Synergistic activity profile of an antimicrobial peptide against multidrug-resistant and extensively drug-resistant strains of Gram-negative bacterial pathogens

1,3,5-Triazine as Branching Connector for the Construction of Novel Antimicrobial Peptide Dendrimers: Synthesis and Biological Characterization

Peptides displaying antimicrobial properties are being regarded as useful tools to evade and combat antimicrobial resistance, a major public health challenge. Here we have addressed dendrimers, attractive molecules in pharmaceutical innovation and development displaying broad biological activity. Triazine-based dendrimers were fully synthesized in the solid phase, and their antimicrobial activity and some insights into their mechanisms of action were explored. Triazine is present in a large number of compounds with highly diverse biological targets with broad biological activities and could be an excellent branching unit to accommodate peptides. Our results show that the novel peptide dendrimers synthesized have remarkable antimicrobial activity against Gram-negative bacteria (E. coli and P. aeruginosa) and suggest that they may be useful in neutralizing the effect of efflux machinery on resistance.

Synergistic effect of two antimicrobial peptides, BP203 and MAP-0403 J-2 with conventional antibiotics against colistin-resistant Escherichia coli and Klebsiella pneumoniae clinical isolates

Drug-resistant Enterobacterales infections are a great health concern due to the lack of effective treatments. Consequently, finding novel antimicrobials or combining therapies becomes a crucial approach in addressing this problem. BP203 and MAP-0403 J-2, novel antimicrobial peptides, have exhibited effectiveness against Gram-negative bacteria. In this study, we assessed the in vitro antibacterial activity of BP203 and MAP-0403 J-2, along with their synergistic interaction with conventional antibiotics including colistin, rifampicin, chloramphenicol, ceftazidime, meropenem, and ciprofloxacin against colistin-resistant Escherichia coli and Klebsiella pneumoniae clinical isolates. The minimal inhibitory concentrations (MIC) of BP203 and MAP-0403 J-2 against tested E. coli isolates were 2-16 and 8-32 μg/mL, respectively. However, for the majority of K. pneumoniae isolates, the MIC of BP203 and MAP-0403 J-2 were >128 μg/mL. Notably, our results demonstrated a synergistic effect when combining BP203 with rifampicin, meropenem, or chloramphenicol, primarily observed in most K. pneumoniae isolates. In contrast, no synergism was evident between BP203 and colistin, chloramphenicol, ceftazidime, rifampicin, or ciprofloxacin when tested against all E. coli isolates. Furthermore, synergistic effects between MAP-0403 J-2 and rifampicin, ceftazidime or colistin were observed against the majority of E. coli isolates. Similarly, the combined effect of MAP-0403 J-2 with rifampicin or chloramphenicol was synergistic in the majority of K. pneumoniae isolates. Importantly, these peptides displayed the stability at high temperatures, across a wide range of pH values, in specific serum concentrations and under physiological salt conditions. Both peptides also showed no significant hemolysis and cytotoxicity against mammalian cells. Our findings suggested that BP203 and MAP-0403 J-2 are promising candidates against colistin-resistant E. coli. Meanwhile, the synergism of these peptides and certain antibiotics could be of great therapeutic value as antimicrobial drugs against infections caused by colistin-resistant E. coli and K. pneumoniae.

The antimicrobial peptide Esc(1-21)-1c increases susceptibility of Pseudomonas aeruginosa to conventional antibiotics by decreasing the expression of the MexAB-OprM efflux pump

Introduction: The increase in bacterial strains resistant to conventional antibiotics is an alarming problem for human health and could lead to pandemics in the future. Among bacterial pathogens responsible for a large variety of severe infections there is Pseudomonas aeruginosa. Therefore, there is an urgent need for new molecules with antimicrobial activity or that can act as adjuvants of antibiotics already in use. In this scenario, antimicrobial peptides (AMPs) hold great promise. Recently, we characterized a frog-skin AMP derived from esculentin-1a, namely Esc(1-21)-1c, endowed with antipseudomonal activity without being cytotoxic to human cells. Methods: The combinatorial effect of the peptide and antibiotics was investigated through the checkerboard assay, differential proteomic and transcriptional analysis. Results: Here, we found that Esc(1-21)-1c can synergistically inhibit the growth of P. aeruginosa cells with three different antibiotics, including tetracycline. We therefore investigated the underlying mechanism implemented by the peptide using a differential proteomic approach. The data revealed a significant decrease in the production of three proteins belonging to the MexAB-OprM efflux pump upon treatment with sub-inhibitory concentration of Esc(1-21)-1c. Down-regulation of these proteins was confirmed by transcriptional analysis and direct measurement of their relative levels in bacterial cells by tandem mass spectrometry analysis in multiple reaction monitoring scan mode. Conclusion: These evidences suggest that treatment with Esc(1-21)-1c in combination with antibiotics would increase the intracellular drug content making bacteria more susceptible to the antibiotic. Overall, these results highlight the importance of characterizing new molecules able to synergize with conventional antibiotics, paving the way for the development of alternative therapeutic strategies based on AMP/antibiotic formulations to counteract the emergence of resistant bacterial strains and increase the use of "old" antibiotics in medical practice.

