Activity of an antimicrobial peptide mimetic against planktonic and biofilm cultures of oral pathogens

Integration of Lysin into Chitosan Nanoparticles for Improving Bacterial Biofilm Inhibition

Bacterial biofilms (BBFs) exhibit high drug resistance, antiphagocytosis, and extremely strong adhesion, and therefore can cause various diseases. They are also one of the important causes of bacterial infections. Thus, the effective removal of BBFs has attracted considerable research interest. Endolysins, which are efficient antibacterial bioactive macromolecules, have recently been receiving increasing attention. In this study, we overcame the deficiencies of endolysins via immobilization on chitosan nanoparticles (CS-NPs) by preparing LysST-3-CS-NPs using the ionic cross-linking reaction between CS-NPs and LysST-3, an endolysin purified using phage ST-3 expression. The obtained LysST-3-CS-NPs were verified and thoroughly characterized, their antimicrobial activity was investigated using microscopy, and their antibacterial efficacy on polystyrene surfaces was studied. The results obtained suggested that LysST-3-CS-NPs exhibit enhanced bactericidal properties and increased stability and can serve as reliable biocontrol agents for the prevention and treatment of Salmonella biofilm infections.

Effects of synthetic peptide RP557 and its origin, LL-37, on carbapenem-resistant Pseudomonas aeruginosa

Pseudomonas aeruginosa is a common bacterium in nosocomial infection. The biofilm-forming ability and antimicrobial resistance make P. aeruginosa biofilm infection refractory to patients requiring hospitalization, especially patients in the intensive care unit. Therefore, many alternative compounds have been developed. A newly synthesized peptide, RP557, derived from human cathelicidin LL-37, was evaluated for its antimicrobial and antibiofilm effect toward carbapenem-resistant P. aeruginosa (CRPA). The results showed that regardless of the resistance to carbapenems, the minimal inhibition concentrations of RP557 and LL-37 against P. aeruginosa were 32 µg/mL and 256 µg/mL, respectively. Both RP557 and LL-37 significantly reduced the P. aeruginosa biofilm mass at subMICs, while subMICs of carbapenems induced biofilm formation. RP557 could also remove approximately 50% of the mature biofilm at a concentration of 64 µg/mL, while 256 µg/mL LL-37 was needed to remove it. A quarter MIC of RP557 and LL-37 was used together with carbapenems (ertapenem, imipenem, and meropenem). The results show that both RP-557 and LL-37 might increase the susceptibility to CRPA by 4-16 times. Significant gene expression level changes were observed in RP557- or LL-37-treated CRPA. Confocal images showed that biofilm structures and biofilm cell viability were significantly reduced in the LL-37- or RP557-treated groups. Therefore, RP557 and its structural origin, LL-37, could be potential treatments for carbapenem-resistant P. aeruginosa infection, especially for chronic biofilm infection. IMPORTANCE Pseudomonas aeruginosa is one of the major pathogens of nosocomial infection. Combined its biofilm-forming ability with carbapenem-resistance, it is hard to handle P. aeruginosa infection, especially for patients requiring hospitalization. Antimicrobial peptide is a type of potential compound for bacterial infection treatment. Among these, RP557 was found effective in inhibiting biofilm previously. By assessing its effect on both carbapenem-resistant P. aeruginosa planktonic cells and biofilm, our results offered a potential treatment for carbapenem-resistant P. aeruginosa infection. It could be helpful to treat severe nosocomial infection related to carbapenem-resistant bacteria and increase the patients' survival rate.

The efficacy of biosynthesized silver nanoparticles against Pseudomonas aeruginosa isolates from cystic fibrosis patients

The high antibiotic resistance of Pseudomonas aeruginosa (PA) makes it critical to develop alternative antimicrobial agents that are effective and affordable. One of the many applications of silver nanoparticles (Ag NPs) is their use as an antimicrobial agent against bacteria resistant to common antibiotics. The key purpose of this research was to assess the antibacterial and antibiofilm effectiveness of biosynthesized Ag NPs against six biofilm-forming clinically isolated strains of PA and one reference strain (ATCC 27853). Ag NPs were biosynthesized using a seed extract of Peganum harmala as a reducing agent. Ag NPs were characterized by Ultraviolet-visible (UV-Vis) spectroscopy and scanning transmission electron microscopy (STEM). The effect of Ag NPs on biofilm formation and eradication was examined through micro-titer plate assays, and the minimal inhibitory (MIC) and minimum bactericidal (MBC) concentrations determined. In addition, real-time polymerase chain reactions (RT-PCR) were performed to examine the effects of Ag NPs on the expression of seven PA biofilm-encoding genes (LasR, LasI, LssB, rhIR, rhII, pqsA and pqsR). The biosynthesized Ag NPs were spherically-shaped with a mean diameter of 11 nm. The MIC for each PA strain was 15.6 µg/ml, while the MBC was 31.25 µg/ml. All PA strains exposed to Ag NPs at sub-inhibitory concentrations (0.22-7.5 µg/ml) showed significant inhibitory effects on growth and biofilm formation. Biomass and biofilm metabolism were reduced dependent on Ag NP concentration. The expression of the quorum-sensing genes of all strains were significantly reduced at an Ag NP concentration of 7.5 µg/ml. The results demonstrate the extensive in-vitro antibacterial and antibiofilm performance of Ag NPs and their potential in the treatment of PA infection. It is recommended that future studies examine the possible synergy between Ag NPs and antibiotics.

Down-regulation of β-lactam antibiotics resistance and biofilm formation by Staphylococcus epidermidis is associated with isookanin

Introduction

Biofilm formation is the major pathogenicity of Staphylococcus epidermidis (S. epidermidis), which enhances bacterial resistance to antibiotics. Isookanin has potential inhibitory activity on biofilm.

Method

The inhibiting mechanisms of isookanin against biofilm formation through surface hydrophobicity assay, exopolysaccharides, eDNA, gene expression analysis, microscopic visualization, and molecular docking were explored. Additionally, the combination of isookanin and β-lactam antibiotics were evaluated by the broth micro-checkerboard assay.

Results

The results showed that isookanin could decrease the biofilm formation of S. epidermidis by ≥85% at 250 μg/mL. The exopolysaccharides, eDNA and surface hydrophobicity were reduced after treatment with isookanin. Microscopic visualization analysis showed that there were fewer bacteria on the surface of the microscopic coverslip and the bacterial cell membrane was damaged after treatment with isookanin. The down-regulation of icaB and up-regulation of icaR were observed after treatment with isookanin. Additionally, the RNAIII gene was significantly up-regulated (p < 0.0001) at the mRNA level. Molecular docking showed that isookanin could bind to biofilm-related proteins. This indicated that isookanin can affect biofilm formation at the initial attachment phase and the aggregation phase. The FICI index showed that the combination of isookanin and β-lactam antibiotics were synergistic and could reduce doses of antibiotics by inhibiting biofilm formation.

Discussion

This study improved the antibiotic susceptibility of S. epidermidis through inhibition of the biofilm formation, and provided a guidance for the treatment of antibiotic resistance caused by biofilm.

