A novel membrane-disruptive antimicrobial peptide from frog skin secretion against cystic fibrosis isolates and evaluation of anti-MRSA effect using Galleria mellonella model

Understanding the biofilm development of Acinetobacter baumannii and novel strategies to combat infection

Acinetobacter baumannii (A. baumannii) is a Gram-negative, nonmotile, and aerobic bacillus emerged as a superbug, due to increasing the possibility of infection and accelerating rates of antimicrobial agents. It is recognized as a nosocomial pathogen due to its ability to form biofilms. These biofilms serve as a defensive barrier, increase antibiotic resistance, and make treatment more difficult. As a result, the current situation necessitates the rapid emergence of novel therapeutic approaches to ensure successful treatment outcomes. This review explores the intricate relationship between biofilm formation and antibiotic resistance in A. baumannii, emphasizing the role of key virulence factors and quorum sensing (QS) mechanisms that will lead to infections and facilitate insight into developing innovative method to control A. baumannii infections. Furthermore, the review article looks into promising approaches for preventing biofilm formation on medically important surfaces and potential therapeutic methods for eliminating preformed biofilms, which can address biofilm-associated A. baumannii infections. Modern advances in emerging therapeutic options such as antimicrobial peptide (AMPs), nanoparticles (NPs), bacteriophage therapy, photodynamic therapy (PDT), and other biofilm inhibitors can assist readers understand the current landscape and future prospects for effectively treating A. baumannii biofilm infections.

Bactericidal and biofilm eradication efficacy of a fluorinated benzimidazole derivative, TFBZ, against methicillin-resistant Staphylococcus aureus

Methicillin-resistant Staphylococcus aureus (MRSA) is a major inducement of nosocomial infections and its biofilm formation render the high tolerance to conventional antibiotics, which highlights the requirement to develop new antimicrobial agents urgently. In this study, we identified a fluorinated benzimidazole derivative, TFBZ, with potent antibacterial efficacy toward planktonic MRSA (MIC = 4 μg/mL, MBC = 8 μg/mL) and its persistent biofilms (≥99%, MBEC = 8 μg/mL). TFBZ manifested significant irreversible time-dependent killing against MRSA as characterized by diminished cell viability, bacterial morphological change and protein leakage. Furthermore, the results from CBD devices, crystal violet assay in conjunction with live/dead staining and scanning electron microscopy confirmed that TFBZ was capable of eradicating preformed MRSA biofilms with high efficiency. Simultaneously, TFBZ reduced the bacterial invasiveness and exerted negligible hemolysis and cytotoxicity toward mammalian cells, which ensuring the robust therapeutic effect on mouse skin abscess model. The transcriptome profiling and quantitative RT-PCR revealed that a set of encoding genes associated with cell adhesion, biofilm formation, translation process, cell wall biosynthesis was consistently downregulated in MRSA biofilms upon exposure to TFBZ. In conclusion, TFBZ holds promise as a valuable candidate for therapeutic applications against MRSA chronic infections.

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

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

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

Background

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

Methods

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

Results

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

Conclusion

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

A saposin domain-containing protein of tongue sole Cynoglossus semilaevis: Antimicrobial activity and mechanism

Prosaposin is a precursor that can be processed into four different saposins, designated as A, B, C, and D, which have multiple functions in mammals, including neuroprotection and immune modulation. The immune function of saposin in teleost remains largely unknown. In the present study, a saposin (SAP) domain-containing protein was identified in half-smooth tongue sole Cynoglossus semilaevis and named CsSDP. CsSDP harbors one SAP A domain and two SAP B domains. When expressed in HEK293T cells, CsSDP was specifically localized in the lysosome. When overexpressed in Escherichia coli, CsSDP markedly inhibited bacterial growth, and the inhibitory effect depended on two specific regions in the SAP A and SAP B domains. Two polypeptides (P32 and P30) derived from the above SAP A and B domains could bind to and inhibit the growth of both Gram-negative and Gram-positive bacteria. The ultrastructural analysis revealed that P32 and P30 killed target bacteria by disrupting the bacterial cell wall and inducing substantial release of cytoplasmic contents. These results shed new lights on the immune function of saposin domain-containing protein in teleost.

