antibiotics

Globally, infections due to multi-drug resistant (MDR) Gram-negative bacterial (GNB) pathogens are on the rise, negatively impacting morbidity and mortality, necessitating urgent treatment alternatives. Herein, we report a detailed bio-evaluation of an ultrashort, cationic lipopeptide 'SVAP9I' that demonstrated potent antibiotic activity and acted as an adjuvant to potentiate existing antibiotic classes towards GNBs. Newly synthesized lipopeptides were screened against ESKAPE pathogens and cytotoxicity assays were performed to evaluate the selectivity index (SI). SVAP9I exhibited broad-spectrum antibacterial activity against critical MDR-GNB pathogens including members of Enterobacteriaceae (MIC 4-8 mg/L), with a favorable CC50 value of ≥100 mg/L and no detectable resistance even after 50th serial passage. It demonstrated fast concentration-dependent bactericidal action as determined via time-kill analysis and also retained full potency against polymyxin B-resistant E. coli, indicating distinct mode of action. SVAP9I targeted E. coli's outer and inner membranes by binding to LPS and phospholipids such as cardiolipin and phosphatidylglycerol. Membrane damage resulted in ROS generation, depleted intracellular ATP concentration and a concomitant increase in extracellular ATP. Checkerboard assays showed SVAP9I's synergism with narrow-spectrum antibiotics like vancomycin, fusidic acid and rifampicin, potentiating their efficacy against MDR-GNB pathogens, including carbapenem-resistant Acinetobacter baumannii (CRAB), a WHO critical priority pathogen. In a murine neutropenic thigh infection model, SVAP9I and rifampicin synergized to express excellent antibacterial efficacy against MDR-CRAB outcompeting polymyxin B. Taken together, SVAP9I's distinct membrane-targeting broad-spectrum action, lack of resistance and strong in vitro andin vivopotency in synergism with narrow spectrum antibiotics like rifampicin suggests its potential as a novel antibiotic adjuvant for the treatment of serious MDR-GNB infections.

Non-canonical amino acid bioincorporation into antimicrobial peptides and its challenges

The rise of antimicrobial resistance and multi-drug resistant pathogens has necessitated explorations for novel antibiotic agents as the discovery of conventional antibiotics is becoming economically less viable and technically more challenging for biopharma. Antimicrobial peptides (AMPs) have emerged as a promising alternative because of their particular mode of action, broad spectrum and difficulty that microbes have in becoming resistant to them. The AMPs bacitracin, gramicidin, polymyxins and daptomycin are currently used clinically. However, their susceptibility to proteolytic degradation, toxicity profile, and complexities in large-scale manufacture have hindered their development. To improve their proteolytic stability, methods such as integrating non-canonical amino acids (ncAAs) into their peptide sequence have been adopted, which also improves their potency and spectrum of action. The benefits of ncAA incorporation have been made possible by solid-phase peptide synthesis. However, this method is not always suitable for commercial production of AMPs because of poor yield, scale-up difficulties, and its non-'green' nature. Bioincorporation of ncAA as a method of integration is an emerging field geared towards tackling the challenges of solid-phase synthesis as a green, cheaper, and scalable alternative for commercialisation of AMPs. This review focusses on the bioincorporation of ncAAs; some challenges associated with the methods are outlined, and notes are given on how to overcome these challenges. The review focusses particularly on addressing two key challenges: AMP cytotoxicity towards microbial cell factories and the uptake of ncAAs that are unfavourable to them. Overcoming these challenges will draw us closer to a greater yield and an environmentally friendly and sustainable approach to make AMPs more druggable.

Dimeric peptoids as antibacterial agents

Building upon our previous study on peptoid-based antibacterials which showed good activity against Gram-positive bacteria only, herein we report the synthesis of 34 dimeric peptoid compounds and the investigation of their activity against Gram-positive and Gram-negative pathogens. The newly designed peptoids feature a di-hydrophobic moiety incorporating phenyl, bromo-phenyl, and naphthyl groups, combined with variable lengths of cationic units such as amino and guanidine groups. The study also underscores the pivotal interplay between hydrophobicity and cationicity in optimizing efficacy against specific bacteria. The bromophenyl dimeric guanidinium peptoid compound 10j showed excellent activity against S. aureus 38 and E. coli K12 with MIC of 0.8 μg mL-1 and 6.2 μg mL-1, respectively. Further investigation into the mechanism of action revealed that the antibacterial effect might be attributed to the disruption of bacterial cell membranes, as suggested by tethered bilayer lipid membranes (tBLMs) and cytoplasmic membrane permeability studies. Notably, these promising antibacterial agents exhibited negligible toxicity against mammalian red blood cells. Additionally, the study explored the potential of 12 active compounds to disrupt established biofilms of S. aureus 38. The most effective biofilm disruptors were ethyl and octyl-naphthyl guanidinium peptoids (10c and 10 k). These compounds 10c and 10 k disrupted the established biofilms of S. aureus 38 with 51 % at 4x MIC (MIC = 17.6 μg mL-1 and 11.2 μg mL-1) and 56 %-58 % at 8x MIC (MIC = 35.2 μg mL-1 and 22.4 μg mL-1) respectively. Overall, this research contributes insights into the design principles of cationic dimeric peptoids and their antibacterial activity, with implications for the development of new antibacterial compounds.

Evaluation of Bacteriophage ϕ11 host recognition protein and its host-binding peptides for diagnosing/targeting Staphylococcus aureus infections

BACKGROUND

Evaluating the potential of using both synthetic and biological products as targeting agents for the diagnosis, imaging, and treatment of infections due to particularly antibiotic-resistant pathogens is important for controlling infections. This study examined the interaction between Gp45, a receptor-binding protein of the ϕ11 lysogenic phage, and its host Staphylococcus aureus (S. aureus), a common cause of nosocomial infections.

METHODS

Using molecular dynamics and docking simulations, this study identified the peptides that bind to S. aureus wall teichoic acids via Gp45. It compared the binding affinity of Gp45 and the two highest-scoring peptide sequences (P1 and P3) and their scrambled forms using microscopy, spectroscopy, and ELISA.

RESULTS

It was found that rGp45 (recombinant Gp45) and chemically synthesised P1 had a higher binding affinity for S. aureus compared with all other peptides, except for Escherichia coli. Furthermore, rGp45 had a capture efficiency of > 86%; P1 had a capture efficiency of > 64%.

CONCLUSION

These findings suggest that receptor-binding proteins such as rGp45, which provide a critical initiation of the phage life cycle for host adsorption, might play an important role in the diagnosis, imaging, and targeting of bacterial infections. Studying such proteins could accordingly enable the development of effective strategies for controlling infections.

Protecting Orthopaedic Implants from Infection: Antimicrobial Peptide Mel4 Is Non-Toxic to Bone Cells and Reduces Bacterial Colonisation When Bound to Plasma Ion-Implanted 3D-Printed PAEK Polymers

Even with the best infection control protocols in place, the risk of a hospital-acquired infection of the surface of an implanted device remains significant. A bacterial biofilm can form and has the potential to escape the host immune system and develop resistance to conventional antibiotics, ultimately causing the implant to fail, seriously impacting patient well-being. Here, we demonstrate a 4 log reduction in the infection rate by the common pathogen S. aureus of 3D-printed polyaryl ether ketone (PAEK) polymeric surfaces by covalently binding the antimicrobial peptide Mel4 to the surface using plasma immersion ion implantation (PIII) treatment. The surfaces with added texture created by 3D-printed processes such as fused deposition-modelled polyether ether ketone (PEEK) and selective laser-sintered polyether ketone (PEK) can be equally well protected as conventionally manufactured materials. Unbound Mel4 in solution at relevant concentrations is non-cytotoxic to osteoblastic cell line Saos-2. Mel4 in combination with PIII aids Saos-2 cells to attach to the surface, increasing the adhesion by 88% compared to untreated materials without Mel4. A reduction in mineralisation on the Mel4-containing surfaces relative to surfaces without peptide was found, attributed to the acellular portion of mineral deposition.

The activity of antimicrobial peptoids against multidrug-resistant ocular pathogens

BACKGROUND

Ocular infections caused by antibiotic-resistant pathogens can result in partial or complete vision loss. The development of pan-resistant microbial strains poses a significant challenge for clinicians as there are limited antimicrobial options available. Synthetic peptoids, which are sequence-specific oligo-N-substituted glycines, offer potential as alternative antimicrobial agents to target multidrug-resistant bacteria.