The impact of biosynthesized ZnO nanoparticles from Olea europaea (Common Olive) on Pseudomonas aeruginosa growth and biofilm formation

There is a limitation in the range of effectual antibiotics due to the Pseudomonas aeruginosa (PA) infection due to its innate antimicrobial resistance. Researchers have therefore been concentrating their efforts to discover advanced and cost effective antibacterial agents among the ever-increasing PA bacterial resistance strains. It has been discovered that various nanoparticles can be employed as antimicrobial agents. Here, we evaluated the antibacterial properties of the Zinc Oxide nanoparticles (ZnO NPs), which was biosynthesized, being examined on six hospital strains of PA alongside a reference strain (ATCC 27853). A chemical approach was applied to biosynthesize the ZnO NPs from Olea europaea was performed, and confirmed by using X-ray diffraction and Scanning Electron Microscopes. The nanoparticles then applied their antibacterial properties to examine them against six clinically isolated PA strains alongside the reference strain. This process tested for the results of the minimum inhibitory concentration (MIC) and the minimum bactericidal concentration (MBC). The Growth, biofilm formation and eradication were analyzed. The influence of the differentiating degrees ZnO NPs in regard to Quorom sensing gene expression were further examined. The ZnO NPs exhibited a crystalline size and diameter (Dc) of 40–60 nm and both the MIC and MBC tests revealed positive outcomes of concentrations of 3 and 6 mg/ml for each PA strain, respectively. At sub inhibitory concentration, The ZnO NPs were found to significantly inhibit the growth and biofilm formation of all PA strains and decreases in the biomass and metabolic behavior of PA established biofilms; these decreases varied depending on the dosage. At ZnO NPs concentrations of 900 µg/ml, the expression of majority of quorum sensing genes of all strains were significantly reduced, at ZnO NPs concentrations of 300 µg/ml, few genes were significantly impacted. In conclusion, the treatment of PA and could be other antibiotic resistant bacteria can therefore be approached by using ZnO NPs as it has been uncovered that they withhold advanced antibacterial properties.

Peptide Designs for Use in Caries Management: A Systematic Review

The objective of this study was to review the design methods that have been used to create peptides for use in caries management. Two independent researchers systematically reviewed many in vitro studies in which peptides were designed for use in caries management. They assessed the risk of bias in the included studies. This review identified 3592 publications, of which 62 were selected. Forty-seven studies reported 57 antimicrobial peptides. Among them, 31 studies (66%, 31/47) used the template-based design method; 9 studies (19%, 9/47) used the conjugation method; and 7 studies (15%, 7/47) used other methods, such as the synthetic combinatorial technology method, the de novo design method and cyclisation. Ten studies reported mineralising peptides. Seven of these (70%, 7/10) used the template-based design method, two (20%, 2/10) used the de novo design method, and one study (10%, 1/10) used the conjugation method. In addition, five studies developed their own peptides with antimicrobial and mineralising properties. These studies used the conjugation method. Our assessment for the risk of bias in the 62 reviewed studies showed that 44 publications (71%, 44/62) had a medium risk and that 3 publications had a low risk (5%, 3/62). The two most common methods for developing peptides for use in caries management that were used in these studies were the template-based design method and the conjugation method.

Assessment of New Strategies to Improve the Performance of Antimicrobial Peptides

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

Bioselectivity of silk protein-based materials and their bio-inspired applications

Adhesion to material surfaces is crucial for almost all organisms regarding subsequent biological responses. Mammalian cell attachment to a surrounding biological matrix is essential for maintaining their survival and function concerning tissue formation. Conversely, the adhesion and presence of microbes interferes with important multicellular processes of tissue development. Therefore, tailoring bioselective, biologically active, and multifunctional materials for biomedical applications is a modern focus of biomaterial research. Engineering biomaterials that stimulate and interact with cell receptors to support binding and subsequent physiological responses of multicellular systems attracted much interest in the last years. Further to this, the increasing threat of multidrug resistance of pathogens against antibiotics to human health urgently requires new material concepts for preventing microbial infestation and biofilm formation. Thus, materials exhibiting microbial repellence or antimicrobial behaviour to reduce inflammation, while selectively enhancing regeneration in host tissues are of utmost interest. In this context, protein-based materials are interesting candidates due to their natural origin, biological activity, and structural properties. Silk materials, in particular those made of spider silk proteins and their recombinant counterparts, are characterized by extraordinary properties including excellent biocompatibility, slow biodegradation, low immunogenicity, and non-toxicity, making them ideally suited for tissue engineering and biomedical applications. Furthermore, recombinant production technologies allow for application-specific modification to develop adjustable, bioactive materials. The present review focusses on biological processes and surface interactions involved in the bioselective adhesion of mammalian cells and repellence of microbes on protein-based material surfaces. In addition, it highlights the importance of materials made of recombinant spider silk proteins, focussing on the progress regarding bioselectivity.

Molecular Research on Oral Diseases and Related Biomaterials: A Journey from Oral Cell Models to Advanced Regenerative Perspectives

Oral diseases such as gingivitis, periodontitis, and oral cancer affect millions of people worldwide. Much research has been conducted to understand the pathogenetic mechanisms of these diseases and translate this knowledge into therapeutics. This review aims to take the reader on a journey from the initial molecular discoveries to complex regenerative issues in oral medicine. For this, a semi-systematic literature search was carried out in Medline and Web of Science databases to retrieve the primary literature describing oral cell models and biomaterial applications in oral regenerative medicine. First, an in vitro cell model of gingival keratinocytes is discussed, which illustrates patho- and physiologic principles in the context of oral epithelial homeostasis and carcinogenesis and represents a cellular tool to understand biomaterial-based approaches for periodontal tissue regeneration. Consequently, a layered gradient nonwoven (LGN) is described, which demonstrates that the key features of biomaterials serve as candidates for oral tissue regeneration. LGN supports proper tissue formation and obeys the important principles for molecular mechanotransduction. Furthermore, current biomaterial-based tissue regeneration trends, including polymer modifications, cell-based treatments, antimicrobial peptides and optogenetics, are introduced to represent the full spectrum of current approaches to oral disease mitigation and prevention. Altogether, this review is a foray through established and new concepts in oral regenerative medicine and illustrates the process of knowledge translation from basic molecular and cell biological research to future clinical applications.

RW-BP100-4D, a Promising Antimicrobial Candidate With Broad-Spectrum Bactericidal Activity

With the rapid emergence and dissemination of antimicrobial resistance (AMR) genes in bacteria from animal, animal-derived food and human clinic, it is of great significance to develop new approaches to combat the multidrug-resistant bacteria. This study presented a short linear antimicrobial peptide RW-BP100-4D, which was derived from RW-BP100 (RRLFRRILRWL-NH2) by transforming the N-terminal 4th amino acid from L- to D-enantiomer. This modification remarkably reduced the peptide cytotoxicity to mammalian cells, as indicated by hemolytic and cytotoxicity assays. Meanwhile, the antimicrobial activity of RW-BP100-4D was improved against a more variety of Gram-positive and Gram-negative bacteria (sensitive and resistant) as well as fungi. Also, RW-BP100-4D showed strong in vitro anti-biofilm activity in a concentration-dependent manner, including inhibition of the biofilm-formation and dispersion of the mature biofilms of Staphylococcus aureus. RW-BP100-4D could be efficiently uptaken by bovine mammary epithelial cells (MAC-T) cells to eliminate the intracellular S. aureus ATCC29213 and Salmonella enterica ATCC13076. Moreover, RW-BP100-4D was highly effective in food disinfection of multiple bacterial contamination (including S. aureus, Listeria monocytogenesis, Escherichia coli O157: H7, Campylobacter jejuni, S. enterica, and Shewanella putrefaction, 3.61 ± 0.063 log reduction) on chicken meat, and could kill 99.99% of the methicillin-resistant Staphylococcus aureus (MRSA) strain in the mouse skin infection model. In summary, RW-BP100-4D is a promising antimicrobial candidate for application on food disinfection and local infection treatment. However, the protease-sensitivity of RW-BP100-4D and toxic effect at higher doses reduced the therapeutic effect of the candidate peptide in vivo and should be improved in the future studies.

