Elucidation of the Mode of Action of a New Antibacterial Compound Active against Staphylococcus aureus and Pseudomonas aeruginosa

A small-molecule membrane fluidizer re-sensitizes methicillin-resistant Staphylococcus aureus (MRSA) to β-lactam antibiotics

Novel antibacterial agents and strategies are urgently needed to fight against the ongoing global antibiotic resistance problem. While natural products remain the main source in antibiotic discovery, synthetic antibacterials provide an attractive alternative and may evade the ancient antibiotic resistance. Herein, we report a small molecule that re-sensitizes methicillin-resistant Staphylococcus aureus to β-lactam antibiotics with extremely low potential for resistance development. It belongs to a new class of broad-spectrum antibacterials, trypyricins, which share similar structural characteristics and mechanism of action to the cationic antimicrobial peptides. Mechanistic studies indicated that trypyricins fluidize and disrupt bacterial cytoplasmic membrane. These results suggested that trypyricins represent a promising new class of antibacterials and may be further developed as antibiotic adjuvants to fight against resistant bacteria in the clinic.

Identification of 5-(Aryl/Heteroaryl)amino-4-quinolones as Potent Membrane-Disrupting Agents to Combat Antibiotic-Resistant Gram-Positive Bacteria

Nosocomial infections caused by resistant Gram-positive organisms are on the rise, presumably due to a combination of factors including prolonged hospital exposure, increased use of invasive procedures, and pervasive antibiotic therapy. Although antibiotic stewardship and infection control measures are helpful, newer agents against multidrug-resistant (MDR) Gram-positive bacteria are urgently needed. Here, we describe our efforts that led to the identification of 5-amino-4-quinolone 111 with exceptionally potent Gram-positive activity with minimum inhibitory concentrations (MICs) ≤0.06 μg/mL against numerous clinical isolates. Preliminary mechanism of action and resistance studies demonstrate that the 5-amino-4-quinolones are bacteriostatic, do not select for resistance, and selectively disrupt bacterial membranes. While the precise molecular mechanism has not been elucidated, the lead compound is nontoxic displaying a therapeutic index greater than 500, is devoid of hemolytic activity, and has attractive physicochemical properties (clog P = 3.8, molecular weight (MW) = 441) that warrant further investigation of this promising antibacterial scaffold for the treatment of Gram-positive infections.

Elucidating the Mechanisms of Action of Antimicrobial Agents

Despite the ever-growing antibiotic resistance crisis, the rate at which new antimicrobials are being discovered and approved for human use has rapidly declined over the past 75 years. A barrier for advancing newly identified antibiotics beyond discovery is elucidating their mechanism(s) of action.

PaP1, a Broad-Spectrum Lysin-Derived Cationic Peptide to Treat Polymicrobial Skin Infections

Most skin infections, including those complicating burns, are polymicrobial involving multiple causative bacteria. Add to this the fact that many of these organisms may be antibiotic-resistant, and a simple skin lesion or burn could soon become life-threatening. Membrane-acting cationic peptides from Gram-negative bacteriophage lysins can potentially aid in addressing the urgent need for alternative therapeutics. Such peptides natively constitute an amphipathic region within the structural composition of these lysins and function to permit outer membrane permeabilization in Gram-negative bacteria when added externally. This consequently allows the lysin to access and degrade the peptidoglycan substrate, resulting in rapid hypotonic lysis and bacterial death. When separated from the lysin, some of these cationic peptides kill sensitive bacteria more effectively than the native molecule via both outer and cytoplasmic membrane disruption. In this study, we evaluated the antibacterial properties of a modified cationic peptide from the broad-acting lysin PlyPa01. The peptide, termed PaP1, exhibited potent in vitro bactericidal activity toward numerous high priority Gram-positive and Gram-negative pathogens, including all the antibiotic-resistant ESKAPE pathogens. Both planktonic and biofilm-state bacteria were sensitive to the peptide, and results from time-kill assays revealed PaP1 kills bacteria on contact. The peptide was bactericidal over a wide temperature and pH range and could withstand autoclaving without loss of activity. However, high salt concentrations and complex matrices were found to be largely inhibitory, limiting its use to topical applications. Importantly, unlike other membrane-acting antimicrobials, PaP1 lacked cytotoxicity toward human cells. Results from a murine burn wound infection model using methicillin-resistant Staphylococcus aureus or multidrug-resistant Pseudomonas aeruginosa validated the in vivo antibacterial efficacy of PaP1. In these studies, the peptide enhanced the potency of topical antibiotics used clinically for treating chronic wound infections. Despite the necessity for additional preclinical drug development, the collective data from our study support PaP1 as a potential broad-spectrum monotherapy or adjunctive therapy for the topical treatment of polymicrobial infections and provide a foundation for engineering future lysin-derived peptides with improved antibacterial properties.

