Biochemical property and membrane-peptide interactions of de novo antimicrobial peptides designed by helix-forming units

Antibacterial activity of a short de novo designed peptide against fish bacterial pathogens

In the face of increasing antimicrobial resistance in aquaculture, researchers are exploring novel substitutes to customary antibiotics. One potential solution is the use of antimicrobial peptides (AMPs). We aimed to design and evaluate a novel, short, and compositionally simple AMP with potent activity against various bacterial pathogens in aquaculture. The resulting peptide, KK12YW, has an amphipathic nature and net charge of + 7. Molecular docking experiments disclosed that KK12YW has a strong affinity for aerolysin, a virulence protein produced by the bacterial pathogen Aeromonas sobria. KK12YW was synthesized using Fmoc chemistry and tested against a range of bacterial pathogens, including A. sobria, A. salmonicida, A. hydrophila, Edwardsiella tarda, Vibrio parahaemolyticus, Pseudomonas aeruginosa, Escherichia coli, Staphylococcus epidermidis, and methicillin-resistant S. aureus. The AMP showed promising antibacterial activity, with MIC and MBC values ranging from 0.89 to 917.1 µgmL-1 and 3.67 to 1100.52 µgmL-1, respectively. In addition, KK12YW exhibited resistance to high temperatures and remained effective even in the presence of serum and salt, indicating its stability. The peptide also demonstrated minimal hemolysis toward fish RBCs, even at higher concentrations. Taken together, these findings indicate that KK12YW could be a highly promising and viable substitute for conventional antibiotics to combat microbial infections in aquaculture.

pH/Hyal-Responsive Surface-Charge Switchable Electrostatic Complexation for Efficient Elimination of MRSA Infection

Methicillin-resistant Staphylococcus aureus (MRSA) has become a great threat to human health worldwide, making new effective antibacterial strategies urgently desired. In this study, a cationic pH-responsive delivery system (pHSM) was developed based on poly(β-amino esters)-methoxy poly(ethylene glycol), by which linezolid (LZD) could be encapsulated to form pHSM/LZD. The biocompatibility and stability of pHSM/LZD were further enhanced by adding low-molecular-weight hyaluronic acid (LWT HA) on the surface through electrostatic interaction to form pHSM/LZD@HA, of which the positive surface charges were neutralized by LWT HA under physiological conditions. LWT HA can be degraded by hyaluronidase (Hyal) after arriving at the infection site. In vitro, pHSM/LZD@HA could rapidly change to being positively charged on the surface within 0.5 h under acidic conditions, especially when Hyal was present, thus promoting bacterial binding and biofilm penetration of pHSM/LZD@HA. In addition, the pH/Hyal-dependent accelerated drug release behavior was also observed and it is beneficial for the comprehensive treatment of MRSA infection in vitro and in vivo. Our study provides a novel strategy to develop a pH/Hyal-responsive drug delivery system for the treatment of MRSA infection.

Albumin-coated pH-responsive dimeric prodrug-based nano-assemblies with high biofilm eradication capacity

Pseudomonas aeruginosa (PA) biofilms cause many persistent chronic infections in humans, especially in cystic fibrosis (CF) patients. The biofilms form a strong barrier which may inhibit antimicrobial agents from penetrating the biofilms and killing PA bacteria that reside deep within the biofilms. Concomitant therapies based on tobramycin (TOB) and azithromycin (AZM) have demonstrated beneficial effects in CF patients with chronic PA infections. However, the co-delivery of TOB and AZM has rarely been reported. In this study, we constructed a self-assembled pH-sensitive nano-assembly (DPNA) based on a dimeric prodrug (AZM-Cit-TOB) by simply inserting citraconic amide bonds between AZM and TOB. Moreover, the cationic surface of DPNA was further modified with anionic albumin (HSA) via electrostatic interactions to form an electrostatic complex (termed HSA@DPNA) for better biocompatibility. Upon arrival at the infected tissues, the citraconic amide bonds would be cleaved at acidic pH, resulting in the release of TOB and AZM for bacteria killing and biofilm eradication. As expected, HSA@DPNA showed comparable antibacterial abilities against the P. aeruginosa strain PAO1 in both planktonic and biofilm modes of growth compared to the TOB/AZM mixture in vitro. Moreover, HSA@DPNA exhibited excellent therapeutic efficacy on mice with PAO1-induced lung infection compared to the TOB/AZM mixture, and no detectable toxicity to mammalian cells/animals was observed during the therapeutic process. In summary, our study provides a promising method for the co-delivery of AZM and TOB in concomitant therapies against PAO1-related infection.

Hydrogen Peroxide-Activated Nitric Oxide-Releasing Vancomycin-Loaded Electrostatic Complexation for Efficient Elimination of Methicillin-Resistant Staphylococcus aureus Abscesses

The treatment of subcutaneous abscesses has been greatly hindered due to the spread of drug-resistant strains such as methicillin-resistant Staphylococcus aureus (MRSA). Thus, alternative strategies are highly desired to complement conventional antibiotic therapies and surgical intervention. As one of such strategies, applications of nitric oxide (NO) have shown great potential in the treatment of bacteria-induced subcutaneous abscesses by improving the efficacy of many therapeutic methods. However, it is extremely challenging to achieve precise delivery and controlled release because of its gaseous nature. In the present study, an effective strategy was reported in which on demand hydrogen peroxide (H₂O₂)-activated nitric oxide-releasing vancomycin (Van)-loaded electrostatic complexation (Lipo/Van@Arg) was fabricated. In this system, Van was encapsulated into a negative-charged DSPG/Chol liposome (Lipo/Van) and electrostatically bound with the positive-charged l-arginine (l-Arg). As expected, Lipo/Van@Arg exhibited superior bacterial binding and biofilm penetration abilities. After being in the interior of the biofilms, Lipo/Van@Arg could be triggered by the endogenous H₂O₂ and effectively release NO. The released NO could exhibit combined antibacterial and biofilm eradication effects with Van. Moreover, an in vivo evaluation using a BALB/c mouse model of subcutaneous abscesses indicated that the combination treatment of NO and Van based on Lipo/Van@Arg could effectively eliminate MRSA from the abscesses, thereby preventing abscess recurrence. In summary, the Lipo/Van@Arg system developed in this study realized controlled delivery and precise release of NO, which had significant clinical implications in the efficient treatment of abscesses.