Antimicrobial peptides for combating drug-resistant bacterial infections

The problem of drug resistance due to long-term use of antibiotics has been a concern for years. As this problem grows worse, infections caused by multiple bacteria are expanding rapidly and are extremely detrimental to human health. Antimicrobial peptides (AMPs) are a good alternative to current antimicrobials with potent antimicrobial activity and unique antimicrobial mechanisms, which have advantages over traditional antibiotics in fighting against drug-resistant bacterial infections. Currently, researchers have conducted clinical investigations on AMPs for drug-resistant bacterial infections while integrating new technologies in the development of AMPs, such as changing amino acid structure of AMPs and using different delivery methods for AMPs. This article introduces the basic properties of AMPs, deliberates the mechanism of drug resistance in bacteria and the therapeutic mechanism of AMPs. The current disadvantages and advances of AMPs in combating drug-resistant bacterial infections are also discussed. This article provides important insights into the research and clinical application of new AMPs for drug-resistant bacterial infections.

Inspiration from cruzioseptin-1: membranolytic analogue with improved antibacterial properties

Peptide engineering has gained attraction as a source of new cationicity-enhanced analogues with high potential for the design of next-generation antibiotics. In this context, cruzioseptin-1 (CZS-1), a peptide identified from Cruziohyla calcarifer, is recognized for its antimicrobial potency. However, this amidated-peptide is moderately hemolytic. In order to reduce toxicity and increase antimicrobial potency, 3 peptide analogues based on cruzioseptin-1 were designed and evaluated. [K4K15]CZS-1, an analogue with increased cationicity and reduced hydrophobicity, showed antibacterial, antifungal and antiproliferative properties. In addition, [K4K15]CZS-1 is less hemolytic than CZS-1. The in silico and scanning electron microscopy analysis reveal that [K4K15]CZS-1 induces a membranolytic effect on bacteria. Overall, these results confirm the potential of CZS-1 as source of inspiration for design new selective antimicrobial analogues useful for development of new therapeutic agents.

Biofilm Formation by Staphylococcus aureus in the Specific Context of Cystic Fibrosis

Staphylococcus aureus is a major human pathogen whose characteristics support its success in various clinical settings including Cystic Fibrosis (CF). In CF, S. aureus is indeed the most commonly identified opportunistic pathogen in children and the overall population. S. aureus colonization/infection, either by methicillin-susceptible or methicillin-resistant strains, will become chronic in about one third of CF patients. The persistence of S. aureus in CF patients' lungs, despite various eradication strategies, is favored by several traits in both host and pathogen. Among the latter, living in biofilm is a highly protective way to survive despite deleterious environmental conditions, and is a common characteristic shared by the main pathogens identified in CF. This is why CF has earned the status of a biofilm-associated disease for several years now. Biofilm formation by S. aureus, and the molecular mechanisms governing and regulating it, have been extensively studied but have received less attention in the specific context of CF lungs. Here, we review the current knowledge on S. aureus biofilm in this very context, i.e., the importance, study methods, molecular data published on mono- and multi-species biofilm and anti-biofilm strategies. This focus on studies including clinical isolates from CF patients shows that they are still under-represented in the literature compared with studies based on reference strains, and underlines the need for such studies. Indeed, CF clinical strains display specific characteristics that may not be extrapolated from results obtained on laboratory strains.

Molecular Cloning and Characterization of a Novel Antimicrobial Peptide from the Skin of Kaloula pulchra

BACKGROUND

Bacterial resistance to all currently available conventional antibiotics has caused a global public health crisis and led to an imperative search for new agents. Antimicrobial peptides (AMPs) are essential components of host innate immune defense against microbial invasions.

OBJECTIVES

The objective of this study was to report a novel AMP, brevinin-2KP, from the skin of the black Kaloula pulchra frog and describe its structural and biological characterization.

MATERIALS AND METHODS

The physical and chemical parameters of brevinin-2KP were predicted with the ExPASy Bioinformatics Resource Portal. The assembled sequences were aligned with ClustalW, and the phylogenetic tree was constructed using Mega. Circular dichroism (CD) experiments were carried out to identify the secondary structure and the stability of peptide in different solvent environments. The cytotoxicity of brevinin-2KP was evaluated by the MTT test. To determine antibacterial activity of brevinin- 2KP, a standard two-fold broth dilution method was used. SEM was carried out to observe the morphological change in the bacterial treated by brevinin-2KP. The live/dead bacterial viability was measured with a LIVE/DEAD® BacLight kit. Histamine release and mast cell degranulation assays were performed.

