Synergistic effects and antibiofilm properties of chimeric peptides against multidrug-resistant Acinetobacter baumannii strains

Antimicrobial peptide-based strategies to overcome antimicrobial resistance

Antibiotic resistance has emerged as a global threat, rendering the existing conventional treatment strategies ineffective. In view of this, antimicrobial peptides (AMPs) have proven to be potent alternative therapeutic interventions with a wide range of applications in clinical health. AMPs are small peptides produced naturally as a part of the innate immune responses against a broad range of bacterial, fungal and viral pathogens. AMPs present a myriad of advantages over traditional antibiotics, including their ability to target multiple sites, reduced susceptibility to resistance development, and high efficacy at low doses. These peptides have demonstrated notable potential in inhibiting microbes resistant to traditional antibiotics, including the notorious ESKAPE pathogens, recognized as the primary culprits behind nosocomial infections. AMPs, with their multifaceted benefits, emerge as promising candidates in the ongoing efforts to combat the escalating challenges posed by antibiotic resistance. This in-depth review provides a detailed discussion on AMPs, encompassing their classification, mechanism of action, and diverse clinical applications. Focus has been laid on combating newly emerging drug-resistant organisms, emphasizing the significance of AMPs in mitigating this pressing challenge. The review also illuminates potential future strategies that may be implemented to improve AMP efficacy, such as structural modifications and using AMPs in combination with antibiotics and matrix-inhibiting compounds.

Antibacterial and antibiofilm potentials of vancomycin-loaded niosomal drug delivery system against methicillin-resistant Staphylococcus aureus (MRSA) infections

The threat of methicillin-resistant Staphylococcus aureus (MRSA) is increasing worldwide, making it significantly necessary to discover a novel way of dealing with related infections. The quick spread of MRSA isolates among infected individuals has heightened public health concerns and significantly limited treatment options. Vancomycin (VAN) can be applied to treat severe MRSA infections, and the indiscriminate administration of this antimicrobial agent has caused several concerns in medical settings. Owing to several advantageous characteristics, a niosomal drug delivery system may increase the potential of loaded antimicrobial agents. This work aims to examine the antibacterial and anti-biofilm properties of VAN-niosome against MRSA clinical isolates with emphasis on cytotoxicity and stability studies. Furthermore, we aim to suggest an effective approach against MRSA infections by investigating the inhibitory effect of formulated niosome on the expression of the biofilm-associated gene (icaR). The thin-film hydration approach was used to prepare the niosome (Tween 60, Span 60, and cholesterol), and field emission scanning electron microscopy (FE-SEM), an in vitro drug release, dynamic light scattering (DLS), and entrapment efficiency (EE%) were used to investigate the physicochemical properties. The physical stability of VAN-niosome, including hydrodynamic size, polydispersity index (PDI), and EE%, was analyzed for a 30-day storage time at 4 °C and 25 °C. In addition, the human foreskin fibroblast (HFF) cell line was used to evaluate the cytotoxic effect of synthesized niosome. Moreover, minimum inhibitory and bactericidal concentrations (MICs/MBCs) were applied to assess the antibacterial properties of niosomal VAN formulation. Also, the antibiofilm potential of VAN-niosome was investigated by microtiter plate (MTP) and real-time PCR methods. The FE-SEM result revealed that synthesized VAN-niosome had a spherical morphology. The hydrodynamic size and PDI of VAN-niosome reported by the DLS method were 201.2 nm and 0.301, respectively. Also, the surface zeta charge of the prepared niosome was - 35.4 mV, and the EE% ranged between 58.9 and 62.5%. Moreover, in vitro release study revealed a sustained-release profile for synthesized niosomal formulation. Our study showed that VAN-niosome had acceptable stability during a 30-day storage time. Additionally, the VAN-niosome had stronger antibacterial and anti-biofilm properties against MRSA clinical isolates compared with free VAN. In conclusion, the result of our study demonstrated that niosomal VAN could be promising as a successful drug delivery system due to sustained drug release, negligible toxicity, and high encapsulation capacity. Also, the antibacterial and anti-biofilm studies showed the high capacity of VAN-niosome against MRSA clinical isolates. Furthermore, the results of real-time PCR exhibited that VAN-niosome could be proposed as a powerful strategy against MRSA biofilm via down-regulation of icaR gene expression.

Discovery of Melittin as Triple-Action Agent: Broad-Spectrum Antibacterial, Anti-Biofilm, and Potential Anti-Quorum Sensing Activities

The development of antibiotic-resistant microorganisms is a major global health concern. Recently, there has been an increasing interest in antimicrobial peptides as a therapeutic option. This study aimed to evaluate the triple-action (broad-spectrum antibacterial, anti-biofilm, and anti-quorum sensing activities) of melittin, a membrane-active peptide present in bee venom. The minimum inhibitory concentration and minimum bactericidal concentration of the melittin were determined using the microdilution method and agar plate counting. Growth curve analysis revealed that melittin showed a concentration-dependent antibacterial activity. Scanning electron microscope analysis revealed that melittin treatment altered the morphology. Confocal laser scanning microscope revealed that melittin increased the membrane permeability and intracellular ROS generation in bacteria, all of which contribute to bacterial cell death. In addition, the crystal violet (CV) assay was used to test the anti-biofilm activity. The CV assay demonstrated that melittin inhibited biofilm formation and eradicated mature biofilms. Biofilm formation mediated by quorum sensing (QS) plays a major role in this regard, so molecular docking and molecular dynamics analysis confirmed that melittin interacts with LasR receptors through hydrogen bonds, and further evaluates the anti-QS activity of melittin through the production of virulence factors (pyocyanin, elastase, and rhamnolipid), exopolysaccharides secretion, and bacterial motility, that may be the key to inhibiting the biofilm formation mechanism. The present findings highlight the promising role of melittin as a broad-spectrum antibacterial, anti-biofilm agent, and potential QS inhibitor, providing a new perspective and theoretical basis for the development of alternative antibiotics.

Combination antimicrobial therapy: in vitro synergistic effect of anti-staphylococcal drug oxacillin with antimicrobial peptide nisin against Staphylococcus epidermidis clinical isolates and Staphylococcus aureus biofilms

The ability of Staphylococcus epidermidis and S. aureus to form strong biofilm on plastic devices makes them the major pathogens associated with device-related infections (DRIs). Biofilm-embedded bacteria are more resistant to antibiotics, making biofilm infections very difficult to effectively treat. Here, we evaluate the in vitro activities of anti-staphylococcal drug oxacillin and antimicrobial peptide nisin, alone and in combination, against methicillin-resistant S. epidermidis (MRSE) clinical isolates and the methicillin-resistant S. aureus ATCC 43,300. The minimum inhibitory concentrations (MIC) and minimum biofilm eradication concentrations (MBEC) of oxacillin and nisin were determined using the microbroth dilution method. The anti-biofilm activities of oxacillin and nisin, alone or in combination, were evaluated. In addition, the effects of antimicrobial agents on the expression of icaA gene were examined by quantitative real-time PCR. MIC values for oxacillin and nisin ranged 4-8 µg/mL and 64-128 µg/mL, respectively. Oxacillin and nisin reduced biofilm biomass in all bacteria in a dose-dependent manner and this inhibitory effect was enhanced with combinatorial treatment. MBEC ranges for oxacillin and nisin were 2048-8192 µg/mL and 2048-4096 µg/mL, respectively. The addition of nisin significantly decreased the oxacillin MBECs from 8- to 32-fold in all bacteria. At the 1× MIC and 1/2× MIC, both oxacillin and nisin decreased significantly the expression of icaA gene in comparison with untreated control. When two antimicrobial agents were combined at 1/2× MIC concentration, the expression of icaA were significantly lower than when were used alone. Nisin/conventional oxacillin combination showed considerable anti-biofilm effects, including inhibition of biofilm formation, eradication of mature biofilm, and down-regulation of biofilm-related genes, proposing its applications for treating or preventing staphylococcal biofilm-associated infections, including device-related infections.