Evaluating the Antibacterial Activity and Mode of Action of Thymol-Loaded Chitosan Nanoparticles Against Plant Bacterial Pathogen Xanthomonas campestris pv. campestris

The bacterium Xanthomonas campestris pv. campestris (Xcc) causes black rot disease in cruciferous crops, resulting in severe yield loss worldwide. The excessive use of chemical pesticides in agriculture to control diseases has raised significant concern about the impact on the environment and human health. Nanoparticles have recently gained significant attention in agriculture owing to their promising application in plant disease control, increasing soil fertility and nutrient availability. In the current study, we synthesized thymol-loaded chitosan nanoparticles (TCNPs) and assessed their antibacterial activity against Xcc. The synthesis of TCNPs was confirmed by using ultraviolet–visible spectroscopy. Fourier-transform infrared spectroscopy, transmission electron microscopy, and scanning electron microscopy analysis revealed the functional groups, size, and shape of TCNPs, with sizes ranging from 54 to 250 nm, respectively. The antibacterial activity of TCNPs against Xcc was investigated in vitro by liquid broth, cell viability, and live dead staining assay, and all of them demonstrated the antibacterial activity of TCNPs. Furthermore, TCNPs were found to directly inhibit the growth of Xcc by suppressing the growth of biofilm formation and the production of exopolysaccharides and xanthomonadin. The ultrastructure studies revealed membrane damage in TCNP-treated Xcc cells, causing a release of intracellular contents. Headspace/gas chromatography (GC)–mass spectrometry (MS) analysis showed changes in the volatile profile of Xcc cells treated with TCNPs. Increased amounts of carbonyl components (mainly ketones) and production of new volatile metabolites were observed in Xcc cells incubated with TCNPs. Overall, this study reveals TCNPs as a promising antibacterial candidate against Xcc.

Synergistic evaluation of Moringa oleifera extract and ß-lactam antibiotic to restore the susceptibility of methicillin-resistant Staphylococcus aureus

INTRODUCTION

The antibiotic resistance has become a major threat to global health. The combinatorial use of two or more compounds to develop a new formulation may overcome the emerging cases of drug resistance. Moringa oleifera has been utilized as a strong nutritional, immunomodulator and therapeutic agent for decades. In this study, different parts of Moringa oleifera were screened for bioactive compounds that can act as a resistance modifying agent for multi-drug resistant organisms (MDROs).

METHODOLOGY

Initially, the combined effect of stem bark extracts and ampicillin was calculated by checkerboard assay. Active compounds of effective extract were assessed by High Performance Liquid Chromatography (HPLC). Minimal Inhibitory Concentration (MIC) and Fractional Inhibitory Concentration Index (FICI) were calculated to evaluate the synergistic behavior of stem bark extract with ampicillin. To study the blocking of resistance pathways of Methicillin-Resistant Staphylococcus aureus (MRSA) western blot was performed.

RESULTS

The results revealed that stem bark has significant anti-MRSA activity. The methanolic extract of stem bark in combination with ampicillin showed the highest synergistic effect (FICI value ≤ 0.237) against MRSA. Killing kinetics and membrane potential of ampicillin alone and in combination revealed an increase in the inhibitory potential of ampicillin against MRSA. Decolourization in iodometric assay confirmed the inhibition of β-lactamase, western blot results confirmed the blocking of penicillin-binding protein (PBP2a) expression with the restoration of MRSA sensitivity against β-lactams.

CONCLUSION

It can be concluded that methanolic extract of Moringa oleifera stem bark has bioactive compounds and can be used as an adjuvant with antibiotics to modify the resistance of MDROs.

Using membrane perturbing small molecules to target chronic persistent infections

After antibiotic treatment, a subpopulation of bacteria often remains and can lead to recalcitrant infections. This subpopulation, referred to as persisters, evades antibiotic treatment through numerous mechanisms such as decreased uptake of small molecules and slowed growth. Membrane perturbing small molecules have been shown to eradicate persisters as well as render these populations susceptible to antibiotic treatment. Chemotype similarities have emerged suggesting amphiphilic heteroaromatic compounds possess ideal properties to increase membrane fluidity and such molecules warrant further investigation as effective agents or potentiators against persister cells.