RESULTS

The precursor of brevinin-2KP contains 72 amino acid residues, including a conserved signal peptide, acidic propeptide with KR residues, and mature peptide with a sequence of GVITDALKGAAKTVAAELLKKAHCKLTNSC. Phylogenetic analysis based on the amino acid sequences of 34 brevinin-2 peptides from 30 anuran species demonstrates that K. pulchra is genetically closely related to the genus Hylarana. The CD spectra analysis indicates that brevinin-2KP adopts random coil in the water and an organized α-helical conformation in SDS solution. Further, this secondary structure is stable under high salt and high-temperature conditions. Brevinin-2KP is weakly active towards the tested Gram-positive and Gram-negative bacteria as well as fungi due to its membranolytic action. Moreover, brevinin-2KP inhibits the proliferation of several mammal cells with IC₅₀ values ranging from 3.27 to 59.75 μM. In addition, brevinin-2KP promotes degranulation and histamine release of mast cells, indicating that it is involved in the inflammatory response.

CONCLUSION

This is the first report on AMP identified from the skin of K. pulchra. Brevinin-2KP adopts a typical amphipathic α-helix conformation in membrane mimic environment and shows antimicrobial and antitumor activities by potential membranolytic mechanism. In addition, brevinin-2KP can promote degranulation and histamine release of mast cells. Brevinin-2KP is expected to become a good drug temple molecule.

The synthetic antimicrobial peptide IKR18 displays anti-infectious properties in Galleria mellonella in vivo model

Antimicrobial peptides (AMPs) are promising tools for developing new antibiotics. We described the design of IKR18, an AMP designed with the aid of computational tools. IKR18 showed antimicrobial activity against Gram-negative and Gram-positive bacteria, including methicillin-resistant Staphylococcus aureus (MRSA). CD studies revealed that IKR18 assumes an alpha-helical structure in the membrane-mimetic environment. The action mechanism IKR18 involves damage to the bacteria membrane, as demonstrated by Sytox green uptake. Furthermore, IKR18 displayed synergic and additive effects in combination with antibiotics ciprofloxacin and vancomycin. The peptide showed anti-biofilm activity in concentration and efficiency compared with commercial antibiotics, involving the direct death of bacteria, as confirmed by scanning electron microscopy. The anti-infective activity of IKR18 was demonstrated in the Galleria mellonella model infected with S. aureus, MRSA, and Acinetobacter baumannii. The novel bioinspired peptide, IKR18, proved to be effective in the control of bacterial infection, opening opportunities for the development of further assays, including preclinical models.

Prevention of hospital pathogen biofilm formation by antimicrobial peptide KWI18

This study investigated the antimicrobial and antibiofilm activity of KWI18, a new synthetic peptide. KWI18 was tested against planktonic cells and Pseudomonas aeruginosa and Candida parapsilosis biofilms. Time-kill and synergism assays were performed. Sorbitol, ergosterol, lipid peroxidation, and protein oxidation assays were used to gain insight into the mechanism of action of the peptide. Toxicity was evaluated against erythrocytes and Galleria mellonella. KWI18 showed antimicrobial activity, with minimum inhibitory concentration (MIC) values ranging from 0.5 to 10 μM. KWI18 at 10 × MIC reduced P. aeruginosa and C. parapsilosis biofilm formation and cell viability. Time-kill assays revealed that KWI18 inhibited the growth of P. aeruginosa in 4 h and that of C. parapsilosis in 6 h. The mechanism of action was related to ergosterol as well as induction of oxidative damage in cells and biofilms. Furthermore, KWI18 demonstrated low toxicity to erythrocytes and G. mellonella. KWI18 proved to be an effective antibiofilm agent, opening opportunities for the development of new antimicrobials.