Complementary Activities of Host Defence Peptides and Antibiotics in Combating Antimicrobial Resistant Bacteria

Due to their ability to eliminate antimicrobial resistant (AMR) bacteria and to modulate the immune response, host defence peptides (HDPs) hold great promise for the clinical treatment of bacterial infections. Whereas monotherapy with HDPs is not likely to become an effective first-line treatment, combinations of such peptides with antibiotics can potentially provide a path to future therapies for AMR infections. Therefore, we critically reviewed the recent literature regarding the antibacterial activity of combinations of HDPs and antibiotics against AMR bacteria and the approaches taken in these studies. Of the 86 studies compiled, 56 featured a formal assessment of synergy between agents. Of the combinations assessed, synergistic and additive interactions between HDPs and antibiotics amounted to 84.9% of the records, while indifferent and antagonistic interactions accounted for 15.1%. Penicillin, aminoglycoside, fluoro/quinolone, and glycopeptide antibiotic classes were the most frequently documented as interacting with HDPs, and Staphylococcus aureus, Pseudomonas aeruginosa, Escherichia coli, and Enterococcus faecium were the most reported bacterial species. Few studies formally evaluated the effects of combinations of HDPs and antibiotics on bacteria, and even fewer assessed such combinations against bacteria within biofilms, in animal models, or in advanced tissue infection models. Despite the biases of the current literature, the studies suggest that effective combinations of HDPs and antibiotics hold promise for the future treatment of infections caused by AMR bacteria.

Transforming Cross-Linked Cyclic Dimers of KR-12 into Stable and Potent Antimicrobial Drug Leads

Is it possible to enhance structural stability and biological activity of KR-12, a truncated antimicrobial peptide derived from the human host defense peptide LL-37? Based on the mapping of essential residues in KR-12, we have designed backbone-cyclized dimers, cross-linked via a disulfide bond to improve peptide stability, while at the same time improving on-target activity. Circular dichroism showed that each of the dimers adopts a primarily alpha-helical conformation (55% helical content) when bound to lyso-phosphatidylglycerol micelles, indicating that the helical propensity of the parent peptide is maintained in the new cross-linked cyclic form. Compared to KR-12, one of the cross-linked dimers showed 16-fold more potent antimicrobial activity against human pathogens Pseudomonas aeruginosa, Staphylococcus aureus, and Candida albicans and 8-fold increased activity against Escherichia coli. Furthermore, these peptides retained antimicrobial activity at physiologically relevant conditions, including in the presence of salts and in human serum, and with selective Gram-negative antibacterial activity in rich growth media. In addition to giving further insight into the structure-activity relationship of KR-12, the current work demonstrates that by combining peptide stabilization strategies (dimerization, backbone cyclization, and cross-linking via a disulfide bond), KR-12 can be engineered into a potent antimicrobial peptide drug lead with potential utility in a therapeutic context.

Nanotechnology: a contemporary therapeutic approach in combating infections from multidrug-resistant bacteria

In the 20th century, the discovery of antibiotics played an essential role in the fight against infectious diseases, including meningitis, typhoid fever, pneumonia and Mycobacterium tuberculosis. The development of multidrug resistance in microflora due to improper antibiotic use created significant public health issues. Antibiotic resistance has increased at an alarming rate in the past few decades. Multidrug-resistant bacteria (superbugs) such as methicillin-resistant Staphylococcus aureus (MRSA) as well as drug-resistant tuberculosis pose serious health implications. Despite the continuous increase in resistant microbes, the discovery of novel antibiotics is constrained by the cost and complexities of discovery of drugs. The nanotechnology has given new hope in combating this problem. In the present review, recent developments in therapeutics utilizing nanotechnology for novel antimicrobial drug development are discussed. The nanoparticles of silver, gold and zinc oxide have proved to be efficient antimicrobial agents against multidrug-resistant Klebsiella, Pseudomonas, Escherichia Coli and MRSA. Using nanostructures as carriers for antimicrobial agents provides better bioavailability, less chances of sub-therapeutic drug accumulation and less drug-related toxicity. Nanophotothermal therapy using fullerene and antibody functionalized nanostructures are other strategies that can prove to be helpful.

Synergistic effect and antibiofilm activity of the antimicrobial peptide K11 with conventional antibiotics against multidrug-resistant and extensively drug-resistant Klebsiella pneumoniae

Introduction

Infections caused by drug-resistant Klebsiella pneumoniae are now a serious problem for public health, associated with high morbidity and mortality due to limited treatment options. Therefore, new antibacterial agents or a combination of agents as the first line of treatment are urgently needed. K11 is a novel antimicrobial peptide (AMP) that has demonstrated in vitro antimicrobial activity against several types of bacteria. Additionally, K11 has previously shown no hemolytic activity. Herein, the antibacterial activity, the synergistic action of K11 in combination with different conventional antibiotics and the antibiofilm activity of K11 against multidrug-resistant (MDR) and extensively drug-resistant (XDR) K. pneumoniae were investigated. Meanwhile, the stability and ability to induce the bacterial resistance of K11 were also tested.

Methods

Fifteen clinical isolates of MDR/XDR K. pneumoniae were used in this study. The minimum inhibitory concentration (MIC) of K11 against these isolates was determined by the broth microdilution method. In vitro synergy between K11 and antibiotics was evaluated using the checkerboard methodology. The antibiofilm activity of K11 against K. pneumoniae strong biofilm producers were explored by the crystal violet staining. The stability in different environments and resistance induction of K11 were evaluated by MIC determination.

Results

The MIC values of K11 against MDR/XDR K. pneumoniae isolates were 8-512 μg/mL. Intriguingly, the synergistic effects were clearly observed for K11 in combination with chloramphenicol, meropenem, rifampicin, or ceftazidime, whereas no synergy was observed when K11 was combined with colistin. Besides, K11 effectively prevented biofilm formation against K. pneumoniae strong biofilm producers in a concentration-dependent manner starting at 0.25×MIC and exerted an enhancing effect when administered in combination with meropenem, chloramphenicol, or rifampicin. Additionally, K11 demonstrated high thermal and wide pH stability along with good stability in serum and physiological salts. Significantly, K. pneumoniae showed no induction of resistance even after prolonged exposure to a sub-inhibitory concentration of K11.

