LL-37-derived membrane-active FK-13 analogs possessing cell selectivity, anti-biofilm activity and synergy with chloramphenicol and anti-inflammatory activity

LC-AMP-I1, a novel venom-derived antimicrobial peptide from the wolf spider Lycosa coelestis

Antibiotic resistance has become a critical concern in recent years, and antimicrobial peptides may function as innovative antibacterial agents to address this issue. In this work, we identified a novel antimicrobial peptide, LC-AMP-I1, derived from the venom of Lycosa coelestis, demonstrating substantial antibacterial properties and minimal hemolytic activity. LC-AMP-I1 was subjected to additional assessment for antibacterial efficacy, anti-biofilm properties, drug resistance, stability, and cytotoxicity in vitro. It exhibited comparable antibacterial efficacy to melittin against six common clinical multidrug-resistant bacteria, effectively inhibiting biofilm formation and disrupting established biofilms. Additionally, LC-AMP-I1 demonstrated minimal bacterial resistance, excellent stability, negligible mammalian cell toxicity, low hemolytic activity, and appropriate selectivity for both normal and tumor cells. When combined with traditional antibiotics, LC-AMP-I1 exhibited additive or synergistic therapeutic effects. In a neutropenic mouse thigh infection model, LC-AMP-I1 exhibited a therapeutic effect in inhibiting bacterial proliferation in vivo. The mechanistic investigation indicated that LC-AMP-I1 could influence bacterial cell membrane permeability at low concentrations and directly disrupt structure-function at high concentrations. The results of this work indicate that LC-AMP-I1 may function as a viable alternative to traditional antibiotics in addressing multidrug-resistant bacteria.

Novel Leech Antimicrobial Peptides, Hirunipins: Real-Time 3D Monitoring of Antimicrobial and Antibiofilm Mechanisms Using Optical Diffraction Tomography

Antimicrobial peptides (AMPs) are promising agents for treating antibiotic-resistant bacterial infections. Although discovering novel AMPs is crucial for combating multidrug-resistant bacteria and biofilm-related infections, their clinical potential relies on precise, real-time evaluation of efficacy, toxicity, and mechanisms. Optical diffraction tomography (ODT), a label-free imaging technology, enables real-time visualization of bacterial morphological changes, membrane damage, and biofilm formation over time. Here, a computational analysis of the leech transcriptome using an advanced AI-based peptide screening strategy with ODT to identify potential AMPs is employed. Among the 19 potential AMPs identified, hirunipin 2 demonstrates potent antibacterial activity, low mammalian cytotoxicity, and minimal hemolytic effects. It demonstrates efficacy comparable to melittin, resistance to physiological salts and human serum, and a low likelihood of inducing bacterial resistance. Microscopy and 3D-ODT confirm its disruption of bacterial membranes and intracellular aggregation, leading to cell death. Notably, hirunipin 2 effectively inhibits biofilm formation, eradicates preformed biofilms, and synergizes with antibiotics against multidrug-resistant Acinetobacter baumannii (MDRAB) by enhancing membrane permeability. Additionally, hirunipin 2 significantly suppresses pro-inflammatory cytokine expression in LPS-stimulated macrophages, highlighting its anti-inflammatory properties. These findings highlight hirunipin 2 as a strong candidate for developing novel antibacterial, anti-inflammatory, and antibiofilm therapies, particularly against multidrug-resistant bacterial infections.

Combined antimicrobial activity of short peptide and phage-derived endolysin against antibiotic-resistant Salmonella Typhimurium

This study was designed to evaluate the combination effects of antimicrobial peptides (FK13 and FK16) and phage-encoded endolysin (LysPB32) on the inhibition of growth of polymyxin B-resistant Salmonella Typhimurium ATCC 19585 (STPMB). The inhibitory effects of FK13, FK16, and LysPB32 against STPMB were evaluated by using antimicrobial susceptibility, membrane permeability, biofilm reduction, cross-resistance, and mutant frequency assay. The minimum inhibitory concentrations (MICs) of FK13 and FK16 treated with LysPB32 (FK13+LysPB32 and FK16+LysPB32) against STPMB were decreased from more than 512 to 128 μg/ml and from 64 to 32 μg/ml, respectively. Compared to the control, the number of STPMB in the growing culture was reduced by 4.2 and 5.2 log CFU/ml, respectively, for FK13+LysPB32 and FK16+LysPB32 after 12-h incubation at 37 °C. All treatments (FK13, FK16, FK13+LysPB32, FK16+LysPB32) significantly increased the permeability of the outer membrane of STPMB. Biofilms were significantly decreased from OD600 of 0.6 to 0.16 for FK13+LysPB32 and from 0.6 to 0.13 for FK16+LysPB32. The ratios of MICs of erythromycin, ceftriaxone, polymyxin B, and ciprofloxacin to MIC of the control against STPMB were decreased to 0.50 for FK13+LysPB32 and FK16+LysPB32. The bactericidal activities of amikacin and gentamicin were enhanced for FK13+LysPB32 and FK16+LysPB32 (2-fold < MBC/MIC ratio). The mutant frequencies of STPMB to antibiotics were decreased when treated with FK13+LysPB32 and FK16+LysPB32. The results suggest that the combination of antimicrobial peptides and endolysins can be a promising strategy to control polymyxin B-resistant S. Typhimurium.

Therapeutic potential of isoallolithocholic acid in methicillin-resistant Staphylococcus Aureus peritoneal infection

A significant increase in multidrug-resistant Methicillin-resistant Staphylococcus aureus (MRSA) infections has made it crucial to explore new antimicrobial drugs and strategies. Emerging evidence suggests that the bile acid metabolite isoallolithocholic acid (isoallo-LCA) may contribute to reducing the risk of infection among centenarians. However, its precise role remains somewhat ambiguous and necessitates further investigation. This study aims to investigate the roles of isoallo-LCA in MRSA-associated peritoneal infection. The effects of isoallo-LCA on peritoneal infection are examined in a MRSA-induced peritoneal infected model. Antibacterial activity, biofilm formation assay, and bacterial membrane permeability experiments are conducted to explore the mechanisms involved. Our findings demonstrate that isoallo-LCA effectively suppresses the replication of MRSA with minimal adverse effects on mammalian cells. Furthermore, isoallo-LCA significantly inhibits the formation of bacterial biofilms and eradicates existing bacterial biofilms of MRSA. Administration of isoallo-LCA reduces MRSA colonization in peritoneal organs and alleviates peritonitis-related inflammation and damage in a MRSA-infected peritonitis mice. Mechanistically, isoallo-LCA exhibits potent bactericidal activity against MRSA by disrupting the integrity and permeability of bacterial cells. In addition, isoallo-LCA also enhances the macrophage phagocytosis. In conclusion, our results suggest that isoallo-LCA could be an effective treatment for infections caused by MRSA.

Novel Nano Drug-Loaded Hydrogel Coatings for the Prevention and Treatment of CAUTI

Catheter-associated urinary tract infection (CAUTI) is a prevalent type of hospital-acquired infection, affecting approximately 15% to 25% of patients with urinary catheters. Long-term use of the catheter can lead to colonization of microorganisms and biofilm formation, and may develop into bacterial CAUTI. However, the frequent replacement of catheters in clinical settings can result in tissue damage, inflammation, ulceration, and additional complications, causing discomfort and pain for patients. In light of these challenges, a novel nanodrug-supported hydrogel coating called NP-AM/FK@OMV-P/H has been developed in this study. Through in vitro experiments, it is confirmed that OMV nano-loaded liquid gel coating has an effective reaction against E.coli HAase and releases antibacterial drugs. This coating has also demonstrated strong inhibition of E.coli and has shown the ability to inhibit the formation of bacterial biofilm. These findings highlight the potential of the OMV nanoparticle gel coating in preventing and treating bacterial infections. Notably, NP-AM/FK@OMV-P/H has exhibited greater efficacy against multidrug-resistant E.coli associated with UTIs compared to coatings containing single antimicrobial peptides or antibiotics. Additionally, it has demonstrated good biosecurity. In conclusion, the NP-AM/FK@OMV-P/H coating holds great potential in providing benefits to patients with CAUTI.