Synthetic peptides bioinspired in temporin-PTa with antibacterial and antibiofilm activity

Several antimicrobial peptides (AMPs) have been reported in amphibian toxins, as temporin-PTa from Hylarana picturata. The amino acid distribution within a helical structure of AMPs favors the design of new bioactive peptides. Therefore, this work reports the rational design of two new synthetic peptides denominated Hp-MAP1 and Hp-MAP2 derived from temporin-PTa. These peptides present an amphipathic helix with positive charges of +4 and +5, hydrophobic moment (<µH>) of 0.66 and 0.72 and hydrophobicity (<H>) of 0.49 and 0.41, respectively. Hp-MAP1 and Hp-MAP2 displayed in vitro activity against Gram-negative and Gram-positive bacteria from 2.8 to 92 µM, without presenting hemolytic effects. Molecular dynamics simulation suggested that the parent and designed temporin-like peptides lack structural stability in an aqueous solution. By contrast, α-helical structures were predicted in hydrophobic and anionic environments. Additionally, the peptides were simulated on mimetic membranes composed of anionic and neutral phospholipids 1,2-dipalmitoylsn-glycerol-3-phosphatidylglycerol (DPPG-anionic), 1,2-dipalmitoyl-sn-lyco-3 phosphatidylethanolamine (DPPE-neutral). When in contact with DPPG/DPPE (90:10) and DPPG/DPPE (50:50) temporin-PTa, Hp-MAP1 and Hp-MAP2 established interactions guided by hydrogen and saline bounds. Therefore, the findings described here reveal that the optimization of the amphipathic α-helical cationic peptides Hp-MAP1 and Hp-MAP2 enabled the generation of new synthetic antimicrobial agents to combat pathogenic microorganisms.

A novel antimicrobial peptide found in Pelophylax nigromaculatus

Background

Many active peptides have been found in frog skin secretions. In this paper, our research focused on Pelophylax nigromaculatus and found a broad-spectrum antimicrobial peptide Nigrocin-PN based on the molecular cloning technique. Thereafter, the “Rana box” function was briefly studied by two mutated peptides (Nigrocin-M1 and Nigrocin-M2). Furthermore, in vitro and in vivo assays were used to characterize the peptide’s biofunctions, and the peptide’s function in treating multidrug-resistant pathogens was also studied.

Results

Nigrocin-PN not only displayed potent antimicrobial abilities in vitro but also significantly ameliorated pulmonary inflammation induced by Klebsiella pneumoniae in vivo. By comparing, leucine-substituted analogue Nigrocin-M1 only displayed bactericidal abilities towards gram-positive bacteria, while the shorter analogue Nigrocin-M2 lost this function. More strikingly, Nigrocin-PN exhibited synergistic effects with commonly used antibiotics; in vitro evolution experiments revealed that coadministration between Nigrocin-PN and ampicillin could delay Staphylococcus aureus antibiotic resistance acquisition. Kinetics and morphology studies indicate that antibacterial mechanisms involved membrane destruction. Furthermore, toxicities and anticancer abilities of these peptides were also studied; compared to two analogues, Nigrocin-PN showed mild haemolytic activity and indistinctive cytotoxicity towards normal cell lines HMEC-1 and HaCaT.

Conclusions

A broad-spectrum antimicrobial peptide Nigrocin-PN was discovered from the skin secretion of Pelophylax nigromaculatus. Structurally, “Rana box” played a crucial role in reducing toxicities without compromising antibacterial abilities, and Nigrocin-PN could be a desired therapeutic candidate.

Graphical abstract

• For AMPs, disulphide bond can affect their biofunction and cytotoxicity.

• Frog skin secretion is a reservoir to delve valuable peptides.

• AMPs-antibiotics coadministration could be a strategy to delay drug resistance.

In Vitro & In Vivo Studies on Identifying and Designing Temporin-1CEh from the Skin Secretion of Rana chensinensis as the Optimised Antibacterial Prototype Drug

Amphibian skin secretion is an ideal source of antimicrobial peptides that are difficult to induce drug resistance to due to their membrane-targeting mechanism as a new treatment scheme. In this study, a natural antimicrobial peptide Temporin-1CEh was identified by molecular cloning and mass spectrometry from the skin secretions of the Chinese forest frog (Rana chensinensis). Through the study of the structure and biological activity, it was found that Temporin-1CEh was a helical peptide from the Temporin family, and possessed good anti-Gram-positive bacteria activity through the mechanism of membrane destruction. Seven analogues were further designed to obtain broad-spectrum antimicrobial activity and higher stability in different physiological conditions. The results showed that T1CEh-KKPWW showed potent antibacterial activity with significantly increasing the activity against Gram-negative bacteria in vitro and in vivo with low haemolysis. In addition, T1CEh-KKPWW2 showed high sensitivity to the pH, serum or salts conditions, which applied a branched structure to allow the active units of the peptide to accumulate. Even though the haemolytic activity was increased, the stable antibacterial activity made this novel analogue meet the conditions to become a potential candidate in future antimicrobial and antibiofilm applications.