Conclusion

These findings indicate that K11 is a promising candidate with potent antibacterial and antibiofilm activities without inducing resistance and acts synergistically with conventional antibiotics against drug-resistant K. pneumoniae.

R R, L. F L, M.C.G DR, N.B C, S.C D, O. L F. Advances and perspectives for antimicrobial peptide and combinatory therapies

Antimicrobial peptides (AMPs) have shown cell membrane-directed mechanisms of action. This specificity can be effective against infectious agents that have acquired resistance to conventional drugs. The AMPs' membrane-specificity and their great potential to combat resistant microbes has brought hope to the medical/therapeutic scene. The high death rate worldwide due to antimicrobial resistance (AMR) has pushed forward the search for new molecules and product developments, mainly antibiotics. In the current scenario, other strategies including the association of two or more drugs have contributed to the treatment of difficult-to-treat infectious diseases, above all, those caused by bacteria. In this context, the synergistic action of AMPs associated with current antibiotic therapy can bring important results for the production of new and effective drugs to overcome AMR. This review presents the advances obtained in the last 5 years in medical/antibiotic therapy, with the use of products based on AMPs, as well as perspectives on the potentialized effects of current drugs combined with AMPs for the treatment of bacterial infectious diseases.

The development of a novel ultrashort antimicrobial peptide nanoparticles with potent antimicrobial effect

Conventional antibiotics are facing significant microbial resistance, which has recently reached previously unnoticed critical levels. As a result of this situation, a large proportion of antimicrobial agents currently used in the clinic have significantly reduced therapeutic potential. Antimicrobial peptides (AMPs) may offer the medical community an alternative strategy to traditional antibiotics in the fight against microbial resistance. Current research efforts are focused on developing technologies that may reduce AMP toxicity while retaining their potent antimicrobial activity and possibly improving their delivery. The ionotropic gelation method was used to encapsulate a novel in-house designed potent ultrashort antimicrobial peptide (USAMP) into chitosan-based nanoparticles (CS-NPs) in this study. WRWRWR -CS-NPs were tested for antibacterial kinetics against two strains of Staphylococcus aureus for four days, and the developed WRWRWR -CS-NPs showed a 3-log decrease in the number of colonies when compared to CS-NP and a 5-log decrease when compared to control bacteria. Loaded WRWRWR into CS-NPs could represent an innovative approach to develop delivery systems based on NPs technology for achieving potent antimicrobial effects against multi-drug resistant and biofilm forming bacteria with negligible systemic toxicity and reduced synthetic costs that are obstructing the clinical development of AMPs generally.

Antimicrobial Peptides Epinecidin-1 and Beta-Defesin-3 Are Effective against a Broad Spectrum of Antibiotic-Resistant Bacterial Isolates and Increase Survival Rate in Experimental Sepsis

The antimicrobial peptides human Beta-defensin-3 (hBD-3) and Epinecidin-1 (Epi-1; by Epinephelus coioides) could be a promising tool to develop novel antibacterials to combat antibiotic resistance. The antibacterial activity of Epi-1 + vancomycin against methicillin-resistant Staphylococcus aureus (22 isolates) and Epi-1 + hBD-3 against carbapenem-resistant isolates of Klebsiella pneumoniae (n = 23), Klebsiella aerogenes (n = 17), Acinetobacter baumannii (n = 9), and Pseudomonas aeruginosa (n = 13) was studied in vitro. To evaluate the in vivo efficacy of hBD-3 and Epi-1, ICR (CD-1) mice were injected intraperitoneally with a lethal dose of K. pneumoniae or P. aeruginosa. The animals received a single injection of either sterile saline, hBD-3 monotherapy, meropenem monotherapy, hBD-3 + meropenem, or hBD-3 + Epi-1. Studied peptides showed antibacterial activity in vitro against all studied clinical isolates in a concentration of 2 to 32 mg/L. In both experimental models of murine sepsis, an increase in survival rate was seen with hBD-3 monotherapy, hBD-3 + meropenem, and hBD-3 + Epi-1. For K. pneumoniae-sepsis, hBD-3 was shown to be a promising option in overcoming the resistance of Klebsiella spp. to carbapenems, though more research is needed. In the P. aeruginosa-sepsis model, the addition of Epi-1 to hBD-3 was found to have a slightly reduced mortality rate compared to hBD-3 monotherapy.

Combination of AgNPs and Domiphen is Antimicrobial Against Biofilms of Common Pathogens

Purpose

The aim was to evaluate the antimicrobial potential of AgNPs synthesized with Artemisia argyi leaf extract and investigate the antimicrobial synergistic effects of AgNPs combined with domiphen and provide an efficient and broad-spectrum combination drug strategy.

Methods

AgNPs synthesized with Artemisia argyi leaf extract were studied using UV–vis spectroscopy, FTIR spectroscopy and particle size analysis. Then, Artemisia argyi leaf extract-synthesized AgNPs and domiphen were tested against Acinetobacter baumannii (ATCC 19606), Staphylococcus aureus (ATCC 6538), Escherichia coli (8099) and Candida albicans (ATCC 10231), respectively. Then, we explore synergistic antimicrobial effect and synergistic anti-biofilm effect through combined drug susceptibility test and combined drug minimum biofilm eradication concentration (MBEC₅₀) test.