A supramolecular host for phosphatidylglycerol (PG) lipids with antibacterial activity

Lipids fulfill a variety of important physiological functions, such as energy storage, providing a hydrophobic barrier, and signal transduction. Despite this plethora of biological roles, lipids are rarely considered a potential target for medical applications. Here, we report a set of neutral small molecules that contain boronic acid and urea functionalities to selectively recognize the bacterial lipid phosphatidylglycerol (PG). The affinity and selectivity was determined using ¹H NMR titrations and a liposome-based Alizarin Red S assay. Minimum inhibitory concentrations (MIC) were determined to assess antibacterial activity. The most potent compounds display an association constant with PG in liposomes of at least 5 × 10³ M^-1, function as antibacterial agents against Gram-positive bacteria (MIC = 12.5-25 μM), and show little hemolytic activity. Mode of action studies suggest that the boronic acids bind to the headgroup of the PG lipids, which leads to a change in membrane fluidity and ultimately causes membrane depolarization and cell death.

Antimicrobial and Antibiofilm Properties of Bioceramic Materials in Endodontics

Microbes are prevalent in the root canals of necrotic teeth, and they are the cause of primary and post-treatment apical periodontitis. Bacteria can dwell within the infected root canal system as surface-adherent biofilm structures, which exhibit high resistance to antimicrobial agents. Bioceramic materials, with their biocompatible nature and excellent physico-chemical properties, have been widely used in dental applications, including endodontics. This review focuses on the application of bioceramic technology in endodontic disinfection and the antibiofilm effects of endodontic bioceramic materials. Different bioceramic materials have shown different levels of antibiofilm effects. New supplements have emerged to potentially enhance the antibiofilm properties of bioceramics aiming to achieve the goal of microbial elimination in the root canal system.

Approaches to inhibit biofilm formation applying natural and artificial silk-based materials

The discovery of penicillin started a new era of health care since it allowed the effective treatment of formerly deadly infections. As a drawback, its overuse led to a growing number of multi-drug resistant pathogens. Challenging this arising threat, material research focuses on the development of microbe-killing or microbe repellent agents implementing such functions directly into materials. Due to their biocompatibility, non-immunogenicity and mechanical strength, silk-based materials are attractive candidates for applications in the biomedical field. Furthermore, it has been observed that silks display high persistency in their natural environment giving reason to suspect that they might be attractive candidates to prevent microbial infestation. The current review describes the process of biofilm formation on medical devices and the most common strategies to prevent it, divided into effects of surface topography, material modification and integrated additives. In this context, recent state of the art developments in the field of natural and artificial silk-based materials with microbe-repellant or antimicrobial properties are addressed. These silk properties are controversially discussed and conclusions are drawn as to which parameters will be decisive for the successful design of new bio-functional materials based on the blueprint of silk proteins.

Chemical modifications to increase the therapeutic potential of antimicrobial peptides

The continued use of antibiotics has been accompanied by the rapid emergence and spread of antibiotic-resistant strains of bacteria. Antimicrobial peptides (AMPs), also known as host defense peptides, show multiple features as an ideal antimicrobial agent, including potent, rapid, and broad-spectrum antimicrobial activity, low promotion of antimicrobial resistance, potent anti-biofilm activity, and lethality against metabolically inactive microorganisms. However, several crucial drawbacks constrain the use of AMPs as clinical drugs, e.g., liability in vivo, toxicity when used systemically, and high production costs. Based on recent findings and our own experiences, here we summarize some chemical modifications and key design strategies to increase the therapeutic potential of AMPs, including 1) enhancing antimicrobial activities, 2) improving in vivo effectiveness, and 3) reduction in toxicity, which may facilitate the design and optimization of AMPs for the development of drug candidates. We also discuss the present challenges in the optimization of AMPs and future concerns about the resistance and cross-resistance to AMPs in the development of AMPs as therapeutic drugs.

The structure of nontypeable Haemophilus influenzae SapA in a closed conformation reveals a constricted ligand-binding cavity and a novel RNA binding motif

Nontypeable Haemophilus influenzae (NTHi) is a significant pathogen in respiratory disease and otitis media. Important for NTHi survival, colonization and persistence in vivo is the Sap (sensitivity to antimicrobial peptides) ABC transporter system. Current models propose a direct role for Sap in heme and antimicrobial peptide (AMP) transport. Here, the crystal structure of SapA, the periplasmic component of Sap, in a closed, ligand bound conformation, is presented. Phylogenetic and cavity volume analysis predicts that the small, hydrophobic SapA central ligand binding cavity is most likely occupied by a hydrophobic di- or tri- peptide. The cavity is of insufficient volume to accommodate heme or folded AMPs. Crystal structures of SapA have identified surface interactions with heme and dsRNA. Heme binds SapA weakly (Kd 282 μM) through a surface exposed histidine, while the dsRNA is coordinated via residues which constitute part of a conserved motif (estimated Kd 4.4 μM). The RNA affinity falls within the range observed for characterized RNA/protein complexes. Overall, we describe in molecular-detail the interactions of SapA with heme and dsRNA and propose a role for SapA in the transport of di- or tri-peptides.

Multifaceted MRGPRX2: New insight into the role of mast cells in health and disease

Cutaneous mast cells (MCs) express Mas-related G protein-coupled receptor-X2 (MRGPRX2; mouse ortholog MrgprB2), which is activated by an ever-increasing number of cationic ligands. Antimicrobial host defense peptides (HDPs) generated by keratinocytes contribute to host defense likely by 2 mechanisms, one involving direct killing of microbes and the other via MC activation through MRGPRX2. However, its inappropriate activation may cause pseudoallergy and likely contribute to the pathogenesis of rosacea, atopic dermatitis, allergic contact dermatitis, urticaria, and mastocytosis. Gain- and loss-of-function missense single nucleotide polymorphisms in MRGPRX2 have been identified. The ability of certain ligands to serve as balanced or G protein-biased agonists has been defined. Small-molecule HDP mimetics that display both direct antimicrobial activity and activate MCs via MRGPRX2 have been developed. In addition, antibodies and reagents that modulate MRGPRX2 expression and signaling have been generated. In this article, we provide a comprehensive update on MrgprB2 and MRGPRX2 biology. We propose that harnessing MRGPRX2's host defense function by small-molecule HDP mimetics may provide a novel approach for the treatment of antibiotic-resistant cutaneous infections. In contrast, MRGPRX2-specific antibodies and inhibitors could be used for the modulation of allergic and inflammatory diseases that are mediated via this receptor.

Effect of Dermaseptin S4 on C. albicans Growth and EAP1 and HWP1 Gene Expression

Increasing resistance and changes in the spectrum of Candida infections have generated considerable interest in the development of new antifungal molecules. The use of antimicrobial peptides (AMPs) appears to be a promising approach. Frog skin AMPs (such as dermaseptins) have shown antimicrobial activity against several pathogens. In this study, we aimed to test the antimicrobial efficacy of dermaseptin S4 (DS4) against C. albicans. We determined the minimal inhibitory concentration (MIC) of DS4, and investigated the effects of the DS4 at low concentrations on human primary gingival fibroblasts. Additionally, we evaluated the effect of DS4 on C. albicans growth, form changes, and biofilm formation, as well as the expression of certain virulent genes. Our data show that DS4 completely inhibits C. albicans growth at a concentration of 32 μg/mL referring to the MIC of DS4. It should be noted that even with low concentrations (below 16 μg/mL), DS4 still have significant growth reduction of C. albicans, but were not toxic to human gingival fibroblasts. DS4 inhibited the transition from yeast to hyphae, and decreased the biofilm formation by reducing the biofilm mass weight. Surface morphological changes in the yeast cell membrane were observed following exposure to DS4. The gene expression analyses revealed that DS4 significantly decreased the expression of EAP1 and HWP1 genes. Overall results suggest the potential use of DS4 as an antifungal therapy to prevent C. albicans pathogenesis.

