The human antimicrobial peptide LL-37 and its fragments possess both antimicrobial and antibiofilm activities against multidrug-resistant Acinetobacter baumannii

Decoding antimicrobial resistance: unraveling molecular mechanisms and targeted strategies

Antimicrobial resistance poses a significant global health threat, necessitating innovative approaches for combatting it. This review explores various mechanisms of antimicrobial resistance observed in various strains of bacteria. We examine various strategies, including antimicrobial peptides (AMPs), novel antimicrobial materials, drug delivery systems, vaccines, antibody therapies, and non-traditional antibiotic treatments. Through a comprehensive literature review, the efficacy and challenges of these strategies are evaluated. Findings reveal the potential of AMPs in combating resistance due to their unique mechanisms and lower propensity for resistance development. Additionally, novel drug delivery systems, such as nanoparticles, show promise in enhancing antibiotic efficacy and overcoming resistance mechanisms. Vaccines and antibody therapies offer preventive measures, although challenges exist in their development. Non-traditional antibiotic treatments, including CRISPR-Cas systems, present alternative approaches to combat resistance. Overall, this review underscores the importance of multifaceted strategies and coordinated global efforts to address antimicrobial resistance effectively.

Current treatment strategies for targeting virulence factors and biofilm formation in Acinetobacter baumannii

A higher prevalence of Acinetobacter baumannii infections and mortality rate has been reported recently in hospital-acquired infections (HAI). The biofilm-forming capability of A. baumannii makes it an extremely dangerous pathogen, especially in device-associated hospital-acquired infections (DA-HAI), thereby it resists the penetration of antibiotics. Further, the transmission of the SARS-CoV-2 virus was exacerbated in DA-HAI during the epidemic. This review specifically examines the complex interconnections between several components and genes that play a role in the biofilm formation and the development of infections. The current review provides insights into innovative treatments and therapeutic approaches to combat A. baumannii biofilm-related infections, thereby ultimately improving patient outcomes and reducing the burden of HAI.

Interaction and simulation studies suggest the possible molecular targets of intrinsically disordered amyloidogenic antimicrobial peptides in Acinetobacter baumannii

Acinetobacter baumannii is one of the causing agents of nosocomial infections. A wide range of antibiotics fails to work against these pathogens. Hence, there is an urgent requirement to develop other therapeutics to solve this problem. Antimicrobial peptides (AMPs) are a diverse group of naturally occurring peptides that have the ability to kill diverse groups of microorganisms. The major challenge of using AMPs as therapeutics is their unstable nature and the fact that most of their molecular targets are still unknown. In this study, we have selected intrinsically disordered and amyloidogenic AMPs, showing activity against A. baumannii, that is, Bactenecin, Cath BF, Citropin 1.1, DP7, NA-CATH, Tachyplesin, and WAM-1. To identify the probable target of these AMPs in A. baumannii, calculation of docking score, binding energy, dissociation constant, and molecular dynamics analysis was performed with selected seventeen possible molecular targets. The result showed that the most probable molecular targets of most of the intrinsically disordered amyloidogenic AMPs were UDP-N-acetylenol-pyruvoyl-glucosamine reductase (MurB), followed by 33-36 kDa outer membrane protein (Omp 33-36), UDP-N-acetylmuramoyl-l-alanyl-d-glutamate-2,6-diaminopimelate ligase (MurE), and porin Subfamily Protein (PorinSubF). Further, molecular dynamics analysis concluded that the target of antimicrobial peptide Bactenecin is MurB of A. baumannii, and identified other molecular targets of selected AMPs. Additionally, the oligomerization capacity of the selected AMPs was also investigated, and it was shown that the selected AMPs form oligomeric states, and interact with their molecular targets in that state. Experimental validation using purified AMPs and molecular targets needs to be done to confirm the interaction.Communicated by Ramaswamy H. Sarma.

Biomaterials for immunomodulation in wound healing

The substantial economic impact of non-healing wounds, scarring, and burns stemming from skin injuries is evident, resulting in a financial burden on both patients and the healthcare system. This review paper provides an overview of the skin's vital role in guarding against various environmental challenges as the body's largest protective organ and associated developments in biomaterials for wound healing. We first introduce the composition of skin tissue and the intricate processes of wound healing, with special attention to the crucial role of immunomodulation in both acute and chronic wounds. This highlights how the imbalance in the immune response, particularly in chronic wounds associated with underlying health conditions such as diabetes and immunosuppression, hinders normal healing stages. Then, this review distinguishes between traditional wound-healing strategies that create an optimal microenvironment and recent peptide-based biomaterials that modulate cellular processes and immune responses to facilitate wound closure. Additionally, we highlight the importance of considering the stages of wounds in the healing process. By integrating advanced materials engineering with an in-depth understanding of wound biology, this approach holds promise for reshaping the field of wound management and ultimately offering improved outcomes for patients with acute and chronic wounds.

Molecular Aspects of the Functioning of Pathogenic Bacteria Biofilm Based on Quorum Sensing (QS) Signal-Response System and Innovative Non-Antibiotic Strategies for Their Elimination

One of the key mechanisms enabling bacterial cells to create biofilms and regulate crucial life functions in a global and highly synchronized way is a bacterial communication system called quorum sensing (QS). QS is a bacterial cell-to-cell communication process that depends on the bacterial population density and is mediated by small signalling molecules called autoinducers (AIs). In bacteria, QS controls the biofilm formation through the global regulation of gene expression involved in the extracellular polymeric matrix (EPS) synthesis, virulence factor production, stress tolerance and metabolic adaptation. Forming biofilm is one of the crucial mechanisms of bacterial antimicrobial resistance (AMR). A common feature of human pathogens is the ability to form biofilm, which poses a serious medical issue due to their high susceptibility to traditional antibiotics. Because QS is associated with virulence and biofilm formation, there is a belief that inhibition of QS activity called quorum quenching (QQ) may provide alternative therapeutic methods for treating microbial infections. This review summarises recent progress in biofilm research, focusing on the mechanisms by which biofilms, especially those formed by pathogenic bacteria, become resistant to antibiotic treatment. Subsequently, a potential alternative approach to QS inhibition highlighting innovative non-antibiotic strategies to control AMR and biofilm formation of pathogenic bacteria has been discussed.

Coassembled Multicomponent Protein Nanoparticles Elicit Enhanced Antibacterial Activity

The waning pipeline of the useful antibacterial arsenal has necessitated the urgent development of more effective antibacterial strategies with distinct mechanisms to rival the continuing emergence of resistant pathogens, particularly Gram-negative bacteria, due to their explicit drug-impermeable, two-membrane-sandwiched cell wall envelope. Herein, we have developed multicomponent coassembled nanoparticles with strong bactericidal activity and simultaneous bacterial cell envelope targeting using a peptide coassembly strategy. Compared to the single-component self-assembled nanoparticle counterparts or cocktail mixtures of these at a similar concentration, coassembled multicomponent nanoparticles showed higher bacterial killing efficiency against Acinetobacter baumannii, Pseudomonas aeruginosa, and Escherichia coli by several orders of magnitude (about 100-1,000,000-fold increase). Comprehensive confocal and electron microscopy suggest that the superior antibacterial activity of the coassembled nanoparticles proceeds via multiple complementary mechanisms of action, including membrane destabilization, disruption, and cell wall hydrolysis, actions that were not observed with the single nanoparticle counterparts. To understand the fundamental working mechanisms behind the improved performance of coassembled nanoparticles, we utilized a "dilution effect" system where the antibacterial components are intermolecularly mixed and coassembled with a non-antibacterial protein in the nanoparticles. We suggest that coassembled nanoparticles mediate enhanced bacterial killing activity by attributes such as optimized local concentration, high avidity, cooperativity, and synergy. The nanoparticles showed no cytotoxic or hemolytic activity against tested eukaryotic cells and erythrocytes. Collectively, these findings reveal potential strategies for disrupting the impermeable barrier that Gram-negative pathogens leverage to restrict antibacterial access and may serve as a platform technology for potential nano-antibacterial design to strengthen the declining antibiotic arsenal.

