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

QSAR Reveals Decreased Lipophilicity of Polar Residues Determines the Selectivity of Antimicrobial Peptide Activity

Antimicrobial resistance has increased rapidly, causing daunting morbidity and mortality rates worldwide. Antimicrobial peptides (AMPs) have emerged as promising alternatives to traditional antibiotics due to their broad range of targets and low tendency to elicit resistance. However, potent antimicrobial activity is often accompanied by excessive cytotoxicity toward host cells, leading to a halt in AMP therapeutic development. Here, we present multivariate analyses that correlate 28 peptide properties to the activity and toxicity of 46 diverse African-derived AMPs and identify the negative lipophilicity of polar residues as an essential physiochemical property for selective antimicrobial activity. Twenty-seven active AMPs are identified, of which the majority are of scorpion or frog origin. Of these, thirteen are novel with no previously reported activities. Principal component analysis and quantitative structure-activity relationships (QSAR) reveal that overall hydrophobicity, lipophilicity, and residue side chain surface area affect the antimicrobial and cytotoxic activity of an AMP. This has been well documented previously, but the present QSAR analysis additionally reveals that a decrease in the lipophilicity, contributed by those amino acids classified as polar, confers selectivity for a peptide to pathogen over mammalian cells. Furthermore, an increase in overall peptide charge aids selectivity toward Gram-negative bacteria and fungi, while selectivity toward Gram-positive bacteria is obtained through an increased number of small lipophilic residues. Finally, a conservative increase in peptide size in terms of sequence length and molecular weight also contributes to improved activity without affecting toxicity. Our findings suggest a novel approach for the rational design or modification of existing AMPs to increase pathogen selectivity and enhance therapeutic potential.

Tryptophan End-Tagging Confers Antifungal Activity on a Tick-Derived Peptide by Triggering Reactive Oxygen Species Production

WHO has identified several Candida species including Candida albicans as critical priority fungal pathogens due to greater infection prevalence and formation of recalcitrant biofilms. Novel antifungal agents are urgently needed, and antimicrobial peptides (AMPs) are being considered as potential alternatives, but inactivity in physiological salt environments, serum, and plasma often limits further therapeutic development. Tryptophan end-tagging is a strategy to overcome these limitations and is thought to selectively enhance membrane permeabilization in both fungal and bacterial plasma membranes. Here, we show that C-terminal tryptophan end-tagging of the tick-derived peptide Os-C transforms an inactive peptide into Os-C(W5), an antifungal peptide capable of preventing the formation of C. albicans biofilms. Mechanistic insight is provided by circular dichroism spectroscopy and molecular dynamics simulations, which demonstrate that tryptophan end-tagging alters the secondary structure of Os-C, while the latter reveals that end-tagging reduces interactions with, and insertion into, a model C. albicans membrane but promotes peptide aggregation on its surface. Interestingly, this leads to the induction of reactive oxygen species production rather than membrane permeabilization, and consequently, oxidative stress leads to cell wall damage. Os-C(W5) does not induce the hemolysis of human erythrocytes. Reduced cell adhesion and viability contribute to decreased biofilm extracellular matrix formation which, although reduced, is retained in the serum-containing medium. In this study, tryptophan end-tagging was identified as a promising strategy for enhancing the antifungal activity, including the biofilm inhibitory activity of Os-C against C. albicans in physiological salt environments.

Solving polymicrobial puzzles: evolutionary dynamics and future directions

Polymicrobial infections include various microorganisms, often necessitating different treatment methods than a monomicrobial infection. Scientists have been puzzled by the complex interactions within these communities for generations. The presence of specific microorganisms warrants a chronic infection and impacts crucial factors such as virulence and antibiotic susceptibility. Game theory is valuable for scenarios involving multiple decision-makers, but its relevance to polymicrobial infections is limited. Eco-evolutionary dynamics introduce causation for multiple proteomic interactions like metabolic syntropy and niche segregation. The review culminates both these giants to form evolutionary dynamics (ED). There is a significant amount of literature on inter-bacterial interactions that remain unsynchronised. Such raw data can only be moulded by analysing the ED involved. The review culminates the inter-bacterial interactions in multiple clinically relevant polymicrobial infections like chronic wounds, CAUTI, otitis media and dental carries. The data is further moulded with ED to analyse the niche colonisation of two notoriously competitive bacteria: S.aureus and P.aeruginosa. The review attempts to develop a future trajectory for polymicrobial research by following recent innovative strategies incorporating ED to curb polymicrobial infections.