Isolation, characterization, and molecular identification of soil bacteria showing antibacterial activity against human pathogenic bacteria

BACKGROUND

The present study dealt with the screening of soil bacteria with antibacterial activity from different locations in Bangalore, India. Antibiotics play the role of self-defense mechanism for the bacteria and are produced as secondary metabolites to protect themselves from other competitive microorganisms. The need for new antibiotics arose as the pathogenic bacteria acquire resistance to various antibiotics meant for treating human diseases. Given the importance of antibiotics of bacterial origin, standard techniques have been used to isolate and characterize the soil bacteria which showed antibacterial activity.

RESULTS

The isolated bacteria were tested against human pathogenic bacteria like Staphylococcus aureus, Escherichia coli, Pseudomonas aeruginosa, and Klebsiella pneumoniae by primary and secondary screening methods. The isolates PR1, PR2, and PR3 were confirmed to have antibacterial activity against S. aureus, E. coli, P. aeruginosa, and K. pneumoniae by both methods. Studies on the effect of filter sterilization, autoclaving, and proteinase K treatment on culture filtrates showed filter sterilization as the best method. The effect of different carbon and nitrogen sources on the antibacterial activity showed that preference by each isolate differed for carbon and nitrogen requirements. The isolates PR1, PR2, and PR3 were identified as Bacillus aryabhattai strain PR-D07, Arthrobacter humicola strain PR-F07, and Neomicrococcus lactis strain PR-F11 through 16S rRNA sequencing.

CONCLUSION

Findings from this research work are encouraging and could proceed further to applied aspects. Only 3 bacterial isolates out of 263 isolates from soil samples displayed antibacterial activity against human pathogens S. aureus, E. coli, P. aeruginosa, and K. pneumoniae. They were identified as B. aryabhattai, A. humicola, and N. lactis by 16S rRNA studies and all of them are Gram-positive. Each isolate preferred different carbon and nitrogen sources for their enhanced antibacterial activity. Efficacy of the culture filtrates of these isolates was tested by filter sterilization, autoclaving, and proteinase K treatment. Filter-sterilized culture filtrates showed higher antibacterial activity than other treatments. A comparison of the antibacterial activity of culture filtrates and antibiotic streptomycin produced an inhibition zone of 18.5 mm and 15.5 mm respectively. This is the first report on the antibacterial activity of all the 3 bacterial strains (B. aryabhattai strain PR-D07, A. humicola strain PR-F07, and N. lactis strain PR-F11), against all the human pathogens, mentioned earlier. It is also found that the antibiotic factor is proteinaceous as proteinase K considerably reduced the antibacterial activity of the culture filtrates. With the above significant results, these 3 bacteria are considered to be promising candidates for the isolation of new antibacterial agents.

Effect of naturally isolated hydroquinone in disturbing the cell membrane integrity of Pseudomonas aeruginosa MTCC 741 and Staphylococcus aureus MTCC 740

Nosocomial pathogens cause various health problems in human and many novel drugs are under investigation to combat the pathogens. The present study explains the naturally derived hydroquinone possible mode of action against Pseudomonas aeruginosa MTCC 741 and Staphylococcus aureus MTCC 740. Time kill studies, cell viability assays, membrane potential assays, and potassium release assays were carried out to study the mode of action. Time kill studies revealed the rapid death of bacterial pathogens exposed to 4X MIC (Minimum inhibitory concentration) of the hydroquinone. Cell viability assay results showed that nearly half of the cell destruction of test pathogens occurred within one hour. Transmission electron microscopic (TEM) observations revealed the disruption of the cell membrane, which caused severe ultrastructural changes in both test pathogens. Hydroquinone dissipated the membrane potential of test pathogens, as confirmed by the depolarization of membrane potential, increases in permeability and leakage of intracellular potassium ions. At the concentration of 2X MIC hydroquinone in 5 min, about 91.41% and 84.85% potassium ions were released from P. aeruginosa MTCC 741 and S. aureus MTCC 740, respectively. This is the first report on the mode of action of naturally derived hydroquinone against clinical pathogens.

