Synergistic effect of clinically used antibiotics and peptide antibiotics against Gram-positive and Gram-negative bacteria

Dual Antibiotic Approach: Synthesis and Antibacterial Activity of Antibiotic-Antimicrobial Peptide Conjugates

In recent years, bacterial resistance to conventional antibiotics has become a major concern in the medical field. The global misuse of antibiotics in clinics, personal use, and agriculture has accelerated this resistance, making infections increasingly difficult to treat and rendering new antibiotics ineffective more quickly. Finding new antibiotics is challenging due to the complexity of bacterial mechanisms, high costs and low financial incentives for the development of new molecular scaffolds, and stringent regulatory requirements. Additionally, innovation has slowed, with many new antibiotics being modifications of existing drugs rather than entirely new classes. Antimicrobial peptides (AMPs) are a valid alternative to small-molecule antibiotics offering several advantages, including broad-spectrum activity and a lower likelihood of inducing resistance due to their multifaceted mechanisms of action. However, AMPs face challenges such as stability issues in physiological conditions, potential toxicity to human cells, high production costs, and difficulties in large-scale manufacturing. A reliable strategy to overcome the drawbacks associated with the use of small-molecule antibiotics and AMPs is combination therapy, namely the simultaneous co-administration of two or more antibiotics or the synthesis of covalently linked conjugates. This review aims to provide a comprehensive overview of the literature on the development of antibiotic-AMP conjugates, with a particular emphasis on critically analyzing the design and synthetic strategies employed in their creation. In addition to the synthesis, the review will also explore the reported antibacterial activity of these conjugates and, where available, examine any data concerning their cytotoxicity.

Evaluation of deoxythymidine-based cationic amphiphiles as antimicrobial, antibiofilm, and anti-inflammatory agents

OBJECTIVES

We recently designed a series of cationic deoxythymidine-based amphiphiles that mimic the cationic amphipathic structure of antimicrobial peptides (AMPs). Among these amphiphiles, ADG-2e and ADL-3e displayed the highest selectivity against bacterial cells. In this study, ADG-2e and ADL-3e were evaluated for their potential as novel classes of antimicrobial, antibiofilm, and anti-inflammatory agents.

METHODS

Minimum inhibitory concentrations of ADG-2e and ADL-3e against bacteria were determined using the broth microdilution method. Proteolytic resistance against pepsin, trypsin, α-chymotrypsin, and proteinase K was determined by radial diffusion and HPLC analysis. Biofilm activity was investigated using the broth microdilution and confocal microscopy. The antimicrobial mechanism was investigated by membrane depolarization, cell membrane integrity analysis, scanning electron microscopy (SEM), genomic DNA influence and genomic DNA binding assay. Synergistic activity was evaluated using checkerboard method. Anti-inflammatory activity was investigated using ELISA and RT-PCR.

RESULTS

ADG-2e and ADL-3e showed good resistance to physiological salts and human serum, and a low incidence of drug resistance. Moreover, they exhibit proteolytic resistance against pepsin, trypsin, α-chymotrypsin, and proteinase K. ADG-2e and ADL-3e were found to kill bacteria by an intracellular target mechanism and bacterial cell membrane-disrupting mechanism, respectively. Furthermore, ADG-2e and ADL-3e showed effective synergistic effects when combined with several conventional antibiotics against methicillin-resistant Staphylococcus aureus (MRSA) and multidrug-resistant Pseudomonas aeruginosa (MDRPA). Importantly, ADG-2e and ADL-3e not only suppressed MDRPA biofilm formation but also effectively eradicated mature MDRPA biofilms. Furthermore, ADG-2e and ADL-3e drastically decreased tumor necrosis factor-α (TNF-α) and interleukin-6 (IL-6) gene expression and protein secretion in lipopolysaccharide (LPS)-stimulated macrophages, implying potent anti-inflammatory activity in LPS-induced inflammation.

CONCLUSION

Our findings suggest that ADG-2e and ADL-3e could be further developed as novel antimicrobial, antibiofilm, and anti-inflammatory agents to combat bacterial infections.

