Analogs of the Frog-skin Antimicrobial Peptide Temporin 1Tb Exhibit a Wider Spectrum of Activity and a Stronger Antibiofilm Potential as Compared to the Parental Peptide

Coinfections in Tuberculosis in Low- and Middle-Income Countries: Epidemiology, Clinical Implications, Diagnostic Challenges, and Management Strategies-A Narrative Review

Tuberculosis (TB) continues to be a major public health challenge in low- and middle-income countries (LMICs), where high burdens of coinfections exacerbate the disease's impact. In 2023, an estimated 8.2 million people were newly diagnosed with tuberculosis worldwide, reflecting an increase from 7.5 million in 2022 and 7.1 million in 2019. In LMICs, limited access to healthcare, inadequate nutrition, and poor living conditions contribute to higher coinfection rates among TB patients, leading to delayed diagnosis and treatment, which in turn exacerbates disease severity and facilitates transmission. This narrative review synthesizes the epidemiology, clinical implications, diagnostic challenges, and management strategies related to TB coinfections with viral pathogens including HIV, SARS-CoV-2, and influenza, bacteria such as Streptococcus pneumoniae, Staphylococcus aureus, Klebsiella pneumoniae, and Pseudomonas aeruginosa, fungi such as Aspergillus and Candida species, and parasites. This review highlights that overlapping symptoms, immune system compromise, and socioeconomic barriers in LMICs lead to delayed diagnoses and suboptimal treatment outcomes, while also addressing the challenges of managing drug interactions particularly in HIV-TB coinfections and underscoring the need for integrated diagnostic approaches, improved treatment regimens, and strengthened healthcare systems, thereby consolidating current evidence to inform future research priorities and policy interventions aimed at reducing the overall burden of TB and its coinfections in resource-limited settings.

Dual-function antimicrobial-antibiofilm peptide hybrid to tackle biofilm-forming Staphylococcus epidermidis

BACKGROUND

Due to their resistance and difficulty in treatment, biofilm-associated infections are problematic among hospitalized patients globally and account for 60% of all bacterial infections in humans. Antibiofilm peptides have recently emerged as an alternative treatment since they can be effectively designed and exert a different mode of biofilm inhibition and eradication.

METHODS

A novel antibiofilm peptide, BiF, was designed from the conserved sequence of 18 α-helical antibiofilm peptides by template-assisted technique and its activity was improved by hybridization with a lipid binding motif (KILRR). Novel antibiofilm peptide derivatives were modified by substituting hydrophobic amino acids at positions 5 or 7, and both, with positively charged lysines (L5K, L7K). These peptide derivatives were tested for antibiofilm and antimicrobial activities against biofilm-forming Staphylococcus epidermidis and multiple other microbes using crystal violet and broth microdilution assays, respectively. To assess their impact on mammalian cells, the toxicity of peptides was determined through hemolysis and cytotoxicity assays. The stability of candidate peptide, BiF2_5K7K, was assessed in human serum and its secondary structure in bacterial membrane-like environments was analyzed using circular dichroism. The action of BiF2_5K7K on planktonic S. epidermidis and its effect on biofilm cell viability were assessed via viable counting assays. Its biofilm inhibition mechanism was investigated through confocal laser scanning microscopy and transcription analysis. Additionally, its ability to eradicate mature biofilms was examined using colony counting. Finally, a preliminary evaluation involved coating a catheter with BiF2_5K7K to assess its preventive efficacy against S. epidermidis biofilm formation on the catheter and its surrounding area.

RESULTS

BiF2_5K7K, the modified antibiofilm peptide, exhibited dose-dependent antibiofilm activity against S. epidermidis. It inhibited biofilm formation at subinhibitory concentrations by altering S. epidermidis extracellular polysaccharide production and quorum-sensing gene expression. Additionally, it exhibited broad-spectrum antimicrobial activity and no significant hemolysis or toxicity against mammalian cell lines was observed. Its activity is retained when exposed to human serum. In bacterial membrane-like environments, this peptide formed an α-helix amphipathic structure. Within 4 h, a reduction in the number of S. epidermidis colonies was observed, demonstrating the fast action of this peptide. As a preliminary test, a BiF2_5K7K-coated catheter was able to prevent the development of S. epidermidis biofilm both on the catheter surface and in its surrounding area.

CONCLUSIONS

Due to the safety and effectiveness of BiF2_5K7K, we suggest that this peptide be further developed to combat biofilm infections, particularly those of biofilm-forming S. epidermidis.

Exploring the Potential of Bioactive Peptides: From Natural Sources to Therapeutics

Bioactive peptides, specific protein fragments with positive health effects, are gaining traction in drug development for advantages like enhanced penetration, low toxicity, and rapid clearance. This comprehensive review navigates the intricate landscape of peptide science, covering discovery to functional characterization. Beginning with a peptidomic exploration of natural sources, the review emphasizes the search for novel peptides. Extraction approaches, including enzymatic hydrolysis, microbial fermentation, and specialized methods for disulfide-linked peptides, are extensively covered. Mass spectrometric analysis techniques for data acquisition and identification, such as liquid chromatography, capillary electrophoresis, untargeted peptide analysis, and bioinformatics, are thoroughly outlined. The exploration of peptide bioactivity incorporates various methodologies, from in vitro assays to in silico techniques, including advanced approaches like phage display and cell-based assays. The review also discusses the structure-activity relationship in the context of antimicrobial peptides (AMPs), ACE-inhibitory peptides (ACEs), and antioxidative peptides (AOPs). Concluding with key findings and future research directions, this interdisciplinary review serves as a comprehensive reference, offering a holistic understanding of peptides and their potential therapeutic applications.

ZnO/chitosan nanocomposites as a new approach for delivery LL37 and evaluation of the inhibitory effects against biofilm-producing Methicillin-resistant Staphylococcus aureus isolated from clinical samples

Modification surface of chitosan nanoparticles using ZnO nanoparticles is important interest in drug delivery because of the beneficial properties. In this study, we proposed a chitosan/ZnO nanocomposite for the targeted delivery of antibacterial peptide (LL37). Synthesized LL37-loaded chitosan/ZnO nanocomposite (CS/ZnO/LL37-NCs) was based on the ionotropic gelation method. The antibacterial activity of the synthesized platform versus Methicillin-resistant Staphylococcus aureus (MRSA) was determined by the microdilution method in 10 mM sodium phosphate buffer. The biofilm formation inhibitory was also evaluated using microtiter plate method. In addition, the ability of CS/ZnO/LL37-NCs on the icaA gene expression level was assessed by the Real-Time PCR. The loading and release investigations confirmed the suitability of CS/ZnO-NCs for LL37 encapsulation. Results showed 6 log10 CFU/ml reduction in MRSA treated with the CS/ZnO/LL37-NPs. Moreover, CS/ZnO/LL37-NCs showed 81 % biofilm formation inhibition than LL37 alone. Also, icaA gene expression decreased 1-fold in the face of CS/ZnO/LL37-NCs. In conclusion, the modification surface of chitosan nanoparticles with ZnO nanoparticles is a suitable chemical platform for the delivery of LL37 that could be used as a promising nanocarrier for enhancing the delivery of antibacterial peptide and improving the antibacterial activity of LL37.