METHODS

The antimicrobial activity of synthesised peptoids against multidrug-resistant (MDR) ocular pathogens was evaluated using the microbroth dilution method. Hemolytic propensity was assessed using mammalian erythrocytes. Peptoids were also incubated with proteolytic enzymes, after which their minimum inhibitory activity against bacteria was re-evaluated.

RESULTS

Several alkylated and brominated peptoids showed good inhibitory activity against multidrug-resistant Pseudomonas aeruginosa strains at concentrations of ≤15 μg mL-1 (≤12 µM). Similarly, most brominated compounds inhibited the growth of methicillin-resistant Staphylococcus aureus at 1.9 to 15 μg mL-1 (12 µM). The N-terminally alkylated peptoids caused less toxicity to erythrocytes. The peptoid denoted as TM5 had a high therapeutic index, being non-toxic to either erythrocytes or corneal epithelial cells, even at 15 to 22 times its MIC. Additionally, the peptoids were resistant to protease activity.

CONCLUSIONS

Peptoids studied here demonstrated potent activity against various multidrug-resistant ocular pathogens. Their properties make them promising candidates for controlling vision-related morbidity associated with eye infections by antibiotic-resistant strains.

DP1, a multifaceted synthetic peptide: Mechanism of action, activity and clinical potential

AIMS

Microbial infections remain a leading cause of mortality worldwide, with Staphylococcus aureus (S. aureus) being a prominent etiological agent, responsible for causing persistent bacterial infections in humans. It is a nosocomial, opportunistic pathogen, capable to propagate within the bloodstream and withstand therapeutic interventions. In the current study, a novel, indigenously designed synthetic antimicrobial peptide (sAMP) has been evaluated for its antimicrobial potential to inhibit the growth and proliferation of S. aureus.

MAIN METHODS

The sAMP, designed peptide (DP1) was evaluated for its minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) against a panel of pathogenic bacterial strains. Membrane mechanistic studies were performed by measuring membrane conductivity via dielectric spectroscopy and visualizing changes in bacterial membrane structure through field emission scanning electron microscopy (FE-SEM). Further, DP1 was tested for its in vivo antimicrobial potential in an S. aureus-induced systemic infection model.

KEY FINDINGS

The results indicated that DP1 has the potential to inhibit the growth and proliferation of a broad spectrum of Gram-positive, Gram-negative and multidrug-resistant (MDR) bacterial strains. Strong bactericidal effect attributed to change in electrical conductivity of the bacterial cells leading to membrane disruption was observed through dielectric spectroscopy and FE-SEM micrographs. Further, in the in vivo murine systemic infection study, 50 % reduction in S. aureus bioburden was observed within 1 day of the administration of DP1.

SIGNIFICANCE

The results indicate that DP1 is a multifaceted peptide with potent bactericidal, antioxidant and therapeutic properties. It holds significance as a novel drug candidate to effectively combat S. aureus-mediated systemic infections.

Antimicrobial peptides: An alternative to traditional antibiotics

As antibiotic-resistant bacteria and genes continue to emerge, the identification of effective alternatives to traditional antibiotics has become a pressing issue. Antimicrobial peptides are favored for their safety, low residue, and low resistance properties, and their unique antimicrobial mechanisms show significant potential in combating antibiotic resistance. However, the high production cost and weak activity of antimicrobial peptides limit their application. Moreover, traditional laboratory methods for identifying and designing new antimicrobial peptides are time-consuming and labor-intensive, hindering their development. Currently, novel technologies, such as artificial intelligence (AI) are being employed to develop and design new antimicrobial peptide resources, offering new opportunities for the advancement of antimicrobial peptides. This article summarizes the basic characteristics and antimicrobial mechanisms of antimicrobial peptides, as well as their advantages and limitations, and explores the application of AI in antimicrobial peptides prediction amd design. This highlights the crucial role of AI in enhancing the efficiency of antimicrobial peptide research and provides a reference for antimicrobial drug development.

The Study of Antistaphylococcal Potential of Omiganan and Retro-Omiganan Under Flow Conditions

Staphylococcus aureus is considered one of the leading pathogens responsible for infections in humans and animals. The heterogeneous nature of diseases caused by these bacteria is due to the occurrence of multiple strains, differentiated by several mechanisms of antibiotic resistance and virulence factors. One of these is the ability to form biofilm. Biofilm-associated bacteria exhibit a different phenotype that protects them from external factors such as the activity of immune system or antimicrobial substances. Moreover, it has been shown that the majority of persistent and recurrent infections are associated with the presence of the biofilm. Omiganan, an analog of indolicidin - antimicrobial peptide (AMP) derived from bovine neutrophil granules, was found to exhibit high antistaphylococcal and antibiofilm potential. Furthermore, its analog with a reversed sequence (retro-omiganan) was found to display enhanced activity against a variety of pathogens. Based on experience of our group, we found out that counterion exchange can improve the antistaphylococcal activity of AMPs. The aim of this study was to investigate the activity of both compounds against S. aureus biofilm under flow conditions. The advantage of this approach was that it offered the opportunity to form and characterize the biofilm under more controlled conditions. To do this, unique flow cells made of polydimethylsiloxane (PDMS) were developed. The activity against pre-formed biofilm as well as AMPs-treated bacteria was measured. Also, the incorporation of omiganan and retro-omiganan into the channels was conducted to learn whether or not it would inhibit the development of biofilm. The results of the microbiological tests ultimately confirmed the high potential of the omiganan and its retro-analog as well as the importance of counterion exchange in terms of antimicrobial examination. We found out that retro-omiganan trifluoroacetate had the highest biofilm inhibitory properties, however, acetates of both compounds exhibited the highest activity against planktonic and biofilm cultures. Moreover, the developed methodology of investigation under flow conditions allows the implementation of the studies under flow conditions to other compounds.

Repurposing of the antimalarial agent tafenoquine to combat MRSA

IMPORTANCE

This study represents the first investigation into the antimicrobial effect of TAF against S. aureus and its potential mechanisms. Our data highlighted the effects of TAF against MRSA planktonic cells, biofilms, and persister cells, which is conducive to broadening the application of TAF. Through mechanistic studies, we revealed that TAF targets bacterial cell membranes. In addition, the in vivo experiments in mice demonstrated the safety and antimicrobial efficacy of TAF, suggesting that TAF could be a potential antibacterial drug candidate for the treatment of infections caused by multiple drug-resistant S. aureus.

Anti-methicillin-resistant Staphylococcus aureus and antibiofilm activity of new peptides produced by a Brevibacillus strain

Background

Methicillin-resistant Staphylococcus aureus (MRSA) is listed as a highly prioritized pathogen by the World Health Organization (WHO) to search for effective antimicrobial agents. Previously, we isolated a soil Brevibacillus sp. strain SPR19 from a botanical garden, which showed anti-MRSA activity. However, the active substances were still unknown.

Methods

The cell-free supernatant of this bacterium was subjected to salt precipitation, cation exchange, and reversed-phase chromatography. The antimicrobial activity of pure substances was determined by broth microdilution assay. The peptide sequences and secondary structures were characterized by tandem mass spectroscopy and circular dichroism (CD), respectively. The most active anti-MRSA peptide underwent a stability study, and its mechanism was determined through scanning electron microscopy, cell permeability assay, time-killing kinetics, and biofilm inhibition and eradication. Hemolysis was used to evaluate the peptide toxicity.

Results

The pure substances (BrSPR19-P1 to BrSPR19-P5) were identified as new peptides. Their minimum inhibition concentration (MIC) and minimum bactericidal concentration (MBC) against S. aureus and MRSA isolates ranged from 2.00 to 32.00 and 2.00 to 64.00 µg/mL, respectively. The sequence analysis of anti-MRSA peptides revealed a length ranging from 12 to 16 residues accompanied by an amphipathic structure. The physicochemical properties of peptides were predicted such as pI (4.25 to 10.18), net charge at pH 7.4 (-3 to +4), and hydrophobicity (0.12 to 0.96). The CD spectra revealed that all peptides in the water mainly contained random coil structures. The increased proportion of α-helix structure was observed in P2-P5 when incubated with SDS. P2 (NH2-MFLVVKVLKYVV-COOH) showed the highest antimicrobial activity and high stability under stressed conditions such as temperatures up to 100 °C, solution of pH 3 to 10, and proteolytic enzymes. P2 disrupted the cell membrane and caused bacteriolysis, in which its action was dependent on the incubation time and peptide concentration. Antibiofilm activity of P2 was determined by which the half-maximal inhibition of biofilm formation was observed at 2.92 and 4.84 µg/mL for S. aureus TISTR 517 and MRSA isolate 2468, respectively. Biofilm eradication of tested pathogens was found at the P2 concentration of 128 µg/mL. Furthermore, P2 hemolytic activity was less than 10% at concentrations up to 64 µg/mL, which reflected the hemolysis index thresholds of 32.