Survey of probable synergism between melittin and ciprofloxacin, rifampicin, and chloramphenicol against multidrug-resistant Pseudomonas aeruginosa

Background

The emergence of multidrug-resistant bacteria and also biofilm-associated infections is a great health concern due to the failure of available antibiotics. This has alerted scientists to developing alternative antibiotics. Melittin as an antimicrobial peptide has antibacterial synergistic activity in combining with conventional antibiotics against pathogenic bacteria. Accordingly, this study aimed to assess the synergistic effect of melittin in combination with Ciprofloxacin, Rifampicin, and Chloramphenicol against MDR strains of P. aeruginosa.

Materials and methods

Fifty strains of P. aeruginosa were isolated from clinical specimens. The antibiotic susceptibility of isolates was evaluated by the disk diffusion method. The MIC and MBC of melittin and melittin-antibiotics combination against isolated strains were examined by microdilution method. The probable synergism between melittin and antibiotics was assayed using the FIC protocol. Time-killing kinetics and anti-biofilm effects of melittin and melittin-antibiotics combination were evaluated using time-kill kinetics and crystal violet staining method, respectively. The toxicity of the melittin-antibiotics combination on the HEK293 cell line was also assessed by the MTT assay method.

Results

Out of 50 isolates of P. aeruginosa, 15 strains are considered to be multidrug strains. Among MDR strains of P. aeruginosa, 42.85% were resistant to cefepime and ceftazidime and all urine-originate isolates were resistant to cotrimoxazole. A combination of MIC dose of ciprofloxacin and melittin decreased resistance against ciprofloxacin up to 33%. The ciprofloxacin-melittin combination showed a favorable synergism and anti-biofilm effect and was also 30.3% less toxic than melittin alone at 4 μg/ml against the HEK293 cell line. In contrast to ciprofloxacin, with the melittin-rifampicin and melittin-chloramphenicol combinations, an addition effect occurred, respectively, in 86.66 and 53.33% of MDR strains of P. aeruginosa.

Conclusion

Combining melittin's antibacterial and anti-biofilm properties with traditional antibiotics may offer a novel strategy to address antibiotic resistance in P. aeruginosa. The simultaneous administration of melittin and ciprofloxacin in a single dose has shown a marked increase in antibacterial effectiveness while minimizing toxicity to the HEK293 cell line. It is advisable to conduct additional research to explore the combined antibacterial effects of melittin and ciprofloxacin in a wider range of clinical samples, animal models, and clinical trial settings.

Novel Tree Shrew-Derived Antimicrobial Peptide with Broad-Spectrum Antibacterial Activity

The number of cationic residues and net charge are critical for the activity of antimicrobial peptides (AMPs) due to their role in facilitating initial electrostatic interactions with negatively charged bacterial membranes. A cathelicidin AMP (TC-33) has been identified from the Chinese tree shrew in our previous work, which exhibited weak antimicrobial activity, likely due to its moderately cationic nature. In the current study, based on TC-33, we designed a novel AMP by peptide truncation and Glu substitutions to increase its net cationic charge from +4 to +8. The resulting peptide, TC-LAR-18, showed 4-128-fold enhanced antimicrobial activity relative to TC-33 without causing hemolysis and cytotoxicity within 100 μg/mL. TC-LAR-18 effectively eliminated both planktonic and biofilm-associated bacteria, demonstrating rapid bactericidal effects due to its ability to quickly penetrate and disrupt bacterial cell membranes with a low propensity to induce resistance. Notably, TC-LAR-18 provided substantial protection against skin bacterial infection in mice, underscoring its therapeutic potential. These findings not only highlight the importance of positively charged residues for the antibacterial activity of AMPs but also present a useful drug candidate for combating multidrug-resistant bacteria.

Visualized and pH-responsive hydrogel antibacterial coating for ventilator-associated pneumonia

Ventilator-associated pneumonia (VAP) is a common healthcare-acquired infection often arising during artificial ventilation using endotracheal intubation (ETT), which offers a platform for bacterial colonization and biofilm development. In particular, the effects of prolonged COVID-19 on the respiratory system. Herein, we developed an antimicrobial coating (FK-MEM@CMCO-CS) capable of visualizing pH changes based on bacterial infection and releasing meropenem (MEM) and FK13-a1 in a controlled manner. Using a simple dip-coating process with controlled loading, chitosan was cross-linked with sodium carboxymethyl cellulose oxidation (CMCO) and coated onto PVC-based ETT to form a hydrogel coating. Subsequently, the coated segments were immersed in an indicator solution containing bromothymol blue (BTB), MEM, and FK13-a1 to fabricate the FK-MEM@CMCO-CS coating. In vitro studies have shown that MEM and FK13-a1 can be released from coatings in a pH-responsive manner. Moreover, anti-biofilm and antibacterial adhesion results showed that FK-MEM@CMCO-CS coating significantly inhibited biofilm formation and prevented their colonization of the coating surface. In the VAP rat model, the coating inhibited bacterial growth, reduced lung inflammation, and had good biocompatibility. The coating can be applied to the entire ETT and has the potential for industrial production.

Antimicrobial Properties and Cytotoxicity of LL-37-Derived Synthetic Peptides to Treat Orthopedic Infections

Open fractures and prosthetic joints are prone to bacterial infections, especially those involving biofilms, and are worsened by antibiotic inefficacy and resistance. This highlights the need for targeted treatments against orthopedic infections. LL-37, a human cathelicidin, is known for its antimicrobial properties. This study aimed to synthesize and evaluate LL-37-derived antimicrobial peptides (AMPs) for antibacterial efficacy and toxicity. Several truncated LL-37 analogues were created and tested against 18 bacterial strains, both ATCC and orthopedic clinical isolates, using MIC and MBC assays. Synergy with antibiotics and resistance development were also analyzed, alongside cytotoxicity on NIH-3T3 fibroblasts and hemolytic activity assessments. Six AMPs were synthesized, with FK-16 and GF-17 emerging as the most effective. The MIC values ranged from 4.69 to 18.75 µg/mL and 2.34 to 18.75 µg/mL, respectively, against S. epidermidis and S. aureus, with the MBC values matching the MIC values. Cytotoxicity tests showed no toxicity at concentrations below 75 µg/mL for GF-17 and 150 µg/mL for FK-16. Hemolytic activity was below 1% at 18.75 µg/mL for GF-17 and 75 µg/mL for FK-16. These AMPs showed no synergistic effects with antibiotics and no resistance development. FK-16 and GF-17 effectively removed biofilms, particularly against S. epidermidis. Incorporating these AMPs into surgical materials (hydrogels, cements, etc.) could enhance infection control in orthopedic procedures, warranting further in vivo studies.

Development of a defibrinated human blood hemolysis assay for rapid testing of hemolytic activity compared to computational prediction

Cytotoxicity studies determining hemolytic properties of antimicrobial peptides or other drugs are an important step in the development of novel therapeutics for clinical use. Hemolysis is an affordable, accessible, and rapid method for initial assessment of cellular toxicity for all drugs under development. However, variability in species of red blood cells and protocols used may result in significant differences in results. AMPs generally possess higher selectivity for bacterial cells but can have toxicity against host cells at high concentrations. Knowing the hemolytic activity of the peptides we are developing contributes to our understanding of their potential toxicity. Computational approaches for predicting hemolytic activity of AMPs exist and were tested head-to-head with our experimental results.

RESULTS

Starting with an observation of high hemolytic activity of LL-37 peptide against human red blood cells that were collected in EDTA, we explored alternative approaches to develop a more robust, accurate and simple hemolysis assay using defibrinated human blood. We found significant differences between the sensitivity of defibrinated red blood cells and EDTA treated red blood cells.

SIGNIFICANCE

Accurately determining the hemolytic activity using human red blood cells will allow for a more robust calculation of the therapeutic index of our potential antimicrobial compounds, a critical measure in their pre-clinical development.