Study of the ultrastructure of Enterococcus faecalis and Streptococcus mutans incubated with salivary antimicrobial peptides

OBJECTIVES

Enterococcus faecalis has been associated with root canal infections, while Streptococcus mutans has a central role in the etiology of dental caries. One of the main reasons of endodontic failure has been associated to the presence of E. faecalis and the formation of biofilms. S. mutans inhabits the oral cavity, specifically the dental plaque, which is a multispecies biofilm formed on the hard surfaces of the tooth. The biofilm formation is the main factor determining the pathogenicity of numerous bacteria. Natural antimicrobial peptides in the saliva protect against pathogenic bacteria and biofilms. The aim of this study was to assess the ultrastructural damage induced by salivary peptides in bacteria involved in biofilms has not been previously studied.

MATERIAL AND METHODS

Enterococcus faecalis and S. mutans incubated with cystatin C, chromogranin A, or histatin 5 were morphologically analyzed and counted. The ultrastructural damage was evaluated by transmission electron microscopy (TEM).

RESULTS

A decrease in bacterial numbers was observed after incubation with cystatin C, chromogranin A, or histatin 5, compared to the control group (P <  0.001). Ultrastructural damage in E. faecalis and S. mutans incubated with salivary peptides was found in the cell wall, plasma membrane with a decreased distance between the bilayers, a granular pattern in the cytoplasm, and pyknotic nucleoids.

CONCLUSIONS

This study demonstrated that salivary peptides exert antibacterial activity and induce morphological damage on E. faecalis and S. mutans. Knowledge on the ultrastructural damage inflicted by salivary antimicrobial peptides on two important bacteria causing dental caries and root canal infections could aid the design of new therapeutic approaches to facilitate the elimination of these bacteria.

Antimicrobial peptides for the prevention and treatment of dental caries: A concise review

The objective of this study was to perform a comprehensive review of the use of antimicrobial peptides for the prevention and treatment of dental caries. The study included publications in the English language that addressed the use of antimicrobial peptides in the prevention and treatment of caries. These publications were also searchable on PubMed, Web of Science, Embase, Scopus, the Collection of Anti-Microbial Peptides and the Antimicrobial Peptide Database. A total of 3,436 publications were identified, and 67 publications were included. Eight publications reported seven natural human antimicrobial peptides as bactericidal to Streptococcus mutans. Fifty-nine publications reported 43 synthetic antimicrobial peptides developed to mimic natural antimicrobial peptides, fusing peptides with functional sequences and implementing new designs. The 43 synthetic antimicrobial peptides were effective against Streptococcus mutans, and nine peptides specifically targeted Streptococcus mutans. Ten antimicrobial peptides had an affinity for hydroxyapatite to prevent bacterial adhesion. Six antimicrobial peptides were also antifungal. Four antimicrobial peptides promoted remineralisation or prevented the demineralisation of teeth by binding calcium to hydroxyapatite. In conclusion, this study identified 67 works in the literature that reported seven natural and 43 synthetic antimicrobial peptides for the prevention and treatment of caries. Most of the antimicrobial peptides were bactericidal, and some prevented bacterial adhesion. A few antimicrobial peptides displayed remineralising properties with hydroxyapatite.

Host Defense Peptides as Templates for Antifungal Drug Development

Current treatment for invasive fungal diseases is limited to three classes of antifungal drugs: azoles, polyenes, and echinocandins. The most recently introduced antifungal class, the echinocandins, was first approved nearly 30 years ago. The limited antifungal drug portfolio is rapidly losing its clinical utility due to the inexorable rise in the incidence of invasive fungal infections and the emergence of multidrug resistant (MDR) fungal pathogens. New antifungal therapeutic agents and novel approaches are desperately needed. Here, we detail attempts to exploit the antifungal and immunoregulatory properties of host defense peptides (HDPs) in the design and evaluation of new antifungal therapeutics and discuss historical limitations and recent advances in this quest.

Improved osteogenic activity and inhibited bacterial biofilm formation on andrographolide-loaded titania nanotubes

Background

Delivery of local drugs with a titania nanotube is an attractive approach to combat implant-related infection. Our earlier study has confirmed that nanotubes loaded with gentamicin could significantly improve the antibacterial ability. On this basis, the used andrographolide in this paper has a high antibacterial activity, which cannot only avoid the evolution of antibiotic-resistant bacteria but also has simultaneously excellent biocompatibility with osteogenic cells.

Methods

Two mg of andrographolide was loaded into titania nanotubes, which were fabricated into different diameters (50 and 100 nm) and 200 nm length by the method of lyophilization and vacuum drying. We chose a standard strain, Staphylococcus epidermidis (American Type Culture Collection 35984), and two clinical isolates, S. aureus 376 and S. epidermidis 389 to research the bacterial adhesion at 6, 12 and 24 hours and biofilm formation at 48, and 72 hours on the andrographolide-loaded nanotubes (NT-A) using the diffusion plate method. Smooth titanium (smooth Ti) and nanotubes with no drug loading (NT) were also inclusive and analyzed. Furthermore, the Sprague-Dawley (SD) rats mesenchymal stem cells were used to assess the influence of nanotubular topographies on the osteogenic differentiation of mesenchymal stem cells.

Results

Our results showed that NT-A could inhibit bacterial adhesion and biofilm formation on implant surfaces. NT-A and NT, especially those with 100 nm diameters, were found to significantly promoted cell attachment, proliferation, diffusion, and osteogenic differentiation when compared with smooth Ti, while the same diameter in NT-A and NT did not differ.

Conclusions

Titania nanotube modification and andrographolide loading can significantly improve the antibacterial ability and osteogenic activity of orthopedic implants. Nanotubes-based local delivery could be a promising strategy for combating implant-associated infection.

Cationic amphiphiles against Gardnerella vaginalis resistant strains and bacterial vaginosis-associated pathogens

Antibiotic resistance and infection recurrence are critical issues in treating bacterial vaginosis, the most common vaginal disorder in women of reproductive age. Novel alternatives to traditional antibiotics, such as peptidomimetics, have the potential to address this challenge. Previously, two series of cationic amphiphiles (CAms) were developed with both hydrophilic head groups and non-polar domains, giving them the ability to self-assemble into supramolecular nanostructures with membrane-lytic properties. Those CAms were shown to be effective against biofilms of Gardnerella vaginalis while preserving the commensal microbiota. Two new series of CAms were designed with varying levels of flexibility between the hydrophilic head groups and the hydrophobic domains. Activities against the vaginal pathogen G. vaginalis ranged from 1.3 to 18.5 µM, while the tested vaginal lactobacilli were significantly more tolerant of CAms, with minimal inhibitory concentration values as high as 208 µM. Minimal biofilm bactericidal concentrations of the tested CAms ranged from 21.47 to <388.3 µM, and were lowest against resistant forms of G. vaginalis, while Lactobacillus biofilms were tolerant of concentrations ≥687 µM. Safety aspects of the CAms were also investigated, and they were found to be safe for use against vaginal ectocervical tissue.