Bisdemethoxycurcumin Reduces Methicillin-Resistant Staphylococcus aureus Expression of Virulence-Related Exoproteins and Inhibits the Biofilm Formation

Methicillin-resistant Staphylococcus aureus (MRSA) is a major pathogen of nosocomial infection, which is resistant to most antibiotics. Presently, anti-virulence therapy and anti-biofilm therapy are considered to be promising alternatives. In the current work, we investigated the influence of bisdemethoxycurcumin (BDMC) on the virulence-related exoproteins and the biofilm formation using a reference strain and clinic isolated strains. Western blotting, quantitative RT-PCR, and tumor necrosis factor (TNF) release assay were performed to assess the efficacy of BDMC in reducing the expression of Staphylococcus enterotoxin-related exoproteins (enterotoxin A, enterotoxin B) and α-toxin in MRSA. The anti-biofilm activity of BDMC was evaluated through a biofilm inhibition assay. The study suggests that sub-inhibitory concentrations of BDMC significantly inhibited the expression of sea, seb, and hla at the mRNA level in MRSA. Moreover, the expression of virulence-related exoproteins was significantly decreased by down-regulating accessory gene regulator agr, and the inhibition of biofilms formation was demonstrated by BDMC at sub-inhibitory concentrations. Consequently, the study suggests that BDMC may be a potential natural antibacterial agent to release the pressure brought by antibiotic resistance.

Galleria mellonella as an infection model: an in-depth look at why it works and practical considerations for successful application

The larva of the greater wax moth Galleria mellonella is an increasingly popular model for assessing the virulence of bacterial pathogens and the effectiveness of antimicrobial agents. In this review, we discuss details of the components of the G. mellonella larval immune system that underpin its use as an alternative infection model, and provide an updated overview of the state of the art of research with G. mellonella infection models to study bacterial virulence, and in the evaluation of antimicrobial efficacy. Emphasis is given to virulence studies with relevant human and veterinary pathogens, especially Escherichia coli and bacteria of the ESKAPE group. In addition, we make practical recommendations for larval rearing and testing, and overcoming potential limitations of the use of the model, which facilitate intra- and interlaboratory reproducibility.

Structure-Activity Relationship and Molecular Docking of a Kunitz-Like Trypsin Inhibitor, Kunitzin-AH, from the Skin Secretion of Amolops hainanensis

Kunitz-like trypsin inhibitors are one of the most noteworthy research objects owing to their significance in pharmacological studies, including anticarcinogenic activity, obesity regulation and anticoagulation. In the current study, a novel Kunitz-like trypsin inhibitor, Kunitzin-AH, was isolated from the skin secretion of Amolops hainanensis. The novel peptide displayed a modest trypsin inhibitory activity with the inhibitor constant (K_i) value of 1.18 ± 0.08 µM without inducing damage to healthy horse erythrocytes. Then, a series of shortened variants of Kunitzin-AH were designed by truncating a peptide loop and site mutation inside the loop to illustrate the structure–activity relationship of the trypsin inhibition function. Among the variants, a significant decrease was observed for the Cys-Cys loop domain, while the extension of an Arg at N-terminus (RCKAAFC) retained the inhibitory activity, indicating that the -RCK-motif is essential in forming the reactive domain for exerting the inhibitory activity. Furthermore, substitutions of Ala by hydrophobic or hydrophilic residues decreased the activity, indicating suitable steric hindrance provides convenience for the combination of trypsin. Additionally, the conformational simulation of the analogues processed with Chimera and Gromacs and further combination simulations between the peptides and trypsin conducted with HDOCK offered a potential opportunity for the natural trypsin inhibitory drug design. The truncated sequence, AH-798, may be a good replacement for the full-length peptide, and can be optimized via cyclization for further study.