CONCLUSION

We introduce a standardized, more accurate protocol for assessing hemolytic activity using defibrinated human red blood cells. This approach, facilitated by the increased commercial availability of de-identified human blood and defibrination methods, offers a robust tool for evaluating toxicity of emerging drug compounds, especially AMPs.

pH- and Matrix Metalloproteinase-Responsive Multifunctional Bilayer Microneedles Platform for Treatment of Tinea Pedis

Persistent foot odor and itchiness are common symptoms of tinea pedis, significantly disrupting the daily life of those affected. The cuticular barrier at the site of the tinea pedis is thickened, which impedes the effective penetration of antifungal agents. Additionally, fungi can migrate from the skin surface to deeper tissues, posing challenges in the current clinical treatment for tinea pedis. To effectively treat tinea pedis, we developed a platform of bilayer gelatin methacrylate (GelMA) microneedles (MNs) loaded with salicylic acid (SA) and FK13-a1 (SA/FK13-a1@GelMA MNs). SA/FK13-a1@GelMA MNs exhibit pH- and matrix metalloproteinase (MMP)-responsive properties for efficient drug delivery. The MNs are designed to deliver salicylic acid (SA) deep into the stratum corneum, softening the cuticle and creating microchannels. This process enables the antibacterial peptide FK13-a1 to penetrate through the stratum corneum barrier, facilitating intradermal diffusion and exerting antifungal and anti-inflammatory effects. In severe cases of tinea pedis, heightened local pH levels and MMP activity further accelerate drug release. Our research demonstrates that SA/FK13-a1@GelMA MNs are highly effective against Trichophyton mentagrophytes, Trichophyton rubrum, and Candida albicans. They also reduced stratum corneum thickness, fungal burden, and inflammation in a guinea pig model of tinea pedis induced by T. mentagrophytes. Furthermore, it was discovered that SA/FK13-a1@GelMA MNs exhibit excellent biocompatibility. These findings suggest that SA/FK13-a1@GelMA MNs have significant potential for the clinical treatment of tinea pedis as well as other fungal skin disorders.

Antibacterial activity of the antimicrobial peptide PMAP-36 in combination with tetracycline against porcine extraintestinal pathogenic Escherichia coli in vitro and in vivo

The increase in the emergence of antimicrobial resistance has led to great challenges in controlling porcine extraintestinal pathogenic Escherichia coli (ExPEC) infections. Combinations of antimicrobial peptides (AMPs) and antibiotics can synergistically improve antimicrobial efficacy and reduce bacterial resistance. In this study, we investigated the antibacterial activity of porcine myeloid antimicrobial peptide 36 (PMAP-36) in combination with tetracycline against porcine ExPEC PCN033 both in vitro and in vivo. The minimum bactericidal concentrations (MBCs) of AMPs (PMAP-36 and PR-39) against the ExPEC strains PCN033 and RS218 were 10 μM and 5 μM, respectively. Results of the checkerboard assay and the time-kill assay showed that PMAP-36 and antibiotics (tetracycline and gentamicin) had synergistic bactericidal effects against PCN033. PMAP-36 and tetracycline in combination led to PCN033 cell wall shrinkage, as was shown by scanning electron microscopy. Furthermore, PMAP-36 delayed the emergence of PCN033 resistance to tetracycline by inhibiting the expression of the tetracycline resistance gene tetB. In a mouse model of systemic infection of PCN033, treatment with PMAP-36 combined with tetracycline significantly increased the survival rate, reduced the bacterial load and dampened the inflammatory response in mice. In addition, detection of immune cells in the peritoneal lavage fluid using flow cytometry revealed that the combination of PMAP-36 and tetracycline promoted the migration of monocytes/macrophages to the infection site. Our results suggest that AMPs in combination with antibiotics may provide more therapeutic options against multidrug-resistant porcine ExPEC.

Anti-Staphylococcus aureus effects of natural antimicrobial peptides and the underlying mechanisms

Staphylococcus aureus can cause localized infections such as abscesses and pneumonia, as well as systemic infections such as bacteremia and sepsis. Especially, methicillin-resistant S. aureus often presents multidrug resistance, which becomes a major clinical challenge. One of the most common reasons for methicillin-resistant S. aureus antibiotic resistance is the presence of biofilms. Natural antimicrobial peptides derived from different species have shown effectiveness in combating S. aureus biofilms. In this review, we summarize the inhibitory activity of antimicrobial peptides against S. aureus planktonic cells and biofilms. We also summarize the possible inhibitory mechanisms, involving cell adhesion inhibition, membrane fracture, biofilm disruption and DNA disruption. We believe this can provide the basis for further research against S. aureus biofilm-associated infections.

Novel antimicrobial peptides against Cutibacterium acnes designed by deep learning

The increasing prevalence of antibiotic resistance in Cutibacterium acnes (C. acnes) requires the search for alternative therapeutic strategies. Antimicrobial peptides (AMPs) offer a promising avenue for the development of new treatments targeting C. acnes. In this study, to design peptides with the specific inhibitory activity against C. acnes, we employed a deep learning pipeline with generators and classifiers, using transfer learning and pretrained protein embeddings, trained on publicly available data. To enhance the training data specific to C. acnes inhibition, we constructed a phylogenetic tree. A panel of 42 novel generated linear peptides was then synthesized and experimentally evaluated for their antimicrobial selectivity and activity. Five of them demonstrated their high potency and selectivity against C. acnes with MIC of 2-4 µg/mL. Our findings highlight the potential of these designed peptides as promising candidates for anti-acne therapeutics and demonstrate the power of computational approaches for the rational design of targeted antimicrobial peptides.

Study of the Antimicrobial Activity of the Human Peptide SQQ30 against Pathogenic Bacteria

Given the continuous increase in antibiotic resistance, research has been driven towards the isolation of new antimicrobial molecules. Short, charged, and very hydrophobic antimicrobial peptides have a direct action against biological membranes, which are less prone to developing resistance. Using a bioinformatic tool, we chose the SQQ30 peptide, isolated from the human SOGA1 protein. The antimicrobial activity of this peptide against various Gram-negative and Gram-positive bacterial strains and against a fungal strain was studied. A mechanism of action directed against biological membranes was outlined. When administered in combination with the antibiotic ciprofloxacin and with the TRS21 (buforin II), another antimicrobial peptide, SQQ30 can be used with a lower MIC, showing additivity and synergism, respectively. Particularly interesting is the ability of SQQ30 to bind LPS in Gram-negative strains, preventing the eukaryotic cell from releasing inflammatory mediators. Our study indicates SQQ30 as a novel and promising antimicrobial agent.

What can AlphaFold do for antimicrobial amyloids?

Amyloids, protein, and peptide assemblies in various organisms are crucial in physiological and pathological processes. Their intricate structures, however, present significant challenges, limiting our understanding of their functions, regulatory mechanisms, and potential applications in biomedicine and technology. This study evaluated the AlphaFold2 ColabFold method's structure predictions for antimicrobial amyloids, using eight antimicrobial peptides (AMPs), including those with experimentally determined structures and AMPs known for their distinct amyloidogenic morphological features. Additionally, two well-known human amyloids, amyloid-β and islet amyloid polypeptide, were included in the analysis due to their disease relevance, short sequences, and antimicrobial properties. Amyloids typically exhibit tightly mated β-strand sheets forming a cross-β configuration. However, certain amphipathic α-helical subunits can also form amyloid fibrils adopting a cross-α structure. Some AMPs in the study exhibited a combination of cross-α and cross-β amyloid fibrils, adding complexity to structure prediction. The results showed that the AlphaFold2 ColabFold models favored α-helical structures in the tested amyloids, successfully predicting the presence of α-helical mated sheets and a hydrophobic core resembling the cross-α configuration. This implies that the AI-based algorithms prefer assemblies of the monomeric state, which was frequently predicted as helical, or capture an α-helical membrane-active form of toxic peptides, which is triggered upon interaction with lipid membranes.

Gut-Antimicrobial Peptides: Synergistic Co-Evolution with Antibiotics to Combat Multi-Antibiotic Resistance

Due to huge diversity and dynamic competition, the human gut microbiome produces a diverse array of antimicrobial peptides (AMPs) that play an important role in human health. The gut microbiome has an important role in maintaining gut homeostasis by the AMPs and by interacting with other human organs via established connections such as the gut-lung, and gut-brain axis. Additionally, gut AMPs play a synergistic role with other gut microbiota and antimicrobials to maintain gut homeostasis by fighting against multi-antibiotic resistance (MAR) bacteria. Further, conventional antibiotics intake creates a synergistic evolutionary pressure for gut AMPs, where antibiotics and gut AMPs fight synergistically against MAR. Overall, gut AMPs are evolving under a complex and highly synergistic co-evolutionary pressure created by the various interactions between gut microbiota, gut AMPs, and antibiotics; however, the complete mechanism is not well understood. The current review explores the synergistic action of gut AMPs and antibiotics along with possibilities to fight against MAR bacteria.