The bactericidal and antibiofilm effects of a lysine-substituted hybrid peptide, CM-10K14K, on biofilm-forming Staphylococcus epidermidis

Staphylococci, notably biofilm-forming Staphylococcus epidermidis, have been recognized as global nosocomial pathogens in medical device-related infections. Their potential to attach to and form biofilm on indwelling catheters are significant factors impeding conventional treatment. Due to their extensive antimicrobial and antibiofilm actions, antimicrobial peptides (AMPs) have attracted interest as promising alternative compounds for curing difficult-to-treat, biofilm-forming bacterial infections. Cecropin A-melittin or CM, a well-known hybrid peptide, exhibits broad-spectrum antimicrobial activity, however it also possesses high toxicity. In the current study, a series of hybrid CM derivatives was designed using an amino acid substitution strategy to explore potential antibacterial and antibiofilm peptides with low toxicity. Among the derivatives, CM-10K14K showed the least hemolysis along with potent antibacterial activity against biofilm-forming S. epidermidis (MICs = 3.91 μg/mL) and rapid killing after 15 min exposure (MBCs = 7.81 μg/mL). It can prevent the formation of S. epidermidis biofilm and also exhibited a dose-dependent eradication activity on mature or established S. epidermidis biofilm. In addition, it decreased the development of biofilm by surviving bacteria, and formation of biofilm on the surface of CM-10K14K-impregnated catheters. Released CM-10K14K decreased planktonic bacterial growth and inhibited biofilm formation by S. epidermidis in a dose-dependent manner for 6 and 24 h post-exposure. Impregnation of CM-10K14K prevented bacterial attachment on catheters and thus decreased formation of extensive biofilms. SEM images supported the antibiofilm activity of CM-10K14K. Flow cytometry analysis and TEM images demonstrated a membrane-active mechanism of CM-10K14K, inducing depolarization and permeabilization, and subsequent membrane rupture leading to cell death. The presence of an interaction with bacterial DNA was verified by gel retardation assay. These antibacterial and antibiofilm activities of CM-10K14K suggest its potential application to urinary catheters for prevention of biofilm-forming colonization or for treatment of medical devices infected with S. epidermidis.

Antimicrobial peptide 2K4L disrupts the membrane of multidrug-resistant Acinetobacter baumannii and protects mice against sepsis

Antimicrobial peptides represent a promising therapeutic alternative for the treatment of antibiotic-resistant bacterial infections. 2K4L is a rationally-designed analog of a short peptide temporin-1CEc, a natural peptide isolated and purified from the skin secretions of the Chinese brown frog Rana chensinensis by substituting amino acid residues. 2K4L adopt an α-helical confirm in a membrane-mimetic environment and displayed an improved and broad-spectrum antibacterial activity against sensitive and multidrug-resistant Gram-negative and Gram-positive bacterial strains. Here, the action mechanism of 2K4L on multidrug resistant Acinetobacter baumannii (MRAB) and protection on MRAB-infected mice was investigated. The results demonstrated high bactericidal activity of 2K4L against both a multidrug resistant A. baumannii 0227 strain (MRAB 0227) and a sensitive A. baumannii strain (AB 22934), indicating a potential therapeutic advantage of this peptide. Strong positively-charged residues significantly promoted the electrostatic interaction on 2K4L with lipopolysaccharides (LPS) of the bacterial outer membrane. High hydrophobicity and an α-helical confirm endowed 2K4L remarkably increase the permeability of A. baumannii cytoplasmic membrane by depolarization of membrane potential and disruption of membrane integration, as well as leakage of fluorescein from the liposomes. Additionally, 2K4L at low concentrations inhibited biofilm formation and degraded mature 1-day-old MRAB 0227 biofilms by reducing the expression of biofilm-related genes. In an invasive A. baumannii infection model, 2K4L enhanced the survival of sepsis mice and decreased the production of the proinflammatory cytokines downregulating the phosphorylation level of signaling protein in MAPK and NF-κB signaling pathways, indicating that 2K4L represents a novel therapeutic antibiotic candidate against invasive multidrug-resistant bacterial strain infections.

Temporins: Multifunctional Peptides from Frog Skin

Temporins are short peptides secreted by frogs from all over the world. They exert antimicrobial activity, mainly against Gram-positive bacteria, including resistant pathogens; recent studies highlight other possible applications of these peptides as anticancer or antiviral agents. This review is meant to describe the main features of temporins produced by different ranid genera. Due to the abundance of published papers, we focus on the most widely investigated peptides. We report studies on their mechanism of action and three-dimensional structure in model systems mimicking bacterial membranes or in the presence of cells. The design and the antimicrobial activity of peptide analogues is also described, with the aim of highlighting elements that are crucial to improve the bioactivity of peptides while reducing their toxicity. Finally, a short section is dedicated to the studies aimed at applying these peptides as drugs, to produce new antimicrobial materials or in other technological uses.

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

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

Evaluation of the Synthetic Multifunctional Peptide Hp-MAP3 Derivative of Temporin-PTa

In recent years, antimicrobial peptides isolated from amphibian toxins have gained attention as new multifunctional drugs interacting with different molecular targets. We aimed to rationally design a new peptide from temporin-PTa. Hp-MAP3 (NH2-LLKKVLALLKKVL-COOH), net charge (+4), hydrophobicity (0.69), the content of hydrophobic residues (69%), and hydrophobic moment (0.73). For the construction of the analog peptide, the physicochemical characteristics were reorganized into hydrophilic and hydrophobic residues with the addition of lysines and leucines. The minimum inhibitory concentration was 2.7 to 43 μM against the growth of Gram-negative and positive bacteria, and the potential for biofilm eradication was 173.2 μM. Within 20 min, the peptide Hp-MAP3 (10.8 μM) prompted 100% of the damage to E. coli cells. At 43.3 μM, eliminated 100% of S. aureus within 5 min. The effects against yeast species of the Candida genus ranged from 5.4 to 86.6 μM. Hp-MAP3 presents cytotoxic activity against tumor HeLa at a concentration of 21.6 μM with an IC50 of 10.4 µM. Furthermore, the peptide showed hemolytic activity against murine erythrocytes. Structural studies carried out by circular dichroism showed that Hp-MAP3, while in the presence of 50% trifluoroethanol or SDS, an α-helix secondary structure. Finally, Amphipathic Hp-MAP3 building an important model for the design of new multifunctional molecules.