Novel Antibacterial Agents SAAP-148 and Halicin Combat Gram-Negative Bacteria Colonizing Catheters

The antibiotic management of catheter-related infections (CRIs) often fails owing to the emergence of antimicrobial-resistant strains and/or biofilm/persister apparitions. Thus, we investigated the efficacy of two novel antimicrobial agents, i.e., the synthetic peptide SAAP-148 and the novel antibiotic halicin, against Gram-negative bacteria (GNB) colonizing catheters. The antibacterial, anti-biofilm, and anti-persister activities of both agents were evaluated against Acinetobacter baumannii, Escherichia coli, and Klebsiella pneumoniae strains. The enrolled strains were isolated from catheters and selected based on their resistance to at least three antibiotic classes and biofilm formation potential. Furthermore, the hemolysis and endotoxin neutralization abilities of these agents were explored. The bactericidal activity of both agents was reduced in urine and plasma as compared to buffered saline. In a dose-dependent manner, SAAP-148 and halicin reduced bacterial counts in 24 h preformed biofilms on silicone elastomer discs and eliminated persisters originating from antibiotic-exposed mature 7-day biofilms, with halicin being less effective than SAAP-148. Importantly, SAAP-148 and halicin acted synergistically on E. coli and K. pneumoniae biofilms but not on A. baumannii biofilms. The peptide, but not halicin, decreased the production of IL-12p40 upon exposure to UV-killed bacteria. This preliminary study showed that SAAP-148 and halicin alone/in combination are promising candidates to fight GNB colonizing catheters.

Effect of Camel Peptide on the Biofilm of Staphylococcus epidermidis and Staphylococcus haemolyticus Formed on Orthopedic Implants

The increasing bacterial drug resistance and the associated challenges in the treatment of infections warrant the search for alternative therapeutic methods. Hope is placed in antimicrobial peptides, which have a broad spectrum of action and are effective against strains which are resistant to conventional antibiotics. Antimicrobial peptides are also tested for their efficacy in the treatment of infections associated with the formation of biofilm. The aim of the present study was to examine the effect of Camel peptide on S. epidermidis and S. haemolyticus adhesion to and formation of biofilm on steel cortical bone screws and also on the process of reducing mature biofilm in orthopedic implants. The tests were performed on steel implants for osteosynthesis. The MIC value and MBEC values of the peptide were determined using the microdilution method in microtiter plates. The effect of the peptide on adhesion and biofilm formation, as well as on the activity on the preformed biofilm, was evaluated using quantitative methods and confocal microscopy. The presented research results indicate that the peptide exhibits very good antimicrobial properties against the analyzed strains. Concentrations above MIC reduced biofilm in the range of 90-99%.

An Update on the Therapeutic Potential of Antimicrobial Peptides against Acinetobacter baumannii Infections

The rise in antibiotic-resistant strains of clinically important pathogens is a major threat to global health. The World Health Organization (WHO) has recognized the urgent need to develop alternative treatments to address the growing list of priority pathogens. Antimicrobial peptides (AMPs) rank among the suggested options with proven activity and high potential to be developed into effective drugs. Many AMPs are naturally produced by living organisms protecting the host against pathogens as a part of their innate immunity. Mechanisms associated with AMP actions include cell membrane disruption, cell wall weakening, protein synthesis inhibition, and interference in nucleic acid dynamics, inducing apoptosis and necrosis. Acinetobacter baumannii is a critical pathogen, as severe clinical implications have developed from isolates resistant to current antibiotic treatments and conventional control procedures, such as UV light, disinfectants, and drying. Here, we review the natural AMPs representing primary candidates for new anti-A. baumannii drugs in post-antibiotic-era and present computational tools to develop the next generation of AMPs with greater microbicidal activity and reduced toxicity.

De novo peptides as potential antimicrobial agents

The phenomenon of antimicrobial resistance threatens our ability to treat common infections. The clinical pipeline for new antimicrobials is pretty much dry and hence, there is a need for the development of new antimicrobial agents with low toxicities to help fight resistant microorganisms. This work aimed to design antimicrobial peptides with low toxicities using a database filtering technology and evaluate their bioactivities. The physicochemical properties of the designed peptides were explored with molecular dynamics (MD) simulations. Microbroth dilution and hemolytic assays were used to assess the peptides' antimicrobial activity and toxicity. The activity of combinations of the peptides and some standard antibiotics was tested by the checkerboard method. In general, the designed peptides had a charge of +2, chain length of 13, and hydrophobicity of 61%. The predicted secondary structures of the peptides were either extended conformations or alpha-helices, and these structures were found to fluctuate during the MD simulations, where coils, bends, and helices dominated. , of the peptides, BRG003, BRG004 and BRG002 had the greatest aggregation propensities, whereas BRG001, BRG005, and BRG006 exhibited lower aggregation propensities. The minimum inhibitory concentration (MIC) of the peptides ranged from 0.015 to >1.879 μM, with BRGP-001 exhibiting high activity against MRSA with an MIC of 15 nM. BRGP-005 and BRGP-006 exhibited synergistic effects against Escherichia coliR when used in combination with erythromycin. At the minimum hemolytic concentration, the percentage of lysed erythrocytes was lower for all the peptides in comparison to the reference peptide, indicating low hemolytic activity. The study revealed the importance of peptide self-association in the antimicrobial activity of the peptides. These peptides provide a basis for the design of potent antimicrobial peptides that can further be developed for use in antimicrobial therapy.

Molecular Docking, Anti-Biofilm & Antibacterial Activities and Therapeutic Index of mCM11 Peptide on Acinetobacter baumannii Strains

Despite the huge efforts of microbiologists, infectious diseases have yet remained one of the leading causes of death in humans, further highlighting the research priority for controlling opportunistic pathogens. Many researchers have used antibacterial peptides to solve the problem of antibiotic resistance. This research is thus conducted to investigate the antibacterial and anti-biofilm activity of a novel modified cecropin-melittin 11-peptide with improved therapeutic properties and lower side effects. After synthesis and purification of mCM11 (NH2-WRLFRRILRVL-NH2) by solid-phase synthesis and HPLC methods, respectively, the antibacterial and biofilm inhibitory activities were explored in vitro. TMHMM was used to confirm the reaction of mCM11 on the plasma membrane of the prokaryotic cells. The interaction between mCM11 on Acinetobacter baumannii strains was investigated by molecular docking using ClusPro2.0. Hemolysis and therapeutic indexes were also calculated to quantify the relative safety and adverse effects of mCM11. According to the results, mCM11 has a high inhibitory and lethal effect on A. baumannii strains due to its cationic properties and new specific sequence. Molecular docking revealed the release of a significant amount of energy when mCM11 binds to the surface of A. baumannii in an appropriate site. The findings indicated that mCM11 IC50 (4 μg/mL) lysed 2.78% of RBCs; moreover, 8 strains of Acinetobacter baumannii showed a favorable therapeutic index. The mCM11 exhibits strong antibacterial and antibiofilm activities against A. baumannii strains, suggesting its potential therapeutic role in infections caused by these strains. Similar to its impact on A. baumannii, mCM11 could be a suitable alternative to antibiotics in combat against antibiotic-resistant bacteria in the in vivo experiments.

Strategic modification of low-activity natural antimicrobial peptides confers antibacterial potential in vitro and in vivo

Antimicrobial peptides (AMPs) show great promise for clinical applications, but the utility of naturally occurring AMPs is often limited by their stability. Here, we used a rational design approach to improve the characteristics of a pair of inactive peptides, tilapia piscidin 1 and 2 (TP1 and TP2). From each starting peptide, we generated a series of novel derivatives by substituting residues to adjust cationic charge density, percent hydrophobicity and hydrophilicity/hydrophobicity coefficients. This approach yielded a novel peptide, TP2-5 (KKCIAKAILKKAKKLLKKLVNP), that exhibits significant bactericidal potency, low cytotoxicity and high stability. The designed peptide further showed antibiofilm activity, rapid antibacterial action and a low capacity to induce bacterial resistance. Importantly, we also demonstrated that TP2-5 can protect mice in a Vibrio vulnificus-infected wound model. Therefore, our peptide modification strategy successfully generated a novel AMP with high potential for future clinical application.