Complementary Activities of Host Defence Peptides and Antibiotics in Combating Antimicrobial Resistant Bacteria

Due to their ability to eliminate antimicrobial resistant (AMR) bacteria and to modulate the immune response, host defence peptides (HDPs) hold great promise for the clinical treatment of bacterial infections. Whereas monotherapy with HDPs is not likely to become an effective first-line treatment, combinations of such peptides with antibiotics can potentially provide a path to future therapies for AMR infections. Therefore, we critically reviewed the recent literature regarding the antibacterial activity of combinations of HDPs and antibiotics against AMR bacteria and the approaches taken in these studies. Of the 86 studies compiled, 56 featured a formal assessment of synergy between agents. Of the combinations assessed, synergistic and additive interactions between HDPs and antibiotics amounted to 84.9% of the records, while indifferent and antagonistic interactions accounted for 15.1%. Penicillin, aminoglycoside, fluoro/quinolone, and glycopeptide antibiotic classes were the most frequently documented as interacting with HDPs, and Staphylococcus aureus, Pseudomonas aeruginosa, Escherichia coli, and Enterococcus faecium were the most reported bacterial species. Few studies formally evaluated the effects of combinations of HDPs and antibiotics on bacteria, and even fewer assessed such combinations against bacteria within biofilms, in animal models, or in advanced tissue infection models. Despite the biases of the current literature, the studies suggest that effective combinations of HDPs and antibiotics hold promise for the future treatment of infections caused by AMR bacteria.

Computationally Designed AMPs with Antibacterial and Antibiofilm Activity against MDR Acinetobacter baumannii

The discovery of new antimicrobials is necessary to combat multidrug-resistant (MDR) bacteria, especially those that infect wounds and form prodigious biofilms, such as Acinetobacter baumannii. Antimicrobial peptides (AMPs) are a promising class of new therapeutics against drug-resistant bacteria, including gram-negatives. Here, we utilized a computational AMP design strategy combining database filtering technology plus positional analysis to design a series of novel peptides, named HRZN, designed to be active against A. baumannii. All of the HRZN peptides we synthesized exhibited antimicrobial activity against three MDR A. baumannii strains with HRZN-15 being the most active (MIC 4 µg/mL). This peptide also inhibited and eradicated biofilm of A. baumannii strain AB5075 at 8 and 16 µg/mL, which is highly effective. HRZN-15 permeabilized and depolarized the membrane of AB5075 rapidly, as demonstrated by the killing kinetics. HRZN 13 and 14 peptides had little to no hemolysis activity against human red blood cells, whereas HRZN-15, -16, and -17 peptides demonstrated more significant hemolytic activity. HRZN-15 also demonstrated toxicity to waxworms. Further modification of HRZN-15 could result in a new peptide with an improved toxicity profile. Overall, we successfully designed a set of new AMPs that demonstrated activity against MDR A. baumannii using a computational approach.

Antimicrobial Peptides in Infectious Diseases and Beyond-A Narrative Review

Despite recent medical research and clinical practice developments, the development of antimicrobial resistance (AMR) significantly limits therapeutics for infectious diseases. Thus, novel treatments for infectious diseases, especially in this era of increasing AMR, are urgently needed. There is ongoing research on non-classical therapies for infectious diseases utilizing alternative antimicrobial mechanisms to fight pathogens, such as bacteriophages or antimicrobial peptides (AMPs). AMPs are evolutionarily conserved molecules naturally produced by several organisms, such as plants, insects, marine organisms, and mammals, aiming to protect the host by fighting pathogenic microorganisms. There is ongoing research regarding developing AMPs for clinical use in infectious diseases. Moreover, AMPs have several other non-medical applications in the food industry, such as preservatives, animal husbandry, plant protection, and aquaculture. This review focuses on AMPs, their origins, biology, structure, mechanisms of action, non-medical applications, and clinical applications in infectious diseases.

Synergy between Human Peptide LL-37 and Polymyxin B against Planktonic and Biofilm Cells of Escherichia coli and Pseudomonas aeruginosa

The rise in antimicrobial resistant bacteria is limiting the number of effective treatments for bacterial infections. Escherichia coli and Pseudomonas aeruginosa are two of the pathogens with the highest prevalence of resistance, and with the greatest need for new antimicrobial agents. Combinations of antimicrobial peptides (AMPs) and antibiotics that display synergistic effects have been shown to be an effective strategy in the development of novel therapeutic agents. In this study, we investigated the synergy between the AMP LL-37 and various classes of antibiotics against E. coli and P. aeruginosa strains. Of the six antibiotics tested (ampicillin, tetracycline, ciprofloxacin, gentamicin, aztreonam, and polymyxin B (PMB)), LL-37 displayed the strongest synergy against E. coli MG1655 and P. aeruginosa PAO1 laboratory strains when combined with PMB. Given the strong synergy, the PMB + LL-37 combination was chosen for further examination where it demonstrated synergy against multidrug-resistant and clinical E. coli isolates. Synergy of PMB + LL-37 towards clinical isolates of P. aeruginosa varied and showed synergistic, additive, or indifferent effects. The PMB + LL-37 combination treatment showed significant prevention of biofilm formation as well as eradication of pre-grown E. coli and P. aeruginosa biofilms. Using the Galleria mellonella wax worm model, we showed that the PMB + LL-37 combination treatment retained its antibacterial capacities in vivo. Flow analyses were performed to characterize the mode of action. The results of the present study provide proof of principle for the synergistic response between LL-37 and PMB and give novel insights into a promising new antimicrobial combination against gram-negative planktonic and biofilm cells.

Discovery of structural and functional transition sites for membrane-penetrating activity of sheep myeloid antimicrobial peptide-18

Cathelicidin antimicrobial peptides have an extended and/or unstructured conformation in aqueous solutions but fold into ordered conformations, such as the α-helical structure, when interacting with cellular membranes. These structural transitions can be directly correlated to their antimicrobial activity and its underlying mechanisms. SMAP-18, the N-terminal segment (residues 1-18) of sheep cathelicidin (SMAP-29), is known to kill microorganisms by translocating across membranes and interacting with their nucleic acids. The amino acid sequence of SMAP-18 contains three Gly residues (at positions 2, 7, and 13) that significantly affect the flexibility of its peptide structure. This study investigated the role of Gly residues in the structure, membrane interaction, membrane translocation, and antimicrobial mechanisms of SMAP-18. Five analogs were designed and synthesized through Gly → Ala substitution (i.e., G2A, G7A, G13A, G7,13A, and G2,7,13A); these substitutions altered the helical content of SMAP-18 peptides. We found that G7,13A and G2,7,13A changed their mode of action, with circular dichroism and nuclear magnetic resonance studies revealing that these analogs changed the structure of SMAP-18 from a random coil to an α-helical structure. The results of this experiment suggest that the Gly residues at positions 7 and 13 in SMAP-18 are the structural and functional determinants that control its three-dimensional structure, strain-specific activity, and antimicrobial mechanism of action. These results provide valuable information for the design of novel peptide-based antibiotics.

Vancomycin Resistance in Enterococcus and Staphylococcus aureus

Enterococcus faecalis, Enterococcus faecium and Staphylococcus aureus are both common commensals and major opportunistic human pathogens. In recent decades, these bacteria have acquired broad resistance to several major classes of antibiotics, including commonly employed glycopeptides. Exemplified by resistance to vancomycin, glycopeptide resistance is mediated through intrinsic gene mutations, and/or transferrable van resistance gene cassette-carrying mobile genetic elements. Here, this review will discuss the epidemiology of vancomycin-resistant Enterococcus and S. aureus in healthcare, community, and agricultural settings, explore vancomycin resistance in the context of van and non-van mediated resistance development and provide insights into alternative therapeutic approaches aimed at treating drug-resistant Enterococcus and S. aureus infections.

R R, L. F L, M.C.G DR, N.B C, S.C D, O. L F. Advances and perspectives for antimicrobial peptide and combinatory therapies

Antimicrobial peptides (AMPs) have shown cell membrane-directed mechanisms of action. This specificity can be effective against infectious agents that have acquired resistance to conventional drugs. The AMPs' membrane-specificity and their great potential to combat resistant microbes has brought hope to the medical/therapeutic scene. The high death rate worldwide due to antimicrobial resistance (AMR) has pushed forward the search for new molecules and product developments, mainly antibiotics. In the current scenario, other strategies including the association of two or more drugs have contributed to the treatment of difficult-to-treat infectious diseases, above all, those caused by bacteria. In this context, the synergistic action of AMPs associated with current antibiotic therapy can bring important results for the production of new and effective drugs to overcome AMR. This review presents the advances obtained in the last 5 years in medical/antibiotic therapy, with the use of products based on AMPs, as well as perspectives on the potentialized effects of current drugs combined with AMPs for the treatment of bacterial infectious diseases.