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.

Temporin B Forms Hetero-Oligomers with Temporin L, Modifies Its Membrane Activity, and Increases the Cooperativity of Its Antibacterial Pharmacodynamic Profile

The pharmacodynamic profile of antimicrobial peptides (AMPs) and their in vivo synergy are two factors that are thought to restrict resistance evolution and ensure their conservation. The frog Rana temporaria secretes a family of closely related AMPs, temporins A–L, as an effective chemical dermal defense. The antibacterial potency of temporin L has been shown to increase synergistically in combination with both temporins B and A, but this is modest. Here we show that the less potent temporin B enhances the cooperativity of the in vitro antibacterial activity of the more potent temporin L against EMRSA-15 and that this may be associated with an altered interaction with the bacterial plasma membrane, a feature critical for the antibacterial activity of most AMPs. Addition of buforin II, a histone H2A fragment, can further increase the cooperativity. Molecular dynamics simulations indicate temporins B and L readily form hetero-oligomers in models of Gram-positive bacterial plasma membranes. Patch-clamp studies show transmembrane ion conductance is triggered with lower amounts of both peptides and more quickly when used in combination, but conductance is of a lower amplitude and pores are smaller. Temporin B may therefore act by forming temporin L/B hetero-oligomers that are more effective than temporin L homo-oligomers at bacterial killing and/or by reducing the probability of the latter forming until a threshold concentration is reached. Exploration of the mechanism of synergy between AMPs isolated from the same organism may therefore yield antibiotic combinations with advantageous pharmacodynamic properties.

Antibiofilm and anticancer activities of unripe and ripe Azadirachta indica (neem) seed extracts

BACKGROUND

Antibiotic resistances of pathogens and breast cancer warrant the search for new alternative strategies. Phytoextracts can eradicate microbe-borne diseases as well as cancer with lower side effects compared to conventional antibiotics.

AIM

Unripe and ripe Azadirachta indica (neem) seed extracts were explored as potential antibiofilm and anticancer agents in combating multidrug-resistant infectious bacteria as well as anticancer agents against the MDR breast cancer cell lines.

METHODS

Shed-dried neem seeds (both unripe and ripe) were pulverized and extracted using methanol. The chemical components were identified with FTIR and gas chromatography - mass spectrometry. Antibiofilm activity of neem seed extracts were assessed in terms of minimum biofilm inhibitory concentration (MBIC), minimum biofilm eradication concentration (MBEC), and fluorescence microscopic studies on Staphylococcus aureus and Vibrio cholerae. Bacterial cells were studied by fluorescence microscopy using acridine orange/ethidium bromide as the staining agents. Minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) values were evaluated to observe the antibacterial activities. Cytotoxicity of the extracts against human blood lymphocytes and the anticancer activity against drug-resistant breast cancer cell lines were assessed by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay and fluorescence-activated cell sorting (FACS) studies.

RESULTS

4-Ethyl-2-hydroxy-2-cyclopentene-1-one, phthalic acid, and 2-hexyl-tetrahydro thiophane were the major compounds in unripe neem seed, whereas 3,5-dihydroxy-6-methyl-2,3-dihydro-4-H-pyran-4-one and 4-ethylbenzamide were predominant in ripe neem seed. Triazine derivatives were also common for both the extracts. MBIC values of unripe and ripe neem seed extracts for S. aureus are 75 and 100 µg/mL, respectively, and for V. cholerae, they are 100 and 300 µg/mL, respectively. MBEC values of unripe and ripe seed extracts are 500 and 300 µg/mL, respectively for S. aureus and for V. cholerae the values are 700 and 500 µg/mL, respectively. Fluorescence microscopic studies at 16 and 24 h, after bacterial culture, demonstrate enhanced antibiofilm activity for the ripe seed extract than that of the unripe seeds for both the bacteria. MTT assay reveals lower cytotoxicity of both the extracts towards normal blood lymphocytes, and anticancer activity against breast cancer cell line (MDA-MB-231) with superior activity of ripe seed extract. FACS studies further supported higher anticancer activity for ripe seed extract.

CONCLUSIONS

Methanolic extract of neem seeds could substantially inhibit and eradicate biofilm along with their potent antibacterial and anticancer activities. Both the extracts showed higher antibiofilm and antibacterial activity against S. aureus (gram-positive) than V. cholerae (gram-negative). Moreover, ripe seed extract showed higher antibiofilm and anticancer activity than unripe extracts.

Anti-biofilm potential of kefir-derived Lactobacillus paracasei L10 against Vibrio parahaemolyticus

Vibrio parahaemolyticus, a kind of biofilm-forming foodborne bacterium, presents formidable challenges to the effectiveness of antimicrobial agents. Increasingly, the safety of chemical antimicrobials has aroused the widespread attention of the public. The development of the novel nature antimicrobial agents has become critical for controlling biofilm-related pollution and infections. In this paper, we investigated the antibacterial activity of Lactobacillus paracasei L10, and evaluated the inhibition and eradication effects of the cell-free supernatant (CFS) of the strain on V. parahaemolyticus biofilms in detail. We found that the CFS exhibited marked antibacterial activity against all tested pathogenic bacteria. In co-cultural assay, L. paracasei L10 could notably reduce cell viability in both plankton and biofilm of V. parahaemolyticus and this antagonism effect in biofilm was greater than that in planktonic state. Meanwhile, the growth of V. parahaemolyticus was completely inhibited when 6% (v/v) of the CFS was added, and the supernatant also showed a concentration-dependent manner to inhibit and eradicate the biofilms of V. parahaemolyticus while decreased the metabolic activity of the biofilm in the same way. Moreover, the fluorescence microscopic and confocal laser scanning microscopy images confirmed the anti-biofilm activity of the CFS. This study elucidates that L. paracasei L10 displays a significant anti-biofilm effect on V. parahaemolyticus and the mechanism of its antagonism merits further study, which provides theoretical support for further development and application of L. paracasei L10 as anti-biofilm agents.