Conclusion

Five novel anti-MRSA peptides were identified from SPR19. P2 was the most active peptide and was demonstrated to cause membrane disruption and cell lysis. The P2 activity was dependent on the peptide concentration and exposure time. This peptide had antibiofilm activity against tested pathogens and was compatible with human erythrocytes, supporting its potential use as an anti-MRSA agent in this post-antibiotic era.

Retama monosperma chemical profile, green synthesis of silver nanoparticles, and antimicrobial potential: a study supported by network pharmacology and molecular docking

In this study, Retama monosperma extract (RME) was used for the green synthesis of silver nanoparticles (RME-AgNPs). RME's phenolic profile was identified by liquid chromatography coupled to mass spectroscopy (LC-ESI/MS/MS) technique. A tentative identification of 21 phenolic metabolites from the extract was performed. The produced RME-AgNPs showed UV absorbance at 443 nm. FTIR spectroscopy confirmed the presence of RME functional groups. In addition, XRD analysis confirmed the crystallography of RME-AgNPs via exhibiting peaks with 2θ values at 38.34°, 44.29°, and 64.65°. RME-AgNPs were spherical with particle sizes ranging from 9.87 to 21.16 nm, as determined by SEM and HR-TEM techniques. The zeta potential determined the particle's charge value as -15.25 mv. RME-AgNPs exhibited significantly higher antibacterial activity against Gram-negative (Escherichia coli, Pseudomonas aeruginosa, Serratia marcescens, and Klebsiella pneumoniae) and Gram-positive bacteria (Bacillus subtilis and Staphylococcus aureus) compared to RME. Moreover, the SEM images of green-synthesized nanoparticles revealed severe damage and deformation in the bacterial cell wall of the different strains subjected to the current investigation. The bioinformatics study identified 266 targets, among which only 41 targets were associated with bacterial infections. The PI3K-Akt and Relaxin signaling pathways were the top KEGG signaling pathways. Molecular docking was also performed for the 21 identified compounds at the TNF-α active site; kaempferol-3-O-robinoside-7-O-rhamnoside had a higher binding energy (-6.8084). The findings of this study warrant the use of green-synthesized AgNPs from Retama monosperma as potential antibacterial agents.

Efficient Synergistic Antibacterial Activity of α-MSH Using Chitosan-Based Versatile Nanoconjugates

The application of antimicrobial peptides has emerged as an alternative therapeutic tool to encounter against multidrug resistance of different pathogenic organisms. α-Melanocyte stimulating hormone (α-MSH), an endogenous neuropeptide, is found to be efficient in eradicating infection of various kinds of Staphylococcus aureus, including methicillin-resistant Staphylococcus aureus (MRSA). However, the chemical stability and efficient delivery of these biopharmaceuticals (i.e., α-MSH) to bacterial cells with a significant antibacterial effect remains a key challenge. To address this issue, we have developed a chitosan-cholesterol polymer using a single-step, one-pot, and simple chemical conjugation technique, where α-MSH is loaded with a significantly high amount (37.7%), and the final product is obtained as chitosan-cholesterol α-MSH polymer-drug nanoconjugates. A staphylococcal growth inhibition experiment was performed using chitosan-cholesterol α-MSH and individual controls. α-MSH and chitosan-cholesterol both show bacterial growth inhibition by a magnitude of 50 and 79%, respectively. The killing efficiency of polymer-drug nanoconjugates was very drastic, and almost no bacterial colony was observed (∼100% inhibition) after overnight incubation. Phenotypic alternation was observed in the presence of α-MSH causing changes in the cell structure and shape, indicating stress on Staphylococcus aureus. As a further consequence, vigorous cell lysis with concomitant release of the cellular material in the nearby medium was observed after treatment of chitosan-cholesterol α-MSH nanoconjugates. This vigorous lysis of the cell structure is associated with extensive aggregation of the bacterial cells evident in scanning electron microscopy (SEM). The dose-response experiment was performed with various concentrations of chitosan-cholesterol α-MSH nanoconjugates to decipher the degree of the bactericidal effect. The concentration of α-MSH as low as 1 pM also shows significant inhibition of bacterial growth (∼40% growth inhibition) of Staphylococcus aureus. Despite playing an important role in inhibiting bacterial growth, our investigation on hemolytic assay shows that chitosan-cholesterol α-MSH is significantly nontoxic at a wide range of concentrations. In a nutshell, our analysis demonstrated novel antimicrobial activity of nanoparticle-conjugated α-MSH, which could be used as future therapeutics against multidrug-resistant Staphylococcus aureus and other types of bacterial cells.

Chitosan-based packaging films with an integrated antimicrobial peptide: Characterization, in vitro release and application to fresh pork preservation

Chitosan (CS) films were developed incorporating peptide HX-12C. The films were studied to determine their microstructures, physical properties, release properties of peptide HX-12C and functional properties. The results indicated that there may be hydrogen bonding interactions between CS and peptide HX-12C, thereby creating a homogeneous internal microstructure and lower crystallinity (10.8-12.8 %). Compared with CS film, CS-HX-12C films displayed lower light transmission, MC (20.8-19.9 %), WVP (8.82-8.59 × 10^-11·g·m^-1·s^-1·Pa^-1), OTR (0.015-0.037 cc/(m².day)) and higher WS (15.7-32.4 %) values. Moreover, controlled-release experiments showed that pH, ionic strength and temperature could all significantly affect the release of peptide HX-12C from the films. Finally, the increase of pH value and TVC and lipid oxidation of fresh pork were delayed due to the treatment with CS-2%HX-12C film. However, incorporating peptide HX-12C into CS films did not improve the mechanical properties of the films and their effects against protein oxidation. Our results suggest that the CS-based antimicrobial packaging films integrated with peptide HX-12C exhibit the potential for fresh pork preservation.

Magnolia officinalis L. bark extract and respiratory diseases: From traditional Chinese medicine to western medicine via network target

The understanding of the use of Magnolia officinalis L. (Magnoliaceae) as a possible dietary supplement for supporting the treatment of airway pathologies might be of clinical interest. Two commercially available bark extracts (M. officinalis extract [MOE]) were characterized by quantitation in honokiol and magnolol content by means of high-performance liquid chromatography with UV detection. MOE effects, as well as those of the reference compounds per se, on some targets connected to airway pathologies (antibacterial- and lung and trachea relaxing- activities) were investigated. Results showed that MOE possessed interesting antibacterial activity against Staphylococcus aureus, Pseudomonas aeruginosa, and Streptococcus pneumoniae. This was accompanied by a spasmolytic and antispasmodic activity, possibly owing to its ability to concurrently modulate different targets such as H₁ -, β₂ - and muscarinic receptors and l-type calcium channels involved in bronchodilation. All these effects were directly related to the MOE content in honokiol and magnolol. In conclusion, the properties of MOE highlighted here strongly encourage its application as dietary supplement in the treatment of airway diseases.

Antibiotic-induced accumulation of lipid II synergizes with antimicrobial fatty acids to eradicate bacterial populations

Antibiotic tolerance and antibiotic resistance are the two major obstacles to the efficient and reliable treatment of bacterial infections. Identifying antibiotic adjuvants that sensitize resistant and tolerant bacteria to antibiotic killing may lead to the development of superior treatments with improved outcomes. Vancomycin, a lipid II inhibitor, is a frontline antibiotic for treating methicillin-resistant Staphylococcus aureus and other Gram-positive bacterial infections. However, vancomycin use has led to the increasing prevalence of bacterial strains with reduced susceptibility to vancomycin. Here, we show that unsaturated fatty acids act as potent vancomycin adjuvants to rapidly kill a range of Gram-positive bacteria, including vancomycin-tolerant and resistant populations. The synergistic bactericidal activity relies on the accumulation of membrane-bound cell wall intermediates that generate large fluid patches in the membrane leading to protein delocalization, aberrant septal formation, and loss of membrane integrity. Our findings provide a natural therapeutic option that enhances vancomycin activity against difficult-to-treat pathogens, and the underlying mechanism may be further exploited to develop antimicrobials that target recalcitrant infection.