Turning cationic antimicrobial peptide KR-12 into self-assembled nanobiotics with potent bacterial killing and LPS neutralizing activities

Gram-negative sepsis has become a substantial and escalating global healthcare challenge due to the growing antibiotic resistance crisis and the sluggish development of new antibiotics. LL-37, a unique Cathelicidin species found in humans, exhibits a wide range of bioactive properties, including direct bactericidal effects, inflammation regulation, and LPS neutralization. KR-12, the smallest yet potent peptide fragment of LL-37, has been modified to create more effective antimicrobials. In this study, we designed two myristoylated derivatives of KR-12, referred to as Myr-KR-12N and Myr-KR-12C. These derivatives displayed remarkable ability to spontaneously assemble into nanoparticles when mixed with deionized water. Myristoylated KR-12 derivatives exhibited broad-spectrum and intensified bactericidal activity by disrupting bacterial cell membranes. In particular, Myr-KR-12N showed superior capability to rescue mice from lethal E. coli-induced sepsis in comparison with the conventional antibiotic meropenem. We also confirmed that the myristoylated KR-12 nanobiotic possesses significant LPS binding capacity and effectively reduces inflammation in vitro. In an in vivo context, Myr-KR-12N outperformed polymyxin B in rescuing mice from LPS-induced sepsis. Crucially, toxicological assessments revealed that neither Myr-KR-12N nor Myr-KR-12C nanobiotics induced meaningful hemolysis or caused damage to the liver and kidneys. Collectively, our study has yielded an innovative nanobiotic with dual capabilities of bactericidal action and LPS-neutralization, offering substantial promise for advancing the clinical translation of antimicrobial peptides and the development of novel antibiotics. This addresses the critical need for effective solutions to combat Gram-negative sepsis, a pressing global medical challenge.

GATR-3, a Peptide That Eradicates Preformed Biofilms of Multidrug-Resistant Acinetobacter baumannii

Acinetobacter baumannii is a gram-negative bacterium that causes hospital-acquired and opportunistic infections, resulting in pneumonia, sepsis, and severe wound infections that can be difficult to treat due to antimicrobial resistance and the formation of biofilms. There is an urgent need to develop novel antimicrobials to tackle the rapid increase in antimicrobial resistance, and antimicrobial peptides (AMPs) represent an additional class of potential agents with direct antimicrobial and/or host-defense activating activities. In this study, we present GATR-3, a synthetic, designed AMP that was modified from a cryptic peptide discovered in American alligator, as our lead peptide to target multidrug-resistant (MDR) A. baumannii. Antimicrobial susceptibility testing and antibiofilm assays were performed to assess GATR-3 against a panel of 8 MDR A. baumannii strains, including AB5075 and some clinical strains. The GATR-3 mechanism of action was determined to be via loss of membrane integrity as measured by DiSC3(5) and ethidium bromide assays. GATR-3 exhibited potent antimicrobial activity against all tested multidrug-resistant A. baumannii strains with rapid killing. Biofilms are difficult to treat and eradicate. Excitingly, GATR-3 inhibited biofilm formation and, more importantly, eradicated preformed biofilms of MDR A. baumannii AB5075, as evidenced by MBEC assays and scanning electron micrographs. GATR3 did not induce resistance in MDR A. baumannii, unlike colistin. Additionally, the toxicity of GATR-3 was evaluated using human red blood cells, HepG2 cells, and waxworms using hemolysis and MTT assays. GATR-3 demonstrated little to no cytotoxicity against HepG2 and red blood cells, even at 100 μg/mL. GATR-3 injection showed little toxicity in the waxworm model, resulting in a 90% survival rate. The therapeutic index of GATR-3 was estimated (based on the HC50/MIC against human RBCs) to be 1250. Overall, GATR-3 is a promising candidate to advance to preclinical testing to potentially treat MDR A. baumannii infections.

Novel Antibacterial Agents SAAP-148 and Halicin Combat Gram-Negative Bacteria Colonizing Catheters

The antibiotic management of catheter-related infections (CRIs) often fails owing to the emergence of antimicrobial-resistant strains and/or biofilm/persister apparitions. Thus, we investigated the efficacy of two novel antimicrobial agents, i.e., the synthetic peptide SAAP-148 and the novel antibiotic halicin, against Gram-negative bacteria (GNB) colonizing catheters. The antibacterial, anti-biofilm, and anti-persister activities of both agents were evaluated against Acinetobacter baumannii, Escherichia coli, and Klebsiella pneumoniae strains. The enrolled strains were isolated from catheters and selected based on their resistance to at least three antibiotic classes and biofilm formation potential. Furthermore, the hemolysis and endotoxin neutralization abilities of these agents were explored. The bactericidal activity of both agents was reduced in urine and plasma as compared to buffered saline. In a dose-dependent manner, SAAP-148 and halicin reduced bacterial counts in 24 h preformed biofilms on silicone elastomer discs and eliminated persisters originating from antibiotic-exposed mature 7-day biofilms, with halicin being less effective than SAAP-148. Importantly, SAAP-148 and halicin acted synergistically on E. coli and K. pneumoniae biofilms but not on A. baumannii biofilms. The peptide, but not halicin, decreased the production of IL-12p40 upon exposure to UV-killed bacteria. This preliminary study showed that SAAP-148 and halicin alone/in combination are promising candidates to fight GNB colonizing catheters.

Characterization of LL37 Binding to Collagen through Peptide Modification with a Collagen-Binding Domain

Collagen-based biomaterials loaded with antimicrobial peptides (AMPs) present a promising approach for promoting wound healing while providing protection against infections. In our previous work, we modified the AMP LL37 by incorporating a collagen-binding domain (cCBD) as an anchoring unit for collagen-based wound dressings. We demonstrated that cCBD-modified LL37 (cCBD-LL37) exhibited improved retention on collagen after washing with PBS. However, the binding mechanism of cCBD-LL37 to collagen remained to be elucidated. In this study, we found that cCBD-LL37 showed a slightly higher affinity for collagen compared to LL37. Our results indicated that cCBD inhibited cCBD-LL37 binding to collagen but did not fully eliminate the binding. This suggests that cCBD-LL37 binding to collagen may involve more than just one-site-specific binding through the collagen-binding domain, with non-specific interactions also playing a role. Electrostatic studies revealed that both LL37 and cCBD-LL37 interact with collagen via long-range electrostatic forces, initiating low-affinity binding that transitions to close-range or hydrophobic interactions. Circular dichroism analysis showed that cCBD-LL37 exhibited enhanced structural stability compared to LL37 under varying ionic strengths and pH conditions, implying potential improvements in antimicrobial activity. Moreover, we demonstrated that the release of LL37 and cCBD-LL37 into the surrounding medium was influenced by the electrostatic environment, but cCBD could enhance the retention of peptide on collagen scaffolds. Collectively, these results provide important insights into cCBD-modified AMP-binding mechanisms and suggest that the addition of cCBD may enhance peptide structural stability and retention under varying electrostatic conditions.

An Update on the Therapeutic Potential of Antimicrobial Peptides against Acinetobacter baumannii Infections

The rise in antibiotic-resistant strains of clinically important pathogens is a major threat to global health. The World Health Organization (WHO) has recognized the urgent need to develop alternative treatments to address the growing list of priority pathogens. Antimicrobial peptides (AMPs) rank among the suggested options with proven activity and high potential to be developed into effective drugs. Many AMPs are naturally produced by living organisms protecting the host against pathogens as a part of their innate immunity. Mechanisms associated with AMP actions include cell membrane disruption, cell wall weakening, protein synthesis inhibition, and interference in nucleic acid dynamics, inducing apoptosis and necrosis. Acinetobacter baumannii is a critical pathogen, as severe clinical implications have developed from isolates resistant to current antibiotic treatments and conventional control procedures, such as UV light, disinfectants, and drying. Here, we review the natural AMPs representing primary candidates for new anti-A. baumannii drugs in post-antibiotic-era and present computational tools to develop the next generation of AMPs with greater microbicidal activity and reduced toxicity.