A New Potent Antimicrobial Metalloporphyrin

A series of bis-acryl functionalized porphyrins and their corresponding metalloporphyrins (M=Co, Mn) were synthesized and investigated for their antimicrobial properties through MIC screening and bacteria time-kill kinetic studies. The Mn(III) 4-(bis)methylphenyl-substituted-porphyrins showed superior batericidal activities even in the dark with low hemotoxicity and good cytotoxicity profile.

A Cell-Free Screen for Bacterial Membrane Disruptors Identifies Mefloquine as a Novel Antibiotic Adjuvant

Antibacterial discovery efforts have lagged far behind the need for new antibiotics. An approach that has gained popularity recently is targeting bacterial phospholipid membranes. We leveraged the differences between bacterial and mammalian phospholipid compositions to develop a high-throughput screen that identifies agents that selectively disrupt bacterial membranes while leaving mammalian membranes intact. This approach was used to screen 4480 compounds representing a subset of the Maybridge HitFinderTM V.11 Collection and the Prestwick Chemical Drug Library®. The screen identified 35 "positives" (0.8% hit rate) that preferentially damage bacterial model membranes. Among these, an antimalarial compound, mefloquine, and an aminoglycoside, neomycin, were identified. Further investigation of mefloquine's activity against Staphylococcus aureus showed that it has little antibiotic activity on its own but can alter membrane fluidity, thereby potentiating a β-lactam antibiotic, oxacillin, against both methicillin-susceptible and methicillin-resistant S. aureus. This study indicates that our cell-free screening approach is a promising platform for discovering bacterial membrane disruptors as antibacterials antibiotic adjuvants.

Comparison of Proteomic Responses as Global Approach to Antibiotic Mechanism of Action Elucidation

New antibiotics are urgently needed to address the mounting resistance challenge. In early drug discovery, one of the bottlenecks is the elucidation of targets and mechanisms. To accelerate antibiotic research, we provide a proteomic approach for the rapid classification of compounds into those with precedented and unprecedented modes of action. We established a proteomic response library of Bacillus subtilis covering 91 antibiotics and comparator compounds, and a mathematical approach was developed to aid data analysis.

New antibiotics are urgently needed to address the mounting resistance challenge. In early drug discovery, one of the bottlenecks is the elucidation of targets and mechanisms. To accelerate antibiotic research, we provide a proteomic approach for the rapid classification of compounds into those with precedented and unprecedented modes of action. We established a proteomic response library of Bacillus subtilis covering 91 antibiotics and comparator compounds, and a mathematical approach was developed to aid data analysis. Comparison of proteomic responses (CoPR) allows the rapid identification of antibiotics with dual mechanisms of action as shown for atypical tetracyclines. It also aids in generating hypotheses on mechanisms of action as presented for salvarsan (arsphenamine) and the antirheumatic agent auranofin, which is under consideration for repurposing. Proteomic profiling also provides insights into the impact of antibiotics on bacterial physiology through analysis of marker proteins indicative of the impairment of cellular processes and structures. As demonstrated for trans-translation, a promising target not yet exploited clinically, proteomic profiling supports chemical biology approaches to investigating bacterial physiology.

Identification of the potential biological target of N-benzenesulfonyl-1,2,3,4-tetrahydroquinoline compounds active against gram-positive and gram-negative bacteria

The development of new antibiotics with activity towards a broad spectrum of bacteria, including multiresistant strains, is a very important topic for global public health. As part of previous works, N-benzenesulfonyl-1,2,3,4-tetrahydroquinoline (BSTHQ) derivatives were described as antimicrobial agents active against gram-positive and gram-negative pathogens. In this work, experimental and molecular modelling studies were performed in order to identify their potential biological target in the light of structure-based design efforts towards further BSTHQ derivatives. First, a carboxyfluorescein leakage assay was performed using liposomes to mimic bacterial membranes, which found no significative membrane disruption effects with respect to control samples. These results support a non-surfactant antimicrobial activity of the tested compounds. In a second stage, the inhibition of potential antimicrobial targets was screened using molecular modelling methods, taking into account previously reported druggable targets deposited in the ChEMBL database for Escherichia coli and Staphylococcus aureus. Two enzymes, namely D-glutamic acid-adding enzyme (MurD) and N-acetylglucosamine-1-phophate-uridyltransferase (GlmU), both involved in bacterial membrane synthesis, were identified as potential targets. Pharmacodynamic interaction models were developed using known MurD and GlmU inhibitors by applying state-of-the-art chemoinformatic methods (molecular docking, molecular dynamics and free energy of interaction analyses). These models were further extended to the analysis of the studied BSTHQ derivatives. Overall, our results demonstrated that the studied BSTHQ derivatives elicit their antibacterial activity by interacting with a specific molecular target, GlmU being the highly feasible one. Based on the presented results, further structure-aided design efforts towards the obtaining of novel BSTHQ derivatives are envisioned.Communicated by Ramaswamy H. Sarma.