The Potential of Surface-Immobilized Antimicrobial Peptides for the Enhancement of Orthopaedic Medical Devices: A Review

Due to the well-known phenomenon of antibiotic resistance, there is a constant need for antibiotics with novel mechanisms and different targets respect to those currently in use. In this regard, the antimicrobial peptides (AMPs) seem very promising by virtue of their bactericidal action, based on membrane permeabilization of susceptible microbes. Thanks to this feature, AMPs have a broad activity spectrum, including antibiotic-resistant strains, and microbial biofilms. Additionally, several AMPs display properties that can help tissue regeneration. A possible interesting field of application for AMPs is the development of antimicrobial coatings for implantable medical devices (e.g., orthopaedic prostheses) to prevent device-related infection. In this review, we will take note of the state of the art of AMP-based coatings for orthopaedic prostheses. We will review the most recent studies by focusing on covalently linked AMPs to titanium, their antimicrobial efficacy and plausible mode of action, and cytocompatibility. We will try to extrapolate some general rules for structure-activity (orientation, density) relationships, in order to identify the most suitable physical and chemical features of peptide candidates, and to optimize the coupling strategies to obtain antimicrobial surfaces with improved biological performance.

Purification and Characterization of Novel Anti-MRSA Peptides Produced by Brevibacillus sp. SPR-20

Methicillin-resistant Staphylococcus aureus (MRSA) is listed as a high-priority pathogen because its infection is associated with a high mortality rate. It is urgent to search for new agents to treat such an infection. Our previous study isolated a soil bacterium (Brevibacillus sp. SPR-20), showing the highest antimicrobial activity against S. aureus TISTR 517 and MRSA strains. The present study aimed to purify and characterize anti-MRSA substances produced by SPR-20. The result showed that five active substances (P1-P5) were found, and they were identified by LC-MS/MS that provided the peptide sequences of 14-15 residues. Circular dichroism showed that all peptides contained β-strand and disordered conformations as the major secondary structures. Only P1-P4 adopted more α-helix conformations when incubated with 50 mM SDS. These anti-MRSA peptides could inhibit S. aureus and MRSA in concentrations of 2-32 μg/mL. P1 (NH₂-VVVNVLVKVLPPPVV-COOH) had the highest activity and was identified as a novel antimicrobial peptide (AMP). The stability study revealed that P1 was stable in response to temperature, proteolytic enzymes, surfactant, and pH. The electron micrograph showed that P1 induced bacterial membrane damage when treated at 1× MIC in the first hour of incubation. The killing kinetics of P1 was dependent on concentration and time. Mechanisms of P1 on tested pathogens involved membrane permeability, leakage of genetic material, and cell lysis. The P1 peptide at a concentration up to 32 μg/mL showed hemolysis of less than 10%, supporting its safety for human erythrocytes. This study provides promising anti-MRSA peptides that might be developed for effective antibiotics in the post-antibiotic era.

The Structural Changes in the Membranes of Staphylococcus aureus Caused by Hydrolysable Tannins Witness Their Antibacterial Activity

Polyphenols, including tannins, are phytochemicals with pronounced antimicrobial properties. We studied the activity of two hydrolysable tannins, (i) gallotannin-1,2,3,4,5-penta-O-galloyl-β-D-glucose (PGG) and (ii) ellagitannin-1,2-di-O-galloyl-4,6-valoneoyl-β-D-glucose (dGVG), applied alone and in combination with antibiotics against Staphylococcus aureus strain 8324-4. We also evaluated the effect of these tannins on bacterial membrane integrity and fluidity and studied their interaction with membrane proteins and lipids. A correlation between the antimicrobial activity of the tannins and their membranotropic action depending on the tannin molecular structure has been demonstrated. We found that the antibacterial activity of PGG was stronger than dGVG, which can be associated with its larger flexibility, dipole moment, and hydrophobicity. In addition, we also noted the membrane effects of the tannins observed as an increase in the size of released bacterial membrane vesicles.

Assessment of New Strategies to Improve the Performance of Antimicrobial Peptides

In this research, we constructed a novel engineered tripeptide modified with lipoic acid (LA-RWR), followed by crosslinking of lipoic acid to form nanoparticles (c-LA-RWR). LA-RWR was also modified with phenethylamine (PEA) on the C-terminus to achieve better antibacterial activities. The as-prepared c-LA-RWR and LA-RWR-PEA were effective against E.coli, S.aureus, C.albicans, and methicillin-resistant Staphylococcus aureus, with minimum inhibitory concentration values ranging from 2 to 16 µg/mL, which greatly improved the performance of LA-RWR. Similar antibacterial activities were demonstrated in anti-biofilm activity; there was no matter on the biofilm that was already established or forming. Moreover, c-LA-RWR/LA-RWR-PEA remarkably induced cytoplasmic membrane depolarization and outer membrane permeabilization, resulting in varying degrees of damage to the bacterial morphology, which were consistent with the results obtained via electron microscopy. Thus, our results show that c-LA-RWR/LA-RWR-PEA exhibited excellent efficacy against a variety of microorganisms with good biosafety, providing new strategies by which to improve the performance of antimicrobial peptides.

The antimicrobial effect of a novel peptide LL-1 on Escherichia coli by increasing membrane permeability

Background

The widespread use of antibiotics has led to the emergence of many drug-resistant strains; thus, the development of new antibacterial drugs is essential with antimicrobial peptides becoming the focus of research. This study assessed the antibacterial effect of a novel antimicrobial peptide, named LL-1 on Escherichia coli (E.coli) by determining the minimum inhibitory concentration (MIC) and the antibacterial curve. The interaction between LL-1 and E. coli DNA was then detected by nucleic acid gel electrophoresis. The effect of LL-1 on the E. coli cell membrane was assessed by detecting the leakage of β-galactosidase, nucleic acid and protein. The influence of LL-1 on the intracellular ATP of E. coli was analysed by determining the concentration of intracellular ATP. Finally, the bacteria and colonies of E. coli treated with LL-1 were observed using scanning and transmission electron microscopy.

Results

The results suggested that the MIC value was 3.125 µg/ml, and the antibacterial effect was dose-dependent. LL-1 dose-dependently combined with E. coli DNA. LL-1 resulted in the leakage of intracellular β-galactosidase, nucleic acid and protein, and decreased intracellular ATP concentrations of E. coli. Two MIC of LL-1 caused E. coli to shrink, resulting in a rough surface, plasmolysis, and bacterial adhesion.

Conclusion

This study indicated that LL-1 had a good bactericidal effect on E. coli by mainly increasing the permeability of the cell membrane, leading to leakage of the intracellular content. This will lay the foundation for an in-depth study on the antibacterial mechanism of LL-1 against E. coli and its clinical application.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12866-022-02621-y.

Isoquinoline Antimicrobial Agent: Activity against Intracellular Bacteria and Effect on Global Bacterial Proteome

A new class of alkynyl isoquinoline antibacterial compounds, synthesized via Sonogashira coupling, with strong bactericidal activity against a plethora of Gram-positive bacteria including methicillin- and vancomycin-resistant Staphylococcus aureus (S. aureus) strains is presented. HSN584 and HSN739, representative compounds in this class, reduce methicillin-resistant S. aureus (MRSA) load in macrophages, whilst vancomycin, a drug of choice for MRSA infections, was unable to clear intracellular MRSA. Additionally, both HSN584 and HSN739 exhibited a low propensity to develop resistance. We utilized comparative global proteomics and macromolecule biosynthesis assays to gain insight into the alkynyl isoquinoline mechanism of action. Our preliminary data show that HSN584 perturb S. aureus cell wall and nucleic acid biosynthesis. The alkynyl isoquinoline moiety is a new scaffold for the development of potent antibacterial agents against fatal multidrug-resistant Gram-positive bacteria.