Aggregation and Its Influence on the Bioactivities of a Novel Antimicrobial Peptide, Temporin-PF, and Its Analogues

Temporin is an antimicrobial peptide (AMP) family discovered in the skin secretion of ranid frog that has become a promising alternative for conventional antibiotic therapy. Herein, a novel temporin peptide, Temporin-PF (TPF), was successfully identified from Pelophylax fukienensis. It exhibited potent activity against Gram-positive bacteria, but no effect on Gram-negative bacteria. Additionally, TPF exhibited aggregation effects in different solutions. Three analogs were further designed to study the relationship between the aggregation patterns and bioactivities, and the MD simulation was performed for revealing the pattern of the peptide assembly. As the results showed, all peptides were able to aggregate in the standard culture media and salt solutions, especially CaCl2 and MgCl2 buffers, where the aggregation was affected by the concentration of the salts. MD simulation reported that all peptides were able to form oligomers. The parent peptide assembly depended on the hydrophobic interaction via the residues in the middle domain of the sequence. However, the substitution of Trp/D-Trp resulted in an enhanced inter-peptide interaction in the zipper-like domain and eliminated overall biological activities. Our study suggested that introducing aromaticity at the zipper-like domain for temporin may not improve the bioactivities, which might be related to the formation of aggregates via the inter-peptide contacts at the zipper-like motif domain, and it could reduce the binding affinity to the lipid membrane of microorganisms.

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.

Natural Anti-biofilm Agents: Strategies to Control Biofilm-Forming Pathogens

Pathogenic microorganisms and their chronic pathogenicity are significant concerns in biomedical research. Biofilm-linked persistent infections are not easy to treat due to resident multidrug-resistant microbes. Low efficiency of various treatments and in vivo toxicity of available antibiotics drive the researchers toward the discovery of many effective natural anti-biofilm agents. Natural extracts and natural product-based anti-biofilm agents are more efficient than the chemically synthesized counterparts with lesser side effects. The present review primarily focuses on various natural anti-biofilm agents, i.e., phytochemicals, biosurfactants, antimicrobial peptides, and microbial enzymes along with their sources, mechanism of action via interfering in the quorum-sensing pathways, disruption of extracellular polymeric substance, adhesion mechanism, and their inhibitory concentrations existing in literature so far. This study provides a better understanding that a particular natural anti-biofilm molecule exhibits a different mode of actions and biofilm inhibitory activity against more than one pathogenic species. This information can be exploited further to improve the therapeutic strategy by a combination of more than one natural anti-biofilm compounds from diverse sources.

Broad-Spectrum Antimicrobial Activity and Improved Stability of a D-Amino Acid Enantiomer of DMPC-10A, the Designed Derivative of Dermaseptin Truncates

DMPC-10A (ALWKKLLKK-Cha-NH₂) is a 10-mer peptide derivative from the N-terminal domain of Dermaseptin-PC which has shown broad-spectrum antimicrobial activity as well as a considerable hemolytic effect. In order to reduce hemolytic activity and improve stability to endogenous enzymes, a D-amino acid enantiomer (DMPC-10B) was designed by substituting all L-Lys and L-Leu with their respective D-form amino acid residues, while the Ala¹ and Trp³ remained unchanged. The D-amino acid enantiomer exhibited similar antimicrobial potency to the parent peptide but exerted lower cytotoxicity and hemolytic activity. Meanwhile, DMPC-10B exhibited remarkable resistance to hydrolysis by trypsin and chymotrypsin. In addition to these advantages, DMPC-10B exhibited an outstanding antibacterial effect against Methicillin-resistant Staphylococcus aureus (MRSA) and Klebsiella pneumoniae using the Galleria mellonella larva model and displayed synergistic activities with gentamicin against carbapenem-resistant K. pneumoniae strains. This indicates that DMPC-10B would be a promising alternative for treating antibiotic-resistant pathogens.