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.

Phosphorylcholine and KR12-Containing Corneal Implants in HSV-1-Infected Rabbit Corneas

Severe HSV-1 infection can cause blindness due to tissue damage from severe inflammation. Due to the high risk of graft failure in HSV-1-infected individuals, cornea transplantation to restore vision is often contraindicated. We tested the capacity for cell-free biosynthetic implants made from recombinant human collagen type III and 2-methacryloyloxyethyl phosphorylcholine (RHCIII-MPC) to suppress inflammation and promote tissue regeneration in the damaged corneas. To block viral reactivation, we incorporated silica dioxide nanoparticles releasing KR12, the small bioactive core fragment of LL37, an innate cationic host defense peptide produced by corneal cells. KR12 is more reactive and smaller than LL37, so more KR12 molecules can be incorporated into nanoparticles for delivery. Unlike LL37, which was cytotoxic, KR12 was cell-friendly and showed little cytotoxicity at doses that blocked HSV-1 activity in vitro, instead enabling rapid wound closure in cultures of human epithelial cells. Composite implants released KR12 for up to 3 weeks in vitro. The implant was also tested in vivo on HSV-1-infected rabbit corneas where it was grafted by anterior lamellar keratoplasty. Adding KR12 to RHCIII-MPC did not reduce HSV-1 viral loads or the inflammation resulting in neovascularization. Nevertheless, the composite implants reduced viral spread sufficiently to allow stable corneal epithelium, stroma, and nerve regeneration over a 6-month observation period.

Antibiofilm properties of cathelicidin LL-37: an in-depth review

Notwithstanding ceaseless endeavors toward developing effective antibiofilm chemotherapeutics, biofilm-associated infections continue to be one of the most perplexing challenges confronting medicine today. Endogenous host defense peptides, such as the human cathelicidin LL-37, are being propounded as promising options for treating such infectious diseases. Over the past decennium, LL-37 has duly received tremendous research attention by virtue of its broad-spectrum antimicrobial activity and immunomodulatory properties. No attempt has hitherto been made, as far as we are aware, to comprehensively review the antibiofilm effects of LL-37. Accordingly, the intent in this paper is to provide a fairly all-embracing review of the literature available on the subject. Accumulating evidence suggests that LL-37 is able to prevent biofilm establishment by different bacterial pathogens such as Acinetobacter baumannii, Aggregatibacter actinomycetemcomitans, Bacteroides fragilis, Burkholderia thailandensis, Cutibacterium acnes, Escherichia coli, Francisella tularensis, Helicobacter pylori, Klebsiella pneumoniae, Pseudomonas aeruginosa, Staphylococcus aureus, and Streptococcus pyogenes. Inhibition of bacterial adhesion, downregulation of biofilm-associated genes, suppression of quorum-sensing pathways, degradation of biofilm matrix, and eradication of biofilm-residing cells are the major mechanisms responsible for antibiofilm properties of LL-37. In terms of its efficacy and safety in vivo, there are still many questions to be answered. Undoubtedly, LL-37 can open up new windows of opportunity to prevent and treat obstinate biofilm-mediated infections.

Bioactive compounds: a goldmine for defining new strategies against pathogenic bacterial biofilms?

Most human infectious diseases are caused by microorganisms growing as biofilms. These three-dimensional self-organized communities are embedded in a dense matrix allowing microorganisms to persistently inhabit abiotic and biotic surfaces due to increased resistance to both antibiotics and effectors of the immune system. Consequently, there is an urgent need for novel strategies to control biofilm-associated infections. Natural products offer a vast array of chemical structures and possess a wide variety of biological properties; therefore, they have been and continue to be exploited in the search for potential biofilm inhibitors with a specific or multi-locus mechanism of action. This review provides an updated discussion of the major bioactive compounds isolated from several natural sources - such as plants, lichens, algae, microorganisms, animals, and humans - with the potential to inhibit biofilm formation and/or to disperse established biofilms by bacterial pathogens. Despite the very large number of bioactive products, their exact mechanism of action often remains to be clarified and, in some cases, the identity of the active molecule is still unknown. This knowledge gap should be filled thus allowing development of these products not only as novel drugs to combat bacterial biofilms, but also as antibiotic adjuvants to restore the therapeutic efficacy of current antibiotics.

In Vitro Pharmacokinetics of LL-37 and Oncorhyncin II Combination Against Acinetobacter baumannii

Background: Multidrug-resistant (MDR) Acinetobacter baumannii is one of the most common nosocomial pathogens. Antimicrobial peptides (AMPs) have been introduced as a viable alternative to antibiotics in the treatment of MDR pathogens. Objectives: This study was designed to assess the in vitro pharmacokinetics of the combination of two potent AMPs, LL-37 and oncorhyncin II, against A. baumannii (ATCC19606). Methods: The synthesized genes of oncorhyncin II and LL-37 were introduced into Escherichia coli BL21 as the expression host. The minimum inhibitory concentration (MIC), time-kills, and growth kinetics of these peptides were used to evaluate their antimicrobial efficiencies against A. baumannii (ATCC19606). Results: LL-37 and oncorhyncin II recombinant peptides showed MIC of 30.6 and 95.87 µg/mL against A. baumannii, respectively. Additive action was confirmed by combining the generated AMPs at the checkerboard approach. The combination of LL-37 and oncorhyncin II at 2 × MIC resulted in a rapid drop in log10 CFU/mL of A. baumannii in the time-kill and growth kinetic findings studies. Conclusions: The combination of the produced LL-37 and oncorhyncin II synergizes the bioactivity of the individual peptides. Therefore, these peptides or their combinations might function as novel antibiotics and be used to develop and produce new antimicrobial drugs for the treatment of infections caused by A. baumannii.

Antimicrobial Peptide Cec4 Eradicates Multidrug-Resistant Acinetobacter baumannii in vitro and in vivo

Introduction

Acinetobacter baumannii has become a major difficulty in the treatment of bacteria-associated infection. The previously reported antimicrobial peptide Cec4 exhibited good and stable activity against A. baumannii in vitro, but the mechanisms and effects in vivo are elusive.

Methods

The effects of Cec4 on bacterial membrane permeability, membrane potential and bacterial reactive oxygen species were measured. The cell membrane localization of antimicrobial peptides was studied by fluorescence labelling. The ability of bacteria to develop resistance to antimicrobial peptides was studied by continuous induction, and transcriptome difference was analysed. The in vivo toxicity of Cec4 against nematodes and mice was studied, and the in vivo therapeutic potential of Cec4 against A. baumannii was assessed.

Results

Cec4 effectively cleared multidrug-resistant A. baumannii by altering bacterial cell membrane permeability, changing bacterial cell membrane polarity, and increasing bacterial intracellular reactive oxygen species. Cec4 affected the expression of the secretion system, outer membrane, and efflux pump genes of A. baumannii. In addition, the bacteria did not acquire stable drug-resistant ability. Cec4 at 1.024 mg/mL did not affect the proliferation of HeLa and HepG2 cells, and Cec4 at 45 mg/kg had little effect on the mortality of Caenorhabditis elegans, even the liver and kidney tissues of mouse. Most importantly, Cec4 could effectively improve the survival rates and reduce the bacterial load of various tissues in the mouse model of infection.

Conclusion

In conclusion, Cec4 can damage the cell membrane of bacteria, and the bacteria is not easy to produce resistance to Cec4. Besides, Cec4 has good potential for the treatment of multidrug-resistant A. baumannii infections.