Results

Characteristic absorption bands of AgNPs were found near 430 nm in the UV–vis spectrum. Particle size analysis results revealed that the average particle size of Artemisia argyi leaf extract-synthesized AgNPs was 77.6 nm. Artemisia argyi leaf extract-synthesized AgNPs showed high antimicrobial activity against the above four strains. Minimum inhibitory concentration (MIC) of Artemisia argyi leaf extract-synthesized AgNPs against strains was 1 μg/mL for Acinetobacter baumannii, 2 μg/mL for Staphylococcus aureus, Escherichia coli and Candida albicans. MBEC₅₀ of Artemisia argyi leaf extract-synthesized AgNPs against strains was 2 μg/mL for Acinetobacter baumannii, 4 μg/mL for Staphylococcus aureus, 1/2 μg/mL for Escherichia coli and 2 μg/mL for Candida albicans. The combination of Artemisia argyi leaf extract-synthesized AgNPs and domiphen has synergistic antimicrobial effect and synergistic anti-biofilm effect. Fractional inhibitory concentration (FIC) was ≤0.5.

Conclusion

Artemisia argyi leaf extract-synthesized AgNPs had antimicrobial activity against the above four strains. The combination of Artemisia argyi leaf extract-synthesized AgNPs and domiphen has synergistic antimicrobial effects to reduce the dosage of each antimicrobial drugs. Artemisia argyi leaf extract-synthesized AgNPs and domiphen have synergistic anti-biofilm effects.

Development of Novel Peptides for the Antimicrobial Combination Therapy against Carbapenem-Resistant Acinetobacter baumannii Infection

Carbapenem-resistant Acinetobacter baumannii (CRAB) infection has a high mortality rate, making the development of novel effective antibiotic therapeutic strategies highly critical. Antimicrobial peptides can outperform conventional antibiotics regarding drug resistance and broad-spectrum activity. PapMA, an 18-residue hybrid peptide, containing N-terminal residues of papiliocin and magainin 2, has previously demonstrated potent antibacterial activity. In this study, PapMA analogs were designed by substituting Ala¹⁵ or Phe¹⁸ with Ala, Phe, and Trp. PapMA-3 with Trp¹⁸ showed the highest bacterial selectivity against CRAB, alongside low cytotoxicity. Biophysical studies revealed that PapMA-3 permeabilizes CRAB membrane via strong binding to LPS. To reduce toxicity via reduced antibiotic doses, while preventing the emergence of multi-drug resistant bacteria, the efficacy of PapMA-3 in combination with six selected antibiotics was evaluated against clinical CRAB isolates (C1–C5). At 25% of the minimum inhibition concentration, PapMA-3 partially depolarized the CRAB membrane and caused sufficient morphological changes, facilitating the entry of antibiotics into the bacterial cell. Combining PapMA-3 with rifampin significantly and synergistically inhibited CRAB C4 (FICI = 0.13). Meanwhile, combining PapMA-3 with vancomycin or erythromycin, both potent against Gram-positive bacteria, demonstrated remarkable synergistic antibiofilm activity against Gram-negative CRAB. This study could aid in the development of combination therapeutic approaches against CRAB.

Synergistic effect of two antimicrobial peptides, Nisin and P10 with conventional antibiotics against extensively drug-resistant Acinetobacter baumannii and colistin-resistant Pseudomonas aeruginosa isolates

BACKGROUND

Infections caused by drug-resistant strains of Acinetobacter baumannii and Pseudomonas aeruginosa are now a global problem that requires the immediate development of new antimicrobial drugs. Combination therapy and using antimicrobial peptides are two strategies with high potential to solve this issue. By these strategies, this study aimed to determine the antimicrobial effect of Nisin and P10 antimicrobial peptides on extensively drug-resistant Acinetobacter baumannii and colistin-resistant Pseudomonas aeruginosa isolates, and investigate the most effective combination of an antimicrobial peptide with an antibiotic.

MATERIAL AND METHODS

This study was performed on five resistant clinical isolates and one standard strain for each kind of bacterium. First, the minimum inhibitory concentrations of two antimicrobial peptides (Nisin and P10) and five common antibiotics for the treatment of Gram-negative bacteria (ceftazidime, tobramycin, ciprofloxacin, doripenem, and colistin) was determined using Scanner-Assisted Colorimetric MIC Method. Then, the combination effect of P10+Nisin, P10+antibiotics, Nisin + antibiotics was investigated using checkerboard method.

RESULTS

The MIC value of Nisin and P10 against studied pathogens were 64-256 and 8-32 μg/ml, respectively. P10+Nisin combination showed synergistic effect against standard strains and additive effect against drug-resistant clinical isolates. It was also found that the combination effect of P10+ceftazidim, P10+doripenem, and Nisin + colistin was synergistic in most cases. Nisin + tobramycin combination showed synergistic effect in exposure to standard strains, while the synergy is strain-dependent against drug-resistant clinical isolates.

CONCLUSION

In conclusion, the synergism of Nisin + colistin and P10+ceftazidime/doripenem could be of great therapeutic value as antimicrobial drugs against infections caused by colistin-resistant P.aeruginosa and XDR A. baumannii.

Antibacterial activity of human defensins against Staphylococcus aureus and Escherichia coli

BACKGROUND

The global problem of antibiotic resistance requires the search for and development of new methods of treatment. One of the promising strategies is the use of low doses of antimicrobial peptides, in particular, human defensins HNP-1, hBD-1, and hBD-3, in combination with antibacterial drugs already used in clinical practice. This approach may be used to increase the effectiveness of conventional antibiotics. However, this requires thorough study of the effectiveness of defensins in combination with antibiotics against a large number of bacterial strains with known phenotypes of antibiotic resistance. The aim of this work was to study the antibacterial effect of HNP-1, hBD-1 and hBD-3 in combination with rifampicin or amikacin against clinical isolates of Staphylococcus aureus (n = 27) and Escherichia coli (n = 24) collected from hospitalized patients.