Synergistic effects of short peptides and antibiotics against bacterial and fungal strains

There is a rising tide of concern about the antibiotic resistance issue. To reduce the possibility of antibiotic-resistant infections, a new generation of antimicrobials must be developed. Antimicrobial peptides are potential alternatives to antibiotics that can be used alone or together with conventional antibiotics to combat antimicrobial resistance. In this work, lead compounds LP-23, DP-23, SA4, and SPO from previously published studies were synthesized by solid-phase peptide synthesis and their antimicrobial evaluation was carried out against various bacterial and fungal strains. Peptide combinations with antibiotics were evaluated by using the checkerboard method and their minimal inhibitory concentration (MIC) in combination was calculated by using the fractional inhibitory concentration (FIC) index. Cytotoxicity evaluations of these peptides further confirmed their selectivity toward microbial cells. Based on the FIC values, LP-23, DP-23, and SPO demonstrated synergy in combination with gentamicin against a gentamicin-resistant clinical isolate of Escherichia coli. For Staphylococcus aureus, Escherichia coli, and Salmonella typhimurium, seven combinations exhibited synergistic effects between peptide/peptoids and the tested antibiotics. Additionally, almost all the combinations of peptides/peptoids with amphotericin B and fluconazole also showed effective synergy against Aspergillus niger and Aspergillus flavus. The synergy found between LP-23, DP-23, SA4, and SPO with the selected antibiotics may have the potential to be used as a combination therapy against various microbial infections.

Polycationic peptide R7-G-Aβ25-35 selectively induces cell death in leukemia Jurkat T cells through speedy mitochondrial depolarization, and CASPASE-3 -independent mechanism

Background

Methods

Results

Conclusion

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Polycationic arginine (R) residue bound Aβ25-35 peptide is cytotoxic to Jurkat cells.

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R⁷-G-Aβ_25-35 is more effective killing leukemia cells than Aβ25-35-G-R⁷.

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R⁷-G-Aβ_25-35 induces alteration of cell metabolism, and reduces cell proliferation.

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R⁷-G-Aβ_25-35 provokes loss of ΔΨ_m and produces high amount of ROS.

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R⁷-G-Aβ_25-35 is harmless to normal proliferative mesenchymal stromal cells.

Antibacterial and Anticancer Activities of Pleurocidin-Amide, a Potent Marine Antimicrobial Peptide Derived from Winter Flounder, Pleuronectes americanus

The extensive use of conventional antibiotics has led to the growing emergence of many resistant strains of pathogenic bacteria. Evidence suggests that cationic antimicrobial peptides (AMPs) have the greatest potential to serve as traditional antibiotic substitutes. Recent studies have also reported that certain AMPs have selective toxicity toward various types of cancer cells. The electrostatic attraction between the negatively charged membrane components and AMPs is believed to play a crucial role in the disruption of bacterial and cancer cell membranes. In the current study, we used a potent AMP called Pleurocidin (Ple) derived from winter flounder Pleuronectes americanus and its C-terminal-amidated derivative Pleurocidin-amide (Ple-a), and evaluated their antibacterial and anticancer activities. Our results indicated that both Ple and Ple-a exhibited significant antibacterial activity against a broad spectrum of Gram-positive and Gram-negative bacteria, especially marine pathogens, with MIC values ranging from 0.25 to 32 μg/mL. These peptides are also potent against several multidrug-resistant (MDR) bacterial strains, with MIC values ranging from 2 to 256 μg/mL. When used in combination with certain antibiotics, they exhibited a synergistic effect against MDR E. coli. Ple and Ple-a also showed notable cytotoxicity toward various cancer cell lines, with IC₅₀ values ranging from 11 to 340 μM, while normal mouse fibroblast 3T3 cells were less susceptible to these peptides. Ple-a was then selected to study its anticancer mechanism toward A549 human lung adenocarcinoma cells. Western blot analysis and confocal microscopy showed that Ple-a could inhibit autophagy of A549 cells, and induce apoptosis 48 h after treatment. Our findings provided support for the future application of Ple-a as potential therapeutic agent for bacterial infections and cancer treatment.

New Antimicrobial Peptide with Two CRAC Motifs: Activity against Escherichia coli and Bacillus subtilis

Due to the emergence of multiple antibiotic resistance in many pathogens, the studies on new antimicrobial peptides (AMPs) have become a priority scientific direction in fundamental and applied biology. Diverse mechanisms underlie the antibacterial action of AMPs. Among them are the effects that AMPs cause on bacterial cell membranes. In this work, we studied the antibacterial activity of a peptide named P4 with the following sequence RTKLWEMLVELGNMDKAVKLWRKLKR that was constructed from two alpha-helical fragments of the influenza virus protein M1 and containing two cholesterol-recognizing amino-acid consensus (CRAC) motifs. Previously we have shown that 50 μM of peptide P4 is toxic to cultured mouse macrophages. In the present work, we have found that peptide P4 inhibits the growth of E. coli and B. subtilis strains at concentrations that are significantly lower than the cytotoxic concentration that was found for macrophages. The half-maximal inhibitory concentration (IC50) for B. subtilis and E. coli cells were 0.07 ± 0.01 μM and 1.9 ± 0.4 μM, respectively. Scramble peptide without CRAC motifs did not inhibit the growth of E. coli cells and was not cytotoxic for macrophages but had an inhibitory effect on the growth of B. subtilis cells (IC50 0.4 ± 0.2 μM). A possible involvement of CRAC motifs and membrane sterols in the mechanism of the antimicrobial action of the P4 peptide is discussed. We assume that in the case of the Gram-negative bacterium E. coli, the mechanism of the toxic action of peptide P4 is related to the interaction of CRAC motifs with sterols that are present in the bacterial membrane, whereas in the case of the Gram-positive bacterium B. subtilis, which lacks sterols, the toxic action of peptide P4 is based on membrane permeabilization through the interaction of the peptide cationic domain and anionic lipids of the bacterial membrane. Whatever the mechanism can be, we report antimicrobial activity of the peptide P4 against the representatives of Gram-positive (B. subtilis) and Gram-negative (E. coli) bacteria.