The antimicrobial peptides LL-37, KR-20, FK-13 and KR-12 inhibit the growth of a sensitive and a metronidazole-resistant strain of Trichomonas vaginalis

The parasite Trichomonas vaginalis is the aetiologic agent of trichomoniasis, the most common non-viral sexually transmitted disease worldwide. This infection often remains asymptomatic and is related to several health complications. The traditional treatment for trichomoniasis uses drugs of the 5-nitroimidazole family, such as metronidazole; however, scientific reports indicate an increasing number of drug-resistant strains. Antimicrobial peptides could be an alternative or complementary treatment. In this sense, one attractive candidate is the human cathelicidin, being LL-37 its active form. LL-37 possesses microbicidal activity against many microorganisms such as bacteria, Candida albicans, and Entamoeba histolytica. Shorter sequences derived from this peptide, such as KR-20, FK-13 and KR-12, have been shown to possess a higher microbicidal effect than LL-37. In this study, we determined the activity of LL-37 and its derivatives against T. vaginalis, which was unknown. The results showed that the four peptides (LL-37, KR-20, FK-13-NH₂ and KR-12) decreased the viability of T. vaginalis on a 5-nitroimidazole-sensitive and a 5-nitroimidazole-resistant strain; however, KR-20 was the most effective peptide, followed by FK-13-NH₂. Low concentrations of all peptides showed a better effect when combined with metronidazole in the sensitive and resistant T. vaginalis strains. These results are promising for potential future therapeutic uses.

Effect of NZ2114 against Streptococcus dysgalactiae biofilms and its application in murine mastitis model

Bovine mastitis caused by Streptococcus dysgalactiae (S. dysgalactiae) is usually treated with antibiotics, which may potentially increase drug resistance as the abuse. NZ2114, a variant of fungal defensin plectasin, displayed a potent antibacterial activity against S. dysgalactiae. The inhibition/eradication effect of the antimicrobial peptide NZ2114 on the early/mature biofilm of S. dysgalactiae CVCC 3938 was evaluated, as well as the elimination of bacteria in mature biofilms. In this study, NZ2114 displayed potent antibacterial activity against S. dysgalactiae CVCC 3938 and three clinical isolated S. dysgalactiae strains (0.11-0.45 μM). The early biofilm inhibition of S. dysgalactiae CVCC 3938 was 55.5–85.9% after treatment with NZ2114 at concentrations of 1–16 × MIC, which was better than that of vancomycin at the same concentration. The mature biofilm eradication rate was up to 92.7–97.6% with the increasing concentration (2–16 × MIC) of NZ2114, and the eradication rate did not change significantly with further increase of NZ2114 concentration, while the biofilm eradication rate of vancomycin-treated group at the same concentration remained at 92.5%. NZ2114 reduced the number of persister bacteria in biofilm. Scanning electron microscopy (SEM) and confocal laser scanning microscopy (CLSM) further demonstrated that NZ2114 could effectively reduce the biofilm thickness and bacterial number of S. dysgalactiae CVCC 3938. In vivo therapeutic effect of NZ2114 on murine mastitis model showed that NZ2114 was better than vancomycin in alleviating mammary gland inflammation by regulating cytokines production, inhibiting bacterial proliferation, and reducing the number of mammary gland bacteria. These data suggested that NZ2114 is a potential peptide candidate for the treatment of mastitis.

Designed Multifunctional Peptides for Intracellular Targets

Nature’s way for bioactive peptides is to provide them with several related functions and the ability to cooperate in performing their job. Natural cell-penetrating peptides (CPP), such as penetratins, inspired the design of multifunctional constructs with CPP ability. This review focuses on known and novel peptides that can easily reach intracellular targets with little or no toxicity to mammalian cells. All peptide candidates were evaluated and ranked according to the predictions of low toxicity to mammalian cells and broad-spectrum activity. The final set of the 20 best peptide candidates contains the peptides optimized for cell-penetrating, antimicrobial, anticancer, antiviral, antifungal, and anti-inflammatory activity. Their predicted features are intrinsic disorder and the ability to acquire an amphipathic structure upon contact with membranes or nucleic acids. In conclusion, the review argues for exploring wide-spectrum multifunctionality for novel nontoxic hybrids with cell-penetrating peptides.

Halogenated Pyrrolopyrimidines with Low MIC on Staphylococcus aureus and Synergistic Effects with an Antimicrobial Peptide

Currently, there is a world-wide rise in antibiotic resistance causing burdens to individuals and public healthcare systems. At the same time drug development is lagging behind. Therefore, finding new ways of treating bacterial infections either by identifying new agents or combinations of drugs is of utmost importance. Additionally, if combination therapy is based on agents with different modes of action, resistance is less likely to develop. The synthesis of 21 fused pyrimidines and a structure-activity relationship study identified two 6-aryl-7H-pyrrolo [2,3-d] pyrimidin-4-amines with potent activity towards Staphylococcus aureus. The MIC-value was found to be highly dependent on a bromo or iodo substitution in the 4-benzylamine group and a hydroxyl in the meta or para position of the 6-aryl unit. The most active bromo and iodo derivatives had MIC of 8 mg/L. Interestingly, the most potent compounds experienced a four-fold lower MIC-value when they were combined with the antimicrobial peptide betatide giving MIC of 1–2 mg/L. The front runner bromo derivative also has a low activity towards 50 human kinases, including thymidylate monophosphate kinase, a putative antibacterial target.

Structural Mimicry in Microbial and Antimicrobial Amyloids

The remarkable variety of microbial species of human pathogens and microbiomes generates significant quantities of secreted amyloids, which are structured protein fibrils that serve diverse functions related to virulence and interactions with the host. Human amyloids are associated largely with fatal neurodegenerative and systemic aggregation diseases, and current research has put forward the hypothesis that the interspecies amyloid interactome has physiological and pathological significance. Moreover, functional and molecular-level connections between antimicrobial activity and amyloid structures suggest a neuroimmune role for amyloids that are otherwise known to be pathological. Compared to the extensive structural information that has been accumulated for human amyloids, high-resolution structures of microbial and antimicrobial amyloids are only emerging. These recent structures reveal both similarities and surprising departures from the typical amyloid motif, in accordance with their diverse activities, and advance the discovery of novel antivirulence and antimicrobial agents. In addition, the structural information has led researchers to postulate that amyloidogenic sequences are natural targets for structural mimicry, for instance in host-microbe interactions. Microbial amyloid research could ultimately be used to fight aggressive infections and possibly processes leading to autoimmune and neurodegenerative diseases.

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.

LcCCL28-25, Derived from Piscine Chemokine, Exhibits Antimicrobial Activity against Gram-Negative and Gram-Positive Bacteria In Vitro and In Vivo

Antimicrobial peptides (AMPs) are currently recognized as potentially promising antibiotic substitutes. Fish are an important seawater/freshwater medicinal biological resource, and the antimicrobial peptides and proteins that are key components of their innate immune systems are potential candidates for the development of novel antibacterial agents. The rainbow trout Oncorhynchus mykiss chemokine CK11 (omCK11), classified in the C-C motif chemokine ligand 27/28 (CCL27/28) family, is the only CC-type chemokine reported to play a direct antibacterial role in the immune response; however, its antibacterial domain remains unknown. In this study, we analyzed the structure-activity relationship of omCK11 and identified the antibacterial C-terminal domain. Additionally, we performed structure-function analyses of CCL27/28 proteins from different, representative freshwater and seawater fishes, revealing their shared C-terminal antibacterial domains. Surprisingly, a synthesized cationic peptide (named lcCCL28-25), derived from the large yellow croaker Larimichthys crocea CCL28, exhibited broad-spectrum and the most acceptable bactericidal activity, as well as antibiofilm activity and negligible hemolytic and cytotoxic activity in vitro. Additionally, lcCCL28-25 conferred a protective effect in the thighs of neutropenic mice infected with Staphylococcus aureus. SYTOX green fluorescence and electron microscopy experiments revealed that lcCCL28-25 was capable of rapidly destroying the integrity and permeability of the bacterial cell membrane. Overall, this study aided in the advancement of antibacterial CC-type chemokine research and also suggested a new strategy for exploring novel AMPs. Additionally, the efficacy of lcCCL28-25 in in vivo antibacterial activity in a mammalian model revealed that this compound could be a promising agent for the development of peptide-based antibacterial therapeutics.