METHODS

The standard checkerboard assay was used to determine minimum inhibitory concentrations (MICs) of antimicrobials. The combined microbicidal effects of two substances (defensin + conventional antibiotic) were assessed by the fractional inhibitory concentration index (FICI).

RESULTS

The highest anti-staphylococcal activity (including methicillin-resistant strains) among defensins was demonstrated by hBD-3 that had MIC of 1 (0.5-4) mg/L (hereinafter, MIC values are presented as median and interquartile range). The MIC of HNP-1 against S. aureus was 4 (2-8) mg/L; the MIC of hBD-1 was 8 (4-8) mg/L. Against E. coli, the most effective was also found to be hBD-3 that had MIC of 4 (4-8) mg/L; the MIC of HNP-1 was 12 (4-32) mg/L. The combinations of HNP-1 + rifampicin and hBD-3 + rifampicin demonstrated synergistic effects against S. aureus. Against E. coli, combinations of HNP-1 + amikacin and hBD-3 + amikacin also showed synergy of action.

Exploiting Lactoferricin (17-30) as a Potential Antimicrobial and Antibiofilm Candidate Against Multi-Drug-Resistant Enteroaggregative Escherichia coli

The study evaluated the in vitro antimicrobial and antibiofilm efficacy of an antimicrobial peptide (AMP), lactoferricin (17–30) [Lfcin (17–30)], against biofilm-forming multi-drug-resistant (MDR) strains of enteroaggregative Escherichia coli (EAEC), and subsequently, the in vivo antimicrobial efficacy was assessed in a Galleria mellonella larval model. Initially, minimum inhibitory concentration (MIC; 32 μM), minimum bactericidal concentration (MBC; 32 μM), and minimum biofilm eradication concentration (MBEC; 32 μM) of Lfcin (17–30) were determined against MDR-EAEC field isolates (n = 3). Lfcin (17–30) was tested stable against high-end temperatures (70 and 90°C), physiological concentration of cationic salts (150 mM NaCl and 2 mM MgCl₂), and proteases (proteinase-K and lysozyme). Further, at lower MIC, Lfcin (17–30) proved to be safe for sheep RBCs, secondary cell lines (HEp-2 and RAW 264.7), and beneficial gut lactobacilli. In the in vitro time-kill assay, Lfcin (17–30) inhibited the MDR-EAEC strains 3 h post-incubation, and the antibacterial effect was due to membrane permeation of Lfcin (17–30) in the inner and outer membranes of MDR-EAEC. Furthermore, in the in vivo experiments, G. mellonella larvae treated with Lfcin (17–30) exhibited an increased survival rate, lower MDR-EAEC counts (P < 0.001), mild to moderate histopathological changes, and enhanced immunomodulatory effect and were safe to larval cells when compared with infection control. Besides, Lfcin (17–30) proved to be an effective antibiofilm agent, as it inhibited and eradicated the preformed biofilm formed by MDR-EAEC strains in a significant (P < 0.05) manner both by microtiter plate assay and live/dead bacterial quantification-based confocal microscopy. We recommend further investigation of Lfcin (17–30) in an appropriate animal model before its application in target host against MDR-EAEC strains.

Antimicrobial Susceptibility Testing of Antimicrobial Peptides to Better Predict Efficacy

During the development of antimicrobial peptides (AMP) as potential therapeutics, antimicrobial susceptibility testing (AST) stands as an essential part of the process in identification and optimisation of candidate AMP. Standard methods for AST, developed almost 60 years ago for testing conventional antibiotics, are not necessarily fit for purpose when it comes to determining the susceptibility of microorganisms to AMP. Without careful consideration of the parameters comprising AST there is a risk of failing to identify novel antimicrobials at a time when antimicrobial resistance (AMR) is leading the planet toward a post-antibiotic era. More physiologically/clinically relevant AST will allow better determination of the preclinical activity of drug candidates and allow the identification of lead compounds. An important consideration is the efficacy of AMP in biological matrices replicating sites of infection, e.g., blood/plasma/serum, lung bronchiolar lavage fluid/sputum, urine, biofilms, etc., as this will likely be more predictive of clinical efficacy. Additionally, specific AST for different target microorganisms may help to better predict efficacy of AMP in specific infections. In this manuscript, we describe what we believe are the key considerations for AST of AMP and hope that this information can better guide the preclinical development of AMP toward becoming a new generation of urgently needed antimicrobials.

Engineered Cationic Antimicrobial Peptides (eCAPs) to Combat Multidrug-Resistant Bacteria

The increasing rate of antibiotic resistance constitutes a global health crisis. Antimicrobial peptides (AMPs) have the property to selectively kill bacteria regardless of resistance to traditional antibiotics. However, several challenges (e.g., reduced activity in the presence of serum and lack of efficacy in vivo) to clinical development need to be overcome. In the last two decades, we have addressed many of those challenges by engineering cationic AMPs de novo for optimization under test conditions that typically inhibit the activities of natural AMPs, including systemic efficacy. We reviewed some of the most promising data of the last two decades in the context of the advancement of the field of helical AMPs toward clinical development.