Functional Characterization of Porcine NK-Lysin: A Novel Immunomodulator That Regulates Intestinal Inflammatory Response

Porcine NK-Lysine (PNKL) is a new antimicrobial peptide (AMP) identified in the small intestine. In this study, PNKL protein was obtained through heterologous expression in Escherichia coli and was estimated by SDS-PAGE at 33 kDa. The antibacterial activities of PNKL were determined using various bacterial strains and showed broad-spectrum antimicrobial activity against Gram-negative and Gram-positive bacteria. Furthermore, E. coli K88-challenged IPEC-J2 cells were used to determine PNKL influences on inflammatory responses. Hemolytic assays showed that PNKL had no detrimental impact on cell viability. Interestingly, PNKL elevated the viability of IPEC-J2 cells exposure to E. coli K88. PNKL significantly decreased the cell apoptosis rate, and improved the distribution and abundance of tight junction protein ZO-1 in IPEC-J2 cells upon E. coli K88-challenge. Importantly, PNKL not only down regulated the expressions of inflammatory cytokines such as the IL-6 and TNF-α, but also down regulated the expressions of NF-κB, Caspase3, and Caspase9 in the E. coli K88-challenged cells. These results suggest a novel function of natural killer (NK)-lysin, and the anti-bacterial and anti-inflammatory properties of PNKL may allow it a potential substitute for conventionally used antibiotics or drugs.

New Perspectives in the Antimicrobial Activity of the Amphibian Temporin B: Peptide Analogs Are Effective Inhibitors of Candida albicans Growth

Temporin B (TB) is a short, positively charged peptide secreted by the granular glands of the European frog Rana temporaria. While the antibacterial and antiviral efficacy of TB and some of its improved analogs are well documented, nothing is known about their antifungal potency so far. We dedicated this study to characterize the antifungal potential of the TB analog TB_KKG6K and the newly designed D-Lys_TB_KKG6K, the latter having the L-lysines replaced by the chiral counterpart D-lysines to improve its proteolytic stability. Both peptides inhibited the growth of opportunistic human pathogenic yeasts and killed planktonic and sessile cells of the most prevalent human pathogen, Candida albicans. The anti-yeast efficacy of the peptides coincided with the induction of intracellular reactive oxygen species. Their thermal, cation, pH and serum tolerance were similar, while the proteolytic stability of D-Lys_TB_KKG6K was superior to that of its template peptide. Importantly, both peptides lacked hemolytic activity and showed minimal in vitro cytotoxicity in primary human keratinocytes. The tolerance of both peptides in a reconstructed human epidermis model further supports their potential for topical application. Our results open up an exciting field of research for new anti-Candida therapeutic options based on amphibian TB analogs.

Challenge in the Discovery of New Drugs: Antimicrobial Peptides against WHO-List of Critical and High-Priority Bacteria

Bacterial resistance has intensified in recent years due to the uncontrolled use of conventional drugs, and new bacterial strains with multiple resistance have been reported. This problem may be solved by using antimicrobial peptides (AMPs), which fulfill their bactericidal activity without developing much bacterial resistance. The rapid interaction between AMPs and the bacterial cell membrane means that the bacteria cannot easily develop resistance mechanisms. In addition, various drugs for clinical use have lost their effect as a conventional treatment; however, the synergistic effect of AMPs with these drugs would help to reactivate and enhance antimicrobial activity. Their efficiency against multi-resistant and extensively resistant bacteria has positioned them as promising molecules to replace or improve conventional drugs. In this review, we examined the importance of antimicrobial peptides and their successful activity against critical and high-priority bacteria published in the WHO list.

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.

The Antimicrobial Peptide Temporin G: Anti-Biofilm, Anti-Persister Activities, and Potentiator Effect of Tobramycin Efficacy Against Staphylococcus aureus

Bacterial biofilms are a serious threat for human health, and the Gram-positive bacterium Staphylococcus aureus is one of the microorganisms that can easily switch from a planktonic to a sessile lifestyle, providing protection from a large variety of adverse environmental conditions. Dormant non-dividing cells with low metabolic activity, named persisters, are tolerant to antibiotic treatment and are the principal cause of recalcitrant and resistant infections, including skin infections. Antimicrobial peptides (AMPs) hold promise as new anti-infective agents to treat such infections. Here for the first time, we investigated the activity of the frog-skin AMP temporin G (TG) against preformed S. aureus biofilm including persisters, as well as its efficacy in combination with tobramycin, in inhibiting S. aureus growth. TG was found to provoke ~50 to 100% reduction of biofilm viability in the concentration range from 12.5 to 100 µM vs ATCC and clinical isolates and to be active against persister cells (about 70–80% killing at 50–100 µM). Notably, sub-inhibitory concentrations of TG in combination with tobramycin were able to significantly reduce S. aureus growth, potentiating the antibiotic power. No critical cytotoxicity was detected when TG was tested in vitro up to 100 µM against human keratinocytes, confirming its safety profile for the development of a new potential anti-infective drug, especially for treatment of bacterial skin infections.

A Noncytotoxic Temporin L Analogue with In Vivo Antibacterial and Antiendotoxin Activities and a Nonmembrane-Lytic Mode of Action

Cytotoxic frog antimicrobial peptide Temporin L (TempL) is an attractive molecule for the design of lead antimicrobial agents due to its short size and versatile biological activities. However, noncytotoxic TempL variants with desirable biological activities have rarely been reported. TempL analogue Q3K,TempL is water-soluble and possesses a significant antiendotoxin property along with comparable cytotoxicity to TempL. A phenylalanine residue, located at the hydrophobic face of Q3K,TempL and the "d" position of its phenylalanine zipper sequence, was replaced with a cationic lysine residue. This analogue, Q3K,F8K,TempL, showed reduced hydrophobic moment and was noncytotoxic with lower antimicrobial activity. Interestingly, swapping between tryptophan at the fourth and serine at the sixth positions turned Q3K,F8K,TempL totally amphipathic as reflected by its helical wheel projection with clusters of hydrophobic and hydrophilic residues and the highest hydrophobic moment among these peptides. Surprisingly, this analogue, SW,Q3K,F8K,TempL, was as noncytotoxic as Q3K,F8K,TempL but showed augmented antimicrobial and antiendotoxin properties, comparable to that of TempL and Q3K,TempL. SW,Q3K,F8K,TempL exhibited appreciable survival of mice against P. aeruginosa infection and a lipopolysaccharide (LPS) challenge. Unlike TempL and Q3K,TempL, SW,Q3K,F8K,TempL adopted an unordered secondary structure in bacterial membrane mimetic lipid vesicles and did not permeabilize them or depolarize the bacterial membrane. Overall, the results demonstrate the design of a nontoxic TempL analogue that possesses clusters of hydrophobic and hydrophilic residues with impaired secondary structure and shows a nonmembrane-lytic mechanism and in vivo antiendotoxin and antimicrobial activities. This paradigm of design of antimicrobial peptide with clusters of hydrophobic and hydrophilic residues and high hydrophobic moment but low secondary structure could be attempted further.