IMPORTANCE The primary function of chemokines has been described as recruiting and activating leukocytes to participate in the immune response. Some chemokines are also broad-spectrum antibacterial proteins in mammals. The Oncorhynchus mykiss chemokine CK11 (omCK11) is the first reported and currently the only CC-type antibacterial chemokine. The present study identified the antibacterial domain of omCK11. Structure-function analysis of various fish CCL27/28 proteins identified a novel antibacterial peptide (lcCCL28-25) from Larimichthys crocea CCL28 that exhibited broad-spectrum and the most acceptable bactericidal activity in vitro, as well as a protective effect in a Staphylococcus aureus infection mouse model. The antibacterial mechanisms included membrane disruption and permeation. This study advanced the field of antibacterial chemokine research in fish and also suggested a new strategy for exploring novel AMPs. The novel peptide lcCCL28-25 may prove to be an effective antibacterial agent.

Amphiphilic cyclic peptide [W4KR5]-Antibiotics combinations as broad-spectrum antimicrobial agents

Linear and cyclic amphiphilic peptides, (W₄KR₅) and [W₄KR₅], were evaluated as antibacterial agents against Gram-positive and Gram-negative bacteria, including four multi-drug resistant strains and the corresponding four non-resistant strains. Cyclic peptide [W₄KR₅] showed higher antibacterial activity than the linear (W₄KR₅) counterpart. Cyclic [W₄KR₅] was subjected to combination (physical mixture or covalent conjugation) with meropenem as a model antibiotic to study the impact of the combination on antimicrobial activity. A physical mixture of meropenem and [W₄KR₅] showed synergistic antibacterial activity against Gram-negative P. aeruginosa (ATCC BAA-1744) and P. aeruginosa (ATCC 27883) strains. [W₄KR₅] was further subjected to extensive antibacterial studies against additional 10 bacteria strains, showing significant antibacterial efficacy against Gram-positive bacteria strains. Combinations studies of [W₄KR₅] with an additional 9 commercially available antibiotics showed significant enhancement in antibacterial activity for all tested combinations, especially with tetracycline, tobramycin, levofloxacin, clindamycin, daptomycin, polymyxin, kanamycin, and vancomycin. Time-kill kinetics assay and flow cytometry results exhibited that [W₄KR₅] had a time-dependent synergistic effect and membrane disruption property. These data indicate that [W₄KR₅] improves the antibacterial activity, presumably by facilitating the internalization of antibiotics and their interaction with the intracellular targets. This study introduces a potential strategy for treating multidrug-resistant pathogens by combining [W₄KR₅] and a variety of classical antibiotics to improve the antibacterial effectiveness.

Developing Antimicrobial Synergy With AMPs

Antimicrobial peptides (AMPs) have been extensively studied due to their vast natural abundance and ability to kill microbes. In an era critically lacking in new antibiotics, manipulating AMPs for therapeutic application is a promising option. However, bacterial pathogens resistant to AMPs remain problematic. To improve AMPs antimicrobial efficacy, their use in conjunction with other antimicrobials has been proposed. How might this work? AMPs kill bacteria by forming pores in bacterial membranes or by inhibiting bacterial macromolecular functions. What remains unknown is the duration for which AMPs keep bacterial pores open, and the extent to which bacteria can recover by repairing these pores. In this mini-review, we discuss various antimicrobial synergies with AMPs. Such synergies might arise if the antimicrobial agents helped to keep bacterial pores open for longer periods of time, prevented pore repair, perturbed bacterial intracellular functions at greater levels, or performed other independent bacterial killing mechanisms. We first discuss combinations of AMPs, and then focus on histones, which have antimicrobial activity and co-localize with AMPs on lipid droplets and in neutrophil extracellular traps (NETs). Recent work has demonstrated that histones can enhance AMP-induced membrane permeation. It is possible that histones, histone fragments, and histone-like peptides could amplify the antimicrobial effects of AMPs, giving rise to antimicrobial synergy. If so, clarifying these mechanisms will thus improve our overall understanding of the antimicrobial processes and potentially contribute to improved drug design.

Mammals' humoral immune proteins and peptides targeting the bacterial envelope: from natural protection to therapeutic applications against multidrug‐resistant Gram ‐negatives

Mammalian innate immunity employs several humoral ‘weapons’ that target the bacterial envelope. The threats posed by the multidrug‐resistant ‘ESKAPE’ Gram‐negative pathogens (Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa, and Enterobacter spp.) are forcing researchers to explore new therapeutic options, including the use of these immune elements. Here we review bacterial envelope‐targeting (peptidoglycan and/or membrane‐targeting) proteins/peptides of the mammalian immune system that are most likely to have therapeutic applications. Firstly we discuss their general features and protective activity against ESKAPE Gram‐negatives in the host. We then gather, integrate, and discuss recent research on experimental therapeutics harnessing their bactericidal power, based on their exogenous administration and also on the discovery of bacterial and/or host targets that improve the performance of this endogenous immunity, as a novel therapeutic concept. We identify weak points and knowledge gaps in current research in this field and suggest areas for future work to obtain successful envelope‐targeting therapeutic options to tackle the challenge of antimicrobial resistance.

Helical structure motifs made searchable for functional peptide design

The systematic design of functional peptides has technological and therapeutic applications. However, there is a need for pattern-based search engines that help locate desired functional motifs in primary sequences regardless of their evolutionary conservation. Existing databases such as The Protein Secondary Structure database (PSS) no longer serves the community, while the Dictionary of Protein Secondary Structure (DSSP) annotates the secondary structures when tertiary structures of proteins are provided. Here, we extract 1.7 million helices from the PDB and compile them into a database (Therapeutic Peptide Design database; TP-DB) that allows queries of compounded patterns to facilitate the identification of sequence motifs of helical structures. We show how TP-DB helps us identify a known purification-tag-specific antibody that can be repurposed into a diagnostic kit for Helicobacter pylori. We also show how the database can be used to design a new antimicrobial peptide that shows better Candida albicans clearance and lower hemolysis than its template homologs. Finally, we demonstrate how TP-DB can suggest point mutations in helical peptide blockers to prevent a targeted tumorigenic protein-protein interaction. TP-DB is made available at http://dyn.life.nthu.edu.tw/design/ .

Machine Learning Prediction of Antimicrobial Peptides

Antibiotic resistance constitutes a global threat and could lead to a future pandemic. One strategy is to develop a new generation of antimicrobials. Naturally occurring antimicrobial peptides (AMPs) are recognized templates and some are already in clinical use. To accelerate the discovery of new antibiotics, it is useful to predict novel AMPs from the sequenced genomes of various organisms. The antimicrobial peptide database (APD) provided the first empirical peptide prediction program. It also facilitated the testing of the first machine-learning algorithms. This chapter provides an overview of machine-learning predictions of AMPs. Most of the predictors, such as AntiBP, CAMP, and iAMPpred, involve a single-label prediction of antimicrobial activity. This type of prediction has been expanded to antifungal, antiviral, antibiofilm, anti-TB, hemolytic, and anti-inflammatory peptides. The multiple functional roles of AMPs annotated in the APD also enabled multi-label predictions (iAMP-2L, MLAMP, and AMAP), which include antibacterial, antiviral, antifungal, antiparasitic, antibiofilm, anticancer, anti-HIV, antimalarial, insecticidal, antioxidant, chemotactic, spermicidal activities, and protease inhibiting activities. Also considered in predictions are peptide posttranslational modification, 3D structure, and microbial species-specific information. We compare important amino acids of AMPs implied from machine learning with the frequently occurring residues of the major classes of natural peptides. Finally, we discuss advances, limitations, and future directions of machine-learning predictions of antimicrobial peptides. Ultimately, we may assemble a pipeline of such predictions beyond antimicrobial activity to accelerate the discovery of novel AMP-based antimicrobials.