Spatial Distribution Control of Antimicrobial Peptides through a Novel Polymeric Carrier for Safe and Efficient Cancer Treatment

Antimicrobial peptides (AMPs) hold great potential for use in tumor treatment. However, developing AMP-based antitumor therapies is challenging due to circulatory instability, hemolytic toxicity, low selectivity, and poor cell permeability of AMPs. In this study, a polymeric carrier for AMPs (denoted as PAMP_m -co-PPBE_n /PCA) is presented that effectively enhances their anticancer efficacy while minimizing their potential side effects. By integrating multiple responsive structures at the molecular level, the carrier finely controls the spatial distribution of AMPs in different biological microenvironments, thereby effectively modulating their membranolytic ability. Upon employing KLA as the model AMP, the polymeric carrier's hemolytic toxicity during blood circulation is suppressed, its cellular internalization when reaching tumor tissues facilitated, and its membranolytic toxicity toward the mitochondria upon entering cancer cells restored and further enhanced. Animal studies indicate that this approach significantly improves the antitumor efficacy of KLA and reduces its toxicity. Considering that the loading method for most AMPs is identical to that of KLA, the polymeric carrier reported in this study may provide a feasible approach for the development of AMP-based cancer treatments.

On-Demand Multifunctional Electrostatic Complexation for Synergistic Eradication of MRSA Biofilms

Recently, methicillin-resistant Staphylococcus aureus (MRSA) severely threatened the public health, especially when the biofilms developed. Although the biofilm eradication capability of nanoparticles (NPs) has been proposed and confirmed, efficient biofilm penetration and retention are still a big challenge. To solve this problem, a multifunctional electrostatic complexation (denoted as TDZ-G4@CA) was constructed for biofilm combination therapy. TDZ-G4@CA was composed of a TDZ-grafted amino-ended poly(amidoamine) dendrimer (TDZ-PAMAM) as the inner core and cis-aconitic anhydride-modified d-tyrosine (CA-Tyr) wrapped outside via electrostatic interaction. In our design, TDZ-G4@CA could simultaneously reduce the particle size and reverse the surface charge under an acidic microenvironment, which was designed for efficient biofilm penetration and retention. Meanwhile, the on-demand two-step sequential delivery of biofilm dispersal and antibacterial agents was also obtained. The acid responsiveness of TDZ-G4@CA triggered the immediate release of d-Tyr to damage the matrix of the biofilm. Subsequently, TDZ-G4 could penetrate over the depth of the biofilm and bind tightly to MRSA, which could enhance the permeability of the bacterial membrane for TDZ internalization. Additionally, TDZ exhibited a sustained-release pattern as a response to lipase to maintain an effective bactericidal concentration for a long time. As expected, in vitro experiments demonstrated that surface charge/particle size-adaptive TDZ-G4@CA with a sequential delivery strategy exhibited intensive infiltration in the biofilm matrix and excellent biofilm eradication capabilities. Afterward, in vivo experimental results also confirmed the prolonged circulation time and comprehensive therapeutic efficacy of TDZ-G4@CA against MRSA-induced subcutaneous abscess without any systemic side effects. Based on the comprehensive evaluation of the therapeutic outcome, the electrostatic complexation (TDZ-G4@CA) can serve as a promising strategy for enhanced antibiotic therapy for combating biofilm-associated infections.

Step-by-step dual stimuli-responsive nanoparticles for efficient bacterial biofilm eradication

Biofilm-related bacterial infections are extremely resistant to antibiotics, mainly due to the impermeability of the intensive matrices, which allow the bacteria to survive antibiotic treatment. Herein, step-by-step dual stimuli-responsive azithromycin-loaded nanoparticles (CM/AZM@Tyr) was constructed for efficient biofilm eradication. CM/AZM@Tyr was prepared by the self-assembly of poly(ε-caprolactone)-polyethylene glycol-polyethylenimine (PCL-PEG-PEI) into cationic micelles and simultaneously encapsulated AZM into the hydrophobic core, which is further bound with cis-aconityl-D-tyrosine (CA-Tyr) through electrostatic interaction. Upon initial penetration, CM/AZM@Tyr could show step-by-step dual-response to the microenvironment of biofilms. Firstly, the CA-Tyr shell rapidly responded to the acidic microenvironment and released D-Tyr to disassemble the biofilm mass. Then, the exposed cationic CM/AZM micelles could bind firmly to the negatively-charged bacteria cell membrane. With the enzymolysis of the PCL core, the rapidly releasing AZM could kill the bacteria over the depth of biofilms. Massive accumulation was observed in the infected lungs of biofilms-associated lung infection mice after the i.v. injection of CM/Cy5.5@Tyr under the 3D mode of the in vivo Imaging System. Reduced bacterial burden and alleviated fibrosis in the infected lungs were also obtained after treatment with CM/AZM@Tyr mainly due to its intensive penetration in the biofilm and the orderly release of the biofilm dispersant and antimicrobial agents. In summary, this research developed an effective strategy for the treatment of blood-accessible biofilm-induced infections.

Elaboration on the architecture of pH-sensitive surface charge-adaptive micelles with enhanced penetration and bactericidal activity in biofilms

Background

Biofilm formation is one of the main reasons for persistent bacterial infections. Recently, pH-sensitive copolymers have fascinated incredible attention to tackle biofilm-related infections. However, the proper incorporation of pH-sensitive segments in the polymer chains, which could significantly affect the biofilms targeting ability, has not been particularly investigated. Herein, we synthesized three types of pH-sensitive copolymers based on poly (β-amino ester) (PAE), poly (lactic-co-glycolic acid) (PLA) and polyethylene glycol (PEG), PAE-PLA-mPEG (A-L-E), PLA-PAE-mPEG (L-A-E) and PLA-PEG-PAE (L-E-A) to address this issue.

Results

The three copolymers could self-assemble into micelles (M_A-L-E, M_L-A-E and M_L-E-A) in aqueous medium. Compared with M_A-L-E and M_L-A-E, placing the PAE at the distal PEG end of PLA-PEG to yield PLA-PEG-PAE (M_L-E-A) was characterized with proper triggering pH, fully biofilm penetration, and high cell membrane binding affinity. Further loaded with Triclosan (TCS), M_L-E-A/TCS could efficiently kill the bacteria either in planktonic or biofilm mode. We reasoned that PAE segments would be preferentially placed near the surface and distant from the hydrophobic PLA segments. This would increase the magnitude of surface charge-switching capability, as the cationic PAE⁺ would easily disassociate from the inner core without conquering the additional hydrophobic force arising from covalent linkage with PLA segments, and rapidly rise to the outermost layer of the micellar surface due to the relative hydrophilicity. This was significant in that it could enable the micelles immediately change its surface charge where localized acidity occurred, and efficiently bind themselves to the bacterial surface where they became hydrolyzed by bacterial lipases to stimulate release of encapsulated TCS even a relatively short residence time to prevent rapid wash-out. In vivo therapeutic performance of M_L-E-A/TCS was evaluated on a classical biofilm infection model, implant-related biofilm infection. The result suggested that M_L-E-A/TCS was effective for the treatment of implant-related biofilm infection, which was proved by the efficient clearance of biofilm-contaminated catheters and the recovery of surrounding infected tissues.