Roles of a Mast Cell-Specific Receptor MRGPRX2 in Host Defense and Inflammation

Mast cells are multifunctional immune cells that are found most abundantly at host-environment interfaces, such as the skin, respiratory tract, and oral/gastrointestinal mucosa. Not surprisingly, mast cells act as sentinel cells that sense microbial attacks and initiate a protective immune response and promote healing. Although mast cells share many features with other innate immune effector cells, such as neutrophils and macrophages, they uniquely interact closely with blood vessels and release an extensive set of mediators for the recruitment of innate and adaptive immune cells. A novel human G protein-coupled receptor (GPCR), known as Mas-related GPCR-X2 (MRGPRX2, mouse ortholog, MrgprB2), has recently been identified, which is expressed on mast cells but not neutrophils and macrophages. Interestingly, activation of MrgprB2 by bacteria-derived quorum-sensing peptides inhibits bacterial growth, prevents biofilm formation, and leads to the recruitment of neutrophils to effectively clear bacteria. Furthermore, host defense antimicrobial peptides and small-molecule peptide mimetics also activate mast cells via MRGPRX2/B2. MrgprB2-mediated activation of local mast cells also clears cutaneous bacterial infection, promotes healing, and protects against reinfection. In addition to their role in host defense, mast cells contribute to a number of chronic inflammatory diseases such as periodontitis, neurogenic inflammation, and inflammatory pain likely via the activation of MRGPRX2. In this review, we discuss the roles of MRGPRX2/B2 in the clearance of bacterial infection, wound healing, periodontal disease, neurogenic inflammation, and inflammatory pain. We propose that harnessing mast cells' host defense and immunomodulatory properties via the activation of MRGPRX2 may lead to novel approaches for the treatment of drug-resistant bacterial infections. On the other hand, increased MRGPRX2 expression on mast cells and their inappropriate activation may contribute to periodontitis, neurogenic inflammation, and inflammatory pain. Thus, targeting MRGPRX2 could provide novel approaches to modulate these conditions.

Microbial Degradation of Plastic in Aqueous Solutions Demonstrated by CO2 Evolution and Quantification

The environmental accumulation of plastics worldwide is a consequence of the durability of the material. Alternative polymers, marketed as biodegradable, present a potential solution to mitigate their ecological damage. However, understanding of biodegradability has been hindered by a lack of reproducible testing methods. We developed a novel method to evaluate the biodegradability of plastic samples based on the monitoring of bacterial respiration in aqueous media via the quantification of CO₂ produced, where the only carbon source available is from the polymer. Rhodococcus rhodochrous and Alcanivorax borkumensis were used as model organisms for soil and marine systems, respectively. Our results demonstrate that this approach is reproducible and can be used with a variety of plastics, allowing comparison of the relative biodegradability of the different materials. In the case of low-density polyethylene, the study demonstrated a clear correlation between the molecular weight of the sample and CO₂ released, taken as a measure of biodegradability.

Antiviral Activities of Human Host Defense Peptides

Peptides with broad-spectrum antimicrobial activity are found widely expressed throughout nature. As they participate in a number of different aspects of innate immunity in mammals, they have been termed Host Defense Peptides (HDPs). Due to their common structural features, including an amphipathic structure and cationic charge, they have been widely shown to interact with and disrupt microbial membranes. Thus, it is not surprising that human HDPs have activity against enveloped viruses as well as bacteria and fungi. However, these peptides also exhibit activity against a wide range of non-enveloped viruses as well, acting at a number of different steps in viral infection. This review focuses on the activity of human host defense peptides, including alpha- and beta-defensins and the sole human cathelicidin, LL-37, against both enveloped and non-enveloped viruses. The broad spectrum of antiviral activity of these peptides, both in vitro and in vivo suggest that they play an important role in the innate antiviral defense against viral infections. Furthermore, the literature suggests that they may be developed into antiviral therapeutic agents.

Bacterial Biofilm Eradication Agents: A Current Review

Most free-living bacteria can attach to surfaces and aggregate to grow into multicellular communities encased in extracellular polymeric substances called biofilms. Biofilms are recalcitrant to antibiotic therapy and a major cause of persistent and recurrent infections by clinically important pathogens worldwide (e.g., Pseudomonas aeruginosa, Escherichia coli, and Staphylococcus aureus). Currently, most biofilm remediation strategies involve the development of biofilm-inhibition agents, aimed at preventing the early stages of biofilm formation, or biofilm-dispersal agents, aimed at disrupting the biofilm cell community. While both strategies offer some clinical promise, neither represents a direct treatment and eradication strategy for established biofilms. Consequently, the discovery and development of biofilm eradication agents as comprehensive, stand-alone biofilm treatment options has become a fundamental area of research. Here we review our current understanding of biofilm antibiotic tolerance mechanisms and provide an overview of biofilm remediation strategies, focusing primarily on the most promising biofilm eradication agents and approaches. Many of these offer exciting prospects for the future of biofilm therapeutics for a large number of infections that are currently refractory to conventional antibiotics.

Peptide and non-peptide mimetics as potential therapeutics targeting oral bacteria and oral biofilms

The development of the oral biofilm requires a complex series of interactions between host tissues and the colonizing bacteria as well as numerous interspecies interactions between the organisms themselves. Disruption of normal host-microbe homoeostasis in the oral cavity can lead to a dysbiotic microbial community that contributes to caries or periodontal disease. A variety of approaches have been pursued to develop novel potential therapeutics that are active against the oral biofilm and/or target specific oral bacteria. The structure and function of naturally occurring antimicrobial peptides from oral tissues and secretions as well as external sources such as frog skin secretions have been exploited to develop numerous peptide mimetics and small molecule peptidomimetics that show improved antimicrobial activity, increased stability and other desirable characteristics relative to the parent peptides. In addition, a rational and minimalist approach has been developed to design small artificial peptides with amphipathic α-helical properties that exhibit potent antibacterial activity. Furthermore, with an increased understanding of the molecular mechanisms of beneficial and/or antagonistic interspecies interactions that contribute to the formation of the oral biofilm, new potential targets for therapeutic intervention have been identified and both peptide-based and small molecule mimetics have been developed that target these key components. Many of these mimetics have shown promising results in in vitro and pre-clinical testing and the initial clinical evaluation of several novel compounds has demonstrated their utility in humans.

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.

Small-Molecule Host-Defense Peptide Mimetic Antibacterial and Antifungal Agents Activate Human and Mouse Mast Cells via Mas-Related GPCRs

Host-defense peptides (HDPs) have an important therapeutic potential against microbial infections but their metabolic instability and cellular cytotoxicity have limited their utility. To overcome these limitations, we utilized five small-molecule, nonpeptide HDP mimetics (smHDPMs) and tested their effects on cytotoxicity, antimicrobial activity, and mast cell (MC) degranulation. None of the smHDPMs displayed cytotoxicity against mouse 3T3 fibroblasts or human transformed liver HepG2 cells. However, one compound had both antifungal and antibacterial activity. Surprisingly, all five compounds induced degranulation in a human MC line, LAD2, and this response was substantially reduced in Mas-related G protein-coupled receptor (GPCR)-X2 (MRGPRX2)-silenced cells. Furthermore, all five compounds induced degranulation in RBL-2H3 cells expressing MRGPRX2 but this response was abolished in cells expressing naturally occurring loss-of-function missense variants G165E (rs141744602) and D184H (rs372988289). Mrgprb2 is the likely mouse ortholog of human MRGPRX2, which is expressed in connective tissue MCs (CTMCs) such as cutaneous and peritoneal MCs (PMCs). All five smHDPMs induced degranulation in wild-type PMCs but not in cells derived from Mrgprb2⁻/⁻ mice. These findings suggest that smHDPMs could serve as novel targets for the treatment of drug-resistant fungal and bacterial infections because of their ability to harness CTMCs’ host defense functions.