Non-antibiotic prevention and treatment against Acinetobacter baumannii infection: Are vaccines and adjuvants effective strategies?

Acinetobacter baumannii (A. baumannii) is a Gram-negative opportunistic pathogen widely attached to the surface of medical instruments, making it one of the most common pathogens of nosocomial infection, and often leading to cross-infection and co-infection. Due to the extensive antibiotic and pan-resistance, A. baumannii infection is facing fewer treatment options in the clinic. Therefore, the prevention and treatment of A. baumannii infection have become a tricky global problem. The requirement for research and development of the new strategy is urgent. Now, non-antibiotic treatment strategies are urgently needed. This review describes the research on A. baumannii vaccines and antibacterial adjuvants, discusses the advantages and disadvantages of different candidate vaccines tested in vitro and in vivo, especially subunit protein vaccines, and shows the antibacterial efficacy of adjuvant drugs in monotherapy.

In Vitro Pharmacokinetics of LL-37 and Oncorhyncin II Combination Against Acinetobacter baumannii

Background: Multidrug-resistant (MDR) Acinetobacter baumannii is one of the most common nosocomial pathogens. Antimicrobial peptides (AMPs) have been introduced as a viable alternative to antibiotics in the treatment of MDR pathogens. Objectives: This study was designed to assess the in vitro pharmacokinetics of the combination of two potent AMPs, LL-37 and oncorhyncin II, against A. baumannii (ATCC19606). Methods: The synthesized genes of oncorhyncin II and LL-37 were introduced into Escherichia coli BL21 as the expression host. The minimum inhibitory concentration (MIC), time-kills, and growth kinetics of these peptides were used to evaluate their antimicrobial efficiencies against A. baumannii (ATCC19606). Results: LL-37 and oncorhyncin II recombinant peptides showed MIC of 30.6 and 95.87 µg/mL against A. baumannii, respectively. Additive action was confirmed by combining the generated AMPs at the checkerboard approach. The combination of LL-37 and oncorhyncin II at 2 × MIC resulted in a rapid drop in log10 CFU/mL of A. baumannii in the time-kill and growth kinetic findings studies. Conclusions: The combination of the produced LL-37 and oncorhyncin II synergizes the bioactivity of the individual peptides. Therefore, these peptides or their combinations might function as novel antibiotics and be used to develop and produce new antimicrobial drugs for the treatment of infections caused by A. baumannii.

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.

Synergy by Perturbing the Gram-Negative Outer Membrane: Opening the Door for Gram-Positive Specific Antibiotics

New approaches to target antibacterial agents toward Gram-negative bacteria are key, given the rise of antibiotic resistance. Since the discovery of polymyxin B nonapeptide as a potent Gram-negative outer membrane (OM)-permeabilizing synergist in the early 1980s, a vast amount of literature on such synergists has been published. This Review addresses a range of peptide-based and small organic compounds that disrupt the OM to elicit a synergistic effect with antibiotics that are otherwise inactive toward Gram-negative bacteria, with synergy defined as a fractional inhibitory concentration index (FICI) of <0.5. Another requirement for the inclusion of the synergists here covered is their potentiation of a specific set of clinically used antibiotics: erythromycin, rifampicin, novobiocin, or vancomycin. In addition, we have focused on those synergists with reported activity against Gram-negative members of the ESKAPE family of pathogens namely, Escherichia coli, Pseudomonas aeruginosa, Klebsiella pneumoniae, and/or Acinetobacter baumannii. In cases where the FICI values were not directly reported in the primary literature but could be calculated from the published data, we have done so, allowing for more direct comparison of potency with other synergists. We also address the hemolytic activity of the various OM-disrupting synergists reported in the literature, an effect that is often downplayed but is of key importance in assessing the selectivity of such compounds for Gram-negative bacteria.

An Overview of Biofilm Formation-Combating Strategies and Mechanisms of Action of Antibiofilm Agents

Biofilm formation on surfaces via microbial colonization causes infections and has become a major health issue globally. The biofilm lifestyle provides resistance to environmental stresses and antimicrobial therapies. Biofilms can cause several chronic conditions, and effective treatment has become a challenge due to increased antimicrobial resistance. Antibiotics available for treating biofilm-associated infections are generally not very effective and require high doses that may cause toxicity in the host. Therefore, it is essential to study and develop efficient anti-biofilm strategies that can significantly reduce the rate of biofilm-associated healthcare problems. In this context, some effective combating strategies with potential anti-biofilm agents, including plant extracts, peptides, enzymes, lantibiotics, chelating agents, biosurfactants, polysaccharides, organic, inorganic, and metal nanoparticles, etc., have been reviewed to overcome biofilm-associated healthcare problems. From their extensive literature survey, it can be concluded that these molecules with considerable structural alterations might be applied to the treatment of biofilm-associated infections, by evaluating their significant delivery to the target site of the host. To design effective anti-biofilm molecules, it must be assured that the minimum inhibitory concentrations of these anti-biofilm compounds can eradicate biofilm-associated infections without causing toxic effects at a significant rate.

Charge effect of water-soluble porphyrin derivatives as a prototype to fight infections caused by Acinetobacter baumannii by aPDT approaches

In the last decade, Acinetobacter baumannii has emerged as a pathogen associated with infections in intensive care units worldwide, especially due to its ability to resist an extensive list of antibiotics. In this context, porphyrins have emerged as an important strategy in photodynamic therapy, since they are a group of tetrapyrrolic compounds with important photochemical and photobiological activities. In this study, the antimicrobial photodynamic activity of meso-tetra(4-N-methyl-pyridyl)porphyrin (H₂TMePyP⁺) and meso-tetra(4-sulfonatophenyl)porphyrin (H₂TPPS^‒) was evaluated against A. baumannii by minimum inhibitory concentration (MIC), anti-biofilm activity, and the interaction with antibiotics after exposure to white-light LED irradiation. The cationic derivative H₂TMePyP⁺ was more potent (MIC = 0.61 µM) than H₂TPPS^‒, with anti-biofilm activity and increased the antimicrobial activity of ciprofloxacin and amikacin. Given these findings, the tetra-cationic porphyrins can be assumed as prototypes to optimize and develop new agents by promoting oxidative stress and inducing free radical production.

Highly Synergistic Effects of Melittin With Vancomycin and Rifampin Against Vancomycin and Rifampin Resistant Staphylococcus epidermidis

Methicillin-resistant Staphylococcus epidermidis (MRSE) strains are increasingly emerging as serious pathogens because they can be resistant to many antibiotics called multidrug resistance (MDR) that limit the therapeutic options. In the case of vancomycin- and rifampin-resistant MDR-MRSE, the physicians are not allowed to increase the doses of antibiotics because of severe toxicity. Accordingly, we investigated the synergistic activity of melittin antimicrobial peptide with vancomycin and rifampin against vancomycin-resistant, and rifampin-resistant MDR-MRSE isolates. Minimum inhibitory concentration (MIC), minimum bactericidal concentration (MBC), fractional inhibitory concentration index (FICi), and fractional bactericidal concentration index (FBCi) of antimicrobial agents against isolates were determined. Coagulate activities and serum and salt stability as well as melittin cytotoxicity on the human embryonic kidney (HEK) 293 cells and human red blood cells (RBCs) at their synergistic concentrations. MIC and MBC values for melittin were in the range of 0.312–2.5 and 0.312–5, respectively. Results also showed that the interaction of melittin with drugs was highly synergistic in which the geometric means of FICi and FBCi were < 0.5. Induced synergism led to a decrease in melittin, rifampin, and vancomycin concentrations by 8–1,020, 2–16, and 4–16-folds, respectively. This phenomenon caused a reduction in melittin toxicity by which the synergistic concentration of melittin needed to kill bacteria did not show cytotoxicity and hemolytic activity. Besides, no coagulation activity was found for the synergistic and alone concentrations of melittin in both Prothrombin Time (PT) and Partial Thromboplastin Time (PTT). Interestingly, the antibacterial activity of melittin in Mueller Hinton Broth (MHB) containing human serum did no significant differences between MIC and MBC values of melittin in MHB and MHB containing 10% human serum. The present findings showed that the therapeutic index of melittin was improved by 32.08- and 12.82-folds when combined with vancomycin and rifampin, respectively. Taken together, the obtained data show that melittin alone was effective against MDR-MRSE isolates and this antimicrobial peptide showed highly synergistic effects with vancomycin and rifampin without causing toxicity. Therefore, the combination of melittin and traditional antibiotics could be a promising strategy for the treatment of infections caused by MDR-MRSE.