Defensing role of novel piscidins from largemouth bass (Micropterus salmoides) with evidence of bactericidal activities and inducible expressional delineation

Micropterus salmoides is an economical important species of freshwater-cultured fish, the in-depth knowledge of its immune system is in urgent development to cope with serious infectious diseases. Piscidin is an important antimicrobial peptide (AMP) family existing in almost all teleosts. However, no piscidin has been reported in largemouth bass. In this study, three novel piscidins (MSPiscidin-1, -2, and -3) were firstly identified and characterized from the largemouth bass. The predicted mature peptides of MSPiscidin-1, -2, and -3 (consists of 24, 27, 25 amino acid residues, respectively) all adopted an amphipathic α-helical conformation representative of cationic AMPs that are important for membrane permeabilization and antibacterial activity. MSPiscidin-2 and -3 indeed displayed strong, broad-spectrum, and highly efficient antimicrobial activities in vitro against aquatic pathogens, but MSPiscidin-1 didn't show direct antimicrobial activity. MSPiscidin-2 and -3 killed bacteria mainly by inducing membrane permeabilization, in addition, they also can interact with bacterial genomic DNA, which might influence the DNA replication and transcription. Besides, MSPiscidin-2 and -3 could effectively inhibit the formation of the bacterial biofilm and eliminate the preformed biofilms. In vivo, MSPiscidin-1-3 genes showed an inducible expression pattern in the tested tissues upon Vibrio harveyi infection, which further indicated the key roles of piscidins in innate immunity in largemouth bass. Overall, this study will supplement the understanding of M. salmoides innate immune system and provide candidates for the design of novel peptide antibacterial agents used in aquaculture.

Efficacy of antimicrobial peptide LL-37 against biofilm forming Staphylococcus aureus strains obtained from chronic wound infections

The antimicrobial peptide LL-37 showed inhibitory effects against Staphylococcus aureus strains, which often responsible for wound infections. Understanding the molecular mechanisms of biofilm-containing wound infections is important. Thus, this study aimed to investigate both the antimicrobial and biofilm efficacy of LL-37 against biofilm-positive methicillin-susceptible S. aureus (MSSA) strains and biofilm-positive methicillin-resistant S. aureus (MRSA) strains obtained from chronic wound infections and its effect on different quorum sensing and virulence genes at suboptimal concentrations. Fifteen biofilm-forming MRSA and 15 biofilm-forming MSSA strains were included in this study. The minimum inhibitory concentration (MIC) values and biofilm formation were tested by microdilution methods. Real-time PCR was performed to determine gene expression levels. MIC values for LL-37 were 89.6 mg/L and 132.3 mg/L for MSSA and MRSA strains, respectively. No statistically significant difference was found between MRSA and MSSA strains in terms of the effect of LL-37 on biofilm formation. A statistically significant difference was found between MRSA and MSSA strains for atlA, RNAIII, and agrA gene expression levels following exposure to a suboptimal concentration of LL-37. Ultimately, the required LL-37 antimicrobial concentration was quite high; however, LL-37 antibiofilm concentration may be acceptable for use in humans against biofilm-forming MRSA and MSSA strains. This is the first study to investigate to effect of a suboptimal LL-37 concentration on gene expression levels of biofilm-forming MSSA and MRSA strains. LL-37 affected quorum sensing and biofilm producing mechanisms, even at suboptimal MIC concentrations.

Antiseptic 9-Meric Peptide with Potency against Carbapenem-Resistant Acinetobacter baumannii Infection

Carbapenem-resistant A. baumannii (CRAB) infection can cause acute host reactions that lead to high-fatality sepsis, making it important to develop new therapeutic options. Previously, we developed a short 9-meric peptide, Pro9-3D, with significant antibacterial and cytotoxic effects. In this study, we attempted to produce safer peptide antibiotics against CRAB by reversing the parent sequence to generate R-Pro9-3 and R-Pro9-3D. Among the tested peptides, R-Pro9-3D had the most rapid and effective antibacterial activity against Gram-negative bacteria, particularly clinical CRAB isolates. Analyses of antimicrobial mechanisms based on lipopolysaccharide (LPS)-neutralization, LPS binding, and membrane depolarization, as well as SEM ultrastructural investigations, revealed that R-Pro9-3D binds strongly to LPS and impairs the membrane integrity of CRAB by effectively permeabilizing its outer membrane. R-Pro9-3D was also less cytotoxic and had better proteolytic stability than Pro9-3D and killed biofilm forming CRAB. As an LPS-neutralizing peptide, R-Pro9-3D effectively reduced LPS-induced pro-inflammatory cytokine levels in RAW 264.7 cells. The antiseptic abilities of R-Pro9-3D were also investigated using a mouse model of CRAB-induced sepsis, which revealed that R-Pro9-3D reduced multiple organ damage and attenuated systemic infection by acting as an antibacterial and immunosuppressive agent. Thus, R-Pro9-3D displays potential as a novel antiseptic peptide for treating Gram-negative CRAB infections.

Synthetic Natural Product Inspired Cyclic Peptides

Natural products are a bountiful source of bioactive molecules. Unfortunately, discovery of novel bioactive natural products is challenging due to cryptic biosynthetic gene clusters, low titers, and arduous purifications. Herein, we describe SNaPP (Synthetic Natural Product Inspired Cyclic Peptides), a method for identifying NP-inspired bioactive peptides. SNaPP expedites bioactive molecule discovery by combining bioinformatics predictions of nonribosomal peptide synthetases with chemical synthesis of the predicted natural products (pNPs). SNaPP utilizes a recently discovered cyclase, the penicillin binding protein-like cyclase, as the lynchpin for the development of a library of head-to-tail cyclic peptide pNPs. Analysis of 500 biosynthetic gene clusters allowed for identification of 131 novel pNPs. Fifty-one diverse pNPs were synthesized using solid phase peptide synthesis and solution-phase cyclization. Antibacterial testing revealed 14 pNPs with antibiotic activity, including activity against multidrug-resistant Gram-negative bacteria. Overall, SNaPP demonstrates the power of combining bioinformatics predictions with chemical synthesis to accelerate the discovery of bioactive molecules.

Targeted Delivery of Miconazole Employing LL37 Fragment Mutant Peptide CKR12-Poly (Lactic-Co-Glycolic) Acid Polymeric Micelles

We previously reported that conjugates of antimicrobial peptide fragment analogues and poly (lactic-co-glycolic) acid (PLGA) enhance antimicrobial activity and that the conjugated micelle structure is an effective tool for antimicrobial drug delivery. In recent years, the delivery of antimicrobial peptides to targets for antimicrobial activity has attracted attention. In this study, we targeted Candida albicans, a causative organism of catheter-related bloodstream infections, which is refractory to antimicrobial agents and is currently a problem in medical practice. We evaluated the antifungal activity of CKR12 (a mutant fragment of the human cathelicidin peptide, LL-37)-PLGA-miconazole (MCZ) micelles using nanotechnology with MCZ delivery. The prepared CKR12-PLGA-MCZ micelles were characterised by measuring dynamic light scattering, zeta potential, dilution stability, and drug release. CKR12-PLGA-MCZ micelles showed higher antifungal activity than CKR12-PLGA micelles and MCZ solution. Furthermore, scanning and transmission electron microscopy suggested that CKR12-PLGA-MCZ micelles disrupted both cell wall and cell membrane of C. albicans. Our results revealed a synergistic effect of antifungal activity using a combination of antimicrobial peptide fragment analogues and MCZ, and that MCZ is a promising tool for the delivery to target microorganisms.

Short and Robust Anti-Infective Lipopeptides Engineered Based on the Minimal Antimicrobial Peptide KR12 of Human LL-37

This study aims to push the frontier of the engineering of human cathelicidin LL-37, a critical antimicrobial innate immune peptide that wards off invading pathogens. By sequential truncation of the smallest antibacterial peptide (KR12) of LL-37 and conjugation with fatty acids, with varying chain lengths, a library of lipopeptides is generated. These peptides are subjected to antibacterial activity and hemolytic assays. Candidates (including both forms made of l- and d-amino acids) with the optimal cell selectivity are subsequently fed to the second layer of in vitro filters, including salts, pH, serum, and media. These practices lead to the identification of a miniature LL-37 like peptide (d-form) with selectivity, stability, and robust antimicrobial activity in vitro against both Gram-positive and negative bacteria. Proteomic studies reveal far fewer serum proteins that bind to the d-form than the l-form peptide. C10-KR8d targets bacterial membranes to become helical, making it difficult for bacteria to develop resistance in a multiple passage experiment. In vivo, C10-KR8d is able to reduce bacterial burden of methicillin-resistant Staphylococcus aureus (MRSA) USA300 LAC in neutropenic mice. In addition, this designer peptide prevents bacterial biofilm formation in a catheter-associated mouse model. Meanwhile, C10-KR8d also recruits cytokines to the vicinity of catheters to clear infection. Thus, based on the antimicrobial region of LL-37, this study succeeds in identifying the smallest anti-infective peptide C10-KR8d with both robust antimicrobial, antibiofilm, and immune modulation activities.