Short Tryptamine-Based Peptoids as Potential Therapeutics for Microbial Keratitis: Structure-Function Correlation Studies

Peptoids are peptidomimetics that have attracted considerable interest as a promising class of antimicrobials against multi-drug-resistant bacteria due to their resistance to proteolysis, bioavailability, and thermal stability compared to their corresponding peptides. Staphylococcus aureus is a significant contributor to infections worldwide and is a major pathogen in ocular infections (keratitis). S. aureus infections can be challenging to control and treat due to the development of multiple antibiotic resistance. This work describes short cationic peptoids with activity against S. aureus strains from keratitis. The peptoids were synthesized via acid amine-coupling between naphthyl-indole amine or naphthyl-phenyl amine with different amino acids to produce primary amines (series I), mono-guanidines (series II), tertiary amine salts (series III), quaternary ammonium salts (series IV), and di-guanidine (series V) peptoids. The antimicrobial activity of the peptoids was compared with ciprofloxacin, an antibiotic that is commonly used to treat keratitis. All new compounds were active against Staphylococcus aureus S.aureus 38. The most active compounds against S.aur38 were 20a and 22 with MIC = 3.9 μg mL−1 and 5.5 μg mL−1, respectively. The potency of these two active molecules was investigated against 12 S. aureus strains that were isolated from microbial keratitis. Compounds 20a and 22 were active against 12 strains with MIC = 3.2 μg mL−1 and 2.1 μg mL−1, respectively. There were two strains that were resistant to ciprofloxacin (Sa.111 and Sa.112) with MIC = 128 μg mL−1 and 256 μg mL−1, respectively. Compounds 12c and 13c were the most active against E. coli, with MIC > 12 μg mL−1. Cytoplasmic membrane permeability studies suggested that depolarization and disruption of the bacterial cell membrane could be a possible mechanism for antibacterial activity and the hemolysis studies toward horse red blood cells showed that the potent compounds are non-toxic at up to 50 μg mL−1.

Host Defense Peptides at the Ocular Surface: Roles in Health and Major Diseases, and Therapeutic Potentials

Sight is arguably the most important sense in human. Being constantly exposed to the environmental stress, irritants and pathogens, the ocular surface – a specialized functional and anatomical unit composed of tear film, conjunctival and corneal epithelium, lacrimal glands, meibomian glands, and nasolacrimal drainage apparatus – serves as a crucial front-line defense of the eye. Host defense peptides (HDPs), also known as antimicrobial peptides, are evolutionarily conserved molecular components of innate immunity that are found in all classes of life. Since the first discovery of lysozyme in 1922, a wide range of HDPs have been identified at the ocular surface. In addition to their antimicrobial activity, HDPs are increasingly recognized for their wide array of biological functions, including anti-biofilm, immunomodulation, wound healing, and anti-cancer properties. In this review, we provide an updated review on: (1) spectrum and expression of HDPs at the ocular surface; (2) participation of HDPs in ocular surface diseases/conditions such as infectious keratitis, conjunctivitis, dry eye disease, keratoconus, allergic eye disease, rosacea keratitis, and post-ocular surgery; (3) HDPs that are currently in the development pipeline for treatment of ocular diseases and infections; and (4) future potential of HDP-based clinical pharmacotherapy for ocular diseases.

The effects of Peptide Mel4-coated titanium plates on infection rabbits after internal fixation of open fractures

INTRODUCTION

Infection after internal fixation surgery is an orthopedic serious complication which affect the fracture healing. The primary objective of this study was to verify the effects of a Peptide Mel4-coated titanium plate applied in the treatment of infection after internal fixation of open fracture.

MATERIALS AND METHODS

Eighty-eight rabbits were intravenously inoculated with Staphylococcus aureus or Pseudomonas aeruginosa suspensions. Bacterial cultures were obtained from titanium plates at 1st, 3rd, 5th, 7th and 9th days. Blood samples were collected at 1st, 3rd, 5th, 7th and 9th days after the infection.

RESULTS

Mel4-coated titanium plates have significant inhibitory effects on Staphylococcus aureus and Pseudomonas aeruginosa (P < 0.05), and there are significant differences in serum IL-1 and TNF-α levels (P < 0.05).

CONCLUSION

We suggest that the use of Mel4-coated titanium plates may be a promising way to control postoperative infection of open fracture in vivo.

Bacteriocins from lactic acid bacteria: purification strategies and applications in food and medical industries: a review

Background

Bacteriocins are generally defined as ribosomally synthesized peptides, which are produced by lactic acid bacteria (LAB) that affect the growth of related or unrelated microorganisms. Conventionally, the extracted bacteriocins are purified by precipitation, where ammonium sulphate is added to precipitate out the protein from the solution.

Main text

To achieve the high purity of bacteriocins, a combination with chromatography is used where the hydrophobicity and cationic properties of bacteriocins are employed. The complexity column inside the chromatography can afford to resolve the loss of bacteriocins during the ammonium sulphate precipitation. Recently, an aqueous two-phase system (ATPS) has been widely used in bacteriocins purification due to the several advantages of its operational simplicity, mild process conditions and versatility. It reduces the operation steps and processing time yet provides high recovery products which provide alternative ways to conventional methods in downstream processing. Bacteriocins are widely approached in the food and medical industry. In food application, nisin, which is produced by Lactococcus lactis subsp. has been introduced as food preservative due to its natural, toxicology safe and effective against the gram-positive bacteria. Besides, bacteriocins provide a board range in medical industries where they are used as antibiotics and probiotics.

Short conclusion

In summary, this review focuses on the downstream separation of bacteriocins from various sources using both conventional and recent ATPS techniques. Finally, recommendations for future interesting areas of research that need to be pursued are highlighted.

A Multi-Purpose Approach to the Mechanisms of Action of Two Biocides (Benzalkonium Chloride and Dibromonitrilopropionamide): Discussion of Pseudomonas fluorescens’ Viability and Death

Biocides are widely used in water treatment for microbiological control. The rise of antimicrobial resistance and the need to assure properly managed water systems require a better understanding of the mechanisms of action of biocides and of their impact on cell’s viability as a function of dosage concentrations. The present work addresses these two aspects regarding the biocides benzalkonium chloride (BAC) and dibromonitrilopropionamide (DBNPA)—two biocides commonly found in the water treatment industry. For that, the following parameters were studied: culturability, membrane integrity, metabolic activity, cellular energy, and the structure and morphology of cells. Also, to assess cell’s death, a reliable positive control, consisting of cells killed by autoclave (dead cells), was introduced. The results confirmed that BAC is a lytic biocide and DBNPA a moderate electrophilic one. Furthermore, the comparison between cells exposed to the biocides’ minimum bactericidal concentrations (MBCs) and autoclaved cells revealed that other viability parameters should be taken into consideration as “death indicators.” The present work also shows that only for the concentrations above the MBC the viability indicators reached values statistically similar to the ones observed for the autoclaved cells (considered to be definitively dead). Finally, the importance of considering the biocide mechanism of action in the definition of the viability parameter to use in the viable but non-culturable (VBNC) determination is discussed.

Combinations of Peptide-Protein Extracts from Native Probiotics Suppress the Growth of Multidrug-Resistant Staphylococcus aureus and Citrobacter freundii via Membrane Perturbation and Ultrastructural Changes

The occurrence of multidrug-resistant pathogens in the food chain causes health problems in humans, thus, research for novel antimicrobials to combat their growth is of interest. This study evaluates the antimicrobial potential of several combinations of peptide-protein extracts (PCs) consisting of peptide extracts from three native probiotic strains, Lactiplantibacillus plantarum UTNGt2, Lactococcus lactis UTNGt28, and L. plantarum UTNGt21A, alone or in combination with EDTA (ethylenediaminetetraacetic acid) against multidrug-resistant Staphylococcus aureus ATCC1026 and Citrobacter freundii UTNB3Sm1. Based on the antimicrobial assay, among the 19 tested PCs, two (PC11 and PC17) produced a greater zone of inhibition against both pathogens in vitro. Time-killing assays indicated the rapid death of S. aureus after exposure to PC11 and PC17, while C. freundii was rapidly inhibited by PC11 and PC1 (UTNGt2 only), suggesting that the inhibitory action is pathogen and dose-dependent of a particular molecule present in the extract. A marginal inhibitory effect was observed when the peptides were combined with EDTA. Transmission electron microscopy (TEM) revealed the structural membrane damage of both target strains upon interaction with individual peptide extracts. Different degrees of cell deformation, condensed cytoplasm, membrane blebbing, and ghost cell formation with visibly broken cell walls were observed in S. aureus. Likewise, the separation of the cytoplasmic membrane from the outer membrane, ghost cells, along with ovoid and deformed cells with undulated cell walls were observed for C. freundii. Furthermore, scanning electronic microscopy (SEM) analysis revealed different wrinkled and deformed cells covered by debris. A leakage of aromatic molecules was detected for both pathogens, indicating that PCs disrupted the cell wall integrity, inducing cell death. Given their inhibitory action and capacity to induce damage of the cytoplasmic membrane, the selected PCs may serve to slow bacterial growth in vitro; further research is required to prove their efficiency ex vitro to battle against food poisoning and subsequent human infection.