Conclusions

In summary, elaboration on the architecture of pH-sensitive copolymers was the first step to target biofilm. The M_L-E-A structure may represent an interesting future direction in the treatment of biofilm-relevant infections associated with acidity.

Graphic abstract

Supplementary Information

The online version contains supplementary material available at 10.1186/s12951-021-00980-8.

On-demand pH-sensitive surface charge-switchable polymeric micelles for targeting Pseudomonas aeruginosa biofilms development

Bacterial biofilm is the complicated clinical issues, which usually results in bacterial resistance and reduce the therapeutic efficacy of antibiotics. Although micelles have been drawn attention in treatment of the biofilms, the micelles effectively permeate and retain in biofilms still facing a big challenge. In this study, we fabricated on-demand pH-sensitive surface charge-switchable azithromycin (AZM)-encapsulated micelles (denoted as AZM-SCSMs), aiming to act as therapeutic agent for treating Pseudomonas aeruginosa (P. aeruginosa) biofilms. The AZM-SCSMs was composed of poly(l-lactide)-polyetherimide-hyd-methoxy polyethylene glycol (PLA-PEI-hyd-mPEG). It was noteworthy that the pH-sensitive acylhydrazone bond could be cleaved in acidic biofilm microenvironment, releasing the secondary AZM-loaded cationic micelles based on PLA-PEI (AZM-SCMs) without destroying the micellar integrity, which could tailor drug-bacterium interaction using micelles through electrostatic attraction. The results proved that positively charged AZM-SCMs could facilitate the enhanced penetration and retention inside biofilms, improved binding affinity with bacterial membrane, and added drug internalization, thus characterized as potential anti-biofilm agent. The excellent in vivo therapeutic performance of AZM-SCSMs was confirmed by the targeting delivery to the infected tissue and reduced bacterial burden in the abscess-bearing mice model. This study not only developed a novel method for construction non-depolymerized pH-sensitive SCSMs, but also provided an effective means for the treatment of biofilm-related infections.

Mechanisms underlying the antimicrobial actions of the antimicrobial peptides Asp-Tyr-Asp-Asp and Asp-Asp-Asp-Tyr

The peptides Asp-Tyr-Asp-Asp (DYDD) and Asp-Asp-Asp-Tyr (DDDY) extracted from Dendrobium aphyllum have antimicrobial effects on Escherichia coli, Pseudomonas aeruginosa, and Monilia albicans, but no effects on Bacillus subtilis and Staphylococcus aureus. The effects of a hydrophobic environment on the secondary structures of these molecules were determined using circular dichroism and atomic force microscopy. Although scanning electron microscopy revealed that DDDY was more destructive to membranes than DYDD, both peptides showed antimicrobial effects against three pathogens. The minimum inhibitory concentration (MIC) of DYDD (18.075 mg/mL) against E. coli was higher than that of DDDY (4.519 mg/mL), and the influence of DYDD on the cell surface potential energy of E. coli was also greater (a decrease of 6.4 ± 0.66 mV) than that of DDDY (a decrease of 4.37 ± 0.77 mV). Moreover, the cell membrane damage and content leakage of DYDD-treated E. coli cells were more severe than those observed in the DDDY-treated cells. However, DDDY showed stronger antibacterial activity against P. aeruginosa and M. albicans than DYDD. A molecular dynamic simulation revealed that the mechanisms underlying the interaction between these two peptides and lipid bilayers were remarkably different. Therefore, two separate models were proposed to describe their antimicrobial activities.

Non-perfectly Amphipathic α-Helical Structure Containing the XXYXX Sequence Improves the Biological Activity of Bovine αs2-Casein Antimicrobial Peptides

Non-amphiphilic WIQPKTKVIPYVRYL (WI-6) derived from bovine α_s2-casein f (193-207) was modified by a defined mutation method to obtain five engineered peptides with mirror symmetry structures. The five engineered peptide sequences were WF-1 (WFQVKTRVRTKVQFW), FW-2 (FWRRYKKVKKYRRWF), FW-3 (FWQVIKKVKKIVQWF), FK-4 (FKQFYRRVRRYFQKF), and FR-5 (FRQWYRRVRRYWQRF). However, FW-2, FW-3, FK-4, and FR-5 had obvious XXYXX sequences. Among these, FW-3 was demonstrated to have the highest antibacterial activity, which indicates that the non-perfectly amphipathic α-helical structure containing the XXYXX sequence has a better bactericidal effect. Therefore, peptide FW-3 could be widely used as a substitute for antibiotics in food, medicine, and other fields. These findings provide a potential method for designing novel antimicrobial peptides.

Inhibitory and anti-inflammatory effects of two antimicrobial peptides moronecidin and temporin-1Dra against Propionibacterium acnes in vitro and in vivo

Proliferation of Propionibacterium acnes (P. acnes) is one of the main pathogenetic mechanisms of acne. Antimicrobial peptides with low-drug resistance and nonresidual are potential anti-acne agents. In this study, two antimicrobial peptides named temporin-1Dra and moronecidin were synthesized and tested their antimicrobial activity against P. acnes in vitro and in vivo. These two peptides inhibited the growth of Escherichia coli, Staphylococcus aureus, Candida albicans, and P. acnes. The minimal inhibitory concentrations (MICs) of temporin-1Dra and moronecidin to P. acnes were 30 and 10 μM, respectively. Both peptides exhibited strong resistance to heat and pH, but no obvious cytotoxicity to HaCaT cells. They also displayed persistent antimicrobial activities in the microbial challenge test. In the P. acnes-induced inflammation mouse model, moronecidin significantly decreased the ear swelling thickness in a concentration-dependent manner. At the 14th day after injection, 20 μg/day moronecidin reduced the ear swelling thickness to 46.15 ± 5.23% compared with the normal cream group. Tissue staining showed that moronecidin effectively reduced abscess and thickness of the dermis layer. Our results indicate that the antimicrobial peptide moronecidin could be developed as a potential natural anti-acne agent in the cosmetics or pharmaceutical industries.

The Antibacterial Activity and Mechanism of Action of Luteolin Against Trueperella pyogenes

Purpose

This research aimed to investigate the antibacterial activity and potential mechanism of luteolin against T. pyogenes.

Materials and Methods

The broth microdilution method was used to determine the minimum inhibitory concentrations (MICs) of luteolin against various T. pyogenes strains. The potential mechanism of action of luteolin was elucidated through testing and analysing the luteolin-induced alterations of T. pyogenes in several aspects, including cell wall, cell membrane, protein expression, nucleic acid content, topoisomerase activity and energy metabolism.