Enhancing Morphology and Separation Performance of Polyamide 6,6 Membranes By Minimal Incorporation of Silver Decorated Graphene Oxide Nanoparticles

Nanomaterials can be incorporated in the synthesis of membrane to obtain mixed-matrix membrane with marked improvement in properties and performance. However, stability and dispersion of the nanomaterials in the membrane matrix, as well as the need to use high ratio of nanomaterials for obvious improvement of membrane properties, remain a major hurdle for commercialization. Hence, this study aims to investigate the improvement of polyamide 6,6 membrane properties with the incorporation of silver nanoparticles decorated on graphene oxide (Ag-GO) nanoplates and at the same time focus is given to the issues above. Graphene oxide nanoplates were synthesized using the modified Hummers’ method and decorated with silver before embedded into the polyamide 6,6 matrix. Physicochemical characterizations were conducted on both nanoplates and the mixed-matrix Ag-GO polyamide 6,6 membrane. The issues of Ag agglomeration and leaching were not observed, which could be attributed to the decoration of Ag on GO that helped to disperse the nanomaterials and provided a better anchor point for the attachment of Ag nanoparticles. The synthesized membrane showed marked improvement regarding flux (135% increment) and antifouling (40% lower irreversible fouling), which could be ascribed to the more negative charge of membrane surface (−14 ± 6 to −31 ± 3.8 mV) and hydrophilicity (46% enhancement) of the membranes. With minimal embedment of Ag nanoparticles, the membrane showed superior antibacterial property where the E. coli bacteria could not form a single colony on the membrane surface. Overall, the decoration of Ag on GO nanoplates could be a promising approach to resolve the agglomeration and leaching issues as well as reduce the amount of precious Ag in the synthesis of Ag-GO polyamide 6,6 membrane.

The Effects of Mentha × piperita Essential Oil on C. albicans Growth, Transition, Biofilm Formation, and the Expression of Secreted Aspartyl Proteinases Genes

The rise in resistance and changes in the spectrum of Candida infections have generated enormous interest in developing new antifungal drugs using natural molecules such as plant essential oils (EOs). Antimicrobial activity against foodborne pathogenic and spoilage microorganisms has been reported for EOs. The goal of this study was to assess the effect of Mentha × piperita essential oil (EO) on C. albicans growth, transition (change from blastospore to hyphae forms), and biofilm formation as well as on the expression of certain virulent genes. We show that whole EO and its vapor attenuated the yeast’s growth, compared to that in the control. The effect of the EO was comparable to that of amphotericin-B (AmB). The EO and its vapor significantly decreased the morphological changes of C. albicans, reduced biofilm formation, and disrupted mature C. albicans biofilms. The effect produced by whole EO on biofilm formation/disruption was notably comparable to that observed with AmB. Exposure of C. albicans to EO and its vapor downregulated the expression of various genes, such as secreted aspartyl proteinases (SAP 1, 2, 3, 9, 10) and hyphal wall protein 1 (HWP1). Altogether, these results provide new insight into the efficacy of Mentha × piperita EO against C. albicans and suggest the potential of Mentha × piperita EO for use as an antifungal therapy in multiple applications.

Anti-bacterial activity of mutant chensinin-1 peptide against multidrug-resistant Pseudomonas aeruginosa and its effects on biofilm-associated gene expression

Nosocomial infections with Pseudomonas aeruginosa (PA) are difficult to treat due to the low outer membrane permeability of the bacterium and the development of resistance. In the present study, the anti-microbial peptide (AMP) mutant chensinin-1 (MC1) was revealed to exhibit anti-bacterial activity against a multidrug-resistant PA (MRPA) strain in vitro, and the minimum inhibitory concentration was 25 µM, which was 4-fold higher than that of the native strain. MC1 was able to disrupt the integrity of the cytoplasmic membrane in the native PA strain and MRPA and had a similar membrane depolarization ability in these strains, but the outer membrane permeability of MRPA cells was lower than that of native PA cells, as determined by a 1-N-phenylnaphthylamine assay. In addition, the abundance of the gene Psl encoding for biofilm-associated polysaccharides was detected using Congo red, and a high concentration of MC1 inhibited the formation of MRPA biofilms. Furthermore, the expression levels of biofilm-associated genes affected by the AMP, MC1, were quantified by polymerase chain reaction analysis. The results indicated that MC1 induced biofilm inhibition by downregulating the relative expression of specific biofilm polysaccharide-associated genes, including pelA, algD and pslA. The present results indicated that the AMP MC1 may be an effective antibiotic against MRPA strains.

Development of an antibacterial surface with a self-defensive and pH-responsive function

In this work, a charge conversion mechanism was introduced to build a bacteria responsive antibacterial surface.

AMPs as Anti-biofilm Agents for Human Therapy and Prophylaxis

Microbial cells show a strong natural tendency to adhere to surfaces and to colonize them by forming complex communities called biofilms. In this growth mode, biofilm-forming cells encase themselves inside a dense matrix which efficiently protects them against antimicrobial agents and effectors of the immune system. Moreover, at the physiological level, biofilms contain a very heterogeneous cell population including metabolically inactive organisms and persisters, which are highly tolerant to antibiotics. The majority of human infectious diseases are caused by biofilm-forming microorganisms which are responsible for pathologies such as cystic fibrosis, infective endocarditis, pneumonia, wound infections, dental caries, infections of indwelling devices, etc. AMPs are well suited to combat biofilms because of their potent bactericidal activity of broad spectrum (including resting cells and persisters) and their ability to first penetrate and then to disorganize these structures. In addition, AMPs frequently synergize with antimicrobial compounds and were recently reported to repress the molecular pathways leading to biofilm formation. Finally, there is a very active research to develop AMP-containing coatings that can prevent biofilm formation by killing microbial cells on contact or by locally releasing their active principle. In this chapter we will describe these strategies and discuss the perspectives of the use of AMPs as anti-biofilm agents for human therapy and prophylaxis.

Effects of Antimicrobial Peptide GH12 on the Cariogenic Properties and Composition of a Cariogenic Multispecies Biofilm

Dental caries is a biofilm-mediated disease that occurs when acidogenic/aciduric bacteria obtain an ecological advantage over commensal species. In previous studies, the effects of the antimicrobial peptide GH12 on planktonic bacteria and monospecies biofilms were confirmed. The objectives of this study were to investigate the effects of GH12 on a cariogenic multispecies biofilm and to preliminarily explain the mechanism. In this biofilm model, Streptococcus mutans ATCC 70061 was the representative of cariogenic bacteria, while Streptococcus gordonii ATCC 35105 and Streptococcus sanguinis JCM 5708 were selected as healthy microbiota. The results showed that GH12 was more effective in suppressing S. mutans than the other two species, with lower MIC and minimal bactericidal concentration (MBC) values among diverse type strains and clinical isolated strains. Therefore, GH12, at no more than 8 mg/liter, was used to selectively suppress S. mutans in the multispecies biofilm. GH12 at 4 mg/liter and 8 mg/liter reduced the cariogenic properties of the multispecies biofilm in biofilm formation, glucan synthesis, and lactic acid production. In addition, GH12 suppressed S. mutans within the multispecies biofilm and changed the bacterial composition. Furthermore, 8 mg/liter GH12 showed a selective bactericidal impact on S. mutans, and GH12 promoted hydrogen peroxide production in S. sanguinis and S. gordonii, which improved their ecological advantages. In conclusion, GH12 inhibited the cariogenic properties and changed the composition of the multispecies biofilm through a two-part mechanism by which GH12 directly suppressed the growth of S. mutans as well as enhanced the ecological competitiveness of S. sanguinis and S. gordonii IMPORTANCE Dental caries is one of the most prevalent chronic infectious diseases worldwide, with substantial economic and quality-of-life impacts. Streptococcus mutans has been considered the principal pathogen of dental caries. To combat dental caries, an antimicrobial peptide, GH12, was designed, and its antibacterial effects on planktonic S. mutans and the monospecies biofilm were confirmed. As etiological concepts of dental caries evolved to include microecosystems, the homeostasis between pathogenic and commensal bacteria and a selective action on cariogenic virulence have increasingly become the focus. The novelty of this research was to study the effects of the antimicrobial peptides on a controlled cariogenic multispecies biofilm model. Notably, the role of an antimicrobial agent in regulating interspecific competition and composition shifts within this multispecies biofilm was investigated. With promising antibacterial and antibiofilm properties, the use of GH12 might be of importance in preventing and controlling caries and other dental infections.