The LL-37 Antimicrobial Peptide as a Treatment for Systematic Infection of Acinetobacter baumannii in a Mouse Model

BACKGROUND

The antimicrobial peptides (AMPs) played a critical role in the innate immunity of the host and are considered natural sources illustrating a broad-spectrum antimicrobial activity with high specificity and low cytotoxicity. AMPs generally possess a net positive charge and have amphipathic structures. Thus, AMPs can bind and interact with negatively charged bacterial cell membranes, leading to destructive defects in biomembranes and ending in cell death. LL37 is the only human cathelicidin-derived antimicrobial peptide that shows a broad spectrum of antimicrobial activity.

MATERIALS AND METHODS

To determine the antibacterial efficiency of LL37 in a mouse model of systemic A. baumannii infection, LL37 corresponding gene was expressed in E. coli, purification and refolding situations were optimized. The antimicrobial performance of produced LL-37 against A. baumannii was evaluated in vitro via MIC and Time Kill assays, and its destructive effects on the bacterial cell were confirmed by SEM image.

RESULTS

The recombinant LL37 showed strong antibacterial function against A. baumannii at 1.5 μg/mL concentration. Time kill assay showed a sharp reduction of cell viability during the first period of exposure, and complete cell death was recorded after 40 min exposure.

CONCLUSION

Furthermore, in vivo results represented a significant ability of LL37 in the treatment of systematic infected mouse models, and all infected mice receiving LL37 protein survived without no trace of bacteria in their blood samples.

LL37 and collagen-binding domain-mediated LL37 binding with type I collagen: Quantification via QCM-D

Antimicrobial peptide (AMP)-loaded biomaterials may represent a viable alternative for stimulating wound healing while protecting against infections. Previously, to develop an efficient delivery system for the cathelicidin antimicrobial peptide, LL37, our lab modified LL37 with a collagen-binding domain derived from collagenase (cCBD) as an anchoring unit to collagen-based wound dressings. However, a direct quantification of unmodified LL37 and cCBD-LL37 binding with collagen has not been performed. In this study, we used quartz crystal microbalance with dissipation monitoring (QCM-D), immunohistochemistry (IHC), and atomic force microscopy (AFM) to establish and characterize an adsorbed layer of type I collagen on the QCM-D sensor and quantify peptide-collagen binding. A collagen deposition protocol was successfully established by measuring concentration-dependent deposition of collagen in QCM-D, and collagen self-assembly was observed by IHC and AFM. Hydrophobicity is known to affect the behavior of collagen adsorption. Therefore, we compared the deposition of collagen on hydrophilic SiO₂-coated sensors vs. hydrophobic polystyrene (PS)-coated sensors via QCM-D, and found that the hydrophobic surface yielded more collagen adsorption, which suggested that hydrophobic surfaces are preferable for collagen layer establishment. There was no significant difference between LL37 and cCBD-LL37 binding with collagen, but the cCBD-LL37 showed better retention on the collagen after washing with PBS, indicating that there is an advantage to using cCBD as an anchoring unit to collagen. Collectively, these results provide important information on cCBD-mediated AMP-binding mechanisms and establish an effective method for quantifying peptide-collagen binding.

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.

Functional and toxicological evaluation of the MAA-41: a novel rationally designed antimicrobial peptide using hybridization and modification methods from LL-37 and BMAP-28

Background:

Managing bacterial infections caused by multidrug-resistant (MDR) and biofilm-forming bacteria is a global health concern. Therefore, enormous efforts were directed toward finding potential alternative antimicrobial agents such as antimicrobial peptides (AMPs).

Aim:

We aimed to synthesize a novel modified hybrid peptide designed from natural parents’ peptides with enhanced activity and reduced toxicity profile.

Method:

Rational design was used to hybridize the two antimicrobial peptides, in which the alpha-helical parts of BMAP-28 and LL-37 were combined. Then, several amino acid modifications were applied to generate a modified hybrid peptide named MAA-41. The physicochemical properties were checked using in silico methods. The MAA-41 was evaluated for its antimicrobial and anti-biofilm activities. Synergistic studies were performed with five conventional antibiotics. Finally, the cytotoxicity on mammalian cells and the hemolytic activity were assessed.

Results:

The MAA-41 revealed a broad-spectrum activity against both Gram-positive and Gram-negative bacteria including standard and MDR bacterial strains. The concentration against planktonic cells ranged between 10 and 20 μM with higher potency against Gram-negative bacteria. Additionally, the MAA-41 displayed potent activity in eradicating biofilm-forming cells, and the reported MBECs were equal to the MIC values reported for planktonic cells. This new peptide exhibited reduced toxicity profiles against erythrocyte cells but not against Vero cells. Combining MAA-41 peptides with conventional antibiotics improved the antimicrobial activity of the combined agents. Either synergistic or additive effects were shown as a significant decrease in MIC to 0.25 μM.

Conclusion:

This study proposes the validity of a novel peptide (MAA-41) with enhanced antimicrobial activity and reduced toxicity, especially when used as conventional antibiotic combinations.

The Role of Antimicrobial Peptides as Antimicrobial and Antibiofilm Agents in Tackling the Silent Pandemic of Antimicrobial Resistance

Just over a million people died globally in 2019 due to antibiotic resistance caused by ESKAPE pathogens (Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa, and Enterobacter species). The World Health Organization (WHO) also lists antibiotic-resistant Campylobacter and Helicobacter as bacteria that pose the greatest threat to human health. As it is becoming increasingly difficult to discover new antibiotics, new alternatives are needed to solve the crisis of antimicrobial resistance (AMR). Bacteria commonly found in complex communities enclosed within self-produced matrices called biofilms are difficult to eradicate and develop increased stress and antimicrobial tolerance. This review summarises the role of antimicrobial peptides (AMPs) in combating the silent pandemic of AMR and their application in clinical medicine, focusing on both the advantages and disadvantages of AMPs as antibiofilm agents. It is known that many AMPs display broad-spectrum antimicrobial activities, but in a variety of organisms AMPs are not stable (short half-life) or have some toxic side effects. Hence, it is also important to develop new AMP analogues for their potential use as drug candidates. The use of one health approach along with developing novel therapies using phages and breakthroughs in novel antimicrobial peptide synthesis can help us in tackling the problem of AMR.

Convergence of Biofilm Formation and Antibiotic Resistance in Acinetobacter baumannii Infection

Acinetobacter baumannii (A. baumannii) is a leading cause of nosocomial infections as this pathogen has certain attributes that facilitate the subversion of natural defenses of the human body. A. baumannii acquires antibiotic resistance determinants easily and can thrive on both biotic and abiotic surfaces. Different resistance mechanisms or determinants, both transmissible and non-transmissible, have aided in this victory over antibiotics. In addition, the propensity to form biofilms (communities of organism attached to a surface) allows the organism to persist in hospitals on various medical surfaces (cardiac valves, artificial joints, catheters, endotracheal tubes, and ventilators) and also evade antibiotics simply by shielding the bacteria and increasing its ability to acquire foreign genetic material through lateral gene transfer. The biofilm formation rate in A. baumannii is higher than in other species. Recent research has shown how A. baumannii biofilm-forming capacity exerts its effect on resistance phenotypes, development of resistome, and dissemination of resistance genes within biofilms by conjugation or transformation, thereby making biofilm a hotspot for genetic exchange. Various genes control the formation of A. baumannii biofilms and a beneficial relationship between biofilm formation and "antimicrobial resistance" (AMR) exists in the organism. This review discusses these various attributes of the organism that act independently or synergistically to cause hospital infections. Evolution of AMR in A. baumannii, resistance mechanisms including both transmissible (hydrolyzing enzymes) and non-transmissible (efflux pumps and chromosomal mutations) are presented. Intrinsic factors [biofilm-associated protein, outer membrane protein A, chaperon-usher pilus, iron uptake mechanism, poly-β-(1, 6)-N-acetyl glucosamine, BfmS/BfmR two-component system, PER-1, quorum sensing] involved in biofilm production, extrinsic factors (surface property, growth temperature, growth medium) associated with the process, the impact of biofilms on high antimicrobial tolerance and regulation of the process, gene transfer within the biofilm, are elaborated. The infections associated with colonization of A. baumannii on medical devices are discussed. Each important device-related infection is dealt with and both adult and pediatric studies are separately mentioned. Furthermore, the strategies of preventing A. baumannii biofilms with antibiotic combinations, quorum sensing quenchers, natural products, efflux pump inhibitors, antimicrobial peptides, nanoparticles, and phage therapy are enumerated.