Convergence of Biofilm Formation and Antibiotic Resistance in Acinetobacter baumannii Infection

Acinetobacter baumannii (A. baumannii) is a leading cause of nosocomial infections as this pathogen has certain attributes that facilitate the subversion of natural defenses of the human body. A. baumannii acquires antibiotic resistance determinants easily and can thrive on both biotic and abiotic surfaces. Different resistance mechanisms or determinants, both transmissible and non-transmissible, have aided in this victory over antibiotics. In addition, the propensity to form biofilms (communities of organism attached to a surface) allows the organism to persist in hospitals on various medical surfaces (cardiac valves, artificial joints, catheters, endotracheal tubes, and ventilators) and also evade antibiotics simply by shielding the bacteria and increasing its ability to acquire foreign genetic material through lateral gene transfer. The biofilm formation rate in A. baumannii is higher than in other species. Recent research has shown how A. baumannii biofilm-forming capacity exerts its effect on resistance phenotypes, development of resistome, and dissemination of resistance genes within biofilms by conjugation or transformation, thereby making biofilm a hotspot for genetic exchange. Various genes control the formation of A. baumannii biofilms and a beneficial relationship between biofilm formation and "antimicrobial resistance" (AMR) exists in the organism. This review discusses these various attributes of the organism that act independently or synergistically to cause hospital infections. Evolution of AMR in A. baumannii, resistance mechanisms including both transmissible (hydrolyzing enzymes) and non-transmissible (efflux pumps and chromosomal mutations) are presented. Intrinsic factors [biofilm-associated protein, outer membrane protein A, chaperon-usher pilus, iron uptake mechanism, poly-β-(1, 6)-N-acetyl glucosamine, BfmS/BfmR two-component system, PER-1, quorum sensing] involved in biofilm production, extrinsic factors (surface property, growth temperature, growth medium) associated with the process, the impact of biofilms on high antimicrobial tolerance and regulation of the process, gene transfer within the biofilm, are elaborated. The infections associated with colonization of A. baumannii on medical devices are discussed. Each important device-related infection is dealt with and both adult and pediatric studies are separately mentioned. Furthermore, the strategies of preventing A. baumannii biofilms with antibiotic combinations, quorum sensing quenchers, natural products, efflux pump inhibitors, antimicrobial peptides, nanoparticles, and phage therapy are enumerated.

Development of Bactericidal Peptides against Multidrug-Resistant Acinetobacter baumannii with Enhanced Stability and Low Toxicity

Pathogenic superbugs are the root cause of untreatable complex infections with limited or no treatment options. These infections are becoming more common as clinical antibiotics have lost their effectiveness over time. Therefore, the development of novel antibacterial agents is urgently needed to counter these microbes. Antimicrobial peptides (AMPs) are a viable treatment option due to their bactericidal potency against multiple microbial classes. AMPs are naturally selected physiological microbicidal agents that are found in all forms of organisms. In the present study, we developed two tilapia piscidin 2 (TP2)-based AMPs for antimicrobial application. Unlike the parent peptide, the redesigned peptides showed significant antimicrobial activity against multidrug-resistant bacterial species. These peptides also showed minimal cytotoxicity. In addition, they were significantly active in the presence of physiological salts, 50% human serum and elevated temperature. The designed peptides also showed synergistic activity when combined with clinical antibiotics. The current approach demonstrates a fruitful strategy for developing potential AMPs for antimicrobial application. Such AMPs have potential for progression to further trials and drug development investigations.

Synergistic antimicrobial and antibiofilm activities of piperic acid and 4-ethylpiperic acid amides in combination with ciprofloxacin

In the present work, piperic acid and 4-ethylpiperic acid (EPA) amides with amino acids (C1-C8) were bio-evaluated for their antimicrobial activity and biofilm inhibition against Gram-positive and Gram-negative bacterial strains. Among all, EPA-β^3,3-Pip(Bzl)-OMe, C2 displayed the potent antimicrobial activity with MIC of 6.25 μg ml^-1 against Gram-negative bacteria Escherichia coli. In combination studies, the FIC indices suggested that C1 and C2 have a synergistic effect with ciprofloxacin against E. coli and Bacillus subtilis, whereas C5 exhibited a synergistic effect with ciprofloxacin against all the tested bacteria. The inhibitory effect of amides C1, C2, and C5 on the biofilm formation of test strains was significantly potentiated by co-administration with ciprofloxacin. Furthermore, the effective concentrations of C2 in combination reduced drastically compared to alone for biofilm inhibition. At these concentrations, C2 showed negligible hemolytic and cytotoxic activities.

Prevention, inhibition, and degradation effects of melittin alone and in combination with vancomycin and rifampin against strong biofilm producer strains of methicillin-resistant Staphylococcus epidermidis

Methicillin-resistant Staphylococcus epidermidis (MRSE) bacteria are being recognized as true pathogens as they are able to resist methicillin and commonly form biofilms. Recent studies have shown that antimicrobial peptides (AMPs) are promising agents against biofilm-associated bacterial infections. In this study, we aimed to explore the antibiofilm activity of melittin, either alone or in combination with vancomycin and rifampin, against biofilm-producing MRSE strains. Minimum biofilm preventive concentration (MBPC), minimum biofilm inhibition concentration (MBIC), and minimum biofilm eradication concentration (MBEC), as well as fractional biofilm preventive-, inhibitory-, and eradication concentrations (FBPCi, FBICi, and FBECi), were determined for the antimicrobial agents tested. Cytotoxicity and hemolytic activity of melittin at its synergistic concentration were examined on human embryonic kidney cells (HEK-293) and Red Blood Cells (RBCs), respectively. The effect of melittin on the downregulation of biofilm-associated genes was explored using Real-Time PCR. MBPC, MBIC, and MBEC values for melittin were in the range of 0.625-20, 0.625-20, and 10-40 μg/μL, respectively. Melittin showed high synergy (FBPCi, FBICi and FBECi < 0.5). The synergism resulted in a 64-512-fold, 2-16 and 2-8-fold reduction in melittin, rifampicin and vancomycin concentrations, respectively. The synergistic melittin concentration found to be effective did not manifest either cytotoxicity on HEK-293 or hemolytic activity on RBCs. Results showed that melittin downregulated the expression of biofilm-associated icaA, aap, and psm genes in all isolates tested, ranging from 0.04-folds to 2.11-folds for icaA and from 0.05 to 3.76-folds for aap and psm. The preventive and therapeutic indexes of melittin were improved 8-fold when combined with vancomycin and rifampin. Based on these findings, the combination of melittin with conventional antibiotics could be proposed for treating or preventing biofilm-associated MRSE infections.