Amphiphilic cyclic peptide [W4KR5]-Antibiotics combinations as broad-spectrum antimicrobial agents

Linear and cyclic amphiphilic peptides, (W₄KR₅) and [W₄KR₅], were evaluated as antibacterial agents against Gram-positive and Gram-negative bacteria, including four multi-drug resistant strains and the corresponding four non-resistant strains. Cyclic peptide [W₄KR₅] showed higher antibacterial activity than the linear (W₄KR₅) counterpart. Cyclic [W₄KR₅] was subjected to combination (physical mixture or covalent conjugation) with meropenem as a model antibiotic to study the impact of the combination on antimicrobial activity. A physical mixture of meropenem and [W₄KR₅] showed synergistic antibacterial activity against Gram-negative P. aeruginosa (ATCC BAA-1744) and P. aeruginosa (ATCC 27883) strains. [W₄KR₅] was further subjected to extensive antibacterial studies against additional 10 bacteria strains, showing significant antibacterial efficacy against Gram-positive bacteria strains. Combinations studies of [W₄KR₅] with an additional 9 commercially available antibiotics showed significant enhancement in antibacterial activity for all tested combinations, especially with tetracycline, tobramycin, levofloxacin, clindamycin, daptomycin, polymyxin, kanamycin, and vancomycin. Time-kill kinetics assay and flow cytometry results exhibited that [W₄KR₅] had a time-dependent synergistic effect and membrane disruption property. These data indicate that [W₄KR₅] improves the antibacterial activity, presumably by facilitating the internalization of antibiotics and their interaction with the intracellular targets. This study introduces a potential strategy for treating multidrug-resistant pathogens by combining [W₄KR₅] and a variety of classical antibiotics to improve the antibacterial effectiveness.

Synergistic Antimicrobial Metal Oxide-Doped Phosphate Glasses; a Potential Strategy to Reduce Antimicrobial Resistance and Host Cell Toxicity

The emergence of antimicrobial resistant strains bacteria and a decline in the discovery of new antibiotics has led to the idea of combining various antimicrobials to treat resistant strains and/or polymicrobial infections. Metal oxide-doped glasses have been extensively investigated for their antimicrobial potential; however to date, most experiments have focused on single metal species in isolation. The present study investigates the antimicrobial potential of sodium calcium phosphates (P₂O₅)₅₀(Na₂O)₂₀(CaO)_30-X(MO)_X, where M is cobalt, copper, or zinc as single species. In addition, this work studied the effect of co-doping glasses containing two different metal ions (Co + Cu, Co + Zn, and Cu + Zn). The antimicrobial efficacy of all glasses was tested against Gram-positive (Staphylococcus aureus) and Gram-negative (Escherichia coli) bacterial strains, as well as a fungal strain (Candida albicans). Minimum inhibitory and bactericidal concentrations and time kill/synergy assays were used to assess the antimicrobial activity. An enhanced antimicrobial effect, at 5 mg/mL concentration, was exhibited by cobalt, copper, and zinc oxide glasses alone and in combinations. A synergistic antimicrobial effect was observed by Cu + Co and Cu + Zn against E. coli and Cu + Zn against S. aureus.