Influence of Proline Substitution on the Bioactivity of Mammalian-Derived Antimicrobial Peptide NK-2

Multidrug-resistant bacteria are emerging as a global threat, making the search for alternative compounds urgent. Antimicrobial peptides (AMPs) became a promising hotspot due to their distinct action mechanism and possibility to be used as an alternative or complement to traditional antibiotics. However, gaining a better understanding about the relationship between antimicrobial peptides structure and its bioactivity is crucial for the development of next generation of antimicrobial agents. NK-2, derived from mammalian protein NK-lysin, has potent antitumor and bactericidal abilities. As proline was considered to be an effective α-helix breaker due to its restricted conformation, to better comprehend the effects of proline in the structure-activity relationship of NK-2, we constructed two NK-2 analogs. We examined the biological activities of NK-2 and its proline substitution analogs and analyzed the resulting conformational changes. Our results showed that introducing proline into the primary sequence of NK-2 significantly decreased the antitumor, antibacterial, and cytotoxic effects, as well as DNA binding activity by changing the α-helix content. However, α-helical content was not the only determining factor, the position of proline inserted was also critical. This study will allow for clearer insight into the role of proline in structure and bioactivity of NK-2 and provide a foundation for future studies.

LL-37 disrupts the Kaposi's sarcoma-associated herpesvirus envelope and inhibits infection in oral epithelial cells

Oral epithelial cells (OECs) represent the first line of defense against viruses that are spread via saliva, including Kaposi's sarcoma-associated herpesvirus (KSHV). Infection of humans by KSHV and viral pathogenesis begins by infecting OECs. One method OECs use to limit viral infections in the oral cavity is the production of antimicrobial peptides (AMPs), or host defense peptides (HDPs). However, no studies have investigated the antiviral activities of any HDP against KSHV. The goal of this study was to determine the antiviral activity of one HDP, LL-37, against KSHV in the context of infecting OECs. Our results show that LL-37 significantly decreased KSHV's ability to infect OECs in both a structure- and dose-dependent manner. However, this activity does not stem from affecting OECs, but instead the virions themselves. We found that LL-37 exerts its antiviral activity against KSHV by disrupting the viral envelope, which can inhibit viral entry into OECs. Our data suggest that LL-37 exhibits a marked antiviral activity against KSHV during infection of oral epithelial cells, which can play an important role in host defense against oral KSHV infection. Thus, we propose that inducing LL-37 expression endogenously in oral epithelial cells, or potentially introducing as a therapy, may help restrict oral KSHV infection and ultimately KSHV-associated diseases.

Antifungal Potential of Host Defense Peptide Mimetics in a Mouse Model of Disseminated Candidiasis

Invasive candidiasis caused by Candida albicans and non-albicansCandida (NAC) present a serious disease threat. Although the echinocandins are recommended as the first line of antifungal drug class, resistance to these agents is beginning to emerge, demonstrating the need for new antifungal agents. Host defense peptides (HDP) exhibit potent antifungal activity, but as drugs they are difficult to manufacture efficiently, and they are often inactivated by serum proteins. HDP mimetics are low molecular weight non-peptide compounds that can alleviate these problems and were shown to be membrane-active against C. albicans and NAC. Here, we expand upon our previous works to describe the in vitro and in vivo activity of 11 new HDP mimetics that are active against C. albicans and NAC that are both sensitive and resistant to standard antifungal drugs. These compounds exhibit minimum inhibitory/fungicidal concentration (MIC/MFC) in the µg/mL range in the presence of serum and are inhibited by divalent cations. Rapid propidium iodide influx into the yeast cells following in vitro exposure suggested that these HDP mimetics were also membrane active. The lead compounds were able to kill C. albicans in an invasive candidiasis CD-1 mouse model with some mimetic candidates decreasing kidney burden by 3–4 logs after 24 h in a dose-dependent manner. The data encouraged further development of this new anti-fungal drug class for invasive candidiasis.

Gemini Cationic Amphiphiles Control Biofilm Formation by Bacterial Vaginosis Pathogens

Antibiotic resistance and recurrence of bacterial vaginosis (BV), a polymicrobial infection, justify the need for novel antimicrobials to counteract microbial resistance to conventional antibiotics. Previously, two series of cationic amphiphiles (CAms) which self-assemble into supramolecular nanostructures with membrane-lytic properties were designed with hydrophilic head groups and nonpolar domains. The combination of CAms and commonly prescribed antibiotics is suggested as a promising strategy for targeting microorganisms that are resistant to conventional antibiotics. Activities of the CAms against Gardnerella vaginalis ATCC 14018, a representative BV pathogen, ranged from 1.1 to 24.4 μM. Interestingly, the tested healthy Lactobacillus species, especially Lactobacillus plantarum ATCC 39268, were significantly more tolerant of CAms than the selected pathogens. In addition, CAms prevented biofilm formation at concentrations which did not influence the normal growth ability of G. vaginalis ATCC 14018. Furthermore, the biofilm minimum bactericidal concentration (MBC-Bs) of CAms against G. vaginalis ATCC 14018 ranged from 58.8 to 425.6 μM, while much higher concentrations (≥850 μM) were required to produce ≥3-log reductions in the number of biofilm-associated lactobacilli. The conventional antibiotic metronidazole synergized strongly with all tested CAms against planktonic cells and biofilms of G. vaginalis ATCC 14018. The synergism between CAms and the tested conventional antibiotic may be considered a new, effective, and beneficial method of controlling biofilm-associated bacterial vaginosis.

Antibacterial and Antibiofilm Activities of a Novel Synthetic Cyclic Lipopeptide against Cariogenic Streptococcus mutans UA159

Despite continuous efforts to control cariogenic dental biofilms, very few effective antimicrobial treatments exist. In this study, we characterized the activity of the novel synthetic cyclic lipopeptide 4 (CLP-4), derived from fusaricidin, against the cariogenic pathogen Streptococcus mutans UA159. We determined CLP-4's MIC, minimum bactericidal concentration (MBC), and spontaneous resistance frequency, and we performed time-kill assays. Additionally, we assessed CLP-4's potential to inhibit biofilm formation and eradicate preformed biofilms. Our results demonstrate that CLP-4 has strong antibacterial activity in vitro and is a potent bactericidal agent with low spontaneous resistance frequency. At a low concentration of 5 μg/ml, CLP-4 completely inhibited S. mutans UA159 biofilm formation, and at 50 μg/ml, it reduced the viability of established biofilms by >99.99%. We also assessed CLP-4's cytotoxicity and stability against proteolytic digestion. CLP-4 withstood trypsin or chymotrypsin digestion even after treatment for 24 h, and our toxicity studies showed that CLP-4 effective concentrations had negligible effects on hemolysis and the viability of human oral fibroblasts. In summary, our findings showed that CLP-4 is a potent antibacterial and antibiofilm agent with remarkable stability and low nonspecific cytotoxicity. Hence, CLP-4 is a promising novel antimicrobial peptide with potential for clinical application in the prevention and treatment of dental caries.