Synergism between WLBU2 peptide and antibiotics against methicillin-resistant Staphylococcus aureus and extended-spectrum beta-lactamase-producing Enterobacter cloacae

Infections caused by Methicillin-Resistant Staphylococcus aureus (MRSA) and Extended-Spectrum Beta-Lactamase (ESBL) producing Enterobacter cloacae are considered as major therapeutic challenge due to their multidrug-resistant (MDR) phenotype against conventional antibiotics. WLBU2 is an engineered cationic peptide with potent antimicrobial activity. This in-vitro study aimed to evaluate the effects of WLBU2 against clinical isolates of the aforementioned bacteria and assess whether synergistic effects can be achieved upon combination with conventional antibiotics. The minimum inhibitory concentrations (MICs) of antimicrobial agents against bacterial clinical isolates (n = 30/strain) were determined using the microbroth dilution assay. The minimum bactericidal concentrations (MBCs) of WLBU2 were determined from microbroth dilution (MICs) tests by subculturing to agar plates. MICs of WLBU2 were evaluated in the presence of physiological concentrations of salts including NaCl, CaCl2 and MgCl2. To identify bacterial resistance profile, MRSA were treated with Oxacillin, Erythromycin and Vancomycin, while Ceftazidime, Ceftriaxone, Ciprofloxacin and Imipenem were used against Enterobacter cloacae. Combination treatments of antibiotics and sub-inhibitory concentrations of WLBU2 were conducted when MICs indicated intermediate/resistant susceptibility. The MICs/MBCs of WLBU2 were identical for each respective bacteria with values of 0.78-6.25 μM and 1.5-12.5 μM against MRSA and Enterobacter cloacae, respectively. WLBU2 was found as salt resistant. Combination treatment showed that synergistic and additive effects were achieved in many isolates of MRSA and Enterobacter cloacae. Our data revealed that WLBU2 is a potent peptide with bactericidal activity. In addition, it demonstrated the selective advantage of WLBU2 as a potential therapeutic agent under physiological solutions. Our findings also support the combination of WLBU2 and conventional antibiotics with potential application for treatment of resistant bacteria.

Antimicrobial activity of synthetic antimicrobial peptides loaded in poly-Ɛ-caprolactone nanoparticles against mycobacteria and their functional synergy with rifampicin

Tuberculosis (TB) treatment has become a challenge because of the natural presence of multilayered cell wall rich in lipids which restrict antibiotic permeability within the bacteria. The development of mutations conferring resistance has aggravated the situation. Consequently, maximum pharmaceutical efforts are required to improve the treatment, and antimicrobial peptides (AMPs) with antimycobacterial activity can be exploited as a new treatment strategy against TB. The synergistic interaction between conventional antibiotics and AMPs has broadened its application landscape. To overcome peptide instability and bioavailability issues, encapsulation of these bioactive in biocompatible polymers was adopted. In this study, the effect of synthetic AMPs HHC-8 [KIWWWWRKR] and MM-10 [MLLKKLLKKM] encapsulated in poly (ε-caprolactone) nanoparticles (PCL-NPs) was evaluated against mycobacteria using REMA (Resazurin Microtiter Assay Plate) technique. PCL encapsulation allowed us to load the required amount of peptides, i.e. HHC-8 and MM-10, with an efficiency of ∼ 18.9 ± 5.24 and ∼ 21.1 ± 6.19 % respectively, and sphere size was around 376.5 ± 14.9 nm and 289.87 ± 17.98 nm for PCL-HHC-8-NPs and PCL-MM-10-NPs, respectively. Minimal degradation and sustained release of peptides from nanoparticles improved antimicrobial activity, decreasing the MIC₅₀ from 75 µg/ml to 18.75 µg/ml against M. smegmatis and from 75 µg/ml to 9 µg/ml against M. tuberculosis, respectively. The combinatorial MIC assays of encapsulated AMP with rifampicin antibiotics against M. smegmatis showed synergism between AMP-PCL-NPs and antibiotics with fractional inhibitory concentrations (FICs) around ∼ 0.09. The combinations of AMP NPs also demonstrated synergy against the mycobacteria. Our findings suggest that enhanced efficacy is due to protection offered by AMPs encapsulation resulting in augmentation of membrane permeation by AMPs and enhanced accumulation of antibiotics within mycobacteria resulting in synergy. The study findings might assist in the preclinical development of AMP for the fight against TB.

Bacterial-Specific Aggregation and Killing of Immunomodulatory Host Defense Peptides

This study involves the design and development of disulfide bridge-linked antimicrobial peptides using the host defense protein Angiogenin 4 (chAng4) as a template. The mini peptides derived from chAng4 (mCA4s) were evaluated for their antibacterial efficacies in various pathogenic bacterial strains, and the role of the oxidation state of thiols in the peptide sequence and its implication on antibacterial properties were explored. A remarkable property of these synthetic mCA4 peptides is their capability to flocculate bacteria and mediate bacterial-specific killing, in the absence of any other external stimulus. mCA4s were further evaluated for their cellular uptake, hemolytic activities, toxicities, and immunomodulatory activities in different eukaryotic cell lines. The results indicate that disulfide bridge-containing cationic amphipathic peptides show superior antibacterial efficacies, are nontoxic and nonhemolytic, and mediate bacterial flocculation and killing, in the absence of external stimuli.

Synergism between Rifampicin and Cationic Polyurethanes Overcomes Intrinsic Resistance of Escherichia coli

Antibiotic-resistant Gram-negative bacteria are emergent pathogens, causing millions of infections worldwide. While there are several classes of antibiotics that are effective against Gram-positive bacteria, the outer membrane (OM) of Gram-negative bacteria excludes high-molecular-weight hydrophobic antibiotics, making these species intrinsically resistant to several classes of antibiotics, including polyketides, aminocoumarins, and macrolides. The overuse of antibiotics such as β-lactams has also promoted the spread of resistance genes throughout Gram-negative bacteria, including the production of extended spectrum β-lactamases (ESBLs). The combination of innate and acquired resistance makes it extremely challenging to identify antibiotics that are effective against Gram-negative bacteria. In this study, we have demonstrated the synergistic effect of outer membrane-permeable cationic polyurethanes with rifampicin, a polyketide that would otherwise be excluded by the OM, on different strains of E. coli, including a clinically isolated uropathogenic multidrug-resistant (MDR) E. coli. Rifampicin combined with a low-dose treatment of a cationic polyurethane reduced the MIC in E. coli of rifampicin by up to 64-fold. The compositions of cationic polyurethanes were designed to have low hemolysis and low cell cytotoxicity while maintaining high antibacterial activity. Our results demonstrate the potential to rescue the large number of available OM-excluded antibiotics to target normally resistant Gram-negative bacteria via synergistic action with these cationic polyurethanes, acting as a novel antibiotic adjuvant class.

Cationic Glycosylated Block Co-β-peptide Acts on the Cell Wall of Gram-Positive Bacteria as Anti-biofilm Agents

Antimicrobial resistance is a global threat. In addition to the emergence of resistance to last resort drugs, bacteria escape antibiotics killing by forming complex biofilms. Strategies to tackle antibiotic resistance as well as biofilms are urgently needed. Wall teichoic acid (WTA), a generic anionic glycopolymer present on the cell surface of many Gram-positive bacteria, has been proposed as a possible therapeutic target, but its druggability remains to be demonstrated. Here we report a cationic glycosylated block co-β-peptide that binds to WTA. By doing so, the co-β-peptide not only inhibits biofilm formation, it also disperses preformed biofilms in several Gram-positive bacteria and resensitizes methicillin-resistant Staphylococcus aureus to oxacillin. The cationic block of the co-β-peptide physically interacts with the anionic WTA within the cell envelope, whereas the glycosylated block forms a nonfouling corona around the bacteria. This reduces physical interaction between bacteria-substrate and bacteria-biofilm matrix, leading to biofilm inhibition and dispersal. The WTA-targeting co-β-peptide is a promising lead for the future development of broad-spectrum anti-biofilm strategies against Gram-positive bacteria.

Inhibition of Virulence Factors and Biofilm Formation of Acinetobacter Baumannii by Naturally-derived and Synthetic Drugs

Acinetobacter baumannii is a gram-negative, aerobic, non-motile, and pleomorphic bacillus. A. baumannii is also a highly-infectious pathogen causing high mortality and morbidity rates in intensive care units. The discovery of novel agents against A. baumannii infections is urgently needed due to the emergence of drug-resistant A. baumannii strains and the limited number of efficacious antibiotics available for treatment. In addition to the production of several virulence factors, A. baumannii forms biofilms on the host cell surface as well. Formation of biofilms occurs through initial surface attachment, microcolony formation, biofilm maturation, and detachment stages, and is one of the major drug resistance mechanisms employed by A. baumannii. Several studies have previously reported the efficacy of naturally-derived and synthetic compounds as anti- biofilm and anti-virulence agents against A. baumannii. Here, inhibition of biofilm formation and virulence factors of A. baumannii using naturally-derived and synthetic compounds are reviewed.