Effect of antimicrobial peptides on planktonic growth, biofilm formation and biofilm-derived bacterial viability of Streptococcus pneumoniae

Streptococcus pneumoniae is a leading cause of pneumonia mortality globally. Pneumococcal disease is often associated with prolonged colonisation of hosts and this process is facilitated by biofilm formation that is largely resistant to conventional antibiotics. We investigated the effects of antimicrobial peptides (AMPs) lysozyme, lactoferrin, LL37 and a combination of all three on planktonic growth, biofilm formation and biofilm-derived bacterial viability by S. pneumoniae, serotype 23F. Planktonic growth and biofilm-derived bacterial viability were determined using standard colony-forming techniques, while biofilm formation was measured using a crystal violet based spectrophotometric method. Relative to controls, lysozyme significantly reduced biofilm formation (0.08 OD vs. 0.10 OD at 570 nm, p = 0.01), while LL37 and the AMP combination increased biofilm formation (0.14 OD vs. 0.10 OD at 570 nm, p = 0.01). The combination of AMPs significantly decreased planktonic growth (1.10 × 10⁸ colony-forming units per millilitres [CFU/mL] vs. 2.13 × 10⁸ CFU/mL, p = 0.02). Biofilm-derived bacterial viability was greatly reduced by exposure to a combination of AMPs (1.05 × 10⁵ CFU/mL vs. 1.12 × 10⁶ CFU/mL, p = 3.60 × 10^-8). Streptococcus pneumoniae displays marked resistance to the individual AMPs. A combination of lysozyme, lactoferrin and LL37 effectively inhibited planktonic growth and biofilm-derived bacterial viability; however, persister cell growth was still evident after exposure.

Trp-Containing Antibacterial Peptides Impair Quorum Sensing and Biofilm Development in Multidrug-Resistant Pseudomonas aeruginosa and Exhibit Synergistic Effects With Antibiotics

Pseudomonas aeruginosa uses quorum sensing (QS) to control virulence, biofilm formation and antibiotic efflux pump expression. The development of effective small molecules targeting the QS system and biofilm formation represents a novel attractive strategy. In this present study, the effects of a series of Trp-containing peptides on the QS-regulated virulence and biofilm development of multidrug-resistant P. aeruginosa, as well as their synergistic antibacterial activity with three classes of traditional chemical antibiotics were investigated. The results showed that Trp-containing peptides at low concentrations reduced the production of QS-regulated virulence factors by downregulating the gene expression of both the las and rhl systems in the strain MRPA0108. Biofilm formation was inhibited in a concentration-dependent manner, which was associated with extracellular polysaccharide production inhibition by downregulating pelA, algD, and pslA transcription. These changes correlated with alterations in the extracellular production of pseudomonal virulence factors and swarming motility. In addition, the combination of Trp-containing peptides at low concentration with the antibiotics ceftazidime and piperacillin provided synergistic effects. Notably, L11W and L12W showed the highest synergy with ceftazidime and piperacillin. A mechanistic study demonstrated that the Trp-containing peptides, especially L12W, significantly decreased β-lactamase activity and expression of efflux pump genes OprM, MexX, and MexA, resulting in a reduction in antibiotic efflux from MRPA0108 cells and thus increasing the antibacterial activity of these antibiotics against MRPA0108.

Cryptic Host Defense Peptides: Multifaceted Activity and Prospects for Medicinal Chemistry

Host defense peptides (HDPs) comprise a heterogeneous group of evolutionarily conserved and biologically active small molecules that are produced by different organisms. HDPs are widely researched because they often have multiple biological activities, for example antimicrobial, immunomodulatory and anticancer activity. In this context, in this review we focus on cryptic HDPs, molecules derived specifically from proteolytic processing of endogenous precursor proteins. Here, we explore the biological activity of such molecules and we further discuss the development of optimized sequences based on these natural cryptic HDPs. In addition, we present clinical-phase studies of cryptic HDPs (natural or optimized), and point out the possible applicability of these molecules in medicinal chemistry.

Phytochemical conjugation as a potential semisynthetic approach toward reactive and reuse of obsolete sulfonamides against pathogenic bacteria

The emergence and reemergence of multidrug-resistant (MDR) bacteria and mycobacteria in community and hospital periphery have directly enhanced the hospitalization costs, morbidity and mortality, globally. The appearance of MDR pathogens, the currently used antibiotics, remains insufficient, and the development of potent antibacterial(s) is merely slow. Thus, the development of active antibacterials is the call of the day. The sulfonamides class of antibacterials was the most successful synthesized drug in the 19th century. Mechanically, sulfonamides were targeting bacterial folic acid biosynthesis and today, those are obsolete or clinically inactive. Nevertheless, the magic sulfonamide pharmacophore has been used continuously in several mainstream antibacterial, antidiabetic, antiviral drugs. Concomitantly, thousands of phytochemicals with antimicrobial potencies have been recorded and were commanded as alternate antibacterials toward control of MDR pathogens. However, none/very few isolated phytochemicals have gone up to the pure-drug stage due to the lack of the desired drug-likeness values and the required pharmacokinetic properties. Thus, chemical modification of parent drug remains as the versatile approach in antibacterial drug development. Improvement of clinically inactive sulfa drugs with suitable phytochemicals to develop active, low-toxic drug molecules followed by medicinal chemistry could be prudent. This review highlights such "sulfonamide-phytochemical" hybrid drug development research works for utilizing inactive sulfonamides and phytochemicals; the ingenious cost-effective and resource-saving hybrid drug concept could be a new trend in current antibacterial drug discovery to reactive the obsolete antibacterials.