Mechanistic Insight into the Early Stages of Toroidal Pore Formation by the Antimicrobial Peptide Smp24

The antimicrobial peptide Smp24, originally derived from the venom of Scorpio maurus palmatus, is a promising candidate for further drug development. However, before doing so, greater insight into the mechanism of action is needed to construct a reliable structure-activity relationship. The aim of this study was to specifically investigate the critical early stages of peptide-induced membrane disruption. Single-channel current traces were obtained via planar patch-clamp electrophysiology, with multiple types of pore-forming events observed, unlike those expected from the traditional, more rigid mechanistic models. To better understand the molecular-level structures of the peptide-pore assemblies underlying these observed conductance events, molecular dynamics simulations were used to investigate the peptide structure and orientation both before and during pore formation. The transition of the peptides to transmembrane-like states within disordered toroidal pores occurred due to a peptide-induced bilayer-leaflet asymmetry, explaining why pore stabilization does not always follow pore nucleation in the experimental observations. To fully grasp the structure-activity relationship of antimicrobial peptides, a more nuanced view of the complex and dynamic mechanistic behaviour must be adopted.

Synergy between Winter Flounder antimicrobial peptides

Some antimicrobial peptides (AMPs) have potent bactericidal activity and are being considered as potential alternatives to classical antibiotics. In response to an infection, such AMPs are often produced in animals alongside other peptides with low or no perceivable antimicrobial activity, whose role is unclear. Here we show that six AMPs from the Winter Flounder (WF) act in synergy against a range of bacterial pathogens and provide mechanistic insights into how this increases the cooperativity of the dose-dependent bactericidal activity and potency that enable therapy. Only two WF AMPs have potent antimicrobial activity when used alone but we find a series of two-way combinations, involving peptides which otherwise have low or no activity, yield potent antimicrobial activity. Weakly active WF AMPs modulate the membrane interactions of the more potent WF AMPs and enable therapy in a model of Acinetobacter baumannii burn wound infection. The observed synergy and emergent behaviour may explain the evolutionary benefits of producing a family of related peptides and are attractive properties to consider when developing AMPs towards clinical applications.

Bolaamphiphile Analogues of 12-bis-THA Cl2 Are Potent Antimicrobial Therapeutics with Distinct Mechanisms of Action against Bacterial, Mycobacterial, and Fungal Pathogens

12-Bis-THA Cl2 [12,12'-(dodecane-1,12-diyl)-bis-(9-amino-1,2,3,4-tetrahydroacridinium) chloride] is a cationic bolalipid adapted from dequalinium chloride (DQC), a bactericidal anti-infective indicated for bacterial vaginosis (BV). Here, we used a structure-activity-relationship study to show that the factors that determine effective killing of bacterial, fungal, and mycobacterial pathogens differ, to generate new analogues with a broader spectrum of activity, and to identify synergistic relationships, most notably with aminoglycosides against Acinetobacter baumannii and Pseudomonas aeruginosa, where the bactericidal killing rate was substantially increased. Like DQC, 12-bis-THA Cl2 and its analogues accumulate within bacteria and fungi. More hydrophobic analogues with larger headgroups show reduced potential for DNA binding but increased and broader spectrum antibacterial activity. In contrast, analogues with less bulky headgroups and stronger DNA binding affinity were more active against Candida spp. Shortening the interconnecting chain, from the most lipophilic twelve-carbon chain to six, improved the selectivity index against Mycobacterium tuberculosis in vitro, but only the longer chain analogue was therapeutic in a Galleria mellonella infection model, with the shorter chain analogue exacerbating the infection. In vivo therapy of Escherichia coli ATCC 25922 and epidemic methicillin-resistant Staphylococcus aureus 15 (EMRSA-15) infections in Galleria mellonella was also achieved with longer-chain analogues, as was therapy for an A. baumannii 17978 burn wound infection with a synergistic combination of bolaamphiphile and gentamicin. The present study shows how this class of bolalipids may be adapted further to enable a wider range of potential applications. IMPORTANCE While we face an acute threat from antibiotic resistant bacteria and a lack of new classes of antibiotic, there are many effective antimicrobials which have limited application due to concerns regarding their toxicity and which could be more useful if such risks are reduced or eliminated. We modified a bolalipid antiseptic used in throat lozenges to see if it could be made more effective against some of the highest-priority bacteria and less toxic. We found that structural modifications that rendered the lipid more toxic against human cells made it less toxic in infection models and we could effectively treat caterpillars infected with either Mycobacterium tuberculosis, methicillin resistant Staphylococcus aureus, or Acinetobacter baumannii. The study provides a rationale for further adaptation toward diversifying the range of indications in which this class of antimicrobial may be used.