Bacterial Dye Release Measures in Response to Antimicrobial Peptides

Assessment of bacterial dye release following exposure to antimicrobial peptides (AMPs) provides a detailed understanding regarding their interaction with the inner and outer membrane of bacteria, and the leak of bacterial intracellular materials. This underpins the overall antimicrobial mechanism of these membrane-active peptides. DiSC3(5) is a membrane potential sensitive dye and can characterize the changes in bacterial membrane potential following exposure to AMPs (see Note 1). SYTOX Green is a nucleic acid stain that enters the cell upon loss of membrane integrity after exposure to AMPs and binds to DNA. SYTO9 is another nucleic acid stain, whereas propidium iodide (PI) is a fluorescent intercalating agent that can be used to stain cells and nucleic acids. Both of these stains are widely used to monitor the viability of bacteria following exposure to AMPs. This chapter describes the methods of using these as bacterial dye release experiments for assessment of the antimicrobial mechanism of AMPs.

Potentiality of Melittin-Loaded Niosomal Vesicles Against Vancomycin-Intermediate Staphylococcus aureus and Staphylococcal Skin Infection

Background

Staphylococcus aureus is an important human pathogen, especially causing skin and soft tissue infections (SSTIs). Over the decades, the infections caused by antibiotic-resistant strains have often become life-threatening. Consequently, exploration and development of competent approaches to combat these serious circumstances are urgently required.

Methods

The antibacterial activity of melittin (Mel) on S. aureus, methicillin-resistant S. aureus (MRSA) and clinical isolates of vancomycin-intermediate S. aureus (VISA) was investigated by minimum inhibitory concentration (MIC) and time-killing assays. The localization of Mel on the bacterial cell was visualized by confocal laser scanning microscopy and its effect on the membrane was indicated based on propidium iodide uptake. The non-ionic surfactant vesicle (NISV) or niosome nanocarrier was established for Mel loading (Mel-loaded NISV) by the thin-film hydration method. Physicochemical and in vitro biological properties of Mel-loaded NISVs were characterized. The cellular uptake of Mel-loaded NISVs was evaluated by holotomography analysis. In addition, an ex vivo study was conducted on a porcine ear skin model to assess the permeation ability of Mel-loaded NISVs and their potential to inhibit bacterial skin infection.

Results

The effective inhibitory activity of Mel on skin pathogens was demonstrated. Among the tested strains, VISA was most susceptible to Mel. Regarding to its function, Mel targeted the bacterial cell envelope and disrupted cell membrane integrity. Mel-loaded NISVs were successfully fabricated with a nano-size of 120-200 nm and entrapment efficiency of greater than 90%. Moreover, Mel-loaded NISVs were taken up and accumulated in the intracellular space. Meanwhile, Mel was released and distributed throughout the cytosol and nucleus. Mel-loaded NISVs efficiently inhibited the growth of bacteria, particularly MRSA and VISA. Importantly, they not only penetrated epidermal and dermal skin layers, but also reduced the bacterial growth in infected skin.

Conclusion

Mel-loaded NISVs have a great potential to exhibit antibacterial activity. Therapeutic application of Mel-loaded NISVs could be further developed as an alternative platform for the treatment of skin infection via dermal and transdermal delivery.

Enhancement of Antibiofilm Activity of Ciprofloxacin against Staphylococcus aureus by Administration of Antimicrobial Peptides

Staphylococcus aureus can develop resistance by mutation, transfection or biofilm formation. Resistance was induced in S. aureus by growth in sub-inhibitory concentrations of ciprofloxacin for 30 days. The ability of the antimicrobials to disrupt biofilms was determined using crystal violet and live/dead staining. Effects on the cell membranes of biofilm cells were evaluated by measuring release of dyes and ATP, and nucleic acids. None of the strains developed resistance to AMPs while only S. aureus ATCC 25923 developed resistance (128 times) to ciprofloxacin after 30 passages. Only peptides reduced biofilms of ciprofloxacin-resistant cells. The antibiofilm effect of melimine with ciprofloxacin was more (27%) than with melimine alone at 1X MIC (p < 0.001). Similarly, at 1X MIC the combination of Mel4 and ciprofloxacin produced more (48%) biofilm disruption than Mel4 alone (p < 0.001). Combinations of either of the peptides with ciprofloxacin at 2X MIC released ≥ 66 nM ATP, more than either peptide alone (p ≤ 0.005). At 2X MIC, only melimine in combination with ciprofloxacin released DNA/RNA which was three times more than that released by melimine alone (p = 0.043). These results suggest the potential use of melimine and Mel4 with conventional antibiotics for the treatment of S. aureus biofilms.

Identification, Isolation, and Characterization of Medipeptins, Antimicrobial Peptides From Pseudomonas mediterranea EDOX

The plant microbiome is a vastly underutilized resource for identifying new genes and bioactive compounds. Here, we used Pseudomonas sp. EDOX, isolated from the leaf endosphere of a tomato plant grown on a small farm in the Netherlands. To get more insight into its biosynthetic potential, the genome of Pseudomonas sp. EDOX was sequenced and subjected to bioinformatic analyses. The genome sequencing analysis identified strain EDOX as a member of the Pseudomonas mediterranea. In silico analysis for secondary metabolites identified a total of five non-ribosomally synthesized peptides synthetase (NRPS) gene clusters, related to the biosynthesis of syringomycin, syringopeptin, anikasin, crochelin A, and fragin. Subsequently, we purified and characterized several cyclic lipopeptides (CLPs) produced by NRPS, including some of the already known ones, which have biological activity against several plant and human pathogens. Most notably, mass spectrometric analysis led to the discovery of two yet unknown CLPs, designated medipeptins, consisting of a 22 amino acid peptide moiety with varying degrees of activity against Gram-positive and Gram-negative pathogens. Furthermore, we investigated the mode of action of medipeptin A. The results show that medipeptin A acts as a bactericidal antibiotic against Gram-positive pathogens, but as a bacteriostatic antibiotic against Gram-negative pathogens. Medipeptin A exerts its potent antimicrobial activity against Gram-positive bacteria via binding to both lipoteichoic acid (LTA) and lipid II as well as by forming pores in membranes. Collectively, our study provides important insights into the biosynthesis and mode of action of these novel medipeptins from P. mediterranea EDOX.

Antimicrobial peptides: mechanism of action, activity and clinical potential

The management of bacterial infections is becoming a major clinical challenge due to the rapid evolution of antibiotic resistant bacteria. As an excellent candidate to overcome antibiotic resistance, antimicrobial peptides (AMPs) that are produced from the synthetic and natural sources demonstrate a broad-spectrum antimicrobial activity with the high specificity and low toxicity. These peptides possess distinctive structures and functions by employing sophisticated mechanisms of action. This comprehensive review provides a broad overview of AMPs from the origin, structural characteristics, mechanisms of action, biological activities to clinical applications. We finally discuss the strategies to optimize and develop AMP-based treatment as the potential antimicrobial and anticancer therapeutics.

Molecular Cloning, Functional and Biophysical Characterization of an Antimicrobial Peptide from Rhizosphere Soil

AIM

This study was designed to screen and identify an antimicrobial peptide from rhizosphere soil. The study was further focused towards overexpression, purification and characterization of this antimicrobial peptide, and to functionally validate its efficiency and efficacy as an antimicrobial agent. Yet the study was further aimed at corroborating structural and functional studies using biophysical tools.

BACKGROUND

Antimicrobial resistance is emerging as one of the top 10 global health crisis, it is multifaceted and is the second largest cause of mortality. According to the World Health Organization (WHO), around the world, an estimated 700,000 people die each year from infection caused by antibiotic-resistant microbes. Antimicrobial peptides offers best alternative to combat and overcome this crisis. In this manuscript, we report cloning, expression, purification and characterization of an antimicrobial peptide discovered from rhizosphere soil.