Antibacterial activity of chensinin-1b, a peptide with a random coil conformation, against multiple-drug-resistant Pseudomonas aeruginosa

Nosocomial infections caused by Pseudomonas aeruginosa are difficult to treat due to the low permeability of its outer membrane as well as to its remarkable ability to acquire further resistance to antibiotics. Chensinin-1b exhibited antibacterial activity against the tested multiple-drug-resistant bacteria with a MIC ranging between 1.56 and 50μM, except E. cloacae strain 0320 (MREC0320), P. fluorescens strain 0322 (MRPF0322) and E. aerogenes strain 0320 (MREA0320). However, the MIC (25μM) of chensinin-1b to multiple-drug-resistant P. aeruginosa strain (MRPA 0108) was 16-fold higher than that observed to P. aeruginosa susceptible strain CGMCC 1.860 (PA1860). Chensinin-1b was able to disturb the integration of the cytoplasmic membrane of PA1860 and MRPA0108 cells similarly, but the outer membrane permeability of MRPA0108 cells was significantly lower. This low permeability was associated with increased expression of lipopolysaccharide (LPS) in the outer membrane and a decrease in negatively charged phospholipids in the outer membrane leaflet. In addition, the biofilm of MRPA0108 was responsible for the reduced susceptibility to chensinin-1b. A higher concentration of chensinin-1b (12.5µM) was required to maximally inhibit the formation of MRPA0108 biofilm. Notably, chensinin-1b inhibited the formation of MRPA0108 biofilm at concentrations below its MIC value by down-regulating the level of PelA, algD, and PslA gene transcription. Importantly, chensinin-1b had a significant antibacterial effect against MRPA0108 in vivo. Administration of chensinin-1b to mice infected with MRPA 0108 significantly increased survival by 50-70%. Moreover, chensinin-1b reduced the production of pro-inflammatory mediators and correspondingly reduced lung and liver tissue damage in the mouse model of septic shock induced by MRPA 0108. Collectively, these results suggest that chensinin-1b could be an effective antibiotic against multiple-drug-resistant bacterial strains.

Potent in vitro and in vivo antifungal activity of a small molecule host defense peptide mimic through a membrane-active mechanism

Lethal systemic fungal infections of Candida species are increasingly common, especially in immune compromised patients. By in vitro screening of small molecule mimics of naturally occurring host defense peptides (HDP), we have identified several active antifungal molecules, which also exhibited potent activity in two mouse models of oral candidiasis. Here we show that one such compound, C4, exhibits a mechanism of action that is similar to the parent HDP upon which it was designed. Specifically, its initial interaction with the anionic microbial membrane is electrostatic, as its fungicidal activity is inhibited by cations. We observed rapid membrane permeabilization to propidium iodide and ATP efflux in response to C4. Unlike the antifungal peptide histatin 5, it did not require energy-dependent transport across the membrane. Rapid membrane disruption was observed by both fluorescence and electron microscopy. The compound was highly active in vitro against numerous fluconazole-resistant clinical isolates of C. albicans and non-albicans species, and it exhibited potent, dose-dependent activity in a mouse model of invasive candidiasis, reducing kidney burden by three logs after 24 hours, and preventing mortality for up to 17 days. Together the results support the development of this class of antifungal drug to treat invasive candidiasis.

Antibiofilm peptides against oral biofilms

The oral cavity is a major entry point for bacteria and other microorganisms. Oral biofilms are formed by mixed communities of microorganisms embedded in an exopolysaccharide matrix. Biofilms forming on dental hard or soft tissue are the major cause of caries and endodontic and periodontal disease. Human oral biofilms exhibit high resistance to antimicrobial agents. Antibiofilm peptides constitute a diverse class of host-defense molecules that act to combat invasion and infection with biofilms. Different in vitro and in vivo biofilm models with quantitative analysis have been established to provide predictable platforms for the evaluation of the antibiofilm effect of oral antibiofilm peptides. These peptides have engendered considerable interest in the past decades as potential alternatives to traditional disinfecting agents due to their ability to target bacterial biofilms specifically, leading to the prevention of biofilm formation and destruction of pre-existing biofilms by Gram-positive and -negative bacterial pathogens and fungi. At the same time, challenges associated with the application of these antibiofilm peptides in dental practice also exist. The production of effective, nontoxic, and stable antibiofilm peptides is desired in both academic and industrial fields. This review focuses on the antibiofilm properties of current synthetic peptides and their application in different areas of dentistry.

Antimicrobial Peptides: Mechanisms of Action and Resistance

More than 40 antimicrobial peptides and proteins (AMPs) are expressed in the oral cavity. These AMPs have been organized into 6 functional groups, 1 of which, cationic AMPs, has received extensive attention in recent years for their promise as potential antibiotics. The goal of this review is to describe recent advances in our understanding of the diverse mechanisms of action of cationic AMPs and the bacterial resistance against these peptides. The recently developed peptide GL13K is used as an example to illustrate many of the discussed concepts. Cationic AMPs typically exhibit an amphipathic conformation, which allows increased interaction with negatively charged bacterial membranes. Peptides undergo changes in conformation and aggregation state in the presence of membranes; conversely, lipid conformation and packing can adapt to the presence of peptides. As a consequence, a single peptide can act through several mechanisms depending on the peptide's structure, the peptide:lipid ratio, and the properties of the lipid membrane. Accumulating evidence shows that in addition to acting at the cell membrane, AMPs may act on the cell wall, inhibit protein folding or enzyme activity, or act intracellularly. Therefore, once a peptide has reached the cell wall, cell membrane, or its internal target, the difference in mechanism of action on gram-negative and gram-positive bacteria may be less pronounced than formerly assumed. While AMPs should not cause widespread resistance due to their preferential attack on the cell membrane, in cases where specific protein targets are involved, the possibility exists for genetic mutations and bacterial resistance. Indeed, the potential clinical use of AMPs has raised the concern that resistance to therapeutic AMPs could be associated with resistance to endogenous host-defense peptides. Current evidence suggests that this is a rare event that can be overcome by subtle structural modifications of an AMP.

Potential applications of antimicrobial peptides and their mimics in combating caries and pulpal infections

Antimicrobial peptides (AMPs) are short cationic host-defense molecules that provide the early stage of protection against invading microbes. They also have important modulatory roles and act as a bridge between innate and acquired immunity. The types and functions of oral AMPs were reviewed and experimental reports on the use of natural AMPs and their synthetic mimics in caries and pulpal infections were discussed. Natural AMPs in the oral cavity, predominantly defensins, cathelicidins and histatins, possess antimicrobial activities against oral pathogens and biofilms. Incomplete debridement of microorganisms in root canal space may precipitate an exacerbated immune response that results in periradicular bone resorption. Because of their immunomodulatory and wound healing potentials, AMPs stimulate pro-inflammatory cytokine production, recruit host defense cells and regulate immuno-inflammatory responses in the vicinity of the pulp and periapex. Recent rapid advances in the development of synthetic AMP mimics offer exciting opportunities for new therapeutic initiatives in root canal treatment and regenerative endodontics.

STATEMENT OF SIGNIFICANCE

Identification of new therapeutic strategies to combat antibiotic-resistant pathogens and biofilm-associated infections continues to be one of the major challenges in modern medicine. Despite the presence of commercialization hurdles and scientific challenges, interests in using antimicrobial peptides as therapeutic alternatives and adjuvants to combat pathogenic biofilms have never been foreshortened. Not only do these cationic peptides possess rapid killing ability, their multi-modal mechanisms of action render them advantageous in targeting different biofilm sub-populations. These factors, together with adjunctive bioactive functions such as immunomodulation and wound healing enhancement, render AMPs or their synthetic mimics exciting candidates to be considered as adjuncts in the treatment of caries, infected pulps and root canals.