In Vivo Evaluation of ECP Peptide Analogues for the Treatment of Acinetobacter baumannii Infection

Antimicrobial peptides (AMPs) are alternative therapeutics to traditional antibiotics against bacterial resistance. Our previous work identified an antimicrobial region at the N-terminus of the eosinophil cationic protein (ECP). Following structure-based analysis, a 30mer peptide (ECPep-L) was designed that combines antimicrobial action against Gram-negative species with lipopolysaccharides (LPS) binding and endotoxin-neutralization activities. Next, analogues that contain non-natural amino acids were designed to increase serum stability. Here, two analogues were selected for in vivo assays: the all-D version (ECPep-D) and the Arg to Orn version that incorporates a D-amino acid at position 2 (ECPep-2D-Orn). The peptide analogues retained high LPS-binding and anti-endotoxin activities. The peptides efficacy was tested in a murine acute infection model of Acinetobacter baumannii. Results highlighted a survival rate above 70% following a 3-day supervision with a single administration of ECPep-D. Moreover, in both ECPep-D and ECPep-2D-Orn peptide-treated groups, clinical symptoms improved significantly and the tissue infection was reduced to equivalent levels to mice treated with colistin, used as a last resort in the clinics. Moreover, treatment drastically reduced serum levels of TNF-α inflammation marker within the first 8 h. The present results support ECP-derived peptides as alternative candidates for the treatment of acute infections caused by Gram-negative bacteria.

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.

Anti-Pythium insidiosum activity of MSI-78, LL-37, and magainin-2 antimicrobial peptides

We investigated the anti-Pythium insidiosum activity of the antimicrobial peptides (AMPs) MSI-78, LL-37, and magainin-2. To detect the minimum inhibitory concentration (MIC), fourteen clinical strains were incubated with the AMPs following the CLSI M38-A2 protocol. All three AMPs showed antimicrobial activity with an MIC range of 20-80 mg/L against all strains. We concluded that the evaluated AMPs have great potential as anti-Pythium insidiosum agents, and their activity deserves to be more explored in further research. Antimicrobial peptides were tested against Pythium insidiosum, a microorganism that causes a difficult-to-treat disease in animals and humans. These peptides have been shown to be able to kill P. insidiosum and may be candidates for use in the treatment of this infection.

The Long-Term Effect of a Nine Amino-Acid Antimicrobial Peptide AS-hepc3(48-56) Against Pseudomonas aeruginosa With No Detectable Resistance

The emergence of multidrug-resistant (MDR) pathogens has become a global public health crisis. Among them, MDR Pseudomonas aeruginosa is the main cause of nosocomial infections and deaths. Antimicrobial peptides (AMPs) are considered as competitive drug candidates to address this threat. In the study, we characterized two AMPs (AS-hepc3_(41-71) and AS-hepc3_(48-56)) that had potent activity against 5 new clinical isolates of MDR P. aeruginosa. Both AMPs destroyed the integrity of the cell membrane, induced leakage of intracellular components, and ultimately led to cell death. A long-term comparative study on the bacterial resistance treated with AS-hepc3_(41-71), AS-hepc3_(48-56) and 12 commonly used antibiotics showed that P. aeruginosa quickly developed resistance to the nine antibiotics tested (including aztreonam, ceftazidime, cefepime, imipenem, meropenem, ciprofloxacin, levofloxacin, gentamicin, and piperacillin) as early as 12 days after 150 days of successive culture generations. The initial effective concentration of 9 antibiotics against P. aeruginosa was greatly increased to a different high level at 150 days, however, both AS-hepc3_(41-71) and AS-hepc3_(48-56) maintained their initial MIC unchangeable through 150 days, indicating that P. aeruginosa did not produce any significant resistance to both AMPs. Furthermore, AS-hepc3_(48-56) did not show any toxic effect on mammalian cells in vitro and mice in vivo. AS-hepc3_(48-56) had a therapeutic effect on MDR P. aeruginosa infection using a mouse lung infection model and could effectively increase the survival rate of mice by inhibiting bacterial proliferation and attenuating lung inflammation. Taken together, the short peptide AS-hepc3_(48-56) would be a promising agent for clinical treatment of MDR P. aeruginosa infections.

A Systematic Study of the Stability, Safety, and Efficacy of the de novo Designed Antimicrobial Peptide PepD2 and Its Modified Derivatives Against Acinetobacter baumannii

Searching for new antimicrobials is a pressing issue to conquer the emergence of multidrug-resistant (MDR) bacteria and fungi. Antimicrobial peptides (AMPs) usually have antimicrobial mechanisms different from those of traditional antibiotics and bring new hope in the discovery of new antimicrobials. In addition to antimicrobial activity, stability and target selectivity are important concerns to decide whether an antimicrobial peptide can be applied in vivo. Here, we used a simple de novo designed peptide, pepD2, which contains only three kinds of amino acid residues (W, K, L), as an example to evaluate how the residues and modifications affect the antimicrobial activity against Acinetobacter baumannii, stability in plasma, and toxicity to human HEK293 cells. We found that pepI2 with a Leu→Ile substitution can decrease the minimum bactericidal concentrations (MBC) against A. baumannii by one half (4 μg/mL). A D-form peptide, pepdD2, in which the D-enantiomers replaced the L-enantiomers of the Lys(K) and Leu(L) residues, extended the peptide half-life in plasma by more than 12-fold. PepD3 is 3-residue shorter than pepD2. Decreasing peptide length did not affect antimicrobial activity but increased the IC₅₀ to HEK293 cells, thus increased the selectivity index (SI) between A. baumannii and HEK293 cells from 4.7 to 8.5. The chain length increase of the N-terminal acyl group and the Lys→Arg substitution greatly enhanced the hemolytic activity, hence those modifications are not good for clinical application. Unlike colistin, the action mechanism of our peptides relies on negatively charged lipids rather than lipopolysaccharides. Therefore, not only gram-negative bacteria but also gram-positive bacteria can be killed by our peptides.

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.

Inhibition of Virulence Factors and Biofilm Formation of Acinetobacter Baumannii by Naturally-derived and Synthetic Drugs

Acinetobacter baumannii is a gram-negative, aerobic, non-motile, and pleomorphic bacillus. A. baumannii is also a highly-infectious pathogen causing high mortality and morbidity rates in intensive care units. The discovery of novel agents against A. baumannii infections is urgently needed due to the emergence of drug-resistant A. baumannii strains and the limited number of efficacious antibiotics available for treatment. In addition to the production of several virulence factors, A. baumannii forms biofilms on the host cell surface as well. Formation of biofilms occurs through initial surface attachment, microcolony formation, biofilm maturation, and detachment stages, and is one of the major drug resistance mechanisms employed by A. baumannii. Several studies have previously reported the efficacy of naturally-derived and synthetic compounds as anti- biofilm and anti-virulence agents against A. baumannii. Here, inhibition of biofilm formation and virulence factors of A. baumannii using naturally-derived and synthetic compounds are reviewed.