OBJECTIVE

Objectives of this study includes construction, screening and identification of antimicrobial peptide from metagenome followed by its expression, purification and functional and biophysical investigation. Yet another objective of the study was to determine antimicrobial efficacy and efficiency as an antimicrobial peptide towards MRSA strains.

METHODS

In this study, we used array of molecular biology tools that include genetic engineering, PCR amplification, construction of an expression construct and NI-NTA based purification of the recombinant peptide. We have also carried out antimicrobial activity assay to determine MIC and IC50 values of antimicrobial peptide. To establish structural and functional relationship, circular dichroism, and both extrinsic and intrinsic fluorescence spectroscopy studies were carried out.

RESULTS

Screening of metagenomic library resulted in identification of gene (~500bp) harbouring an open reading frame (ORF) consisting of 282 bp. Open reading frame identified in gene encodes an antimicrobial peptide which had shared ~95% sequence similarity with the antimicrobial peptide of Bacillus origin. Purification of recombinant protein using Ni-NTA column chromatography demonstrated a purified protein band of ~11 kDa on 14% SDS-PAGE which is well corroborated to theoretical deduced molecular weight of peptide from its amino acids sequence. Interestingly, the peptide exhibited antimicrobial activity in broad range of pH and temperature. MIC (minimum inhibitory concentration) determined against gram positive Bacillus sp. was found to be 0.015mg/ml, whereas in case of gram negative E. coli, it was calculated to be 0.062mg/ml. The peptide exhibited IC50 values corresponding to ~0.25mg/ml against Bacillus and ~0.5 mg/ml against E. coli. Antimicrobial susceptibility assay performed against methicillin resistant Staphylococcus aureus strain ATCC 3412 and standard strain of Staphylococcus aureus ATCC 9144 revealed its strong inhibitory activity against MRSA, whereby we observed a ~16mm clearance zone at higher peptide concentrations ~2mg/ml (~181.8µM). Biophysical investigation carried out using Trp fluorescence, ANS fluorescence and circular dichroism spectroscopy further revealed conformational stability in its secondary and tertiary structure at wide range of temperature and pH.

CONCLUSION

Altogether, the peptide discovered from rhizosphere metagenome hold potential in inhibiting the growth of both the gram positive and gram negative bacteria, and was equally effective in inhibiting the multidrug resistant pathogenic strains (MRSA).

Emergent antibacterial activity of N-(thiazol-2-yl)benzenesulfonamides in conjunction with cell-penetrating octaarginine

Hybrid antimicrobials that combine the effect of two or more agents represent a promising antibacterial therapeutic strategy. In this work, we have synthesized N-(4-(4-(methylsulfonyl)phenyl)-5-phenylthiazol-2-yl)benzenesulfonamide derivatives that combine thiazole and sulfonamide, groups with known antibacterial activity. These molecules are investigated for their antibacterial activity, in isolation and in complex with the cell-penetrating peptide octaarginine. Several of the synthesized compounds display potent antibacterial activity against both Gram-negative and Gram-positive bacteria. Compounds with 4-tert-butyl and 4-isopropyl substitutions exhibit attractive antibacterial activity against multiple strains. The isopropyl substituted derivative displays low MIC of 3.9 μg mL⁻¹ against S. aureus and A. xylosoxidans. The comparative antibacterial behaviour of drug–peptide complex, drug alone and peptide alone indicates a distinctive mode of action of the drug–peptide complex, that is not the simple sum total of its constituent components. Specificity of the drug–peptide complex is evident from comparison of antibacterial behaviour with a synthetic intermediate–peptide complex. The octaarginine–drug complex displays faster killing-kinetics towards bacterial cells, creates pores in the bacterial cell membranes and shows negligible haemolytic activity towards human RBCs. Our results demonstrate that mere attachment of a hydrophobic moiety to a cell penetrating peptide does not impart antibacterial activity to the resultant complex. Conversely, the work suggests distinctive modes of antibiotic activity of small molecules when used in conjunction with a cell penetrating peptide.

Hybrid antimicrobials that combine the effect of two or more agents represent a promising antibacterial therapeutic strategy.

Immobilizing Bactericides on Dental Resins via Electron Beam Irradiation

Polymerizable bactericides, such as quaternary ammonium compound-based monomers, have been intensively studied as candidates for immobilizing antibacterial components on dental resin. However, they predominantly exhibit a bacteriostatic behavior, rather than bactericidal, as the immobilized components are left with insufficient molecular movement to disrupt the bacterial surface structure through contact-mediated action. In this study, we developed a novel strategy to increase the density of the immobilized bactericide and enhance its antibacterial/antibiofilm properties by combining a surface-grafting technique with electron beam irradiation. A solution of the quaternary ammonium compound-based monomer, 12-methacryloyloxydodecylpyridinium bromide (MDPB), was coated on polymethyl methacrylate (PMMA) resin specimens at the concentrations of 30, 50, and 80 wt%. The coated resins were subsequently exposed to 10 MeV of electron beam irradiation at 50 and 100 kGy, followed by thermal stabilization at 60 °C. The antibacterial effect was evaluated by inoculating a Streptococcus mutans suspension on the coated PMMA resin samples, which exhibited bactericidal effects even after 28 d of aging (P < 0.05, Tukey's honestly significant difference test). Transmission electron microscopy and bacteriolytic activity evaluation revealed that the S. mutans cells had sustained membrane depolarization. Furthermore, the antibiofilm effects against S. mutans and bacteria collected from human saliva were assessed. The thickness and the percentage of membrane-intact cells of the S. mutans and multispecies biofilms formed on the MDPB-immobilized surfaces were significantly lower than the uncoated PMMA specimens, even after 28-d aging (P < 0.05, Tukey's honestly significant difference test). Thus, the immobilization of antibacterial MDPB via electron beam irradiation induced rapid membrane depolarization, increasing membrane permeability and eventually causing cell death. Our strategy substantially enhances the antibacterial properties of the resinous materials and inhibits biofilm formation, therefore demonstrating significant potential for preventing infectious diseases in the oral environment.

Polyphenylglyoxamide-Based Amphiphilic Small Molecular Peptidomimetics as Antibacterial Agents with Anti-Biofilm Activity

The rapid emergence of drug-resistant bacteria is a major global health concern. Antimicrobial peptides (AMPs) and peptidomimetics have arisen as a new class of antibacterial agents in recent years in an attempt to overcome antibiotic resistance. A library of phenylglyoxamide-based small molecular peptidomimetics was synthesised by incorporating an N-alkylsulfonyl hydrophobic group with varying alkyl chain lengths and a hydrophilic cationic group into a glyoxamide core appended to phenyl ring systems. The quaternary ammonium iodide salts 16d and 17c showed excellent minimum inhibitory concentration (MIC) of 4 and 8 μM (2.9 and 5.6 μg/mL) against Staphylococcus aureus, respectively, while the guanidinium hydrochloride salt 34a showed an MIC of 16 μM (8.5 μg/mL) against Escherichia coli. Additionally, the quaternary ammonium iodide salt 17c inhibited 70% S. aureus biofilm formation at 16 μM. It also disrupted 44% of pre-established S. aureus biofilms at 32 μM and 28% of pre-established E. coli biofilms 64 μM, respectively. A cytoplasmic membrane permeability study indicated that the synthesised peptidomimetics acted via disruption and depolarisation of membranes. Moreover, the quaternary ammonium iodide salts 16d and 17c were non-toxic against human cells at their therapeutic dosages against S. aureus.

Spotlight on the Selected New Antimicrobial Innate Immune Peptides Discovered During 2015-2019

BACKGROUND

Antibiotic resistance is a global issue and new anti-microbials are required.

INTRODUCTION

Anti-microbial peptides are important players of host innate immune systems that prevent infections. Due to their ability to eliminate drug-resistant pathogens, AMPs are promising candidates for developing the next generation of anti-microbials.

METHODS

The anti-microbial peptide database provides a useful tool for searching, predicting, and designing new AMPs. In the period from 2015-2019, ~500 new natural peptides have been registered.