Ranacyclin-NF, a Novel Bowman-Birk Type Protease Inhibitor from the Skin Secretion of the East Asian Frog, Pelophylax nigromaculatus

Serine protease inhibitors are found in plants, animals and microorganisms, where they play important roles in many physiological and pathological processes. Inhibitor scaffolds based on natural proteins and peptides have gradually become the focus of current research as they tend to bind to their targets with greater specificity than small molecules. In this report, a novel Bowman–Birk type inhibitor, named ranacyclin-NF (RNF), is described and was identified in the skin secretion of the East Asian frog, Pelophylax nigromaculatus. A synthetic replicate of the peptide was subjected to a series of functional assays. It displayed trypsin inhibitory activity with an inhibitory constant, Ki, of 447 nM and had negligible direct cytotoxicity. No observable direct antimicrobial activity was found but RNF improved the therapeutic potency of Gentamicin against Methicillin-resistant Staphylococcus aureus (MRSA). RNF shared significant sequence similarity to previously reported and related inhibitors from Odorrana grahami (ORB) and Rana esculenta (ranacyclin-T), both of which were found to be multi-functional. Two analogues of RNF, named ranacyclin-NF1 (RNF1) and ranacyclin-NF3L (RNF3L), were designed based on some features of ORB and ranacyclin-T to study structure–activity relationships. Structure–activity studies demonstrated that residues outside of the trypsin inhibitory loop (TIL) may be related to the efficacy of trypsin inhibitory activity.

Brevinin-2GHk from Sylvirana guentheri and the Design of Truncated Analogs Exhibiting the Enhancement of Antimicrobial Activity

Brevinins are an important antimicrobial peptide (AMP) family discovered in the skin secretions of Ranidae frogs. The members demonstrate a typical C-terminal ranabox, as well as a diverse range of other structural characteristics. In this study, we identified a novel brevinin-2 peptide from the skin secretion of Sylvirana guentheri, via cloning transcripts, and identifying the expressed mature peptide, in the skin secretion. The confirmed amino acid sequence of the mature peptide was designated brevinin-2GHk (BR2GK). Moreover, as a previous study had demonstrated that the N-terminus of brevinin-2 is responsible for exerting antimicrobial activity, we also designed a series of truncated derivatives of BR2GK. The results show that the truncated derivatives exhibit significantly improved antimicrobial activity and cytotoxicity compared to the parent peptide, except a Pro¹⁴ substituted analog. The circular dichroism (CD) analysis of this analog revealed that it did not fold into a helical conformation in the presence of either lipopolysaccharides (LPS) or TFE, indicating that position 14 is involved in the formation of the α-helix. Furthermore, three more analogs with the substitutions of Ala, Lys and Arg at the position 14, respectively, revealed the influence on the membrane disruption potency on bacteria and mammalian cells by the structural changes at this position. Overall, the N-terminal 25-mer truncates demonstrated the potent antimicrobial activity with low cytotoxicity.

Antibacterial and Antibiofilm Activity of Temporin-GHc and Temporin-GHd Against Cariogenic Bacteria, Streptococcus mutans

Temporin-GHc (GHc) and temporin-GHd (GHd) produced by the frog Hylarana guentheri had shown broad-spectrum antibacterial activities against bacteria and fungi. In this study, we investigated whether they exert antibacterial and antibiofilm activities against cariogenic bacteria, Streptococcus mutans. GHc and GHd adopt the random coil conformation in aqueous solution and convert to α-helix in membrane mimetic environments by using circular dichroism spectroscope. They are positively charged by histidine, with the polar and nonpolar amino acids on opposing faces along the helix. The amphipathicity enabled the peptides to target at bacterial membrane, leading to an increase in membrane permeation and disruption of S. mutans, which allowed the peptides to bind with genomic DNA. GHc and GHd completely impeded the initial attachment of biofilm formation and disrupted preformed S. mutans biofilms. The expression of exopolysaccharide (EPS) biosynthesis genes which synthesize glucosyltransferases in S. mutans was downregulated by exposing to GHc or GHd, contributing to the decrease of soluble and insoluble EPS. GHc and GHd exhibited selectivity toward S. mutans in the presence of human erythrocytes, and no cytotoxicity toward human oral epithelial cells was observed at a concentration of 200 μM. These results laid the foundation for the development of GHc and GHd as potential anti-caries agents.