Intramembrane Nanoaggregates of Antimicrobial Peptides Play a Vital Role in Bacterial Killing

Recent developments in antimicrobial peptides (AMPs) have focused on the rational design of short sequences with less than 20 amino acids due to their relatively low synthesis costs and ease of correlation of the structure-function relationship. However, gaps remain in the understanding of how short cationic AMPs interact with the bacterial outer and inner membranes to affect their antimicrobial efficacy and dynamic killing. The membrane-lytic actions of two designed AMPs, G(IIKK)₃ I-NH₂ (G₃ ) and G(IIKK)₄ I-NH₂ (G₄ ), and previously-studied controls GLLDLLKLLLKAAG-NH₂ (LDKA, biomimetic) and GIGAVLKVLTTGLPALISWIKRKR-NH₂ (Melittin, natural) are examined. The mechanistic processes of membrane damage and the disruption strength of the four AMPs are characterized by molecular dynamics simulations and experimental measurements including neutron reflection and scattering. The results from the combined studies are characterized with distinctly different intramembrane nanoaggregates formed upon AMP-specific binding, reflecting clear influences of AMP sequence, charge and the chemistry of the inner and outer membranes. G₃ and G₄ display different nanoaggregation with the outer and inner membranes, and the smaller sizes and further extent of insertion of the intramembrane nanoaggregates into bacterial membranes correlate well with their greater antimicrobial efficacy and faster dynamic killing. This work demonstrates the crucial roles of intramembrane nanoaggregates in optimizing antimicrobial efficacy and dynamic killing.

The Membrane Activity of the Amphibian Temporin B Peptide Analog TB_KKG6K Sheds Light on the Mechanism That Kills Candida albicans

Temporin B (TB) is a 13-amino-acid-long, cationic peptide secreted by the granular glands of the European frog Rana temporaria. We recently showed that the modified TB peptide analog TB_KKG6K rapidly killed planktonic and sessile Candida albicans at low micromolar concentrations and was neither hemolytic nor cytotoxic to mammalian cells in vitro. The present study aimed to shed light into its mechanism of action, with a focus on its fungal cell membrane activity. We utilized different fluorescent dyes to prove that it rapidly induces membrane depolarization and permeabilization. Studies on model membrane systems revealed that the TB analog undergoes hydrophobic and electrostatic membrane interactions, showing a preference for anionic lipids, and identified phosphatidylinositol and cardiolipin as possible peptide targets. Fluorescence microscopy using fluorescein isothiocyanate-labeled TB_KKG6K in the presence of the lipophilic dye FM4-64 indicated that the peptide compromises membrane integrity and rapidly enters C. albicans cells in an energy-independent manner. Peptide-treated cells analyzed by cryo-based electron microscopy exhibited no signs of cell lysis; however, subcellular structures had disintegrated, suggesting that intracellular activity may form part of the killing mechanism of the peptide. Taken together, this study proved that TB_KKG6K compromises C. albicans membrane function, which explains the previously observed rapid, fungicidal mode of action and supports its great potential as a future anti-Candida therapeutic. IMPORTANCE Fungal infections with the opportunistic human pathogen C. albicans are associated with high mortality rates in immunocompromised patients. This is partly due to the yeast's ability to rapidly develop resistance toward currently available antifungals. Small, cationic, membrane-active peptides are promising compounds to fight against resistance development, as many of them effectuate rapid fungal cell death. This fast killing is believed to hamper the development of resistance, as the fungi do not have sufficient time to adapt to the antifungal compound. We previously reported that the synthetic variant of the amphibian TB peptide, TB_KKG6K, rapidly kills C. albicans. In the current study, the mechanism of action of the TB analog was investigated. We show that this TB analog is membrane-active and impairs cell membrane function, highlighting its potential to be developed as an attractive alternative anti-C. albicans therapeutic that may hinder the development of resistance.

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.

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.