Antibacterial and Antibiofilm Activities of Novel Antimicrobial Peptides against Multidrug-Resistant Enterotoxigenic Escherichia Coli

Post-weaning diarrhea due to enterotoxigenic Escherichia coli (ETEC) is a common disease of piglets and causes great economic loss for the swine industry. Over the past few decades, decreasing effectiveness of conventional antibiotics has caused serious problems because of the growing emergence of multidrug-resistant (MDR) pathogens. Various studies have indicated that antimicrobial peptides (AMPs) have potential to serve as an alternative to antibiotics owing to rapid killing action and highly selective toxicity. Our previous studies have shown that AMP GW-Q4 and its derivatives possess effective antibacterial activities against the Gram-negative bacteria. Hence, in the current study, we evaluated the antibacterial efficacy of GW-Q4 and its derivatives against MDR ETEC and their minimal inhibition concentration (MIC) values were determined to be around 2~32 μg/mL. Among them, AMP Q4-15a-1 with the second lowest MIC (4 μg/mL) and the highest minimal hemolysis concentration (MHC, 256 μg/mL), thus showing the greatest selectivity (MHC/MIC = 64) was selected for further investigations. Moreover, Q4-15a-1 showed dose-dependent bactericidal activity against MDR ETEC in time–kill curve assays. According to the cellular localization and membrane integrity analyses using confocal microscopy, Q4-15a-1 can rapidly interact with the bacterial surface, disrupt the membrane and enter cytosol in less than 30 min. Minimum biofilm eradication concentration (MBEC) of Q4-15a-1 is 4× MIC (16 μg/mL), indicating that Q4-15a-1 is effective against MDR ETEC biofilm. Besides, we established an MDR ETEC infection model with intestinal porcine epithelial cell-1 (IPEC-1). In this infection model, 32 μg/mL Q4-15a-1 can completely inhibit ETEC adhesion onto IPEC-1. Overall, these results suggested that Q4-15a-1 may be a promising antibacterial candidate for treatment of weaned piglets infected by MDR ETEC.

Biofilms as Promoters of Bacterial Antibiotic Resistance and Tolerance

Multidrug resistant bacteria are a global threat for human and animal health. However, they are only part of the problem of antibiotic failure. Another bacterial strategy that contributes to their capacity to withstand antimicrobials is the formation of biofilms. Biofilms are associations of microorganisms embedded a self-produced extracellular matrix. They create particular environments that confer bacterial tolerance and resistance to antibiotics by different mechanisms that depend upon factors such as biofilm composition, architecture, the stage of biofilm development, and growth conditions. The biofilm structure hinders the penetration of antibiotics and may prevent the accumulation of bactericidal concentrations throughout the entire biofilm. In addition, gradients of dispersion of nutrients and oxygen within the biofilm generate different metabolic states of individual cells and favor the development of antibiotic tolerance and bacterial persistence. Furthermore, antimicrobial resistance may develop within biofilms through a variety of mechanisms. The expression of efflux pumps may be induced in various parts of the biofilm and the mutation frequency is induced, while the presence of extracellular DNA and the close contact between cells favor horizontal gene transfer. A deep understanding of the mechanisms by which biofilms cause tolerance/resistance to antibiotics helps to develop novel strategies to fight these infections.

Antimicrobial peptide activity is anticorrelated with lipid a leaflet affinity

The activity of antimicrobial peptides (AMPs) has significant bacterial species bias, the mechanisms of which are not fully understood. We employed single-molecule tracking to measure the affinity of three different AMPs to hybrid supported bilayers composed of lipid A extracted from four different Gram negative bacteria and observed a strong empirical anticorrelation between the affinity of a particular AMP to a given lipid A layer and the activity of that AMP towards the bacterium from which that lipid A was extracted. This suggested that the species bias of AMP activity is directly related to AMP interactions with bacterial outer membranes, despite the fact that the mechanism of antimicrobial activity occurs at the inner membrane. The trend also suggested that the interactions between AMPs and the outer membrane lipid A (even in the absence of other components, such as lipopolysaccharides) capture effects that are relevant to the minimum inhibitory concentration.

Antimicrobial Peptide Cec4 Eradicates the Bacteria of Clinical Carbapenem-Resistant Acinetobacter baumannii Biofilm

The drug resistance rate of Acinetobacter baumannii increases year on year, and the drugs available for the treatment of carbapenem-resistant A. baumannii (CRAB) infection are extremely limited. A. baumannii, which forms biofilms, protects itself by secreting substrates such as exopolysaccharides, allowing it to survive under adverse conditions and increasing drug resistance. Antimicrobial peptides are small molecular peptides with broad-spectrum antibacterial activity and immunomodulatory function. Previous studies have shown that the antimicrobial peptide Cec4 has a strong effect on A. baumannii, but the antibacterial and biofilm inhibition of this antimicrobial peptide on clinical carbapenem resistance A. baumannii is not thoroughly understood. In this study, it was indicated that most of the 200 strains of CRAB were susceptible to Cec4 with a MIC of 4 μg/ml. Cec4 has a strong inhibitory and eradication effect on the CRAB biofilm; the minimum biofilm inhibition concentration (MBIC) was 64–128 μg/ml, and the minimum biofilm eradication concentration (MBEC) was 256–512 μg/ml. It was observed that Cec4 disrupted the structure of the biofilm using scanning electron microscopy (SEM) and confocal laser scanning microscopy (CLSM). A comparative transcriptome analysis of the effects of the antimicrobial peptide Cec4 on CRAB biofilm, identified 185 differentially expressed genes, including membrane proteins, bacterial resistance genes, and pilus-related genes. The results show that multiple metabolic pathways, two-component regulation systems, quorum sensing, and antibiotic synthesis-related pathways in A. baumannii biofilms were affected after Cec4 treatment. In conclusion, Cec4 may represent a new choice for the prevention and treatment of clinical infections, and may also provide a theoretical basis for the development of antimicrobial peptide drugs.

Mechanisms Protecting Acinetobacter baumannii against Multiple Stresses Triggered by the Host Immune Response, Antibiotics and Outside-Host Environment

Acinetobacter baumannii is considered one of the most persistent pathogens responsible for nosocomial infections. Due to the emergence of multidrug resistant strains, as well as high morbidity and mortality caused by this pathogen, A. baumannii was placed on the World Health Organization (WHO) drug-resistant bacteria and antimicrobial resistance research priority list. This review summarizes current studies on mechanisms that protect A. baumannii against multiple stresses caused by the host immune response, outside host environment, and antibiotic treatment. We particularly focus on the ability of A. baumannii to survive long-term desiccation on abiotic surfaces and the population heterogeneity in A. baumannii biofilms. Insight into these protective mechanisms may provide clues for the development of new strategies to fight multidrug resistant strains of A. baumannii.

The Addition of a Synthetic LPS-Targeting Domain Improves Serum Stability While Maintaining Antimicrobial, Antibiofilm, and Cell Stimulating Properties of an Antimicrobial Peptide

Multi-drug resistant (MDR) bacteria and their biofilms are a concern in veterinary and human medicine. Protegrin-1 (PG-1), a potent antimicrobial peptide (AMP) with antimicrobial and immunomodulatory properties, is considered a potential alternative for conventional antibiotics. AMPs are less stable and lose activity in the presence of physiological fluids, such as serum. To improve stability of PG-1, a hybrid peptide, SynPG-1, was designed. The antimicrobial and antibiofilm properties of PG-1 and the PG-1 hybrid against MDR pathogens was analyzed, and activity after incubation with physiological fluids was compared. The effects of these peptides on the IPEC-J2 cell line was also investigated. While PG-1 maintained some activity in 25% serum for 2 h, SynPG-1 was able to retain activity in the same condition for up to 24 h, representing a 12-fold increase in stability. Both peptides had some antibiofilm activity against Escherichia coli and Salmonella typhimurium. While both peptides prevented biofilm formation of methicillin-resistant Staphylococcus aureus (MRSA), neither could destroy MRSA's pre-formed biofilms. Both peptides maintained activity after incubation with trypsin and porcine gastric fluid, but not intestinal fluid, and stimulated IPEC-J2 cell migration. These findings suggest that SynPG-1 has much better serum stability while maintaining the same antimicrobial potency as PG-1.