RESULTS

This article highlights a selected set of new AMP members with interesting properties. Teixobactin is a cell wall inhibiting peptide antibiotic, while darobactin inhibits a chaperone and translocator for outer membrane proteins. Remarkably, cOB1, a sex pheromone from commensal enterococci, restricts the growth of multidrug-resistant Enterococcus faecalis in the gut at a picomolar concentration. A novel proline-rich AMP has been found in the plant Brassica napus. A shrimp peptide MjPen- II comprises three different sequence domains: serine-rich, proline-rich, and cysteine-rich regions. Surprisingly, an amphibian peptide urumin specifically inhibits H1 hemagglutinin-bearing influenza A virus. Defensins are abundant and typically consist of three pairs of intramolecular disulfide bonds. However, rat rattusin dimerizes via forming five pairs of intermolecular disulfide bonds. While human LL-37 can be induced by vitamin D, vitamin A induces the expression of resistin-like molecule alpha (RELMα) in mice. The isolation and characterization of an alternative human cathelicidin peptide, TLN-58, substantiates the concept of one gene multiple peptides. The involvement of a fly AMP nemuri in sleep induction may promote the research on the relationship between sleep and infection control.

CONCLUSION

The functional roles of AMPs continue to grow and the general term "innate immune peptides" becomes useful. These discoveries widen our view on the anti-microbial peptides and may open new opportunities for developing novel peptide therapeutics for different applications.

Next-Generation Sequencing for Whole-Genome Characterization of Weissella cibaria UTNGt21O Strain Originated From Wild Solanum quitoense Lam. Fruits: An Atlas of Metabolites With Biotechnological Significance

The whole genome of Weissella cibaria strain UTNGt21O isolated from wild fruits of Solanum quitoense (naranjilla) shrub was sequenced and annotated. The similarity proportions based on the genus level, as a result of the best hits for the entire contig, were 54.84% with Weissella, 6.45% with Leuconostoc, 3.23% with Lactococcus, and 35.48% no match. The closest genome was W. cibaria SP7 (GCF_004521965.1) with 86.21% average nucleotide identity (ANI) and 3.2% alignment coverage. The genome contains 1,867 protein-coding genes, among which 1,620 were assigned with the EggNOG database. On the basis of the results, 438 proteins were classified with unknown function from which 247 new hypothetical proteins have no match in the nucleotide Basic Local Alignment Search Tool (BLASTN) database. It also contains 78 tRNAs, six copies of 5S rRNA, one copy of 16S rRNA, one copy of 23S rRNA, and one copy of tmRNA. The W. cibaria UTNGt21O strain harbors several genes responsible for carbohydrate metabolism, cellular process, general stress responses, cofactors, and vitamins, conferring probiotic features. A pangenome analysis indicated the presence of various strain-specific genes encoded for proteins responsible for the defense mechanisms as well as gene encoded for enzymes with biotechnological value, such as penicillin acylase and folates; thus, W. cibaria exhibited high genetic diversity. The genome characterization indicated the presence of a putative CRISPR-Cas array and five prophage regions and the absence of acquired antibiotic resistance genes, virulence, and pathogenic factors; thus, UTNGt21O might be considered a safe strain. Besides, the interaction between the peptide extracts from UTNGt21O and Staphylococcus aureus results in cell death caused by the target cell integrity loss and the release of aromatic molecules from the cytoplasm. The results indicated that W. cibaria UTNGt21O can be considered a beneficial strain to be further exploited for developing novel antimicrobials and probiotic products with improved technological characteristics.

Ultrabroad-spectrum, multidrug resistant bacteria-killing, and biocompatible quaternized chitin derivative for infected wound healing

Wound infections have consistently been recognized as serious threats to human. The design of antimicrobial and biocompatible wound dressings for infected wounds is an area of constant research. Herein, we homogeneously synthesized an ultrabroad-spectrum antimicrobial and biocompatible quaternized chitin derivative (QC-4) in a high-efficiency and sustainable route using aqueous KOH/urea solution. Particularly, QC-4 displayed powerful multidrug resistant bacteria-killing activities even at a very low antimicrobial concentration range from 500 ng/mL to 5 μg/mL, including clinically prevalent multidrug-resistant Escherichia coli (MDR-E. coli), methicillin resistant Staphylococcus aureus (MRSA), multidrug-resistant Pseudomonas aeruginosa (MRPA), and multidrug-resistant Acinetobacter baumannii (MDR-A. baumannii). With the aim to facilitate clinical translation, we validated the biocompatibility and safety of QC-4 both in vitro and in vivo, and further assessed the effects of QC-4 on infected wound healing in a porcine infectious full-thickness skin wound model. QC-4 demonstrated significant reduction of microbial aggregates and enhanced wound-healing effects by promoted re-epithelialization and collagen deposition, which were quite comparable to that of commercial Alginate-Ag dressing and absolutely superior to commercial Chitoclot Bandage dressing. Additionally, we provided clear evidences that QC-4 had a unique mechanism of action by attracting electrostatically to the negatively charged microbial surface, thus damaging the microbial cell wall and membrane. Findings of this work provided robust preclinical rationale for the future translational applications of QC-4 as a novel ultrabroad-spectrum and multidrug resistant bacteria-killing antimicrobial wound dressing for clinical wound management.

Novel Seleno- and Thio-Urea Containing Dihydropyrrol-2-One Analogues as Antibacterial Agents

The quorum sensing (QS) system in multi-drug-resistant bacteria such as P. aeruginosa is primarily responsible for the development of antibiotic resistance and is considered an attractive target for antimicrobial drug discovery. In this study, we synthesised a series of novel selenourea and thiourea-containing dihydropyrrol-2-one (DHP) analogues as LasR antagonists. The selenium DHP derivatives displayed significantly better quorum-sensing inhibition (QSI) activities than the corresponding sulphur analogues. The most potent analogue 3e efficiently inhibited the las QS system by 81% at 125 µM and 53% at 31 µM. Additionally, all the compounds were screened for their minimum inhibitory concentration (MIC) against the Gram-positive bacterium S. aureus, and interestingly, only the selenium analogues showed antibacterial activity, with 3c and 3e being the most potent with a MIC of 15.6 µM.

Review: Lessons Learned From Clinical Trials Using Antimicrobial Peptides (AMPs)

Antimicrobial peptides (AMPs) or host defense peptides protect the host against various pathogens such as yeast, fungi, viruses and bacteria. AMPs also display immunomodulatory properties ranging from the modulation of inflammatory responses to the promotion of wound healing. More interestingly, AMPs cause cell disruption through non-specific interactions with the membrane surface of pathogens. This is most likely responsible for the low or limited emergence of bacterial resistance against many AMPs. Despite the increasing number of antibiotic-resistant bacteria and the potency of novel AMPs to combat such pathogens, only a few AMPs are in clinical use. Therefore, the current review describes (i) the potential of AMPs as alternatives to antibiotics, (ii) the challenges toward clinical implementation of AMPs and (iii) strategies to improve the success rate of AMPs in clinical trials, emphasizing the lessons we could learn from these trials.

The introduction of L-phenylalanine into antimicrobial peptide protonectin enhances the selective antibacterial activity of its derivative phe-Prt against Gram-positive bacteria

Protonectin was a typical amphiphilic antimicrobial peptide with potent antimicrobial activity against Gram-positive and Gram-negative bacteria. In the present study, when its eleventh amino acid in the sequence was substituted by phenylalanine, the analog named phe-Prt showed potent antimicrobial activity against Gram-positive bacteria, but no antimicrobial activity against Gram-negative bacteria, indicating a significant selectivity between Gram-positive bacteria and Gram-negative bacteria. However, when Gram-negative bacteria were incubated with EDTA, the bacteria were susceptible to phe-Prt. Next, the binding effect of phe-Prt with LPS was determined. Our result showed that LPS could hamper the bactericidal activity of phe-Prt against Gram-positive bacteria. The result of zeta potential assay further confirmed the binding effect of phe-Prt with LPS for it could neutralize the surface charge of E. coli and LPS. Then, the effect of phe-Prt on the integrity of outer membrane of Gram-negative bacteria was determined. Our results showed that phe-Prt had a much weaker disturbance to the outer membrane of Gram-negative bacteria than the parent peptide protonectin. In summary, the introduction of L-phenylalanine into the sequence of antimicrobial peptide protonectin made phe-Prt show significant selectivity against Gram-positive bacteria, which could partly be attributed to the delay effect of LPS for phe-Prt to access to cell membrane. Although further study is still needed to clarify the exact mechanism of selectivity, the present study provided a strategy to develop antimicrobial peptides with selectivity toward Gram-positive and Gram-negative bacteria.