Multiple Antimicrobial Effects of Hybrid Peptides Synthesized Based on the Sequence of Ribosomal S1 Protein from Staphylococcus aureus

The need to develop new antimicrobial peptides is due to the high resistance of pathogenic bacteria to traditional antibiotics now and in the future. The creation of synthetic peptide constructs is a common and successful approach to the development of new antimicrobial peptides. In this work, we use a simple, flexible, and scalable technique to create hybrid antimicrobial peptides containing amyloidogenic regions of the ribosomal S1 protein from Staphylococcus aureus. While the cell-penetrating peptide allows the peptide to enter the bacterial cell, the amyloidogenic site provides an antimicrobial effect by coaggregating with functional bacterial proteins. We have demonstrated the antimicrobial effects of the R23F, R23DI, and R23EI hybrid peptides against Staphylococcus aureus, methicillin-resistant S. aureus (MRSA), Pseudomonas aeruginosa, Escherichia coli, and Bacillus cereus. R23F, R23DI, and R23EI can be used as antimicrobial peptides against Gram-positive and Gram-negative bacteria resistant to traditional antibiotics.

Toward insights on antimicrobial selectivity of host defense peptides via machine learning model interpretation

Host defense peptides are promising candidates for the development of novel antibiotics. To realize their therapeutic potential, high levels of target selectivity is essential. This study aims to identify factors governing selectivity via the use of the random forest algorithm for correlating peptide sequence information with their bioactivity data. Satisfactory predictive models were achieved from out-of-bag prediction that yielded accuracies and Matthew's correlation coefficients in excess of 0.80 and 0.57, respectively. Model interpretation through the use of variable importance metrics and partial dependence plots indicated that the selectivity was heavily influenced by the composition and distribution patterns of molecular charge and solubility related parameters. Furthermore, the three investigated bacterial target species (Escherichia coli, Pseudomonas aeruginosa and Staphylococcus aureus) likely had a significant influence on how selectivity was realized as there appears to be a similar underlying selectivity mechanism on the basis of charge-solubility properties (i.e. but which is tailored according to the target in question).

Computational Methods and Tools in Antimicrobial Peptide Research

The evolution of antibiotic-resistant bacteria is an ongoing and troubling development that has increased the number of diseases and infections that risk going untreated. There is an urgent need to develop alternative strategies and treatments to address this issue. One class of molecules that is attracting significant interest is that of antimicrobial peptides (AMPs). Their design and development has been aided considerably by the applications of molecular models, and we review these here. These methods include the use of tools to explore the relationships between their structures, dynamics, and functions and the increasing application of machine learning and molecular dynamics simulations. This review compiles resources such as AMP databases, AMP-related web servers, and commonly used techniques, together aimed at aiding researchers in the area toward complementing experimental studies with computational approaches.

Nanocapsule designs for antimicrobial resistance

The pressing need of new antimicrobial products is growing stronger, particularly because of widespread antimicrobial resistance, endangering our ability to treat common infections. The recent coronavirus pandemic has dramatically highlighted the necessity of effective antibacterial and antiviral protection. This work explores at the molecular level the mechanism of action of antibacterial nanocapsules assembled in virus-like particles, their stability and their interaction with mammal and antimicrobial model membranes. We use Molecular Dynamics with force-fields of different granularity and protein design strategies to study the stability, self-assembly and membrane poration properties of these nanocapsules.

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.

Therapeutic Potential of Antimicrobial Peptides in Polymicrobial Biofilm-Associated Infections

It is widely recognized that many chronic infections of the human body have a polymicrobial etiology. These include diabetic foot ulcer infections, lung infections in cystic fibrosis patients, periodontitis, otitis, urinary tract infections and even a proportion of systemic infections. The treatment of mixed infections poses serious challenges in the clinic. First, polymicrobial communities of microorganisms often organize themselves as biofilms that are notoriously recalcitrant to antimicrobial therapy and clearance by the host immune system. Secondly, a plethora of interactions among community members may affect the expression of virulence factors and the susceptibility to antimicrobials of individual species in the community. Therefore, new strategies able to target multiple pathogens in mixed populations need to be urgently developed and evaluated. In this regard, antimicrobial or host defense peptides (AMPs) deserve particular attention as they are endowed with many favorable features that may serve to this end. The aim of the present review is to offer a comprehensive and updated overview of studies addressing the therapeutic potential of AMPs in mixed infections, highlighting the opportunities offered by this class of antimicrobials in the fight against polymicrobial infections, but also the limits that may arise in their use for this type of application.

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.

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.

Synthetic Antimicrobial Peptide Tuning Permits Membrane Disruption and Interpeptide Synergy

The ribosomally produced antimicrobial peptides of bacteria (bacteriocins) represent an unexplored source of membrane-active antibiotics. We designed a library of linear peptides from a circular bacteriocin and show that pore-formation dynamics in bacterial membranes are tunable via selective amino acid substitution. We observed antibacterial interpeptide synergy indicating that fundamentally altering interactions with the membrane enables synergy. Our findings suggest an approach for engineering pore-formation through rational peptide design and increasing the utility of novel antimicrobial peptides by exploiting synergy.