Resistance of Gram-Positive Bacteria to Current Antibacterial Agents and Overcoming Approaches

The discovery of antibiotics has created a turning point in medical interventions to pathogenic infections, but unfortunately, each discovery was consistently followed by the emergence of resistance. The rise of multidrug-resistant bacteria has generated a great challenge to treat infections caused by bacteria with the available antibiotics. Today, research is active in finding new treatments for multidrug-resistant pathogens. In a step to guide the efforts, the WHO has published a list of the most dangerous bacteria that are resistant to current treatments and requires the development of new antibiotics for combating the resistance. Among the list are various Gram-positive bacteria that are responsible for serious healthcare and community-associated infections. Methicillin-resistant Staphylococcus aureus, vancomycin-resistant Enterococcus faecium, and drug-resistant Streptococcus pneumoniae are of particular concern. The resistance of bacteria is an evolving phenomenon that arises from genetic mutations and/or acquired genomes. Thus, antimicrobial resistance demands continuous efforts to create strategies to combat this problem and optimize the use of antibiotics. This article aims to provide a review of the most critical resistant Gram-positive bacterial pathogens, their mechanisms of resistance, and the new treatments and approaches reported to circumvent this problem.

Potential combinations of endocannabinoid/endocannabinoid-like compounds and antibiotics against methicillin-resistant Staphylococcus aureus

Infections caused by antibiotic-resistant strains of Staphylococcus aureus have reached epidemic proportions globally. Our previous study showed antimicrobial effects of anandamide (AEA) and arachidonoyl serine (AraS) against methicillin (MET)-resistant S. aureus (MRSA) strains, proposing the therapeutic potential of these endocannabinoid/endocannabinoid-like (EC/EC-like) agents for the treatment of MRSA. Here, we investigated the potential synergism of combinations of AEA and AraS with different types of antibiotics against MRSA grown under planktonic growth or biofilm formation. The most effective combinations under planktonic conditions were mixtures of AEA and ampicillin (AMP), and of AraS and gentamicin (GEN). The combination with the highest synergy in the biofilm formation against all tested bacterial strains was AEA and MET. Moreover, the combination of AraS and MET synergistically caused default of biofilm formation. Slime production of MRSA was also dramatically impaired by AEA or AraS combined with MET. Our data suggest the novel potential activity of combinations of EC/EC-like agents and antibiotics in the prevention of MRSA biofilm formation.

Antibacterial Prodrugs to Overcome Bacterial Resistance

Bacterial resistance to present antibiotics is emerging at a high pace that makes the development of new treatments a must. At the same time, the development of novel antibiotics for resistant bacteria is a slow-paced process. Amid the massive need for new drug treatments to combat resistance, time and effort preserving approaches, like the prodrug approach, are most needed. Prodrugs are pharmacologically inactive entities of active drugs that undergo biotransformation before eliciting their pharmacological effects. A prodrug strategy can be used to revive drugs discarded due to a lack of appropriate pharmacokinetic and drug-like properties, or high host toxicity. A special advantage of the use of the prodrug approach in the era of bacterial resistance is targeting resistant bacteria by developing prodrugs that require bacterium-specific enzymes to release the active drug. In this article, we review the up-to-date implementation of prodrugs to develop medications that are active against drug-resistant bacteria.

Synergistic Antibacterial Activity of Designed Trp-Containing Antibacterial Peptides in Combination With Antibiotics Against Multidrug-Resistant Staphylococcus epidermidis