Anionic food color tartrazine enhances antibacterial efficacy of histatin-derived peptide DHVAR4 by fine-tuning its membrane activity

Here it is demonstrated how some anionic food additives commonly used in our diet, such as tartrazine (TZ), bind to DHVAR4, an antimicrobial peptide (AMP) derived from oral host defense peptides, resulting in significantly fostered toxic activity against both Gram-positive and Gram-negative bacteria, but not against mammalian cells. Biophysical studies on the DHVAR4-TZ interaction indicate that initially large, positively charged aggregates are formed, but in the presence of lipid bilayers, they rather associate with the membrane surface. In contrast to synergistic effects observed for mixed antibacterial compounds, this is a principally different mechanism, where TZ directly acts on the membrane-associated AMP promoting its biologically active helical conformation. Model vesicle studies show that compared to dye-free DHVAR4, peptide-TZ complexes are more prone to form H-bonds with the phosphate ester moiety of the bilayer head-group region resulting in more controlled bilayer fusion mechanism and concerted severe cell damage. AMPs are considered as promising compounds to combat formidable antibiotic-resistant bacterial infections; however, we know very little on their in vivo actions, especially on how they interact with other chemical agents. The current example illustrates how food dyes can modulate AMP activity, which is hoped to inspire improved therapies against microbial infections in the alimentary tract. Results also imply that the structure and function of natural AMPs could be manipulated by small compounds, which may also offer a new strategic concept for the future design of peptide-based antimicrobials.

Antimicrobial Peptide-Loaded Nanoparticles as Inhalation Therapy for Pseudomonas aeruginosa Infections

INTRODUCTION

Antibiotic-resistant bacteria kill 25,000 people every year in the EU. Patients subject to recurrent lung infections are the most vulnerable to severe or even lethal infections. For these patients, pulmonary delivery of antibiotics would be advantageous, since inhalation can achieve higher concentration in the lungs than iv administration and can provide a faster onset of action. This would allow for the delivery of higher doses and hence reduce the number of treatments required. We report here about a new nanosystem (M33-NS) obtained by capturing SET-M33 peptide on single-chain dextran nanoparticles. SET-M33 is a non-natural antimicrobial peptide synthesized in branched form. This form gives the peptide resistance to degradation in biological fluids. SET-M33 has previously shown efficacy in vitro against about one hundred of Gram-negative multidrug and extensively drug-resistant clinical isolates and was also active in preclinical infection models of pneumonia, sepsis and skin infections.

METHODS

The new nanosystem was evaluated for its efficacy in bacteria cells and in a mouse model of pneumonia. Toxicity and genotoxicity were also tested in vitro. Biodistribution and pharmacokinetic studies in healthy rats were carried out using a radiolabeled derivative of the nanosystem.

RESULTS

The M33-nanosystem, studied here, showed to be effective against Pseudomonas aeruginosa in time-kill kinetic experiments. Cytotoxicity towards different animal cell lines was acceptable. Lung residence time of the antimicrobial peptide, administered via aerosol in healthy rats, was markedly improved by capturing SET-M33 on dextran nanoparticles. M33-NS was also efficient in eradicating pulmonary infection in a BALB/c mouse model of pneumonia caused by P. aeruginosa.

DISCUSSION

This study revealed that the encapsulation of the antimicrobial peptide in dextran nanoparticles markedly improved lung residence time of the peptide administered via aerosol. The result has to be considered among the aims of the development of a new therapeutic option for patients suffering recurrent infections, that will benefit from high local doses of persistent antimicrobials.

Antimicrobial Efficacy of Indolicidin Against Multi-Drug Resistant Enteroaggregative Escherichia coli in a Galleria mellonella Model

Antimicrobial resistance against enteroaggregative Escherichia coli (EAEC), an emerging food-borne pathogen, has been observed in an increasing trend recently. In the recent wake of antimicrobial resistance, alternate strategies especially, cationic antimicrobial peptides (AMPs) have attracted considerable attention to source antimicrobial technology solutions. This study evaluated the in vitro antimicrobial efficacy of Indolicidin against multi-drug resistant enteroaggregative Escherichia coli (MDR-EAEC) strains and further to assess its in vivo antimicrobial efficacy in Galleria mellonella larval model. The minimum inhibitory concentration (MIC; 32 μM) and minimum bactericidal concentration (MBC; 64 μM) of Indolicidin against MDR-EAEC was determined by micro broth dilution method. Indolicidin was also tested for its stability (high-end temperatures, physiological concentration of salts and proteases); safety (sheep RBCs; HEp-2 and RAW 264.7 cell lines); effect on beneficial microflora (Lactobacillus rhamnosus and Lactobacillus acidophilus) and its mode of action (flow cytometry; nitrocefin and ONPG uptake). In vitro time-kill kinetic assay of MDR-EAEC treated with Indolicidin was performed. Further, survival rate, MDR-EAEC count, melanization rate, hemocyte enumeration, cytotoxicity assay and histopathological examination were carried out in G. mellonella model to assess in vivo antimicrobial efficacy of Indolicidin against MDR-EAEC strains. Indolicidin was tested stable at high temperatures (70°C; 90°C), physiological concentration of cationic salts (NaCl; MgCl₂) and proteases, except for trypsin and tested safe with sheep RBCs and cell lines (RAW 264.7; HEp-2) at MIC (1X and 2X); the beneficial flora was not inhibited. Indolicidin exhibited outer membrane permeabilization in a concentration- and time-dependent manner. In vitro time-kill assay revealed concentration-cum-time dependent clearance of MDR-EAEC in Indolicidin-treated groups at 120 min, while, in G. mellonella, the infected group treated with Indolicidin revealed an increased survival rate, immunomodulatory effect, reduced MDR-EAEC counts and were tested safe to the larval cells which was concurred histopathologically. To conclude, the results suggests Indolicidin as an effective antimicrobial candidate against MDR-EAEC and we recommend its further investigation in appropriate animal models (mice/piglets) before its application in the target host.