Temporins: An Approach of Potential Pharmaceutic Candidates

Antimicrobial peptides (AMPs), also known as host defense peptides, are small and mostly polycationic molecules that form part of the innate immune response. There are currently more than 3000 experimentally reported AMPs. Particularly in frogs, the temporin family has been discovered as potential AMPs. The aim of this work is to review the latest publications about this class of peptides, discuss their properties, and present an update of the last studies and new discoveries in the field. More than 130 temporins have been identified in this family. The most studied temporins are temporin A (TA), temporin B (TB), and temporin L (TL). These peptides showed antimicrobial activity against gram-negative, gram-positive bacteria and fungi. Since the discovery of temporins in 1996, several groups of researchers isolated different peptides from various species of frogs that were included as members of this family. Although antimicrobial activity of many temporins has not been analyzed yet, most of them showed antimicrobial and antifungal activities. A combination of nanotechnology and AMPs for temporins in different antimicrobial treatments could be a promising alternative for resistant pathogens. These studies demonstrate that, even with the advancement in scientific research on the composition and antimicrobial activity of temporins, further studies are necessary to wholly understand their components and mechanisms of action.

Temporin L and aurein 2.5 have identical conformations but subtly distinct membrane and antibacterial activities

Frogs such as Rana temporaria and Litoria aurea secrete numerous closely related antimicrobial peptides (AMPs) as an effective chemical dermal defence. Damage or penetration of the bacterial plasma membrane is considered essential for AMP activity and such properties are commonly ascribed to their ability to form secondary amphipathic, α-helix conformations in membrane mimicking milieu. Nevertheless, despite the high similarity in physical properties and preference for adopting such conformations, the spectrum of activity and potency of AMPs often varies considerably. Hence distinguishing apparently similar AMPs according to their behaviour in, and effects on, model membranes will inform understanding of primary-sequence-specific antimicrobial mechanisms. Here we use a combination of molecular dynamics simulations, circular dichroism and patch-clamp to investigate the basis for differing anti-bacterial activities in representative AMPs from each species; temporin L and aurein 2.5. Despite adopting near identical, α-helix conformations in the steady-state in a variety of membrane models, these two AMPs can be distinguished both in vitro and in silico based on their dynamic interactions with model membranes, notably their differing conformational flexibility at the N-terminus, ability to form higher order aggregates and the characteristics of induced ion conductance. Taken together, these differences provide an explanation of the greater potency and broader antibacterial spectrum of activity of temporin L over aurein 2.5. Consequently, while the secondary amphipathic, α-helix conformation is a key determinant of the ability of a cationic AMP to penetrate and disrupt the bacterial plasma membrane, the exact mechanism, potency and spectrum of activity is determined by precise structural and dynamic contributions from specific residues in each AMP sequence.

Sarconesin II, a New Antimicrobial Peptide Isolated from Sarconesiopsis magellanica Excretions and Secretions

Antibiotic resistance is at dangerous levels and increasing worldwide. The search for new antimicrobial drugs to counteract this problem is a priority for health institutions and organizations, both globally and in individual countries. Sarconesiopsis magellanica blowfly larval excretions and secretions (ES) are an important source for isolating antimicrobial peptides (AMPs). This study aims to identify and characterize a new S. magellanica AMP. RP-HPLC was used to fractionate ES, using C18 columns, and their antimicrobial activity was evaluated. The peptide sequence of the fraction collected at 43.7 min was determined by mass spectrometry (MS). Fluorescence and electronic microscopy were used to evaluate the mechanism of action. Toxicity was tested on HeLa cells and human erythrocytes; physicochemical properties were evaluated. The molecule in the ES was characterized as sarconesin II and it showed activity against Gram-negative (Escherichia coli MG1655, Pseudomonas aeruginosa ATCC 27853, P. aeruginosa PA14) and Gram-positive (Staphylococcus aureus ATCC 29213, Micrococcus luteus A270) bacteria. The lowest minimum inhibitory concentration obtained was 1.9 μM for M. luteus A270; the AMP had no toxicity in any cells tested here and its action in bacterial membrane and DNA was confirmed. Sarconesin II was documented as a conserved domain of the ATP synthase protein belonging to the Fli-1 superfamily. The data reported here indicated that peptides could be alternative therapeutic candidates for use in infections against Gram-negative and Gram-positive bacteria and eventually as a new resource of compounds for combating multidrug-resistant bacteria.