The Analogs of Temporin-GHa Exhibit a Broader Spectrum of Antimicrobial Activity and a Stronger Antibiofilm Potential against Staphylococcus aureus

The abuse of antibiotics has led to the emergence of multidrug-resistant bacteria, which is becoming a serious worldwide problem people have to face. In our previous study, temporin-GHa (GHa) cloned from Hylarana guentheri showed antimicrobial activity against Gram-positive bacteria. In order to improve its therapeutic potential, we used a template-based and a database-assisted design to obtain three derived peptides by replacing the histidine at both ends of GHa with lysine, which exhibited faster and stronger bactericidal activity and a broader spectrum than the parent peptide. GHaK and GHa4K targeted to the bacterial membrane to exert their antibacterial activities at a faster membrane damage rate. The derived peptides inhibited the initial adhesion and the formation of Staphylococcus aureus biofilms, and eradicated the mature biofilms, which indicated that the derived peptides effectively penetrated the biofilm and killed bacteria. The therapeutic index (TI) and cell selectivity index (CSI) of the derived peptides increased significantly, which means a broader therapeutic window of the derived peptides. The derived peptides with improved activity and cell selectivity have the potential to be the promising candidates for the treatment of S. aureus infections. Our research also provides new insights into the design and development of antimicrobial peptides.

Design of N-Terminal Derivatives from a Novel Dermaseptin Exhibiting Broad-Spectrum Antimicrobial Activity against Isolates from Cystic Fibrosis Patients

Dermaseptins are an antimicrobial peptide family widely identified from the skin secretions of phyllomeudusinae frogs. Here, we identify Dermaseptin-PC (DM-PC), from the skin secretion of Phyllomedusa coelestis, and further investigate the properties of this peptide, and a number of rationally designed truncated derivatives. The truncated 19-mer derived from the N-terminus exhibited similar antimicrobial potency when compared to the parent peptide, but the haemolytic effect of this truncated peptide was significantly decreased. Based on previous studies, the charge and hydrophobicity of truncated derivatives can affect the bioactivity of these peptides and thus we designed a 10-mer derivative with an optimised positive charge and a cyclohexylalanine (Cha) at the C-terminus for enhancing the hydrophobicity, DMPC-10A, which retained the antimicrobial activity of the parent peptide. To further investigate the influence of Cha at the C-terminus on activity, it was substituted by alanine (Ala) to generate another derivative, DMPC-10, but this was found to be much less potent. In addition, DM-PC, DMPC-19 and DMPC-10A not only rapidly killed planktonic bacteria isolated from cystic fibrosis (CF) patient, but also effectively eradicated their biofilm matrices.

Biofilms: Novel Strategies Based on Antimicrobial Peptides

The problem of drug resistance is very worrying and ever increasing. Resistance is due not only to the reckless use of antibiotics but also to the fact that pathogens are able to adapt to different conditions and develop self-defense mechanisms such as living in biofilms; altogether these issues make the search for alternative drugs a real challenge. Antimicrobial peptides appear as promising alternatives but they have disadvantages that do not make them easily applicable in the medical field; thus many researches look for solutions to overcome the disadvantages and ensure that the advantages can be exploited. This review describes the biofilm characteristics and identifies the key features that antimicrobial peptides should have. Recalcitrant bacterial infections caused by the most obstinate bacterial species should be treated with a strategy to combine conventional peptides functionalized with nano-tools. This approach could effectively disrupt high density infections caused by biofilms. Moreover, the importance of using in vivo non mammalian models for biofilm studies is described. In particular, here we analyze the use of amphibians as a model to substitute the rodent model.

Bioevaluation of Ranatuerin-2Pb from the Frog Skin Secretion of Rana pipiens and its Truncated Analogues

Antimicrobial peptides (AMPs) are considered as a promising agent to overcome the drug-resistance of bacteria. Large numbers of AMPs have been identified from the skin secretion of Rana pipiens, including brevinins, ranatuerins, temporins and esculentins. In this study, the cDNA precursor of a broad-spectrum antimicrobial peptide, ranatuerin-2Pb, was cloned and identified. Additionally, two truncated analogues, RPa and RPb, were synthesised to investigate the structure-activity relationship of ranatuerin-2Pb. RPa lost antimicrobial activity against Candida albicans, MRSA, Enterococcus faecalis and Pseudomonas aeruginosa, while RPb retained its broad-spectrum antimicrobial activity. Additionally, ranatuerin-2Pb, RPa and RPb demonstrated inhibition and eradication effects against Staphylococcus aureus biofilm. RPb showed a rapid bacterial killing manner via membrane permeabilization without damaging the cell membrane of erythrocytes. Moreover, RPb decreased the mortality of S. aureus infected Galleria mellonella larvae. Collectively, our results suggested that RPb may pave a novel way for natural antimicrobial drug design.