Synergistic activity of filtrates of Lactobacillus rhamnosus and Saccharomyces boulardii and antibacterial preparations against Corynebacterium spp

We present the results of the first study of the combined influence of the biologically active substances Lactobacillus rhamnosus GG ATCC 53103 and Saccharomyces boulardii, obtained by the author’s method, and antibacterial agents on Corynebacterium spp. The first area of research was the study of increasing the sensitivity of toxigenic microorganisms to antimicrobial drugs due to the consecutive effects of the structural components and metabolites of L. rhamnosus GG and S. boulardii and antibacterial drugs on Corynebacterium spp. tox+. The greatest increase in the sensitivity of test-cultures of corynebacteria to penicillin (by 19.4 mm), imipenem (by 15.0 mm), vancomycin (by 12.0 mm), gentamicin (by 11.0 mm), ciprofloxacin (by 9.8 mm), erythromycin (by 9.6 mm), cefotaxime (by 9.5 mm) occurred due to the products of lactobacteria and a combination of metabolites of lactobacteria and saccharomycetes. The second area of research was the study of the synergic activity of substances L. rhamnosus GG and S. boulardii and traditional antibacterial drugs manifested by their simultaneous effect on Corynebacterium spp. Maximum potentiation of azithromycin (by 4.6 mm), erythromycin (by 4.5 mm), cefotaxime (by 2.2 mm), ceftriaxone (by 1.6 mm) and ampicillin (by 1.0 mm) relative to corynebacteria was also observed under the influence of lactobacteria metabolites and a combination of lactobacteria and saccharomycetes metabolites. Different degrees of manifestation of the combined action of biologically active substances L. rhamnosus GG and S. boulardii with antibiotics were determined, which depended on the selected combinations, the method of influence on the microorganism, the individual sensitivity of the test-cultures, the activity of the test filtrates and the initial concentration of the producers used to obtain the products of vital activity of lactobacteria and saccharomyces. The presented complexes of structural components and metabolites of L. rhamnosus GG and S. boulardii, obtained without the use of traditional nutrient media, by increasing the bioavailability of pathogenic pathogens can reduce the required concentration of the antibiotic, continuing their use, and suspend the likelihood of pathogens developing resistance to microorganisms. This makes them promising candidates both for the development of "accompaniment-preparations" for antibiotics for the additional therapy of infectious diseases of different etiology, and for the creation of a new direction of antimicrobial agents with multifunctional capabilities. Synergistic activity of filtrates L. rhamnosus GG and S. boulardii and antibacterial preparations against Corynebacterium spp.

An antimicrobial molecule mitigates signs of sepsis in vivo and eradicates infections from lung tissue

The peptide sequence KKIRVRLSA was synthesized in a dimeric structure (SET-M33DIM) and evaluated as a candidate drug for infections due to multidrug-resistant (MDR) Gram-negative pathogens. SET-M33DIM showed significant antibacterial activity against MDR strains of Klebsiella pneumoniae, Acinetobacter baumannii, and Escherichia coli (Minimal Inhibitory Concentration [MICs], 1.5-11 µM), and less activity against Pseudomonas aeruginosa (MICs, 11-22 µM). It showed very low toxicity in vitro, ex vivo, and in vivo; in cytotoxicity tests, its EC₅₀ was as much as 22 times better than that of SET-M33, a peptide with the same amino-acid sequence, but synthesized in tetra-branched form (638 vs 28 µM). In in vivo and ex vivo experiments, SET-M33DIM cleared P. aeruginosa infection, significantly reducing signs of sepsis in animals, and restoring cell viability in lung tissue after bacterial challenge. It also quelled inflammation triggered by LPS and live bacterial cells, inhibiting expression of inflammatory mediators in lung tissue, cultured macrophages, and bronchial cells from a cystic fibrosis patient.

In vitro and in vivo Synergistic Effects of Florfenicol and Thiamphenicol in Combination Against Swine Actinobacillus pleuropneumoniae and Pasteurella multocida

Potential synergism between florfenicol (FF) and thiamphenicol (TAP) was investigated for in vitro efficacy against Actinobacillus pleuropneumoniae and/or Pasteurella multocida as well as in vivo efficacy in swine. Among isolates of A. pleuropneumoniae (n = 58) and P. multocida (n = 79) from pigs in Taiwan that were tested, high percentages showed resistance to FF (52 and 53%, respectively) and TAP (57 and 53%, respectively). Checkerboard microdilution assay indicated that synergism [fractional inhibitory concentration index (FICI) ≤ 0.5] was detected in 17% of A. pleuropneumoniae (all serovar 1) and 24% of P. multocida isolates. After reconfirming the strains showing FICI ≤ 0.625 with time kill assay, the synergism increased to around 32% against both bacteria and the number could further increase to 40% against resistant A. pleuropneumoniae and 65% against susceptible P. multocida isolates. A challenge-treatment trial in pigs with P. multocida showed that the FF + TAP dosage at ratios correspondent to their MIC deduction was equally effective to the recommended dosages. Further on the combination, the resistant mutation frequency is very low when A. pleuropneumoniae is grown with FF + TAP and similar to the exposure to sub-inhibitory concentration of FF or TAP alone. The degree of minimum inhibitory concentration (MIC) reduction in FF could reach 75% (1/4 MIC) or more (up to 1/8 MIC for P. multocida, 1/16 for A. pleuropneumoniae) when combined with 1/4 MIC of TAP (or 1/8 for A. pleuropneumoniae). The synergism or FICI ≤ 0.625 of FF with oxytetracycline (47%), doxycycline (69%), and erythromycin (56%) was also evident, and worth further investigation for FF as a central modulator facilitating synergistic effects with these antimicrobials. Taken together, synergistic FF + TAP combination was effective against swine pulmonary isolates of A. pleuropneumoniae and P. multocida both in vitro and in vivo. Thus, this study may offer a potential alternative for the treatment of A. pleuropneumoniae and P. multocida infections and has the potential to greatly reduce drug residues and withdrawal time.