Multidrug resistance among various bacterial strains is leading to worldwide resistance to a wide range of antibiotics. Combination therapy involving current antibiotics and other biological or chemical molecules represents an attractive novel strategy. In this study, we investigated the synergistic antibacterial activity of a series of Trp-containing antimicrobial peptides (AMPs) with four classes of traditional chemical antibiotics that are inactive against multidrug-resistant Staphylococcus epidermidis (MRSE) in vitro and in vivo. Among the antibiotics that we studied, penicillin, ampicillin and erythromycin showed a distinct synergistic effect in combination with all of the Trp-containing AMPs, represented by a fractional inhibitory concentration index (FICI) of <0.5. The antibacterial activities were noticeably improved, with 32-to 64-fold reductions in the MIC values for ampicillin and 16- to 32-fold reductions in the MIC values for erythromycin and penicillin. Tetracycline showed synergistic activity with only I1WL5W but additive activity with L11W, L12W, and I4WL5W. Ceftazidime exhibited additive activity with the Trp-containing peptides. In addition, the antibiotics in combination with the peptide significantly inhibited biofilm formation by MRSE 1208. A mechanistic study demonstrated that the Trp-containing peptides, especially I1WL5W and I4WL5W, which contain two tryptophan residues, disrupted bacterial inner and outer membranes, which promoted antibiotic delivery into the cytoplasm and access to cytoplasmic targets; however, L11W and L12W may have increased intracellular antibiotic concentrations by decreasing blaZ, tet(m) and msrA expression. Importantly, strong synergistic activity against the MRSE 1208 strain was observed for the combination of I1WL5W and penicillin in a mouse infection model. Thus, the combination of AMPs and traditional antibiotics could be a promising option for the prevention of acute and chronic infections caused by MRSE.

Non-Lytic Antibacterial Peptides That Translocate Through Bacterial Membranes to Act on Intracellular Targets

The advent of multidrug resistance among pathogenic bacteria has attracted great attention worldwide. As a response to this growing challenge, diverse studies have focused on the development of novel anti-infective therapies, including antimicrobial peptides (AMPs). The biological properties of this class of antimicrobials have been thoroughly investigated, and membranolytic activities are the most reported mechanisms by which AMPs kill bacteria. Nevertheless, an increasing number of works have pointed to a different direction, in which AMPs are seen to be capable of displaying non-lytic modes of action by internalizing bacterial cells. In this context, this review focused on the description of the in vitro and in vivo antibacterial and antibiofilm activities of non-lytic AMPs, including indolicidin, buforin II PR-39, bactenecins, apidaecin, and drosocin, also shedding light on how AMPs interact with and further translocate through bacterial membranes to act on intracellular targets, including DNA, RNA, cell wall and protein synthesis.

Graphene oxide immobilized surfaces facilitate the sustained release of doxycycline for the prevention of implant related infection

Preventing implant-associated infection, which can lead to implant failure and increased medical costs, is one of the biggest challenges in the orthopaedic surgeons. Therefore, the development of stable and highly effective surface modifications to increase the antimicrobial properties of implants is required. In this study, graphene oxide (GO-)-immobilized titanium dioxide (TiO₂) was developed to efficiently carry and release antimicrobial drugs. Firstly, tyramine-conjugated GO (GOTA) was synthesized and immobilized onto the surfaces of TiO₂ through tyrosinase (Tyr)-catalyzed oxidative reaction (GOTA/TiO₂). Doxycycline hyclate (Dox) was then loaded onto GOTA/TiO₂ via non-covalent interactions between GO and Dox (Dox/GOTA/TiO₂), including electrostatic interaction, π-π stacking, hydrophobic interaction, and hydrogen bonds. The amount of loaded drug was able to be controlled, reaching a maximum of 36 μg/cm². in vitro experiments revealed that the sustained release of Dox from the TiO₂ surfaces continued for over 30 days. Compared with bare TiO₂ and GOTA/TiO₂, Dox/GOTA/TiO₂ exhibited superior antibacterial activity against both gram-negative Escherichia coli and gram-positive Staphylococcus aureus bacteria, without affecting the viability of human dermal fibroblasts. The obtained results indicated that GO-immobilized TiO₂ is an effective carrier for antimicrobial drug delivery to reduce implant-associated infection through the synergistic antimicrobial effect of GO and the prescribed drugs.

Antibiotic resistance in Pseudomonas aeruginosa: mechanisms and alternative therapeutic strategies

Pseudomonas aeruginosa is an opportunistic pathogen that is a leading cause of morbidity and mortality in cystic fibrosis patients and immunocompromised individuals. Eradication of P. aeruginosa has become increasingly difficult due to its remarkable capacity to resist antibiotics. Strains of Pseudomonas aeruginosa are known to utilize their high levels of intrinsic and acquired resistance mechanisms to counter most antibiotics. In addition, adaptive antibiotic resistance of P. aeruginosa is a recently characterized mechanism, which includes biofilm-mediated resistance and formation of multidrug-tolerant persister cells, and is responsible for recalcitrance and relapse of infections. The discovery and development of alternative therapeutic strategies that present novel avenues against P. aeruginosa infections are increasingly demanded and gaining more and more attention. Although mostly at the preclinical stages, many recent studies have reported several innovative therapeutic technologies that have demonstrated pronounced effectiveness in fighting against drug-resistant P. aeruginosa strains. This review highlights the mechanisms of antibiotic resistance in P. aeruginosa and discusses the current state of some novel therapeutic approaches for treatment of P. aeruginosa infections that can be further explored in clinical practice.