Design, Engineering and Discovery of Novel α-Helical and β-Boomerang Antimicrobial Peptides against Drug Resistant Bacteria

In an era where the pipeline of new antibiotic development is drying up, the continuous rise of multi-drug resistant (MDR) and extensively drug resistant (XDR) bacteria are genuine threats to human health. Although antimicrobial peptides (AMPs) may serve as promising leads against drug resistant bacteria, only a few AMPs are in advanced clinical trials. The limitations of AMPs, namely their low in vivo activity, toxicity, and poor bioavailability, need to be addressed. Here, we review engineering of frog derived short α-helical AMPs (aurein, temporins) and lipopolysaccharide (LPS) binding designed β-boomerang AMPs for further development. The discovery of novel cell selective AMPs from the human proprotein convertase furin is also discussed.

In vitro and intracellular activities of frog skin temporins against Legionella pneumophila and its eukaryotic hosts

Temporin-SHa (SHa) is a small cationic host defence peptide (HDP) produced in skin secretions of the Sahara frog Pelophylax saharicus. This peptide has a broad-spectrum activity, efficiently targeting bacteria, parasites and viruses. Noticeably, SHa has demonstrated an ability to kill Leishmania infantum parasites (amastigotes) within macrophages. Recently, an analog of SHa with an increased net positive charge, named [K³]SHa, has been designed to improve those activities. SHa and [K³]SHa were both shown to exhibit leishmanicidal activity mainly by permeabilization of cell membranes but could also induce apoptotis-like death. Temporins are usually poorly active against Gram-negative bacteria whereas many of these species are of public health interest. Among them, Legionella pneumophila, the etiological agent of Legionnaire’s disease, is of major concern. Indeed, this bacterium adopts an intracellular lifestyle and replicate inside alveolar macrophages likewise inside its numerous protozoan hosts. Despite several authors have studied the antimicrobial activity of many compounds on L. pneumophila released from host cells, nothing is known about activity on intracellular L. pneumophila within their hosts, and subsequently mechanisms of action that could be involved. Here, we showed for the first time that SHa and [K³]SHa were active towards several species of Legionella. Both peptides displayed bactericidal activity and caused a loss of the bacterial envelope integrity leading to a rapid drop in cell viability. Regarding amoebae and THP-1-derived macrophages, SHa was less toxic than [K³]SHa and exhibited low half maximal lethal concentrations (LC₅₀). When used at non-toxic concentration (6.25 µM), SHa killed more than 90% L. pneumophila within amoebae and around 50% within macrophages. Using SHa labeled with the fluorescent dye Cy5, we showed an evenly diffusion within cells except in vacuoles. Moreover, SHa was able to enter the nucleus of amoebae and accumulate in the nucleolus. This subcellular localization seemed specific as macrophages nucleoli remained unlabeled. Finally, no modifications in the expression of cytokines and HDPs were recorded when macrophages were treated with 6.25 µM SHa. By combining all data, we showed that temporin-SHa decreases the intracellular L. pneumophila load within amoebae and macrophages without being toxic for eukaryotic cells. This peptide was also able to reach the nucleolus of amoebae but was not capable to penetrate inside vacuoles. These data are in favor of an indirect action of SHa towards intracellular Legionella and make this peptide a promising template for further developments.

Antibacterial, anti-biofilm and in vivo activities of the antimicrobial peptides P5 and P6.2

Cationic antimicrobial peptides (AMPs) are short linear amino acid sequences, which display antimicrobial activity against a wide range of bacterial species. They are promising novel antimicrobials since they have shown bactericidal effects against multiresistant bacteria. Their amphiphilic structure with hydrophobic and cationic regions drives their interaction with anionic bacterial cytoplasmic membranes, which leads to their disruption. In this work two synthetic designed AMPs, P5 and P6.2, which have been previously analyzed in their ability to interact with bacterial or eukaryotic membranes, were evaluated in their anti-biofilm and in vivo antibacterial activity. In a first step, a time-kill kinetic assay against P. aeruginosa and S. aureus and a curve for hemolytic activity were performed in order to determine the killing rate and the possible undesirable toxic effect, respectively, for both peptides. The biofilm inhibitory activity was quantified at sub MIC concentrations of the peptides and the results showed that P5 displayed antibiofilm activity on both strains while P6.2 only on S. aureus. Scanning electron microscopy (SEM) of bacteria treated with peptides at their MIC revealed protruding blisters on Gam-negative P. aeruginosa strain, but almost no visible surface alteration on Gram-positive S. aureus. These micrographs highlighted different manifestations of the membrane-disrupting activity that these kinds of peptides possess. Finally, both peptides were analyzed in vivo, in the lungs of neutropenic mice previously instilled with P. aeruginosa. Mice lungs were surgically extracted and bacteria and pro-inflammatory cytokines (IL-β, IL-6 and TNF-α) were quantified by colony forming units and ELISA, respectively. Results showed that instillation of the peptides produced a significant decrease in the number of living bacteria in the lungs, concomitant with a decrease in pro-inflammatory cytokines. Overall, the results presented here suggest that these two new peptides could be good candidates for future drug development for anti-biofilm and anti-infective therapy.

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.

Synergistic combinations of antimicrobial peptides against biofilms of methicillin-resistant Staphylococcus aureus (MRSA) on polystyrene and medical devices

OBJECTIVES

Antimicrobial research is being focused to look for more effective therapeutics against antibiotic-resistant infections such as those caused by methicillin-resistant Staphylococcus aureus (MRSA). In this regard, antimicrobial peptides (AMPs) appear to be a promising solution. The aim of the present study was to investigate the potential activity of temporin A, citropin 1.1, CA(1-7)M(2-9)NH₂ and Pal-KGK-NH₂ in synergistic activity against MRSA biofilms developed on polystyrene surface (PSS) and central venous catheter (CVC).

METHODS

The study was subdivided into distinct phases to assess the ability of AMPs to inhibit biofilm formation, to identify possible synergy between AMPs, and to eradicate preformed biofilms on PSS and CVC using AMPs alone or in combination.

RESULTS

Activity of the AMPs was particularly evident in the inhibition of biofilm formation on PSS and CVC, whilst the eradication of preformed biofilms was more difficult and was reached only after 24h of contact. The synergistic activity of AMP combinations, selected by their fractional inhibitory concentration index (FICI), led to an improvement in the performance of all of the molecules in the removal of different biofilms.