Antimicrobial activity of two novel antimicrobial peptides AA139 and SET-M33 against clinically and genotypically diverse Klebsiella pneumoniae isolates with differing antibiotic resistance profiles

Colistin is an antimicrobial peptide (AMP) used as a drug of last resort, although plasmid-mediated colistin resistance (MCR) has been reported. AA139 and SET-M33 are novel AMPs currently in development for the treatment of multidrug-resistant (MDR) Gram-negative bacterial infections. As many AMPs have a similar mode of action to colistin, potentially leading to cross-resistance, the antimicrobial activity of AA139 and SET-M33 was investigated against a collection of 50 clinically and genotypically diverse Klebsiella pneumoniae isolates with differing antibiotic resistance profiles, including colistin-resistant strains. The collection was genotypically characterised and susceptibility to clinically relevant antibiotics was determined. Susceptibility to AA139 and SET-M33 did not differ among the collection despite differences in underlying mechanisms of resistance or susceptibility to colistin. For three colistin-susceptible and three colistin-resistant strains with distinct MDR profiles as well as an additional MCR-producing strain, the bactericidal activity of AA139, SET-M33 and colistin during 24 h of exposure was examined. Following 24 h of exposure to AA139, SET-M33 or colistin, the seven strains were tested for changes in susceptibility to the respective AMPs. AA139 and SET-M33 showed a concentration-dependent bactericidal effect irrespective of bacterial susceptibility to colistin. Exposure to low colistin concentrations resulted in the development of colistin resistance in colistin-susceptible strains, whereas susceptibility to AA139 and SET-M33 following exposure to the respective AMPs was maintained. The two novel AMPs remained effective against colistin-resistant strains and may be promising novel drugs for the treatment of clinically and genotypically diverse MDR K. pneumoniae infections, including infections associated with colistin-resistant bacteria.

Application of Antimicrobial Peptides of the Innate Immune System in Combination With Conventional Antibiotics-A Novel Way to Combat Antibiotic Resistance?

Rapidly growing resistance of pathogenic bacteria to conventional antibiotics leads to inefficiency of traditional approaches of countering infections and determines the urgent need for a search of fundamentally new anti-infective drugs. Antimicrobial peptides (AMPs) of the innate immune system are promising candidates for a role of such novel antibiotics. However, some cytotoxicity of AMPs toward host cells limits their active implementation in medicine and forces attempts to design numerous structural analogs of the peptides with optimized properties. An alternative route for the successful AMPs introduction may be their usage in combination with conventional antibiotics. Synergistic antibacterial effects have been reported for a number of such combinations, however, the molecular mechanisms of the synergy remain poorly understood and little is known whether AMPs cytotoxicy for the host cells increases upon their application with antibiotics. Our study is directed to examination of a combined action of natural AMPs with different structure and mode of action (porcine protegrin 1, caprine bactenecin ChBac3.4, human alpha- and beta-defensins (HNP-1, HNP-4, hBD-2, hBD-3), human cathelicidin LL-37), and egg white lysozyme with varied antibiotic agents (gentamicin, ofloxacin, oxacillin, rifampicin, polymyxin B, silver nanoparticles) toward selected bacteria, including drug-sensitive and drug-resistant strains, as well as toward some mammalian cells (human erythrocytes, PBMC, neutrophils, murine peritoneal macrophages and Ehrlich ascites carcinoma cells). Using “checkerboard titrations” for fractional inhibitory concentration indexes evaluation, it was found that synergy in antibacterial action mainly occurs between highly membrane-active AMPs (e.g., protegrin 1, hBD-3) and antibiotics with intracellular targets (e.g., gentamicin, rifampcin), suggesting bioavailability increase as the main model of such interaction. In some combinations modulation of dynamics of AMP-bacterial membrane interaction in presence of the antibiotic was also shown. Cytotoxic effects of the same combinations toward normal eukaryotic cells were rarely synergistic. The obtained data approve that combined application of antimicrobial peptides with antibiotics or other antimicrobials is a promising strategy for further development of new approach for combating antibiotic-resistant bacteria by usage of AMP-based therapeutics. Revealing the conventional antibiotics that increase the activity of human endogenous AMPs against particular pathogens is also important for cure strategies elaboration.

Investigations into the killing activity of an antimicrobial peptide active against extensively antibiotic-resistant K. pneumon iae and P. aeruginosa

SET-M33 is a multimeric antimicrobial peptide active against Gram-negative bacteria in vitro and in vivo. Insights into its killing mechanism could elucidate correlations with selectivity. SET-M33 showed concentration-dependent bactericidal activity against colistin-susceptible and resistant isolates of P. aeruginosa and K. pneumoniae. Scanning and transmission microscopy studies showed that SET-M33 generated cell blisters, blebs, membrane stacks and deep craters in K. pneumoniae and P. aeruginosa cells. NMR analysis and CD spectra in the presence of sodium dodecyl sulfate micelles showed a transition from an unstructured state to a stable α-helix, driving the peptide to arrange itself on the surface of micelles. SET-M33 kills Gram-negative bacteria after an initial interaction with bacterial LPS. The molecule becomes then embedded in the outer membrane surface, thereby impairing cell function. This activity of SET-M33, in contrast to other similar antimicrobial peptides such as colistin, does not generate resistant mutants after 24h of exposure, non-specific interactions or toxicity against eukaryotic cell membranes, suggesting that SET-M33 is a promising new option for the treatment of Gram-negative antibiotic-resistant infections.