Cathelicidin-related Antimicrobial Peptide Contributes to Host Immune Responses Against Pulmonary Infection with Acinetobacter baumannii in Mice

Acinetobacter baumannii is known for its multidrug antibiotic resistance. New approaches to treating drug-resistant bacterial infections are urgently required. Cathelicidin-related antimicrobial peptide (CRAMP) is a murine antimicrobial peptide that exerts diverse immune functions, including both direct bacterial cell killing and immunomodulatory effects. In this study, we sought to identify the role of CRAMP in the host immune response to multidrug-resistant Acinetobacter baumannii. Wild-type (WT) and CRAMP knockout mice were infected intranasally with the bacteria. CRAMP^−/− mice exhibited increased bacterial colony-forming units (CFUs) in bronchoalveolar lavage (BAL) fluid after A. baumannii infection compared to WT mice. The loss of CRAMP expression resulted in a significant decrease in the recruitment of immune cells, primarily neutrophils. The levels of IL-6 and CXCL1 were lower, whereas the levels of IL-10 were significantly higher in the BAL fluid of CRAMP^−/− mice compared to WT mice 1 day after infection. In an in vitro assay using thioglycollate-induced peritoneal neutrophils, the ability of bacterial phagocytosis and killing was impaired in CRAMP^−/− neutrophils compared to the WT cells. CRAMP was also essential for the production of cytokines and chemokines in response to A. baumannii in neutrophils. In addition, the A. baumannii-induced inhibitor of κB-α degradation and phosphorylation of p38 MAPK were impaired in CRAMP^−/− neutrophils, whereas ERK and JNK phosphorylation was upregulated. Our results indicate that CRAMP plays an important role in the host defense against pulmonary infection with A. baumannii by promoting the antibacterial activity of neutrophils and regulating the innate immune responses.

Antimicrobial peptides as a promising treatment option against Acinetobacter baumannii infections

BACKGROUND

With the increasing rate of antibiotic resistance in Acinetobacter, the World Health Organization introduced the carbapenem-resistant isolates in the priority pathogens list for which innovative new treatments are urgently needed. Antimicrobial peptides (AMPs) are one of the antimicrobial agents with high potential to produce new anti-Acinetobacter drugs. This review aims to summarize recent advances and compare AMPs with anti-Acinetobacter baumannii activity.

METHODS

Active AMPs against Acinetobacter were considered, and essential features, including structure, mechanism of action, anti-A. baumannii potent, and other prominent characteristics, were investigated and compared to each other. In this regard, the Google Scholar search engine and databases of PubMed, Scopus, and Web of Science were used.

RESULTS

Forty-six anti-Acinetobacter peptides were identified and classified into ten groups: Cathelicidins, Defensins, Frog AMPs, Melittin, Cecropins, Mastoparan, Histatins, Dermcidins, Tachyplesins, and computationally designed AMPs. According to the Minimum Inhibitory Concentration (MIC) reports, six peptides of Melittin, Histatin-8, Omega76, AM-CATH36, Hymenochirin, and Mastoparan have the highest anti-A. baumannii power against sensitive and antibiotic-resistant isolates. All anti-Acinetobacter peptides except Dermcidin have a net positive charge. Most of these peptides have alpha-helical structure; however, β-sheet and other structures have been observed among them. The mechanism of action of these antimicrobial agents is divided into two categories of membrane-based and intracellular target-based attack.

CONCLUSION

Evidence from this review indicates that AMPs would be likely among the main anti-A. baumannii drugs in the post-antibiotic era. Also, the application of computer science to increase anti-A. baumannii activity and reduce toxicity could be helpful.

Backbone Cyclization and Dimerization of LL-37-Derived Peptides Enhance Antimicrobial Activity and Proteolytic Stability

Can antimicrobial activity and peptide stability of alpha-helical peptides be increased by making them into dimers and macrocycles? Here, we explore that concept by using KR-12 as the starting point for peptide engineering. KR-12 has previously been determined as the minimalized antimicrobial fragment of the human host defense peptide LL-37. Backbone-cyclized KR-12 dimers, tethered by linkers of two to four amino acid residues, were synthesized and their antimicrobial activity, proteolytic stability and structures characterized. A modified KR-12 sequence, with substitutions at previously identified key residues, were also included in the screening panel. The backbone cyclized KR-12 dimers showed improved antimicrobial activity and increased stability compared to monomeric KR-12. The most active cyclic dimer displayed 16-fold higher antibacterial activity compared to KR-12 against Pseudomonas aeruginosa and Staphylococcus aureus, and 8-fold increased fungicidal activity against Candida albicans. It also showed increased hemolytic and cytotoxic activity. Enhanced antimicrobial activity coincided with increased membrane permeabilization of liposomes with one distinct discrepancy: monomeric KR-12 was much less disruptive of liposomes with bacterial lipid composition compared to liposomes from fungal lipid extract. Circular dichroism showed that the four-residue linked most active cyclic dimer had 65% helical content when bound to lyso-phosphatidylglycerol micelles, indicating that the helical propensity of the parent peptide is maintained in the new macrocyclic form. In conclusion, the current work on KR-12 suggests that dimerization together with backbone cyclization is an effective strategy for improving both potency and stability of linear antimicrobial peptides.

The Ultrashort Peptide OW: A New Antibiotic Adjuvant

BACKGROUND

The over use of current antibiotics and low discovery rate of the new ones are leading to rapid development of multidrug-resistant pathogens worldwide. Antimicrobial peptides have shown promising results against multidrug-resistant bacteria.

OBJECTIVE

To investigate the antimicrobial activity of a new ultrashort hexapeptide (OW).

METHODS

The OW hexapeptide was designed and tested against different strains of bacteria with different levels of sensitivity. Bacterial susceptibility assays were performed according to the guidelines of the Clinical and Laboratory Institute (CLSI). The synergistic studies were then conducted using the Checkerboard assay. This was followed by checking the hemolytic effect of the hexapeptide against human blood cells and Human Embryonic Kidney cell line (HEK293). Finally, the antibiofilm activities of the hexapeptide were studied using the Biofilm Calgary method.

RESULTS

Synergistic assays showed that OW has synergistic effects with antibiotics of different mechanisms of action. It showed an outstanding synergism with Rifampicin against methicillin resistant Staphylococcus aureus; ΣFIC value was 0.37, and the MIC value of Rifampicin was decreased by 85%. OW peptide also displayed an excellent synergism with Ampicillin against multidrug-resistant Pseudomonas aeruginosa, with ΣFIC value of less than 0.38 and a reduction of more than 96% in the MIC value of Ampicillin.

CONCLUSION

This study introduced a new ultrashort peptide (OW) with promising antimicrobial potential in the management of drug-resistant infectious diseases as a single agent or in combination with commonly used antibiotics. Further studies are needed to investigate the exact mechanism of action of these peptides.

KR‑12‑a6 promotes the osteogenic differentiation of human bone marrow mesenchymal stem cells via BMP/SMAD signaling

Considering the increased resistance to antibiotics in the clinic and the ideal antibacterial properties of KR‑12, the effects of KR‑12‑a6, an important analogue of KR‑12, on the osteogenic differentiation of human bone marrow mesenchymal stem cells (hBMSCs) were investigated. Osteogenic differentiation‑associated experiments were conducted in hBMSCs, and KR‑12‑a6 was used as an additional stimulating factor during osteogenic induction. Quantitative analysis of alkaline phosphatase (ALP) and alizarin red staining, and reverse transcription‑quantitative PCR analysis of the expression of osteogenesis‑associated genes were performed to determine the effects of KR‑12‑a6 on the osteogenic differentiation of hBMSCs. LDN‑212854 was selected to selectively suppress BMP/SMAD signaling. Western blotting was performed to investigate the underlying mechanisms. The intensity of ALP and alizarin red staining gradually increased with increasing KR‑12‑a6 concentrations. KR‑12‑a6 induced the strongest staining at 40 µg/ml, whereas 60 µg/ml and 80 µg/ml concentrations did not further increase the intensity of staining. The mRNA expression levels of RUNX2 and ALP increased in a dose‑dependent manner as early as 3 days post‑KR‑12‑a6 treatment. The mRNA expression of COL1A1, BSP and BMP2 exhibited significant upregulation from day 7 post‑KR‑12‑a6 treatment. In contrast, the mRNA levels of OSX, OCN and OPN were enhanced dramatically at day 14 following KR‑12‑a6 stimulation. Additionally, KR‑12‑a6 significantly promoted the phosphorylation of Smad1/5. Furthermore, LDN‑212854 suppressed the activation of Smad1/5 and inhibited the upregulation of several osteogenic differentiation‑associated genes in KR‑12‑a6‑treated hBMSCs. KR‑12‑a6 promoted the osteogenic differentiation of hBMSCs via BMP/SMAD signaling.