CONCLUSION

Overall, AMPs could represent the next generation of antimicrobial agents for a prophylactic or therapeutic tool to control biofilms of antibiotic-resistant bacteria and/or biofilm-associated infections on different medical devices.

The Bactericidal Activity of Temporin Analogues Against Methicillin Resistant Staphylococcus aureus

Staphylococcus aureus is a major infectious agent responsible for a plethora of superficial skin infections and systemic diseases, including endocarditis and septic arthritis. Recent epidemiological data revealed the emergence of resistance to commonly used antibiotics, including increased numbers of both hospital- and community-acquired methicillin-resistant S. aureus (MRSA). Due to their potent antimicrobial functions, low potential to develop resistance, and immunogenicity, antimicrobial peptides (AMPs) are a promising alternative treatment for multidrug-resistant strains. Here, we examined the activity of a lysine-rich derivative of amphibian temporin-1CEb (DK5) conjugated to peptides that exert pro-proliferative and/or cytoprotective activity. Analysis of a library of synthetic peptides to identify those with antibacterial potential revealed that the most potent agent against multidrug-resistant S. aureus was a conjugate of a temporin analogue with the synthetic Leu-enkephalin analogue dalargin (DAL). DAL-PEG-DK5 exerted direct bactericidal effects via bacterial membrane disruption, leading to eradication of both planktonic and biofilm-associated staphylococci. Finally, we showed that accumulation of the peptide in the cytoplasm of human keratinocytes led to a marked clearance of intracellular MRSA, resulting in cytoprotection against invading bacteria. Collectively, the data showed that DAL-PEG-DK5 might be a potent antimicrobial agent for treatment of staphylococcal skin infections.

Design and synthesis of new N-terminal fatty acid modified-antimicrobial peptide analogues with potent in vitro biological activity

Developing novel antimicrobial agents is a top priority in fighting against bacterial resistance. Thus, a series of new monomer and dimer peptides were designed and synthesized by conjugating fatty acids at the N-terminus of partial d-amino acid substitution analogues of anoplin and dimerization. The new peptides exhibited more efficient killing of gram-negative and gram-positive bacteria, including methicillin-resistant Staphylococcus aureus compared with the parent peptide anoplin, and the dimer peptides were superior to the monomer peptides. It was important that the new peptides displayed low impact on bacterial resistance development. In addition, the antimicrobial activities were not significantly influenced by a physiological salt environment. They also presented high stability in the presence of protease or serum. Almost all of the new peptides had better selectivity towards anionic bacterial membranes over zwitterionic mammalian cell membranes. Moreover, the new peptides displayed synergistic or additive effects when used together with the antibiotics rifampicin and polymyxin B. These results showed that the new peptides could also prevent the formation of bacterial biofilms. Furthermore, outer/inner membrane permeabilization and cytoplasmic membrane depolarization experiments revealed that the new peptides had strong membrane permeabilization and depolarization. Confocal laser scanning microscopy, flow cytometry analysis and scanning electron microscopy further demonstrated that the new peptides could damage the integrity of the bacterial membrane. Finally, a DNA-binding affinity assay showed that the new peptides could bind to bacterial DNA. In summary, the conjugation of fatty acids at the N-terminus of peptides and dimerization are promising strategies for obtaining potent antimicrobial agents.

Comparative evaluation of antimicrobial activity of different types of ionic liquids

In order to identify most suitable ionic liquids (ILs) for potential applications in infection prevention and control, in the present study we comparatively evaluated the antimicrobial potency and hemolytic activity of 15 ILs, including 11 previously described and four newly synthesized ILs, using standard microbiological procedures against Gram-positive and Gram-negative bacteria. ILs showing the lowest minimum inhibitory concentration (MIC) were tested for their hemolytic activity. Three ILs characterized by low MIC values and low hemolytic activity, namely 1-methyl-3-dodecylimidazolium bromide, 1-dodecyl-1-methylpyrrolidinium bromide, and 1-dodecyl-1-methylpiperidinium bromide were further investigated to determine their minimum bactericidal concentration (MBC), and their ability to inhibit biofilm formation by Staphylococcus aureus or Pseudomonas aeruginosa. Killing kinetics results revealed that both Gram-positive and Gram-negative bacteria are rapidly killed after exposure to MBC of the selected ILs. Furthermore, the selected ILs efficiently inhibited biofilm formation by S. aureus or P. aeruginosa. To our knowledge, this is the first systematic study investigating the antimicrobial potential of different types of ionic liquids using standard microbiological procedures. In the overall, the selected ILs showed low hemolytic and powerful antimicrobial activity, and efficient inhibition of biofilm formation, especially against S. aureus, suggesting their possible application as anti-biofilm agents.

How Melittin Inserts into Cell Membrane: Conformational Changes, Inter-Peptide Cooperation, and Disturbance on the Membrane

Antimicrobial peptides (AMPs), as a key component of the immune defense systems of organisms, are a promising solution to the serious threat of drug-resistant bacteria to public health. As one of the most representative and extensively studied AMPs, melittin has exceptional broad-spectrum activities against microorganisms, including both Gram-positive and Gram-negative bacteria. Unfortunately, the action mechanism of melittin with bacterial membranes, especially the underlying physics of peptide-induced membrane poration behaviors, is still poorly understood, which hampers efforts to develop melittin-based drugs or agents for clinical applications. In this mini-review, we focus on recent advances with respect to the membrane insertion behavior of melittin mostly from a computational aspect. Membrane insertion is a prerequisite and key step for forming transmembrane pores and bacterial killing by melittin, whose occurrence is based on overcoming a high free-energy barrier during the transition of melittin molecules from a membrane surface-binding state to a transmembrane-inserting state. Here, intriguing simulation results on such transition are highlighted from both kinetic and thermodynamic aspects. The conformational changes and inter-peptide cooperation of melittin molecules, as well as melittin-induced disturbances to membrane structure, such as deformation and lipid extraction, are regarded as key factors influencing the insertion of peptides into membranes. The associated intermediate states in peptide conformations, lipid arrangements, membrane structure, and mechanical properties during this process are specifically discussed. Finally, potential strategies for enhancing the poration ability and improving the antimicrobial performance of AMPs are included as well.