The Potential of Human Peptide LL-37 as an Antimicrobial and Anti-Biofilm Agent

The rise in antimicrobial resistant bacteria threatens the current methods utilized to treat bacterial infections. The development of novel therapeutic agents is crucial in avoiding a post-antibiotic era and the associated deaths from antibiotic resistant pathogens. The human antimicrobial peptide LL-37 has been considered as a potential alternative to conventional antibiotics as it displays broad spectrum antibacterial and anti-biofilm activities as well as immunomodulatory functions. While LL-37 has shown promising results, it has yet to receive regulatory approval as a peptide antibiotic. Despite the strong antimicrobial properties, LL-37 has several limitations including high cost, lower activity in physiological environments, susceptibility to proteolytic degradation, and high toxicity to human cells. This review will discuss the challenges associated with making LL-37 into a viable antibiotic treatment option, with a focus on antimicrobial resistance and cross-resistance as well as adaptive responses to sub-inhibitory concentrations of the peptide. The possible methods to overcome these challenges, including immobilization techniques, LL-37 delivery systems, the development of LL-37 derivatives, and synergistic combinations will also be considered. Herein, we describe how combination therapy and structural modifications to the sequence, helicity, hydrophobicity, charge, and configuration of LL-37 could optimize the antimicrobial and anti-biofilm activities of LL-37 for future clinical use.

Osseointegration and antibacterial effect of an antimicrobial peptide releasing mesoporous titania implant

Medical devices such as orthopedic and dental implants may get infected by bacteria, which results in treatment using antibiotics. Since antibiotic resistance is increasing in society there is a need of finding alternative strategies for infection control. One potential strategy is the use of antimicrobial peptides, AMPs. In this study, we investigated the antibiofilm effect of the AMP, RRP9W4N, using a local drug-delivery system based on mesoporous titania covered titanium implants. Biofilm formation was studied in vitro using a safranine biofilm assay and LIVE/DEAD staining. Moreover, we investigated what effect the AMP had on osseointegration of commercially available titanium implants in vivo, using a rabbit tibia model. The results showed a sustained release of AMP with equal or even better antibiofilm properties than the traditionally used antibiotic Cloxacillin. In addition, no negative effects on osseointegration in vivo was observed. These combined results demonstrate the potential of using mesoporous titania as an AMP delivery system and the potential use of the AMP RRP9W4N for infection control of osseointegrating implants.

Amphipathic Peptide Antibiotics with Potent Activity against Multidrug-Resistant Pathogens

The emergence and prevalence of multidrug-resistant (MDR) bacteria have posed a serious threat to public health. Of particular concern are methicillin-resistant Staphylococcus aureus (MRSA) and blaNDM, mcr-1 and tet(X)-positive Gram-negative pathogens. The fact that few new antibiotics have been approved in recent years exacerbates this global crisis, thus, new alternatives are urgently needed. Antimicrobial peptides (AMPs) originated from host defense peptides with a wide range of sources and multiple functions, are less prone to achieve resistance. All these characteristics laid the foundation for AMPs to become potential antibiotic candidates. In this study, we revealed that peptide WW307 displayed potent antibacterial and bactericidal activity against MDR bacteria, including MRSA and Gram-negative bacteria carrying blaNDM-5, mcr-1 or tet(X4). In addition, WW307 exhibited great biofilm inhibition and eradication activity. Safety and stability experiments showed that WW307 had a strong resistance against various physiological conditions and displayed relatively low toxicity. Mechanistic experiments showed that WW307 resulted in membrane damage by selectively targeting bacterial membrane-specific components, including lipopolysaccharide (LPS), phosphatidylglycerol (PG), and cardiolipin (CL). Moreover, WW307 dissipated membrane potential and triggered the production of reactive oxygen species (ROS). Collectively, these results demonstrated that WW307 represents a promising candidate for combating MDR pathogens.

Cross-Strand Interaction, Central Bending, and Sequence Pattern Act as Biomodulators of Simplified β-Hairpin Antimicrobial Amphiphiles

Novel antimicrobial peptides (AMPs) have revolutionarily evolved into formidable candidates for antibiotic substitute materials against pathogenic infections. However, cost, lability, disorderly sequences, systemic toxicology, and biological profiles have plagued the perennial search. Here, a progressive β-hairpin solution with the simplest formulation is implanted into an AMP-based therapeutic strategy to systematically reveal the complex balance between function and toxicity of structural moieties, including cationicity, hydrophobicity, cross-strand interactions, center bending, and sequence pattern. Comprehensive implementation of structural identification, ten microorganisms, eleven in vitro barriers, four mammalian cells, and a diversified membrane operation setup led to the emergence of β-hairpin prototypes from a 24-member library. Lead amphiphiles, WKF-PG and WRF-NG, can tackle bacterial infection through direct antimicrobial efficacy and potential inflammation-limiting capabilities, such as an Escherichia coli challenge in a mouse peritonitis-sepsis model, without observed toxicity after systemic administration. Their optimal states with dissimilar modulators and the unavailable drug resistance related to membrane lytic mechanisms, also provide an usher for renewed innovation among β-sheet peptide-based antimicrobial biomaterials.

Overcoming Planktonic and Intracellular Staphylococcus aureus-Associated Infection with a Cell-Penetrating Peptide-Conjugated Antimicrobial Peptide

Staphylococcus aureus is a primary pathogen responsible for causing postoperative infections as it survives and persists in host cells, including osteoblasts and macrophages. These cells then serve as reservoirs resulting in chronic infections. Most traditional antibiotics have poor effects on intracellular S. aureus because they cannot enter the cell. Herein, a cell-penetrating peptide TAT-KR-12 was derived from the trans-activating transcription (TAT) peptide and KR-12 (residues 18-29 of human cathelicidin LL-37). The TAT acts as a "trojan horse" to deliver KR-12 peptide into the cells to kill S. aureus. Moreover, effective antibacterial properties and biocompatibility were observed in vitro, demonstrating that TAT-KR-12 is effective not only in eliminating planktonic S. aureus, but also in eliminating intracellular S. aureus cells in vitro. TAT-KR-12, as with LL-37, also elicits strong anti-inflammatory activities in LPS-stimulated macrophages, as demonstrated by significant inhibition of NO, TNF-α, and IL-1β expression and secretion from LPS-stimulated RAW264.7 cells. In the subcutaneous infection mouse model of planktonic and intracellular infections, the growth of S. aureus in vivo is evidently inhibited without cytotoxicity. These results suggest that the novel antimicrobial TAT-KR-12 may prove to be an effective treatment option to overcome antibiotic resistance caused by intracellular bacterial infections.

Decanoic acid modification enhances the antibacterial activity of PMAP-23RI-Dec

Antimicrobial peptides are a new type of antibacterial drugs with a broad antibacterial spectrum. Based on our previous research, PMAP-23RI-Dec was designed by modifying the C-terminal of PMAP-23RI with decanoic acid. In this study, we measured the antibacterial activity, stability, hemolysis, and cytotoxicity of PMAP-23RI-Dec. The mechanism of PMAP-23RI-Dec on biofilm and cell membranes were also studied. The results show that PMAP-23RI-Dec exhibited high antibacterial activity and stability, but the hemolytic activity and cytotoxicity of PMAP-23RI-Dec were not enhanced. Moreover, PMAP-23RI-Dec could inhibit biofilm formation at low concentrations, and enhance the killing effect on bacteria by changing the permeability of their cell membranes. Finally, PMAP-23RI-Dec reduced Pseudomonas aeruginosa GIM1.551 and Staphylococcus aureus ATCC25923 damage to organs, and showed superior efficacy against peritonitis. PMAP-23RI-Dec also reduced the scope of abscess and alleviated wound infections. Our research indicated that PMAP-23RI-Dec is a new antibacterial agent with potential clinical application.