An antibiotic potentiator retains its activity after being immobilized on silicone and prevents growth of multidrug-resistant Pseudomonas aeruginosa biofilms

Device-Associated Healthcare-Associated Infections (DA-HAI) are a major threat to public health worldwide since they are associated with increased hospital stays, morbidity, mortality, financial burden, and hospital overload. A strategy to combat DA-HAI involves the use of medical devices endowed with surfaces that can kill or repel pathogens and prevent biofilm formation. We aimed to develop low-toxic protease-resistant anti-biofilm surfaces that can sensitize drug-resistant bacteria to sub-inhibitory concentrations of antibiotics. To this end, we hypothesized that polymyxin B nonapeptide (PMBN) could retain its antibiotic-enhancing potential upon immobilization on a biocompatible polymer, such as silicone. The ability of PMBN-coated silicone to sensitize a multidrug-resistant clinical isolate of Pseudomonas aeruginosa (strain Ps4) to antibiotics and block biofilm formation was assessed by viable counting, confocal microscopy and safranin uptake. These assays demonstrated that covalently immobilized PMBN enhances not only antibiotics added exogenously but also those incorporated into the functionalized coating. As a result, the functionalized surface exerted a potent bactericidal activity that precluded biofilm formation. PMBN-coated silicone displayed a high level of stability and very low cytotoxicity and hemolytic activity in the presence of antibiotics. We demonstrated for the first time that an antibiotic enhancer can retain its activity when covalently attached to a solid surface. These findings may be applied to the development of medical devices resistant to biofilm formation.

Antibiofilm and antivirulence efficacy of myrtenol enhances the antibiotic susceptibility of Acinetobacter baumannii

Acinetobacter baumannii (AB) is rising as a human pathogen of critical priority worldwide as it is the leading cause of chronic opportunistic infections in healthcare settings and the condition is ineradicable with antibiotic therapy. AB possesses the ability to form biofilm on abiotic as well as biotic surfaces which plays a major role in its pathogenesis and resistance in clinical settings. Hence, the demand for an alternative therapy to combat the biofilm-associated infections is increasing. The present study explored the antibiofilm potential of myrtenol, a bicyclic monoterpene present in various plants against reference and clinical strains of AB. Myrtenol (200 μg/mL) exhibited a strong antibiofilm activity without exerting any harmful effect on growth and metabolic viability of AB strains. Microscopic analyses confirmed the reduction in the biofilm thickness and surface coverage upon myrtenol treatment. Especially, myrtenol was found to be effective in disrupting the mature biofilms of tested AB strains. Furthermore, myrtenol inhibited the biofilm-associated virulence factors of AB strains such as extracellular polysaccharide, cell surface hydrophobicity, oxidant resistance, swarming and twitching motility. Transcriptional analysis unveiled the suppression of the biofilm-associated genes such as bfmR, csuA/B, bap, ompA, pgaA, pgaC, and katE by myrtenol. Notably, myrtenol improved the susceptibility of AB strains towards conventional antibiotics such as amikacin, ciprofloxacin, gentamicin and trimethoprim. Thus, the present study demonstrates the therapeutic potential of myrtenol against biofilm-associated infections of AB.

Inhibition of bacterial adherence to biomaterials by coating antimicrobial peptides with anionic surfactant

Medical devices are widely used in modern medicine, but their utilities are often limited by the biofilm formation of bacteria that are tolerant to most antibiotics. In this report, antimicrobial peptides (AMPs) were coated onto biomaterials by the aid of surfactant through hydrophobic interactions. To increase the coating efficiency, stability of AMPs in body fluids and spectrum of antimicrobial activity, pairs of AMPs were coated simultaneously onto various substrates, such as silicone, polyurethane and titanium, which are commonly used components of biomedical devices. These coated AMPs exhibited very low cytotoxicity and hemolytic activities because they were gradually released into urine or serum. The AMP pairs, such as T9W + SAAP159 and T9W + RRIKA, coated onto the silicone discs were able to inhibit in vitro bacterial adherence in urine. Most importantly, AMP pairs coated onto the silicone tubing by surfactant SDBS could prevent bacterial adherence to mouse bladder and the silicone tubing implanted within it. These results provide a promising approach towards circumventing urinary catheter-associated infections caused by bacterial adherence.

Synergistic Effect of Propidium Iodide and Small Molecule Antibiotics with the Antimicrobial Peptide Dendrimer G3KL against Gram-Negative Bacteria

There is an urgent need to develop new antibiotics against multidrug-resistant bacteria. Many antimicrobial peptides (AMPs) are active against such bacteria and often act by destabilizing membranes, a mechanism that can also be used to permeabilize bacteria to other antibiotics, resulting in synergistic effects. We recently showed that G3KL, an AMP with a multibranched dendritic topology of the peptide chain, permeabilizes the inner and outer membranes of Gram-negative bacteria including multidrug-resistant strains, leading to efficient bacterial killing. Here, we show that permeabilization of the outer and inner membranes of Pseudomonas aeruginosa by G3KL, initially detected using the DNA-binding fluorogenic dye propidium iodide (PI), also leads to a synergistic effect between G3KL and PI in this bacterium. We also identify a synergistic effect between G3KL and six different antibiotics against the Gram-negative Klebsiella pneumoniae, against which G3KL is inactive.

Synergistic antibacterial and anti-biofilm activity of nisin like bacteriocin with curcumin and cinnamaldehyde against ESBL and MBL producing clinical strains

Bacteriocins are small peptides that can inhibit the growth of a diverse range of microbes. There is a need to identify bacteriocins that are effective against biofilms of resistant clinical strains. The present study focussed on the efficacy of purified nisin like bacteriocin-GAM217 against extended spectrum β-lactamase (ESBL) and metallo-beta-lactamase (MBL) producing clinical strains. Bacteriocin-GAM217 when combined with curcumin and cinnamaldehyde, synergistically enhanced antibacterial activity against planktonic and biofilm cultures of Staphylococcus epidermidis and Escherichia coli. Bacteriocin-GAM217 and phytochemical combinations inhibited biofilm formation by >80%, and disrupted the biofilm for selected ESBL and MBL producing clinical strains. The anti-adhesion assay showed that these combinatorial compounds significantly lowered the attachment of bacteria to Vero cells and that they elicited membrane permeability and rapid killing as viewed by confocal microscopy. This study demonstrates that bacteriocin-GAM217 in combination with phytochemicals can be a potential anti-biofilm agent and thus has potential for biomedical applications.

Antimicrobial Activity of Protein Fraction from Naja ashei Venom Against Staphylococcus epidermidis

One of the key problems of modern infectious disease medicine is the growing number of drug-resistant and multi-drug-resistant bacterial strains. For this reason, many studies are devoted to the search for highly active antimicrobial substances that could be used in therapy against bacterial infections. As it turns out, snake venoms are a rich source of proteins that exert a strong antibacterial effect, and therefore they have become an interesting research material. We analyzed Naja ashei venom for such antibacterial properties, and we found that a specific composition of proteins can act to eliminate individual bacterial cells, as well as the entire biofilm of Staphylococcus epidermidis. In general, we used ion exchange chromatography (IEX) to obtain 10 protein fractions with different levels of complexity, which were then tested against certified and clinical strains of S. epidermidis. One of the fractions (F2) showed exceptional antimicrobial effects both alone and in combination with antibiotics. The protein composition of the obtained fractions was determined using mass spectrometry techniques, indicating a high proportion of phospholipases A₂, three-finger toxins, and L-amino acids oxidases in F2 fraction, which are most likely responsible for the unique properties of this fraction. Moreover, we were able to identify a new group of low abundant proteins containing the Ig-like domain that have not been previously described in snake venoms.