Results

The MIC values of luteolin against various T. pyogenes isolates and ATCC19411 were 78 µg/mL. The increased cell membrane permeability, destruction of cell wall integrity and TEM images after exposure to luteolin showed that the cell wall and membrane were damaged. The content of total protein and nucleic acid in T. pyogenes decreased significantly after treatment with luteolin (1/2 MIC) for 12, 24, and 36 h. Moreover, a hypochromic effect was observed in the absorption spectrum of luteolin when deoxyribonucleic acid (DNA) was added. In addition, after treatment with luteolin, a decrease in nicked or relaxed DNA content, which was catalysed by T. pyogenes-isolated DNA topoisomerase, was observed. In addition, the adenosine triphosphate (ATP) content in cells and the activity of succinate dehydrogenase (SDH) both decreased when T. pyogenes was exposed to different concentrations (1/4 MIC, 1/2 MIC, 1 MIC, 2 MIC) of luteolin for 1 h.

Conclusion

Luteolin showed distinct antibacterial activity against T. pyogenes by multiple actions, which mainly include destroying the integrity of the cell wall and cell membrane, influencing the expression of proteins, inhibiting nucleic acid synthesis, and interfering with energy metabolism.

Development of polycationic micelles as an efficient delivery system of antibiotics for overcoming the biological barriers to reverse multidrug resistance in Escherichia coli

Highly pathogenic Gram-negative bacteria (G-) are tenacious and pose a serious threat to public health, mainly because of three biological barriers: cell envelope blockages, biofilm protection, and macrophages shelter. One strategy to bypass the biological barriers and consequently achieve a satisfying G- bactericidal effect is to utilize polymeric micelles with superior bacterial recognition and binding capabilities. In the current study, we explored the biological barriers penetration ability of a traditional polycationic micellar system (PP-PEI) based on a copolymer of polylactide-poly (ethylene glycol)-polyethylenimine (PLA5K-PEG2K-PEI2K). Subsequently, tetracycline (TC) with good fluorescence property was encapsulated into the PLA core of the micelle (PP-PEI/TC) through hydrophobic interaction. The combination of a PEI shell and loaded antibiotic drug endowed the polycationic micelles with a greater capacity for killing drug-resistant bacteria, destructing biofilms, and eradicating intracellular bacteria, compared with free TC and micelles without the inoculation of a PEI moiety. Confocal laser scanning microscopy (CLSM) and flow cytometry illustrated that PP-PEI/TC could completely penetrate and accumulate in drug-resistant E. coli, biofilms, and infected macrophages. The efficient biological barrier penetration was elucidated as due to the strong electrostatic interactions between the polycationic PEI block and the anionic composition of the bacterial outer membrane (e.g., LPS), macrophage cell membrane (e.g., phospholipid), and extracellular polymeric substances (e.g., eDNA), which was confirmed by biolayer interferometry (BLI). Once the micellar system was bound to a negatively-charged surface, bacterial and cellular enzymes could degrade the PP-PEI core to release its antibacterial content and finally kill planktonic bacteria, bacteria over the depth of a biofilm, and/or intracellular bacteria. In vivo imaging indicated that fluorescent polycationic micelles accumulated in bacterial infection sites with strong fluorescence. In vivo antibacterial experiments showed that PP-PEI/TC could dramatically reduce the number of drug-resistant E. coli EB1-1 in the peritoneal cavity of acute peritonitis BALB/c mice compared with its counterparts. In conclusion, our study demonstrated that polycationic micelles with a PEI shell could penetrate into drug-resistant bacteria, the biofilm matrix, and infected macrophages and lead to the spatiotemporal release of antibacterial agents for the comprehensive treatment of drug-resistant relevant infections.

High Cell Selectivity and Bactericidal Mechanism of Symmetric Peptides Centered on d-Pro-Gly Pairs

Antimicrobial peptides (AMPs) have a unique action mechanism that can help to solve global problems in antibiotic resistance. However, their low therapeutic index and poor stability seriously hamper their development as therapeutic agents. In order to overcome these problems, we designed peptides based on the sequence template XXRXXRRzzRRXXRXX-NH₂, where X represents a hydrophobic amino acid like Phe (F), Ile (I), and Leu (L), while zz represents Gly-Gly (GG) or d-Pro-Gly (pG). Showing effective antimicrobial activity against Gram-negative bacteria and low toxicity, designed peptides had a tendency to form an α-helical structure in membrane-mimetic environments. Among them, peptide LR_pG (X: L, zz: pG) showed the highest geometric mean average treatment index (GM_TI = 73.1), better salt, temperature and pH stability, and an additive effect with conventional antibiotics. Peptide LR_pG played the role of anti-Gram-negative bacteria through destroying the cell membrane. In addition, peptide LR_pG also exhibited an anti-inflammatory activity by effectively neutralizing endotoxin. Briefly, peptide LR_pG has the potential to serve as a therapeutic agent to reduce antibiotic resistance owing to its high therapeutic index and great stability.

Potent antibacterial activity of MSI-1 derived from the magainin 2 peptide against drug-resistant bacteria

The structural modification of existing AMPs is an effective strategy to develop antimicrobial agents with high-efficiency, low-cost and low-toxicity antimicrobial agents. Methods: Here, we truncated 14-amino-acids at the N-terminus of MSI-78 to obtain MSI and further modified MSI to obtain four peptide analogs: MSI-1, MSI-2, MSI-3 and MSI-4. These peptide mutants were evaluated regarding their antibacterial activity against various sensitive or resistant bacteria; toxicity against mammalian cells or mice; and stability against violent pH, temperature variations and high NaCl concentrations. Finally, we also elucidated the possible mechanisms underlying its mode of action. Results: The results showed that MSI-1 and MSI-3 displayed activity that was superior to that of MSI-78 with MICs of 4-16 μg/ml and MBCs of 8-64 μg/ml, respectively, especially against drug-resistant bacteria, due to the increase in percent helicity and amphiphilicity. However, MSI-3, with higher hydrophobicity and antibacterial activity, had a relatively higher hemolysis rate and toxicity than MSI-1. MSI-1 exerted rapid bactericidal activity and effectively improved the survival rate and wound closure in penicillin-resistant E. coli-infected mice by eliminating bacterial counts in mouse organs or subeschar, further inhibiting the systemic dissemination of bacteria. Additionally, MSI-1 displayed perfect stability against violent pH, temperature variations and high NaCl concentrations and has the ability to circumvent the development of drug resistance. In terms of the mode of action, we found that at the super-MIC level, MSI-1 exhibited direct antimicrobial activity by disrupting the integrity of the bacterial cell membrane, while at the sub-MIC level, it bound to bacterial DNA to inhibit DNA replication and protein expression and ultimately disrupted bacterial biological function. Conclusions: This novel peptide MSI-1 could be a potential candidate for drug development against infection induced by drug-resistant bacteria.