Modulation of antimicrobial potency of human cathelicidin peptides against the ESKAPE pathogens and in vivo efficacy in a murine catheter-associated biofilm model

Antimicrobial peptides are essential components of innate immune systems that protect hosts from infection. They are also useful candidates for developing a new generation of antibiotics to fight antibiotic-resistant pathogens. Human innate immune peptide LL-37 can inhibit biofilm formation, but suffers from high cost due to a long peptide length and rapid protease degradation. To improve the peptide, we previously identified the major active region and changed the peptide backbone structure. This study designed two families of new peptides by altering peptide side chains. Interestingly, these peptides displayed differential potency against various ESKAPE pathogens in vitro and substantially reduced hemolysis. Further potency test in vivo revealed that 17tF-W eliminated the burden of methicillin-resistant Staphylococcus aureus (MRSA) USA300 in both mouse-embedded catheters and their surrounding tissues. In addition, peptide treatment suppressed the level of chemokine TNFα, and boosted the levels of chemokines MCP-1, IL-17A and IL-10 in the surrounding tissues of the infected catheter embedded in mice. In conclusion, we have designed a set of new LL-37 peptides with varying antimicrobial activities, opening the door to potential topical treatment of infections involving different drug-resistant pathogens.

Antimicrobial Activity of Selected Antimicrobial Peptides Against Planktonic Culture and Biofilm of Acinetobacter baumannii

Acinetobacter baumannii is one of the most challenging pathogens, on account of its predisposition to develop resistance leading to severe, difficult-to-treat infections. As these bacteria are more usually isolated from nosocomial infections, the new therapeutic options are demanded. Antimicrobial peptides (AMPs) are compounds likely to find application in the treatment of A. baumannii. These compounds exhibit a wide spectrum of antimicrobial activity and were found to be effective against biofilm. In this study, eight AMPs, namely aurein 1.2, CAMEL, citropin 1.1., LL-37, omiganan, r-omiganan, pexiganan, and temporin A, were tested for their antimicrobial activity. A reference strain of A. baumannii ATCC 19606 was used. Antimicrobial assays included determination of the minimum inhibitory concentration and the minimum biofilm eradication concentration. Considering the fact that the majority of A. baumannii infections are associated with mechanical ventilation and the use of indwelling devices, the activity against biofilm was assessed on both a polystyrene surface and tracheal tube fragments. In addition, cytotoxicity (HaCaT) was determined and in vitro selectivity index was calculated.

The evaluation of the synergistic antimicrobial and antibiofilm activity of AamAP1-Lysine with conventional antibiotics against representative resistant strains of both Gram-positive and Gram-negative bacteria

Background and purpose: Antimicrobial resistance toward antibiotics is reaching historical unprecedented levels. There is an urgent and imminent need to develop novel antimicrobial alternatives. Antimicrobial peptides could prove to be a successful group of antimicrobials for drug development. Recently, we have designed a novel synthetic peptide named AamAP1-Lysine. The peptide displayed potent wide-spectrum antimicrobial activities against Gram-positive and Gram-negative bacteria. The purpose of this study is to evaluate the antimicrobial effect of combining AamAP1-Lysine with five different conventional antibiotics each representing a distinct mechanism of action in order to explore the possibility of producing a synergistic mode of action against a resistant strain of Gram-positive and a resistant strain of Gram-negative bacteria.

Methodology: The antimicrobial activity of AamAP1-Lysine in combination with five different antibiotics were evaluated for their antimicrobial activity employing standard antimicrobial assays, the synergistic activity of the peptide-antibiotic combinations were evaluated using checkerboard technique in addition to real-time time-kill assays. For the antibiofilm studies, the MBEC values were determined by employing the Calgary device.

Results: The combination strategy displayed potent synergistic activities against planktonic bacteria in a significant number of peptide-antibiotic combinations. The synergistic activity managed to reduce the effective minimum inhibitory concentration (MIC) concentrations dramatically with some combinations exhibiting a 64-fold decrease in the effective MIC of AamAP1-Lysine individually. Additionally, the combined synergistic activities of the peptide antibiotics were evaluated, and our results have identified two peptide antibiotic combinations with potent synergistic activities against biofilm growing strains of resistant bacteria.

Conclusion: Our results clearly indicate that peptide-antibiotic combinations could prove to be a very effective strategy in combatting multidrug-resistant bacteria and biofilm caused infections.

Design and characterization of a new hybrid peptide from LL-37 and BMAP-27

Background and purpose: The world is heading to a post-antibiotic era where the treatment of bacterial infections will not be possible even with well-known last-line antibiotics. Unfortunately, the emergence of multidrug resistant bacterial strains is uncontrollable, and the humanity will face a life-threatening fate unless new antimicrobial agents with new bacterial target sites are promptly developed. Herein, we design a hybrid antimicrobial peptide (B1) from helical parts taken from the parent peptides: LL-37 and BMAP-27. The purpose of this design is to improve the potency and enhance the toxicity profile of the parent peptides. Methods: Rational design was used to hybridize two antimicrobial peptides, in which two helical parts from the bovine analog BMAP-27, and the human cathelicidin LL-37 were used to generate a novel peptide (B1). The physicochemical properties were checked using in silico methods. The antimicrobial activities were tested against nine control and resistant strains of Gram-positive and Gram-negative bacteria. On the other hand, the antibiofilm activities were tested against four resistant strains. The cytotoxicity on mammalian cells was tested using HEK293, and the hemolysis activity was also investigated on human blood. Finally, synergistic studies were performed with four conventional antibiotics against four resistant strains of Gram-positive and Gram-negative bacteria. Results: The new peptide B1 exhibited broad-spectrum activities against all tested strains. The concentration against planktonic cells ranged between 10 and 20 µM. However, 40-60 µM were needed to eradicate the biofilms. B1 showed reduced toxicity toward mammalian cells with minimal hemolysis risk. On the other hand, the synergistic studies showed improved activities for the combined conventional antibiotics with a huge reduction in their minimum inhibitory concentration values. The concentrations of B1 peptide combined with the tested antibiotics were also decreased markedly down to 0.5 µM in some cases. Conclusion: B1 is a hybrid peptide from two cathelicidin peptides. It showed an improved activity compared to parent peptides. The hybridization was successful in this study. It generated a new potent broad-spectrum antimicrobial. The toxicity profile was improved, and the synergism with the convention antibiotics showed promising results.

Innate Immune Responses to Acinetobacter baumannii in the Airway

Acinetobacter baumannii is an emerging opportunistic pathogen that has risen to become a serious global threat, prevalent in health care settings and the community, which results in high morbidity and mortality rates. Its alarming expansion of antibiotic resistance is one of the most problematic traits of A. baumannii and as so, this bacterium has been classified as a serious threat and high priority target by the CDC. The most common types of infections induced by this pathogen include pneumonia (both hospital and community acquired), bacteremia, skin and soft tissue, urinary tract infections, endocarditis, and meningitis. Nosocomial pneumonia is the most prevalent of these. This review summarizes the current state of the signaling and innate immune components activated in response to A. baumannii infection in the airway.