Antibiofilm Potential of Arenecarbaldehyde 2-Pyridinylhydrazone Derivatives Against Acinetobacter baumannii

In the last 15 years, Acinetobacter baumannii has received special attention, mainly due to several resistance mechanisms and high rates of morbimortality. The ability to form biofilms contributes to the persistence of this microorganism in the hospital environment and facilitates the occurrence of nosocomial infections. Several studies have highlighted the pharmacological relevance of pyridines in the treatment and control of infectious diseases and others have related the anti-A. baumannii potential of hydrazine derivatives. Considering this scenario, we aimed to evaluate the antimicrobial and antibiofilm activity of 10 pyridinylhydrazone compounds against A. baumannii. The minimum inhibitory concentration of the compounds was determined by broth microdilution method and the antibiofilm activity was evaluated by inhibition and destruction biofilm assays. In addition, the cytotoxicity of the compounds in the J774A.1 cell line was also evaluated, and the selectivity index was calculated. Among the 10 pyridine compounds, the compounds B and D were able to inhibit the formation of biofilms and destroy bacterial biofilms even in a concentration of 12.5 μg/mL. Thus, the pyridine compounds evaluated can be a scaffold for the development of new substances with antimicrobial and antibiofilm activity.

Protegrin-1 cytotoxicity towards mammalian cells positively correlates with the magnitude of conformational changes of the unfolded form upon cell interaction

Porcine protegrin-1 (PG-1) is a broad-spectrum antimicrobial peptide (AMP) with potent antimicrobial activities. We produced recombinant PG-1 and evaluated its cytotoxicity toward various types of mammalian cell lines, including embryonic fibroblasts, retinal cells, embryonic kidney cells, neuroblastoma cells, alveolar macrophage cells, and neutrophils. The sensitivity of the different mammalian cells to cytotoxic damage induced by PG-1 differed significantly among the cell types, with retinal neuron cells and neutrophils being the most significantly affected. A circular dichroism analysis showed there was a precise correlation between conformational changes in PG-1 and the magnitude of cytotoxicity among the various cell type. Subsequently, a green fluorescent protein (GFP) penetration assay using positively charged GFPs indicated there was a close correlation between the degree of penetration of charged GFP into cells and the magnitude of PG-1 cytotoxicity. Furthermore, we also showed that inhibition of the synthesis of anionic sulphated proteoglycans on the cell surface decreases the cytotoxic damage induced by PG-1 treatment. Taken together, the observed cytotoxicity of PG-1 towards different membrane surfaces is highly driven by the membrane’s anionic properties. Our results reveal a possible mechanism underlying cell-type dependent differences in cytotoxicity of AMPs, such as PG-1, toward mammalian cells.

Synergism of cationic antimicrobial peptide WLBU2 with antibacterial agents against biofilms of multi-drug resistant Acinetobacter baumannii and Klebsiella pneumoniae

Purpose

The activity of the cationic antimicrobial peptide WLBU2 was evaluated against planktonic cells and biofilms of multi-drug resistant (MDR) Acinetobacter baumannii and Klebsiella pneumoniae, alone and in combination with classical antimicrobial agents.

Methods

Control American Type Culture Collection (ATCC) strains and MDR clinical isolates of A. baumannii and K. pneumoniae were utilized. The minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) of WLBU2 alone and in combination with antimicrobials were determined by classical methods. The Calgary biofilm device was used to determine the minimum biofilm eradication concentration (MBEC). The MTT assay was used to determine the cytotoxicity of agents on eukaryotic cells. The electrophoretic mobility shift assay was used to evaluate the ability of WLBU2 to bind bacterial DNA.

Results

The WLBU2 MIC and MBC values were identical indicating bactericidal activity. The MIC/MBC values ranged from 1.5625 to 12.5 µM. At these concentrations, Vero cells and human skin fibroblasts were viable. The MBEC of WLBU2 ranged from 25 to 200 µM. A significant loss of eukaryotic cell viability was observed at the MBEC range. The combination of sub-inhibitory concentrations of WLBU2 with amoxicillin-clavulanate or ciprofloxacin for K. pneumoniae, and with tobramycin or imipenem for A. baumannii, demonstrated synergism, leading to a significant decrease in MIC and MBEC values for some isolates and ATCC strains. However, all combinations were associated with considerable loss in eukaryotic cells’ viability. WLBU2 did not demonstrate the ability to bind bacterial plasmid DNA.

Conclusion

WLBU2 in combination with antimicrobials holds promise in eradication of MDR pathogens.

Sulfur-Functionalized Fullerene Nanoparticle as an Inhibitor and Eliminator Agent on Pseudomonas aeruginosa Biofilm and Expression of toxA Gene

Over the last decade, nanotechnology-based therapeutic platforms have been directed toward developing nanoparticles with unique properties to combat biofilms. In this study, we evaluated the antibiofilm activity of the sulfur-functionalized fullerene nanoparticles (SFF Nps) against Pseudomonas aeruginosa and also analyzed the effect of this nanoparticle on the expression of exotoxin A (toxA) gene. The functionalized fullerenes were prepared by chemical vapor deposition method. We assessed the potential of SFF Nps to inhibit biofilm formation and eradicate preformed biofilms. Also, the effect of this nanoparticle on the expression of toxA gene was investigated by real-time PCR. The minimum biofilm inhibitory concentration of SFF Nps was 1 mg/mL. The minimum biofilm-eradication concentration of SFF Nps on the young (24- and 48-hr old) and older (72- and 96-hr old) biofilms was 2 and 4 mg/mL, respectively. Field emission electron scanning microscopy images confirmed the potent ability of SFF Nps to eradicate biofilm of P. aeruginosa. The expression of toxA was downregulated in the presence of SFF Nps. In conclusion, considering the ability of SFF Nps to kill P. aeruginosa biofilm and downregulate the expression of exotoxin A, this nanoparticle can be used for treatment of both chronic and acute P. aeruginosa infections.

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.

AMPs as Anti-biofilm Agents for Human Therapy and Prophylaxis

Microbial cells show a strong natural tendency to adhere to surfaces and to colonize them by forming complex communities called biofilms. In this growth mode, biofilm-forming cells encase themselves inside a dense matrix which efficiently protects them against antimicrobial agents and effectors of the immune system. Moreover, at the physiological level, biofilms contain a very heterogeneous cell population including metabolically inactive organisms and persisters, which are highly tolerant to antibiotics. The majority of human infectious diseases are caused by biofilm-forming microorganisms which are responsible for pathologies such as cystic fibrosis, infective endocarditis, pneumonia, wound infections, dental caries, infections of indwelling devices, etc. AMPs are well suited to combat biofilms because of their potent bactericidal activity of broad spectrum (including resting cells and persisters) and their ability to first penetrate and then to disorganize these structures. In addition, AMPs frequently synergize with antimicrobial compounds and were recently reported to repress the molecular pathways leading to biofilm formation. Finally, there is a very active research to develop AMP-containing coatings that can prevent biofilm formation by killing microbial cells on contact or by locally releasing their active principle. In this chapter we will describe these strategies and discuss the perspectives of the use of AMPs as anti-biofilm agents for human therapy and prophylaxis.

Action of Antimicrobial Peptides against Bacterial Biofilms

Microbes are known to colonize surfaces and form biofilms. These biofilms are communities of microbes encased in a self-produced matrix that often contains polysaccharides, DNA and proteins. Antimicrobial peptides (AMPs) have been used to control the formation and to eradicate mature biofilms. Naturally occurring or synthetic antimicrobial peptides have been shown to prevent microbial colonization of surfaces, to kill bacteria in biofilms and to disrupt the biofilm structure. This review systemically analyzed published data since 1970 to summarize the possible anti-biofilm mechanisms of AMPs. One hundred and sixty-two published reports were initially selected for this review following searches using the criteria ‘antimicrobial peptide’ OR ‘peptide’ AND ‘mechanism of action’ AND ‘biofilm’ OR ‘antibiofilm’ in the databases PubMed; Scopus; Web of Science; MEDLINE; and Cochrane Library. Studies that investigated anti-biofilm activities without describing the possible mechanisms were removed from the analysis. A total of 17 original reports were included which have articulated the mechanism of antimicrobial action of AMPs against biofilms. The major anti-biofilm mechanisms of antimicrobial peptides are: (1) disruption or degradation of the membrane potential of biofilm embedded cells; (2) interruption of bacterial cell signaling systems; (3) degradation of the polysaccharide and biofilm matrix; (4) inhibition of the alarmone system to avoid the bacterial stringent response; (5) downregulation of genes responsible for biofilm formation and transportation of binding proteins.