Simplified Head-to-Tail Cyclic Polypeptides as Biomaterial-Associated Antimicrobials with Endotoxin Neutralizing and Anti-Inflammatory Capabilities

The therapeutic application of antimicrobial peptides (AMPs), a potential type of peptide-based biomaterial, is impeded by their poor antimicrobial activity and potential cytotoxicity as a lack of understanding of their structure–activity relationships. In order to comprehensively enhance the antibacterial and clinical application potency of AMPs, a rational approach was applied to design amphiphilic peptides, including head-to-tail cyclic, linear and D-proline antimicrobial peptides using the template (IR)_nP(IR)_nP (n = 1, 2 and 3). Results showed that these amphiphilic peptides demonstrated antimicrobial activity in a size-dependent manner and that cyclic peptide OIR3, which contained three repeating units (IR)3, had greater antimicrobial potency and cell selectivity than liner peptide IR3, DIR3 with D-Pro and gramicidin S (GS). Surface plasmon resonance and endotoxin neutralization assays indicated that OIR3 had significant endotoxin neutralization capabilities, which suggested that the effects of OIR3 were mediated by binding to lipopolysaccharides (LPS). Using fluorescence spectrometry and electron microscopy, we found that OIR3 strongly promoted membrane disruption and thereby induced cell lysis. In addition, an LPS-induced inflammation assay showed that OIR3 inhibited the pro-inflammatory factor TNF-α in RAW264.7 cells. OIR3 was able to reduce oxazolone-induced skin inflammation in allergic dermatitis mouse model via the inhibition of TNF-α, IL-1β and IL-6 mRNA expression. Collectively, the engineered head-to-tail cyclic peptide OIR3 was considerable potential candidate for use as a clinical therapeutic for the treatment of bacterial infections and skin inflammation.

Characterization of the Bioactivity and Mechanism of Bactenecin Derivatives Against Food-Pathogens

With the emergence of multidrug-resistant bacteria, antimicrobial peptides (AMPs) are regarded as potential alternatives to traditional antibiotics or chemicals. We designed and synthesized six derivatives of bactenecin (L₂C₃V₁₀C₁₁, RLCRIVVIRVCR), including R₂F₃W₁₀L₁₁ (RRFRIVVIRWLR), R₂W₃W₁₀R₁₁ (RRWRIVVIRWRR), K₂W₃V₁₀R₁₁ (RKWRIVVIRVRR), W₂R₃V₁₀R₁₁ (RWRRIVVIRVRR), W₂K₃K₁₀R₁₁ (RWKRIVVIRKRR), and K₂R₃R₁₀K₁₁ (RKRRIVVIRRKR), by amino acid substitution to increase the net charge and reduce hydrophobicity gradually. The bioactivity and mechanisms of action of the designed peptides were investigated. The results indicated that the antimicrobial activity of the designed peptides was higher than that of bactenecin. The hemolytic activity and cytotoxicity of the designed peptides were significantly lower than those of bactenecin. The designed peptides exhibited a wide range of antimicrobial activity against food-pathogens, particularly peptides K₂W₃V₁₀R₁₁ and W₂R₃V₁₀R₁₁; in addition, the activity was maintained under physiological salt and heat conditions. Mechanism studies indicated that AMPs interacted with negatively charged bacterial cell membranes, resulting in the destruction of cell membrane integrity by increasing membrane permeability and changing transmembrane potential, leading to cell death. The present study suggested that peptides K₂W₃V₁₀R₁₁ and W₂R₃V₁₀R₁₁ exhibited potential as alternatives to traditional antibiotics or chemicals for the treatment of food-pathogens. These findings lead to the development of a potential method for the design of novel AMPs.

Highly Stabilized α-Helical Coiled Coils Kill Gram-Negative Bacteria by Multicomplementary Mechanisms under Acidic Condition

Although antimicrobial peptides (AMPs) hold tremendous promise in overcoming the threats of multidrug resistance, the main obstacle to successful therapeutic applications is their poor stability. Various synthetic strategies such as unnatural amino acids and chemical modifications have made advances for improving this problem. However, this complicated synthesis often greatly increases the cost of production. Here, we show that a series of novel peptides, designed by combining an α-helical coiled coil model, knowledge of the specificity of proteolysis and major parameters of AMPs, exhibited efficient activity against all tested Gram-negative bacteria under acidic condition and demonstrate low toxicity. Of these α-helical coiled coil peptides, 3IH3 displayed the highest average therapeutic index (GM_TI = 294.25) with high stability toward salts, serum, extreme pH, heat, and proteases. Electron microscopy and biological analytical technique analyses showed that 3IH3 killed bacterial cells via a multicomplementary mechanism at pH 6.0, with physical membrane disruption as the dominant bactericidal mechanism. These results suggest that 3IH3 shows great stability as an inexpensive and effective antimicrobial activity agent and has the potential for clinical application in the treatment of infections occurring in body sites with acidic pH.

Brain-targeted delivery of PEGylated nano-bacitracin A against Penicillin-sensitive and -resistant Pneumococcal meningitis: formulated with RVG29 and Pluronic® P85 unimers

Pneumococcal meningitis (PM), caused by Streptococcus pneumonia, remains a high-burden disease in developing countries. Antibiotic therapy has been limited due to the inefficiency of drug transport across the blood-brain barrier (BBB) and the emergence of drug-resistant strains. In our preliminary study, PEGylated nano-self-assemblies of bacitracin A (PEGylated Nano-BA_12K) demonstrated a strong antibacterial potency against S. pneumonia. In this study, the potential application of this micelle for the treatment of both Penicillin-sensitive and -resistant PM was studied. To address BBB-targeting and -crossing issues, PEGylated Nano-BA_12K was formulated with a specific brain-targeting peptide (rabies virus glycopeptide-29, RVG₂₉) and a P-glycoprotein inhibitor (Pluronic^® P85 unimers) to construct a mixed micellar system (RVG₂₉-Nano-BA_P85). RVG₂₉-Nano-BA_P85 demonstrated a strong antibacterial potency against 13 clinical isolates of S. pneumonia, even higher than that of Penicillin G, a conventional anti-PM agent. RVG₂₉-Nano-BA_P85 had more cellular uptake in brain capillary endothelial cells (BCECs) and higher BBB-crossing efficiency than single formulated Nano-BAs as shown in an in vitro BBB model. The enhanced BBB-permeability was attributed to the synergetic effect of RVG₂₉ and P85 unimers through receptor-mediated transcytosis, exhaustion of ATP, and reduction in membrane microviscosity. In vivo results further demonstrated that RVG₂₉-Nano-BA_P85 was able to accumulate in brain parenchyma as confirmed by in vivo optical imaging. In addition, RVG₂₉-Nano-BA_P85 exhibited high therapeutic efficiencies in both Penicillin-sensitive and -resistant PM mouse models with negligible systemic toxicity. Collectively, RVG₂₉-Nano-BA_P85 could effectively overcome BBB barriers and suppressed the growth of both drug-sensitive and -resistant S. pneumonia in the brain tissues, which demonstrated its potential for the treatment of PM.