Unveiling the Multifaceted Mechanisms of Antibacterial Activity of Buforin II and Frenatin 2.3S Peptides from Skin Micro-Organs of the Orinoco Lime Treefrog (Sphaenorhynchus lacteus)

Amphibian skin is a rich source of natural compounds with diverse antimicrobial and immune defense properties. Our previous studies showed that the frog skin secretions obtained by skin micro-organs from various species of Colombian anurans have antimicrobial activities against bacteria and viruses. We purified for the first time two antimicrobial peptides from the skin micro-organs of the Orinoco lime treefrog (Sphaenorhynchus lacteus) that correspond to Buforin II (BF2) and Frenatin 2.3S (F2.3S). Here, we have synthesized the two peptides and tested them against Gram-negative and Gram-positive bacteria, observing an effective bactericidal activity at micromolar concentrations. Evaluation of BF2 and F2.3S membrane destabilization activity on bacterial cell cultures and synthetic lipid bilayers reveals a distinct membrane interaction mechanism. BF2 agglutinates E. coli cells and synthetic vesicles, whereas F2.3S shows a high depolarization and membrane destabilization activities. Interestingly, we found that F2.3S is able to internalize within bacterial cells and can bind nucleic acids, as previously reported for BF2. Moreover, bacterial exposure to both peptides alters the expression profile of genes related to stress and resistance response. Overall, these results show the multifaceted mechanism of action of both antimicrobial peptides that can provide alternative tools in the fight against bacterial resistance.

Synergism of fused bicyclic 2-aminothiazolyl compounds with polymyxin B against Klebsiella pneumoniae

A series of fused bicyclic 2-aminothiazolyl compounds were synthesized and evaluated for their synergistic effects with polymyxin B (PB) against Klebsiella pneumoniae (SIPI-KPN-1712). Some of the synthesized compounds exhibited synergistic activity. When 4 μg ml-1 compound B1 was combined with PB, it showed potent antibacterial activity, achieving 64-fold reduction of the MIC of PB. Furthermore, compound B1 showed prominent synergistic efficacy in both concentration gradient and time-kill curves in vitro. In addition, B1 combined with PB also exhibited synergistic and partial synergistic effect against E. coli (ATCC25922 and its clinical isolates), Acinetobacter baumannii (ATCC19606 and its clinical isolates), and Pseudomonas aeruginosa (Pae-1399).

Bactericide, Immunomodulating, and Wound Healing Properties of Transgenic Kalanchoe pinnata Synergize with Antimicrobial Peptide Cecropin P1 In Vivo

Procedure of manufacturing K. pinnata water extracts containing cecropin P1 (CecP1) from the formerly described transgenic plants is established. It included incubation of leaves at +4°C for 7 days, mechanical homogenization of leaves using water as extraction solvent, and heating at +70°C for inactivating plant enzymes. Yield of CecP1 (after heating and sterilizing filtration) was 0.3% of total protein in the extract. The water extract of K. pinnata + CecP1 exhibits favorable effect on healing of wounds infected with S. aureus (equal to Cefazolin) and with a combination of S. aureus with P. aeruginosa (better than Cefazolin). Wild-type K. pinnata extract exhibited evident microbicide activity against S. aureus with P. aeruginosa but it was substantially strengthened in K. pinnata + CecP1 extract. K. pinnata extracts (both wild-type and transgenic) did not exhibit general toxicity and accelerated wound recovery. Due to immunomodulating activity, wild-type K. pinnata extract accelerated granulation of the wound bed and marginal epithelialization even better than K. pinnata + CecP1 extract. Immunomodulating and microbicide activity of K. pinnata synergizes with microbicide activity of CecP1 accelerating elimination of bacteria.