The Antimicrobial Peptide lin-SB056-1 and Its Dendrimeric Derivative Prevent Pseudomonas aeruginosa Biofilm Formation in Physiologically Relevant Models of Chronic Infections

Antimicrobial peptides (AMPs) are promising templates for the development of novel antibiofilm drugs. Despite the large number of studies on screening and optimization of AMPs, only a few of these evaluated the antibiofilm activity in physiologically relevant model systems. Potent in vitro activity of AMPs often does not translate into in vivo effectiveness due to the interference of the host microenvironment with peptide stability/availability. Hence, mimicking the complex environment found in biofilm-associated infections is essential to predict the clinical potential of novel AMP-based antimicrobials. In the present study, we examined the antibiofilm activity of the semi-synthetic peptide lin-SB056-1 and its dendrimeric derivative (lin-SB056-1)₂-K against Pseudomonas aeruginosa in an in vivo-like three-dimensional (3-D) lung epithelial cell model and an in vitro wound model (consisting of an artificial dermis and blood components at physiological levels). Although moderately active when tested alone, lin-SB056-1 was effective in reducing P. aeruginosa biofilm formation in association with 3-D lung epithelial cells in combination with the chelating agent EDTA. The dimeric derivative (lin-SB056-1)₂-K demonstrated an enhanced biofilm-inhibitory activity as compared to both lin-SB056-1 and the lin-SB056-1/EDTA combination, reducing the number of biofilm-associated bacteria up to 3-Log units at concentrations causing less than 20% cell death. Biofilm inhibition by (lin-SB056-1)₂-K was reported both for the reference strain PAO1 and cystic fibrosis lung isolates of P. aeruginosa. In addition, using fluorescence microscopy, a significant decrease in biofilm-like structures associated with 3-D cells was observed after peptide exposure. Interestingly, effectiveness of (lin-SB056-1)₂-K was also demonstrated in the wound model with a reduction of up to 1-Log unit in biofilm formation by P. aeruginosa PAO1 and wound isolates. Overall, combination treatment and peptide dendrimerization emerged as promising strategies to improve the efficacy of AMPs, especially under challenging host-mimicking conditions. Furthermore, the results of the present study underlined the importance of evaluating the biological properties of novel AMPs in in vivo-like model systems representative of specific infectious sites in order to make a more realistic prediction of their therapeutic success, and avoid the inclusion of unpromising peptides in animal studies and clinical trials.

Minor sequence modifications in temporin B cause drastic changes in antibacterial potency and selectivity by fundamentally altering membrane activity

Antimicrobial peptides (AMPs) are a potential source of new molecules to counter the increase in antimicrobial resistant infections but a better understanding of their properties is required to understand their native function and for effective translation as therapeutics. Details of the mechanism of their interaction with the bacterial plasma membrane are desired since damage or penetration of this structure is considered essential for AMPs activity. Relatively modest modifications to AMPs primary sequence can induce substantial changes in potency and/or spectrum of activity but, hitherto, have not been predicted to substantially alter the mechanism of interaction with the bacterial plasma membrane. Here we use a combination of molecular dynamics simulations, circular dichroism, solid-state NMR and patch clamp to investigate the extent to which temporin B and its analogues can be distinguished both in vitro and in silico on the basis of their interactions with model membranes. Enhancing the hydrophobicity of the N-terminus and cationicity of the C-terminus in temporin B improves its membrane activity and potency against both Gram-negative and Gram-positive bacteria. In contrast, enhancing the cationicity of the N-terminus abrogates its ability to trigger channel conductance and renders it ineffective against Gram-positive bacteria while nevertheless enhancing its potency against Escherichia coli. Our findings suggest even closely related AMPs may target the same bacterium with fundamentally differing mechanisms of action.

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.

The Amphibian Antimicrobial Peptide Temporin B Inhibits In Vitro Herpes Simplex Virus 1 Infection

The herpes simplex virus 1 (HSV-1) is widespread in the population, and in most cases its infection is asymptomatic. The currently available anti-HSV-1 drugs are acyclovir and its derivatives, although long-term therapy with these agents can lead to drug resistance. Thus, the discovery of novel antiherpetic compounds deserves additional effort. Naturally occurring antimicrobial peptides (AMPs) represent an interesting class of molecules with potential antiviral properties. To the best of our knowledge, this study is the first demonstration of the in vitro anti-HSV-1 activity of temporin B (TB), a short membrane-active amphibian AMP. In particular, when HSV-1 was preincubated with 20 μg/ml TB, significant antiviral activity was observed (a 5-log reduction of the virus titer). Such an effect was due to the disruption of the viral envelope, as demonstrated by transmission electron microscopy. Moreover, TB partially affected different stages of the HSV-1 life cycle, including the attachment and the entry of the virus into the host cell, as well as the subsequent postinfection phase. Furthermore, its efficacy was confirmed on human epithelial cells, suggesting TB as a novel approach for the prevention and/or treatment of HSV-1 infections.

Mining the tree of life: Host defense peptides as antiviral therapeutics

Discovering new therapeutics for human viral diseases is important for combatting emerging infectious viruses and omnipresent circulating viruses as well as those that can become resistant to the drugs we currently have available. The innate host defense peptide (HDP) repertoire present in animals is a wealth of potential antimicrobial agents that could be mined to meet these needs. While much of the body of research regarding HDPs is in the context of bacteria, there is increasing evidence that they can be an effective source for antivirals. Peptides can be identified in a number of ways, including eco-conservation-minded approaches. Those shown to have antiviral properties can be modified to exhibit desired properties as the relationship between structure and function is elucidated and then developed into therapeutics for human use. This review looks at the discovery and therapeutic potential of HDPs for human viral infections.

Synergistic and antibiofilm properties of ocellatin peptides against multidrug-resistant Pseudomonas aeruginosa

AIM

To test ocellatin peptides (ocellatins-PT2-PT6) for antibacterial and antibiofilm activities and synergy with antibiotics against Pseudomonas aeruginosa.

MATERIALS & METHODS

Normal- and checkerboard-broth microdilution methods were used. Biofilm studies included microtiter plate-based assays and microscopic analysis by confocal laser scanning microscopy and atomic force microscopy.

RESULTS

Ocellatins were more active against multidrug-resistant isolates of P. aeruginosa than against susceptible strains. Ocellatin-PT3 showed synergy with ciprofloxacin and ceftazidime against multidrug-resistant isolates and was capable of preventing the proliferation of 48-h mature biofilms at concentrations ranging from 4 to 8× the MIC. Treated biofilms had low viability and were slightly more disaggregated.

CONCLUSION

Ocellatin-PT3 may be promising as a template for the development of novel antimicrobial peptides against P. aeruginosa. [Formula: see text].