Characterization and Antimicrobial Activity of Amphiphilic Peptide AP3 and Derivative Sequences

The continued emergence of new antibiotic resistant bacterial strains has resulted in great interest in the development of new antimicrobial treatments. Antimicrobial peptides (AMPs) are one of many potential classes of molecules to help meet this emerging need. AMPs are naturally derived sequences, which act as part of the innate immune system of organisms ranging from insects through humans. We investigated the antimicrobial peptide AP3, which is originally isolated from the winter flounder Pleuronectes americanus. This peptide is of specific interest because it does not exhibit the canonical facially amphiphilic orientation of side chains when in a helical orientation. Different analogs of AP3 were synthesized in which length, charge identity, and Trp position were varied to investigate the sequence-structure and activity relationship. We performed biophysical and microbiological characterization using fluorescence spectroscopy, CD spectroscopy, vesicle leakage assays, bacterial membrane permeabilization assays, and minimal inhibitory concentration (MIC) assays. Fluorescence spectroscopy showed that the peptides bind to lipid bilayers to similar extents, while CD spectra show the peptides adopt helical conformations. All five peptides tested in this study exhibited binding to model lipid membranes, while the truncated peptides showed no measurable antimicrobial activity. The most active peptide proved to be the parent peptide AP3 with the highest degree of leakage and bacterial membrane permeabilization. Moreover, it was found that the ability to permeabilize model and bacterial membranes correlated most closely with the ability to predict antimicrobial activity.

Short Symmetric-End Antimicrobial Peptides Centered on β-Turn Amino Acids Unit Improve Selectivity and Stability

Antimicrobial peptides (AMPs) are excellent candidates to combat the increasing number of multi- or pan-resistant pathogens worldwide based on their mechanism of action, which is different from that of antibiotics. In this study, we designed short peptides by fusing an α-helix and β-turn sequence-motif in a symmetric-end template to promote the higher cell selectivity, antibacterial activity and salt-resistance of these structures. The results showed that the designed peptides PQ and PP tended to form an α-helical structure upon interacting with a membrane-mimicking environment. They displayed high cell selectivity toward bacterial cells over eukaryotic cells. Their activities were mostly maintained in the presence of different conditions (salts, serum, heat, and pH), which indicated their stability in vivo. Fluorescence spectroscopy and electron microscopy analyses indicated that PP and PQ killed bacterial cells through membrane pore formation, thereby damaging membrane integrity. This study revealed the potential application of these designed peptides as new candidate antimicrobial agents.

Insights into the antibacterial mechanism of PEGylated nano-bacitracin A against Streptococcus pneumonia: both penicillin-sensitive and penicillin-resistant strains

Background

Multidrug-resistant (MDR) Streptococcus pneumonia constitute a major worldwide public health concern.

Materials and methods

In our preliminary study, PEGylated nano-self-assemblies of bacitracin A (PEGylated Nano-BA_12K) showed strong antibacterial potency against reference S. pneumonia strain (ATCC 49619). In this study, the possibility of applying PEGylated Nano-BA_12K against penicillin-resistant S. pneumonia was further investigated. In addition, the underlying antibacterial mechanism of PEGylated Nano-BA_12K against both sensitive and resistant S. pneumonia was also clarified systematically, since S. pneumonia was naturally resistant to its unassembled counterpart bacitracin A (BA).

Results

PEGylated Nano-BA_12K showed strong antibacterial potency against 13 clinical isolates of S. pneumonia, including five penicillin-resistant strains. Structural changes, partial collapse, and even lysis of both penicillin-sensitive and penicillin-resistant bacteria were observed after incubation with PEGylated Nano-BA_12K via transmission electron microscopy and atomic force microscopy. Thus, the cell wall or/and cell membrane might be the main target of PEGylated Nano-BA_12K against S. pneumonia. PEGylated Nano-BA_12K exhibited limited effect on the permeabilization and peptidoglycan content of cell wall. Surface pressure measurement suggested that PEGylated Nano-BA_12K was much more tensioactive than BA, which was usually translated into a good membranolytic effect, and is helpful to permeabilize the cell membrane and damage membrane integrity, as evidenced by depolarization of the membrane potential, permeabilization of membrane and leakage of calcein from liposomes.

Conclusion

Collectively, great cell membrane permeability and formidable membrane disruption may work together for the strong antibacterial activity of PEGylated Nano-BA_12K against S. pneumonia. Taken together, PEGylated Nano-BA_12K has excellent potential against both penicillin-sensitive and penicillin-resistant S. pneumonia and might be suitable for the treatment of S. pneumonia infectious diseases.

Antibacterial Activity and Mechanism of Action of Bovine Lactoferricin Derivatives with Symmetrical Amino Acid Sequences

In recent years, the overuse of antibiotics has become very serious. Many pathogenic bacteria have become resistant to them, with serious potential health consequences. Thus, it is urgent that we develop new antibiotic drugs. Antimicrobial peptides (AMPs) are important endogenous antibacterial molecules that contribute to immunity. Most have spectral antibacterial properties and do not confer drug resistance. In this paper, an 11-residue peptide (LFcinB18⁻28) with a sequence of KCRRWQWRMKK was modified by amino acid substitution to form a symmetrical amino acid sequence. The antibacterial activities and mechanisms of action of engineered peptides including KW-WK (KWRRWQWRRWK), FP-PF (FPRRWQWRRPF), FW-WF (FWRRWQWRRWF), and KK-KK (KKRRWQWRRKK) were investigated. The four engineered peptides could more effectively inhibit bacteria than the original peptide, LFcinB18⁻28. This suggested that a symmetrical amino acid sequence might enhance the antibacterial activity of AMPs. However, only peptides KW-WK, FP-PF, and KK-KK were safe; FW-WF displayed hemolytic activity. The engineered peptides shared cationic and amphipathic characteristics that facilitated interactions with the anionic microbial membranes, leading to disruption of membrane integrity and permeabilizing microbial membranes, resulting in cell death. Therefore, a symmetrical amino acid sequence and related structural parameters offer an alternative approach to the design of AMPs. This will provide a scientific basis for the design and synthesis of new AMPs.

Pluronic-based nano-self-assemblies of bacitracin A with a new mechanism of action for an efficient in vivo therapeutic effect against bacterial peritonitis

Background

Although assemblies of hydrophobic-modified bacitracin A with PLGA (Nano-BA_PLGA) have demonstrated promising antibacterial activities against both Gram-positive and Gram-negative bacteria, the desirable antibacterial potency has remained challenging due to the low solubility of Nano-BA_PLGA. To address this tissue, a series of Pluronic copolymers (Pluronic^® F127, Pluronic^® P123 and Pluronic^® P85) were selected to link the N-terminus of bacitracin A to construct Pluronic-based nano-self assemblies (Nano-BA_F127, Nano-BA_P123 and Nano-BA_P85).