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

Infection sustained by multidrug-resistant and extensively drug-resistant bacterial pathogens is often untreatable with the standard of care antibiotics, and the combination of anti-infective compounds often represents the only therapeutic strategy to face this major clinical treat. SET-M33 is a novel antimicrobial peptide (AMP) that has demonstrated in vitro and in vivo antimicrobial activity against Gram-negative bacteria and has shown interesting features in preclinical evaluations. Particularly, it showed efficacy against a number of multidrug-resistant and extensively drug-resistant clinical strains of Gram-negative pathogens, in in vitro and in vivo assessments. Here, we explored the potential synergistic activity of SET-M33 in combination with different standard of care antibiotics by the checkerboard method against a panel of six strains of Gram-negative pathogens including multidrug-resistant and extensively drug-resistant Klebsiella pneumoniae, Pseudomonas aeruginosa, and Acinetobacter baumannii. SET-M33 showed synergistic activity with antibiotics of different families against these clinically relevant strains. A synergistic effect was observed for SET-M33 in combination with rifampin, meropenem, aztreonam, and tobramycin mostly on K. pneumoniae and A. baumannii strains, while the SET-M33 plus ciprofloxacin combination was additive with all tested strains. Synergy was not apparently linked to the bacterial species or phenotype but was rather strain-specific, highlighting the need for individual strain testing for synergistic antimicrobial combinations. These findings extend current knowledge on synergistic activity of AMPs in combination with conventional agents and support the potential role of SET-M33 as a novel therapeutic agent against antibiotic-resistant Gram-negative pathogens. Copyright © 2017 European Peptide Society and John Wiley & Sons, Ltd.

Probiotic Diversity Enhances Rhizosphere Microbiome Function and Plant Disease Suppression

Bacterial communities associated with plant roots play an important role in the suppression of soil-borne pathogens, and multispecies probiotic consortia may enhance disease suppression efficacy. Here we introduced defined Pseudomonas species consortia into naturally complex microbial communities and measured the importance of Pseudomonas community diversity for their survival and the suppression of the bacterial plant pathogen Ralstonia solanacearum in the tomato rhizosphere microbiome. The survival of introduced Pseudomonas consortia increased with increasing diversity. Further, high Pseudomonas diversity reduced pathogen density in the rhizosphere and decreased the disease incidence due to both intensified resource competition and interference with the pathogen. These results provide novel mechanistic insights into elevated pathogen suppression by diverse probiotic consortia in naturally diverse plant rhizospheres. Ecologically based community assembly rules could thus play a key role in engineering functionally reliable microbiome applications.

The increasing demand for food supply requires more-efficient control of plant diseases. The use of probiotics, i.e., naturally occurring bacterial antagonists and competitors that suppress pathogens, has recently reemerged as a promising alternative to agrochemical use. It is, however, still unclear how many and which strains we should choose for constructing effective probiotic consortia. Here we present a general ecological framework for assembling effective probiotic communities based on in vitro characterization of community functioning. Specifically, we show that increasing the diversity of probiotic consortia enhances community survival in the naturally diverse rhizosphere microbiome, leading to increased pathogen suppression via intensified resource competition and interference with the pathogen. We propose that these ecological guidelines can be put to the test in microbiome engineering more widely in the future.

Hydraphiles enhance antimicrobial potency against Escherichia coli, Pseudomonas aeruginosa, and Bacillus subtilis

Hydraphiles are synthetic amphiphiles that form ion-conducting pores in liposomal membranes. These pores exhibit open-close behavior when studied by planar bilayer conductance techniques. In previous work, we showed that when co-administered with various antibiotics to the DH5α strain of Escherichia coli, they enhanced the drug's potency. We report here potency enhancements at low concentrations of hydraphiles for the structurally and mechanistically unrelated antibiotics erythromycin, kanamycin, rifampicin, and tetracycline against Gram negative E. coli (DH5α and K-12) and Pseudomonas aeruginosa, as well as Gram positive Bacillus subtilis. Earlier work suggested that potency increases correlated to ion transport function. The data presented here comport with the function of hydraphiles to enhance membrane permeability in addition to, or instead of, their known function as ion conductors.

Cecropin P1 inhibits porcine reproductive and respiratory syndrome virus by blocking attachment

Background

Porcine reproductive and respiratory syndrome virus (PRRSV) is a continuous threat to the pig industry, causing high economic losses worldwide. Current vaccines have specific limitations in terms of their safety and efficacy, so the development of novel antiviral drugs is urgently required. The aim of this study was to evaluate the inhibitory effects and underlying molecular mechanisms of the antimicrobial peptide cecropin P1 (CP1) against PRRSV infection in vitro.

Results

CP1 not only displayed extracellular virucidal activity against PRRSV, but also exerted a potent inhibitory effect when added either before, simultaneously with, or after viral inoculation. The inhibitory effect of CP1 occurred during viral attachment, but not at viral entry into Marc-145 cells. CP1 also inhibited viral particle release and attenuated virus-induced apoptosis during the late phase of infection. CP1 exerted similar inhibitory effects against PRRSV infection in porcine alveolar macrophages, the cells targeted by the virus in vivo during its infection of pigs. The expression of interleukin 6 was elevated by CP1 in porcine alveolar macrophages, which might contribute to its inhibition of PRRSV infection.

Conclusions

Collectively, our findings provide a new direction for the development of potential therapeutic drugs against PRRSV infection.