Combination Strategies to Enhance the Efficacy of Antimicrobial Peptides against Bacterial Biofilms

The great clinical significance of biofilm-associated infections and their inherent recalcitrance to antibiotic treatment urgently demand the development of novel antibiofilm strategies. In this regard, antimicrobial peptides (AMPs) are increasingly recognized as a promising template for the development of antibiofilm drugs. Indeed, owing to their main mechanism of action, which relies on the permeabilization of bacterial membranes, AMPs exhibit a strong antimicrobial activity also against multidrug-resistant bacteria and slow-growing or dormant biofilm-forming cells and are less prone to induce resistance compared to current antibiotics. Furthermore, the antimicrobial potency of AMPs can be highly increased by combining them with conventional (antibiotics) as well as unconventional bioactive molecules. Combination treatments appear particularly attractive in the case of biofilms since the heterogeneous nature of these microbial communities requires to target cells in different metabolic states (e.g., actively growing cells, dormant cells) and environmental conditions (e.g., acidic pH, lack of oxygen or nutrients). Therefore, the combination of different bioactive molecules acting against distinct biofilm components has the potential to facilitate biofilm control and/or eradication. The aim of this review is to highlight the most promising combination strategies developed so far to enhance the therapeutic potential of AMPs against bacterial biofilms. The rationale behind and beneficial outcomes of using AMPs in combination with conventional antibiotics, compounds capable of disaggregating the extracellular matrix, inhibitors of signaling pathways involved in biofilm formation (i.e., quorum sensing), and other peptide-based molecules will be presented and discussed.

PSN-PC: A Novel Antimicrobial and Anti-Biofilm Peptide from the Skin Secretion of Phyllomedusa-camba with Cytotoxicity on Human Lung Cancer Cell

Peptides derived from amphibian skin secretion are promising drug prototypes for combating widespread infection. In this study, a novel peptide belonging to the phylloseptin family of antimicrobial peptides was isolated from the skin secretion of the Phyllomedusa camba, namely phylloseptin-PC (PSN-PC). The biosynthetic precursor was obtained by molecular cloning and the mature peptide sequence was confirmed through tandem mass spectrometry (MS/MS) fragmentation sequencing in the skin secretion. The synthetic replicate exhibited a broad spectrum antimicrobial activity against Staphylococcus aureus, methicillin-resistant Staphylococcus aureus,Escherichia coli, Pseudomonas aeruginosa, Candida albicans at concentrations of 2, 2, 8, 32 and 2 µM, respectively. It also showed the capability of eliminating S. aureus biofilm with a minimal biofilm eradication concentration of 8 µM. The haemolysis of this peptide was not significant at low concentrations but had a considerable increase at high concentrations. Additionally, this peptide showed an anti-proliferation effect on the non-small cell lung cancer cell line (NCI-H157), with low cytotoxicity on the human microvascular endothelial cell line (HMEC-1). The discovery of the novel peptide may provide useful clues for new drug discoveries.

Generation of Persister Cells of Pseudomonas aeruginosa and Staphylococcus aureus by Chemical Treatment and Evaluation of Their Susceptibility to Membrane-Targeting Agents

Persister cells (PCs) are a subset of dormant, phenotypic variants of regular bacteria, highly tolerant to antibiotics. Generation of PCs in vivo may account for the recalcitrance of most chronic infections to antimicrobial treatment and demands for the identification of new antimicrobial agents able to target such cells. The present study explored the possibility to obtain in vitro PCs of Pseudomonas aeruginosa and Staphylococcus aureus at high efficiency through chemical treatment, and to test their susceptibility to structurally different antimicrobial peptides (AMPs) and two clinically used peptide-based antibiotics, colistin and daptomycin. The main mechanism of action of these molecules (i.e., membrane-perturbing activity) renders them potential candidates to act against dormant cells. Exposure of stationary-phase cultures to optimized concentrations of the uncoupling agent cyanide m-chlorophenylhydrazone (CCCP) was able to generate at high efficiency PCs exhibiting an antibiotic-tolerant phenotype toward different classes of antibiotics. The metabolic profile of CCCP-treated bacteria was investigated by monitoring bacterial heat production through isothermal microcalorimetry and by evaluating oxidoreductase activity by flow cytometry. CCCP-pretreated bacteria of both bacterial species underwent a substantial decrease in heat production and oxidoreductase activity, as compared to the untreated controls. After CCCP removal, induced persisters showed a delay in heat production that correlated with a lag phase before resumption of normal growth. The metabolic reactivation of bacteria coincided with their reversion to an antibiotic-sensitive phenotype. Interestingly, PCs generated by CCCP treatment resulted highly sensitive to three different membrane-targeting AMPs at levels comparable to those of CCCP-untreated bacteria. Colistin was also highly active against PCs of P. aeruginosa, while daptomycin killed PCs of S. aureus only at concentrations 32 to 64-fold higher than those of the tested AMPs. In conclusion, CCCP treatment was demonstrated to be a suitable method to generate in vitro PCs of medically important bacterial species at high efficiency. Importantly, unlike conventional antibiotics, structurally different AMPs were able to eradicate PCs suggesting that such molecules might represent valid templates for the development of new antimicrobials active against persisters.

The Semi-Synthetic Peptide Lin-SB056-1 in Combination with EDTA Exerts Strong Antimicrobial and Antibiofilm Activity against Pseudomonas aeruginosa in Conditions Mimicking Cystic Fibrosis Sputum

Pseudomonas aeruginosa is a major cause of chronic lung infections in cystic fibrosis (CF) patients. The ability of the bacterium to form biofilms and the presence of a thick and stagnant mucus in the airways of CF patients largely contribute to antibiotic therapy failure and demand for new antimicrobial agents able to act in the CF environment. The present study investigated the anti-P. aeruginosa activity of lin-SB056-1, a recently described semi-synthetic antimicrobial peptide, used alone and in combination with the cation chelator ethylenediaminetetraacetic acid (EDTA). Bactericidal assays were carried out in standard culture conditions and in an artificial sputum medium (ASM) closely resembling the CF environment. Peptide’s structure and interaction with large unilamellar vesicles in media with different ionic strengths were also investigated through infrared spectroscopy. Lin-SB056-1 demonstrated fast and strong bactericidal activity against both mucoid and non-mucoid strains of P. aeruginosa in planktonic form and, in combination with EDTA, caused significant reduction of the biomass of P. aeruginosa mature biofilms. In ASM, the peptide/EDTA combination exerted a strong bactericidal effect and inhibited the formation of biofilm-like structures of P. aeruginosa. Overall, the results obtained highlight the potential of the lin-SB056-1/EDTA combination for the treatment of P. aeruginosa lung infections in CF patients.