Results

Impressively, all the newly designed Pluronic-based Nano-BAs possessed higher solubility and stronger effectiveness against both Gram-positive and Gram-negative bacteria compared with Nano-BA_PLGA, especially the modification with Pluronic^® P85. Surface tension measurements indicated that Nano-BA_P85 was much more tensioactive than Nano-BA_PLGA, which usually translated into a good membranolytic effect. Fluorescence spectroscopy and electron microscopy analyses confirmed the speculation that the cell wall/membrane might be the main action target of Nano-BA_P85 by permeabilizing the cell membrane and damaging the membrane integrity. In vivo results further demonstrated that Nano-BA_P85 significantly suppressed bacterial growth and prolonged survival time in the bacterial peritonitis mouse model with negligible toxicity.

Conclusions

Collectively, the membrane targeting mechanism of action is entirely distinct from those of clinically used antibacterial agents. Furthermore, the new approach of construction nanoantibiotics based on the modification of commercially available antibiotics with Pluronic copolymers is demonstrated to have an efficient therapeutic effect against bacterial infection.

Electronic supplementary material

The online version of this article (10.1186/s12951-018-0397-3) contains supplementary material, which is available to authorized users.

Synthesis, construction, and evaluation of self-assembled nano-bacitracin A as an efficient antibacterial agent in vitro and in vivo

Bacitracin A (BA) is an excellent polypeptide antibiotic that is active against gram-positive bacteria without triggering multidrug resistance. However, BA is inactive against gram-negative bacteria because of its inability to cross the outer membrane of these cells, and it has strong nephrotoxicity, thus limiting its clinical applications. Nanoantibiotics can effectively localize antibiotics to the periplasmic space of bacteria while decreasing the adverse effects of antibiotics. In this study, biodegradable hydrophobic copolymers of poly (d,l-lactide-co-glycolide) (PLGA) were attached to the N-termini of BA to design a novel class of self-assembled nano-bacitracin A (nano-BAs), and their potential as antibacterial agents was evaluated in vitro and in vivo. Nano-BAs had a core-shell structure with a mean diameter <150 nm. Impressively, nano-BAs had strong antibacterial properties against both gram-positive and gram-negative bacteria, and the distribution of antibacterial activity as a function of PLGA block length was skewed toward longer PLGA chains. No cytotoxicity against HK-2 cells or human red blood cells (hRBCs) was observed in vitro, suggesting good biocompatibility. A high local density of BA mass on the surface promoted endocytotic cellular uptake, and hydrophobic interactions between the PLGA block and lipopolysaccharide (LPS) facilitated the uptake of nano-BAs, thereby leading to greater antibacterial activities. In addition, Nano-BA_5K was found to be effective in vivo, and it served as an anti-infective agent for wound healing. Collectively, this study provides a cost-effective means of developing self-assembling nano-polypeptide antibiotic candidates with a broader antibacterial spectrum and a lower toxicity than commercially available peptide antibiotics, owing to their modification with biodegradable copolymers.

Cathelicidin-trypsin inhibitor loop conjugate represents a promising antibiotic candidate with protease stability

Cathelicidins are regarded as promising antibiotics due to their capability against antibiotic-resistant bacteria without cytotoxicity. However, some concerns about the balance of cytotoxicity and antimicrobial activity, weak stability and enzymatic susceptibility sually restrict their therapeutic use. Here, we designed a series of shortened variants, Hc1~15, based on our previously characterized Hc-CATH. Hc3, the one with the best activity, after point mutation was engineered with a trypsin inhibitor loop, ORB-C, to obtain four hybrid peptides: H3TI, TIH3, H3TIF and TIH3F. All four except TIH3 were found possessing an appreciable profile of proteases inhibitory and antimicrobial characteristics without increase in cytotoxicity. Among them, TIH3F exhibited the most potent and broad-spectrum antimicrobial and anti-inflammatory activities. Fluorescence spectroscopy has demonstrated a quick induction of bacterial membrane permeability by TIH3F leading to the cell death, which also accounts for its fast anti-biofilm activity. Such mode of antimicrobial action was mainly attributed to peptides’ amphiphilic and helical structures determined by CD and homology modeling. Besides, TIH3F exhibited good tolerance to salt, serum, pH, and temperature, indicating a much better physiological stability in vitro than Hc3, Most importantly, in the case of resistance against proteases hydrolysis, current hybrid peptides displayed a remarkable enhancement than their original templates.

The C-terminal sequences of porcine thrombin are active as antimicrobial peptides

The C-terminal sequences of porcine thrombin encode a series of peptides with the characteristics of net positive charge and hydrophobicity, suggesting antimicrobial potential. In this study, we synthesized truncated C-terminal peptides to explore their antimicrobial potency and structure-activity relationship. The results showed that some peptides exerted antimicrobial activity against Gram-positive and Gram-negative bacteria, with selectivity for microbial membranes. The antimicrobial potency of the peptides increased with the extension of chain length. Considering toxicity to red blood cells, the 21-mer peptide T-6 displayed the highest therapeutic index of 43.4, suggesting its higher cell selectivity. Typical α-helical conformations were observed upon binding to a bacteria-mimicking environment. The derivatives tended to interact preferentially with negatively charged vesicles compared to zwitterionic vesicles. Flow cytometry and electron microscopy revealed that the peptides targeted bacterial cell membranes and disrupted cytoplasmic membrane integrity, thereby causing the release of cellular contents leading to cell death. Peptide-membrane interaction experiments provided evidence that the peptides killed bacteria via a membrane-mediating mechanism. In summary, the C-terminal sequence of porcine thrombin has antimicrobial functions.

High specific selectivity and Membrane-Active Mechanism of the synthetic centrosymmetric α-helical peptides with Gly-Gly pairs

We used a template-assisted approach to develop synthetic antimicrobial peptides, which differ from naturally occurring antimicrobial peptides that can compromise host natural defenses. Previous researches have demonstrated that symmetrical distribution patterns of amino acids contribute to the antimicrobial activity of natural peptides. However, there is little research describing such design ideas for synthetic α-helical peptides. Therefore, here, we established a centrosymmetric α-helical sequence template (y + hhh + y)_n (h, hydrophobic amino acid; +, cationic amino acid; y, Gly or hydrophobic amino acid), which contributed to amphipathicity, and a series of centrosymmetric peptides was designed with pairs of small amino acids (Ala and Gly), which were utilized to modulate the biological activity. The centrosymmetric peptides with 3 repeat units exhibited strong antimicrobial activity; in particular, the Gly-rich centrosymmetric peptide GG3 showed stronger selectivity for gram-negative bacteria without hemolysis. Furthermore, beyond our expectation, fluorescence spectroscopy and electron microscopy analyses indicated that the GG3, which possessed poor α-helix conformation, dramatically exhibited marked membrane destruction via inducing bacterial membrane permeabilization, pore formation and disruption, even bound DNA to further exert antimicrobial activity. Collectively, the Gly-rich centrosymmetric peptide GG3 was an ideal candidate for commercialization as a clinical therapeutic to treat gram-negative bacterial infections.