The effects of antimicrobial peptides WAM-1 and LL-37 on multidrug-resistant Acinetobacter baumannii

Increasing multidrug resistance (MDR) in Acinetobacter baumannii warrants therapeutic alternatives, and the bactericidal nature of antimicrobial peptides (AMPs) offers a possible approach. In this study, we examined the interaction of cathelicidin AMPs WAM-1, a marsupial AMP, and LL-37, a human AMP, with A. baumannii clinical isolates. We characterized the antibiotic resistance of the isolates, the bacteriostatic and bactericidal effects of these AMPs, synergistic activity with antibiotics, and their effects on biofilm formation and dispersal. All clinical isolates were resistant to commonly prescribed antibiotics, with four of seven isolates showing MDR. WAM-1 and LL-37 showed variable activity in clinical isolates, with WAM-1 having a stronger bacteriostatic effect than LL-37 and showing rapid bactericidal activity against clinical isolates. Furthermore, synergistic bactericidal activity was observed with WAM-1 and commonly prescribed antibiotics. Both peptides were able to inhibit biofilm formation in all clinical isolates at some concentrations, and WAM-1 dispersed mature biofilm in most isolates. LL-37 was unable to disperse mature biofilms in any strains. Further studies must be done to elucidate the true value of these alternative treatments, but these results suggest that MDR A. baumannii's susceptibility to AMPs may result in innovative therapeutics to prevent or treat these infections.

Strategies for combating bacterial biofilms: A focus on anti-biofilm agents and their mechanisms of action

Biofilm refers to the complex, sessile communities of microbes found either attached to a surface or buried firmly in an extracellular matrix as aggregates. The biofilm matrix surrounding bacteria makes them tolerant to harsh conditions and resistant to antibacterial treatments. Moreover, the biofilms are responsible for causing a broad range of chronic diseases and due to the emergence of antibiotic resistance in bacteria it has really become difficult to treat them with efficacy. Furthermore, the antibiotics available till date are ineffective for treating these biofilm related infections due to their higher values of minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC), which may result in in-vivo toxicity. Hence, it is critically important to design or screen anti-biofilm molecules that can effectively minimize and eradicate biofilm related infections. In the present article, we have highlighted the mechanism of biofilm formation with reference to different models and various methods used for biofilm detection. A major focus has been put on various anti-biofilm molecules discovered or tested till date which may include herbal active compounds, chelating agents, peptide antibiotics, lantibiotics and synthetic chemical compounds along with their structures, mechanism of action and their respective MICs, MBCs, minimum biofilm inhibitory concentrations (MBICs) as well as the half maximal inhibitory concentration (IC₅₀) values available in the literature so far. Different mode of action of anti biofilm molecules addressed here are inhibition via interference in the quorum sensing pathways, adhesion mechanism, disruption of extracellular DNA, protein, lipopolysaccharides, exopolysaccharides and secondary messengers involved in various signaling pathways. From this study, we conclude that the molecules considered here might be used to treat biofilm-associated infections after significant structural modifications, thereby investigating its effective delivery in the host. It should also be ensured that minimum effective concentration of these molecules must be capable of eradicating biofilm infections with maximum potency without posing any adverse side effects on the host.

Peptide based antimicrobials: Design strategies and therapeutic potential

Therapeutic activity of antibiotics is noteworthy, as they are used in the treatment of microbial infections. Regardless of their utility, there has been a steep decrease in the number of drug candidates due to antibiotic resistance, an inevitable consequence of noncompliance with the full therapeutic regimen. A variety of resistant species like MDR (Multi-Drug Resistant), XDR (Extensively Drug-Resistant) and PDR (Pan Drug-Resistant) species have evolved, but discovery pipeline has already shown signs of getting dried up. Therefore, the need for newer antibiotics is of utmost priority to combat the microbial infections of future times. Peptides have some interesting features like minimal side effect, high tolerability and selectivity towards specific targets, which would help them successfully comply with the stringent safety standards set for clinical trials. In this review, we attempt to present the state of the art in the discovery of peptide-based antimicrobials from a design perspective.

The inhibitory effect of the combination of two new peptides on biofilm formation by Acinetobacter baumannii

The emergence of extensively drug-resistant (XDR) Acinetobacter baumannii strains and the limited number of efficacious antibiotics demonstrate an urgent need to develop novel agents to treat infections caused by this dangerous pathogen. To find antimicrobial peptides against A. baumannii growing either in planktonic or in biofilm mode, biopanning was carried out with a peptide library on five XDR A. baumannii strains grown in the medium containing human blood (blood biopanning) and biofilms formed by these strains (biofilm biopanning). Two groups of peptides were identified, among which two peptides N10 (from blood biopanning) and NB2 (from biofilm biopanning) were selected and synthesized for more assessments. The selected peptides showed significant binding to A. baumannii rather than to the human cell line Caco-2. Both peptides were effective against A. baumannii and showed antibacterial activities (minimum inhibitory concentration (MIC) 500 μg/ml). In the biofilm inhibition assay, NB2 reduced biofilm more efficiently (75%) than N10 (50%). The combination of the two peptides could function better than each peptide alone to prevent biofilm formation by A. baumannii. Supplementation of conventional therapy with a mixture of peptides targeting A. baumannii or using peptides to deliver antibiotics specifically to the site of infection may be promising to control A. baumannii-related diseases.

Inhibitory effect of α-mangostin on Acinetobacter baumannii biofilms - an in vitro study

The present study was designed to investigate the anti-biofilm potential of alpha-mangostin (α-MG) against Acinetobacter baumannii (AB). The biofilm inhibitory concentration (BIC) of α-MG against AB was found to be 2 μg ml^-1. α-MG (0.5, 1 and 2 μg ml^-1) exhibited non-bactericidal concentration-dependent anti-biofilm activities against AB. However, α-MG failed to disintegrate the mature biofilms of AB even at a 10-fold increased concentration from its BIC. Results from qRT-PCR and in vitro bioassays further demonstrated that α-MG downregulated the expression of bfmR, pgaA, pgaC, csuA/B, ompA, bap, katE, and sodB genes, which correspondingly affects biofilm formation and its associated virulence traits. The present study suggests that α-MG exerts its anti-biofilm property by interrupting initial biofilm formation and the cell-to-cell signaling mechanism of AB. Additional studies are required to understand the mode of action responsible for the anti-biofilm property.

4-Hydroxybenzaldehyde sensitizes Acinetobacter baumannii to amphenicols

Bacterial metabolism modulated by environmental chemicals could alter antibiotic susceptibility. 4-Hydroxybenzaldehyde (4-HBA), which cannot support the growth of Acinetobacter baumannii, exhibited synergism only with amphenicol antibiotics including chloramphenicol (CAM) and thiamphenicol. Interestingly, this synergistic effect was not observed with other growth-supporting, structurally similar compounds such as 4-hydroxybenzoate. Transcriptomic analysis demonstrated that genes involved in protocatechuate metabolism (pca genes) and osmotic stress (bet genes) were significantly upregulated by 4-HBA and CAM treatment. The 14C-labeled CAM influx was lower in a pcaK1 (encoding a transporter of protocatechuate) deletion mutant and was higher in the pcaK1 overexpressing cells relative to that in the wild type upon 4-HBA treatment. Our kinetic data using 14C-labeled CAM clearly showed that CAM uptake is possibly through facilitated diffusion. Deletion of pcaK1 did not result in the elimination of CAM influx, indicating that CAM does not enter only through PcaK1. The amount of 4-HBA in the culture supernatant was, however, unaffected during the test conditions, validating that it was not metabolized by the bacteria. CAM resistant A. baumannii cells derived by serial passages through CAM-amended media exhibited lower level of pcaK1 gene expression. These results led us to conclude that the activation of PcaK1 transporter is probably linked to cellular CAM susceptibility. This is the first report showing a relationship between CAM influx and aromatic compound metabolism in A. baumannii.

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

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