Antimicrobial activity and membrane-active mechanism of tryptophan zipper-like β-hairpin antimicrobial peptides

Antimicrobial peptides (AMPs) with amphipathic β-hairpin structures have been demonstrated to possess potent antimicrobial activities and great cell selectivities. However, our understanding of β-hairpin antimicrobial peptides lags behind that of α-helices, mainly because it is difficult for short peptides to form robust β-hairpin structures. Tryptophan zipper (trpzip) peptides are among the most stable β-hairpin peptides known to fold spontaneously without requiring covalent disulfide constraint or metal binding. To develop model β-hairpin AMPs with small size and remarkable stability, a series of amphiphilic linear peptides were designed based on the trpzip motif. The sequence of designed peptides is (WK) n (D) PG(KW) n -NH2 (n = 1, 2, 3, 4, 5), and the antimicrobial activity and membrane interaction mechanism of the peptides were evaluated. The results showed that these peptides readily fold into β-hairpin structures in aqueous and membrane-mimicking environments and exhibit broad-spectrum antimicrobial activities against both gram-positive and gram-negative bacteria. The antibacterial potency of the peptides initially increased and then decreased with increasing chain length. WK3, a 14-residue peptide, displayed excellent antimicrobial activity with minimal hemolytic activity and cytotoxicity, suggesting that it possesses great cell selectivity. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), fluorescence spectroscopy, and flow cytometry indicated that representative peptides WK3 and WK4 exert their activities by permeabilizing the microbial membrane and damaging cell membrane integrity. This study reveals the application potential of the designed peptides as promising antimicrobial agents for the control of infectious diseases, and it also provides new insights into the design and optimization of highly stable β-hairpin AMPs with great antimicrobial activities and cell selectivities.

Characterization of cell selectivity, physiological stability and endotoxin neutralization capabilities of α-helix-based peptide amphiphiles

While naturally occurring antimicrobial peptides (AMPs) have been of increasing interest as alternative antibiotics due to their broad-spectrum antimicrobial activity and reduced possibility for the development of bacterial drug-resistance, some concerns such as potential cytotoxicity, poor antimicrobial activity and weak physiological stability may ultimately weaken their development as antimicrobial agents. To generate AMPs with enhanced therapeutic potential, we designed α-helical hybrid peptides based on PRW4, Fowlicidin-2, Protegrin-3 and Tritrpticin sequences to gain insights into their selectivities, physiological stabilities and endotoxin neutralization capabilities. The designed hybrid peptides PR-FO, PR-PG and PR-TR exhibited high cell selectivity towards bacterial cells over human red blood cells (hRBCs). Their activities were maintained in the presence of physiological concentrations of salts or serum, indicating a high stability in vitro. The results from fluorescence spectroscopy, flow cytometry, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) showed that these designed peptides killed microbial cells by increasing membrane permeability and damaging membrane envelope integrity. Moreover, the hybrid peptides effectively neutralized endotoxins while causing minimal cytotoxicities. Collectively, our results suggest that these hybrid peptides, in particular PR-FO, have tremendous potential for use as novel antimicrobial and antisepsis agents.

Efficacy of the designer antimicrobial peptide SHAP1 in wound healing and wound infection

Infected wounds cause delay in wound closure and impose significantly negative effects on patient care and recovery. Antimicrobial peptides (AMPs) with antimicrobial and wound closure activities, along with little opportunity for the development of resistance, represent one of the promising agents for new therapeutic approaches in the infected wound treatment. However, therapeutic applications of these AMPs are limited by their toxicity and low stability in vivo. Previously, we reported that the 19-amino-acid designer peptide SHAP1 possessed salt-resistant antimicrobial activities. Here, we analyzed the wound closure activities of SHAP1 both in vitro and in vivo. SHAP1 did not affect the viability of human erythrocytes and keratinocytes up to 200 μM, and was not digested by exposure to proteases in the wound fluid, such as human neutrophil elastase and Staphylococcus aureus V8 proteinase for up to 12 h. SHAP1 elicited stronger wound closure activity than human cathelicidin AMP LL-37 in vitro by inducing HaCaT cell migration, which was shown to progress via transactivation of the epidermal growth factor receptor. In vivo analysis revealed that SHAP1 treatment accelerated closure and healing of full-thickness excisional wounds in mice. Moreover, SHAP1 effectively countered S. aureus infection and enhanced wound healing in S. aureus-infected murine wounds. Overall, these results suggest that SHAP1 might be developed as a novel topical agent for the infected wound treatment.

Design of imperfectly amphipathic α-helical antimicrobial peptides with enhanced cell selectivity

Antimicrobial peptides (AMPs), which are produced by multicellular organisms as a defense mechanism against competing pathogenic microbes, appear to be excellent candidates for the development of novel antimicrobial agents. Amphipathicity is traditionally believed to be crucial to the de novo design or systematic optimization of AMPs. In this study, we designed a series of short α-helical AMPs with imperfect amphipathicity to augment the arsenal of strategies and to gain further insights into their antimicrobial and hemolytic activity. These imperfectly amphipathic α-helical AMPs were designed by replacing the paired charged amino acid residues on the polar face of an amphipathic peptide with tryptophan residues on the basis of α-helical protein folding principles. PRW4, an imperfectly amphipathic α-helical AMP with hydrogen bonds formed by paired tryptophan residues, was observed to be more selective towards bacterial cells than toward human red blood cells. PRW4 was also effective against Gram-negative and Gram-positive bacteria, and fluorescence spectroscopy, flow cytometry, scanning electron microscopy and transmission electron microscopy indicated that PRW4 killed microbial cells by permeabilizing the cell membrane and damaging their membrane integrity. Therefore, disruptive amphipathicity has excellent potential for the rational design and optimization of AMPs with promising antimicrobial activities.

Structure-activity relations of myxinidin, an antibacterial peptide derived from the epidermal mucus of hagfish

The structure-activity relations of myxinidin, a peptide derived from epidermal mucus of hagfish, Myxine glutinosa L., were investigated. Analysis of key residues allowed us to design new peptides with increased efficiency. Antimicrobial activity of native and modified peptides demonstrated the key role of uncharged residues in the sequence; the loss of these residues reduces almost entirely myxinidin antimicrobial activity, while insertion of arginine at charged and uncharged position increases antimicrobial activity compared with that of native myxinidin. Particularly, we designed a peptide capable of achieving a high inhibitory effect on bacterial growth. Experiments were conducted using both Gram-negative and Gram-positive bacteria. Nuclear magnetic resonance (NMR) studies showed that myxinidin is able to form an amphipathic α-helical structure at the N terminus and a random coil region at the C terminus.