The Antistaphylococcal Lysin, CF-301, Activates Key Host Factors in Human Blood To Potentiate Methicillin-Resistant Staphylococcus aureus Bacteriolysis

Bacteriophage-derived lysins are cell-wall-hydrolytic enzymes that represent a potential new class of antibacterial therapeutics in development to address burgeoning antimicrobial resistance. CF-301, the lead compound in this class, is in clinical development as an adjunctive treatment to potentially improve clinical cure rates of Staphylococcus aureus bacteremia and infective endocarditis (IE) when used in addition to antibiotics. In order to profile the activity of CF-301 in a clinically relevant milieu, we assessed its in vitro activity in human blood versus in a conventional testing medium (cation-adjusted Mueller-Hinton broth [caMHB]). CF-301 exhibited substantially greater potency (32 to ≥100-fold) in human blood versus caMHB in three standard microbiologic testing formats (e.g., broth dilution MICs, checkerboard synergy, and time-kill assays). We demonstrated that CF-301 acted synergistically with two key human blood factors, human serum lysozyme (HuLYZ) and human serum albumin (HSA), which normally have no nascent antistaphylococcal activity, against a prototypic methicillin-resistant S. aureus (MRSA) strain (MW2). Similar in vitro enhancement of CF-301 activity was also observed in rabbit, horse, and dog (but not rat or mouse) blood. Two well-established MRSA IE models in rabbit and rat were used to validate these findings in vivo by demonstrating comparable synergistic efficacy with standard-of-care anti-MRSA antibiotics at >100-fold lower lysin doses in the rabbit than in the rat model. The unique properties of CF-301 that enable bactericidal potentiation of antimicrobial activity via activation of "latent" host factors in human blood may have important therapeutic implications for durable improvements in clinical outcomes of serious antibiotic-resistant staphylococcal infections.

Potential of Biocellulose Carrier Impregnated with Essential Oils to Fight Against Biofilms Formed on Hydroxyapatite

In this research, bacterial cellulose (BC), one of the most promising biopolymers of the recent years, was saturated with thyme, eucalyptus and clove essential oils (EOs) and applied against staphylococcal and pseudomonal biofilms formed on hydroxyapatite (HA). BC dressings were thoroughly analyzed with regard to their physical properties. Moreover, the exact composition and ability of particular EO molecules to adhere to HA was assessed. Additionally, cytotoxicity of oil-containing, cellulose-based dressings towards osteoblasts and fibroblasts as well as their impact on reactive oxygen species (ROS) production by macrophages was assessed. The results revealed the high ability of BC dressings to absorb and subsequently release EOs from within their microstructure; the highest number of compounds able to adhere to HA was found in the thyme EO. The eucalyptus EO displayed low, while thyme and clove EOs displayed high cytotoxicity towards fibroblast and osteoblast cell lines. The clove EO displayed the highest eradication ability toward staphylococcal, while the thyme EO against pseudomonal biofilm. Taken together, the results obtained indicate the suitability of EO-saturated BC dressings to eradicate pseudomonal and staphylococcal biofilm on HA surface and moreover, to not trigger reactive oxygen species production by immune system effector cells. However, due to cytotoxic effects of thyme and clove EOs towards cell lines in vitro, the eucalyptus EO-saturated BC dressing is of highest potential to be further applied.

A novel bi-functional chalcone inhibits multi-drug resistant Staphylococcus aureus and potentiates the activity of fluoroquinolones

Staphylococcus aureus is the leading cause of bacteraemia and the dwindling supply of effective antibacterials has exacerbated the problem of managing infections caused by this bacterium. Isoliquiritigenin (ISL) is a plant flavonoid that displays therapeutic potential against S. aureus. The present study identified a novel mannich base derivatives of ISL, IMRG4, active against Vancomycin intermediate S. aureus (VISA). IMRG4 damages the bacterial membranes causing membrane depolarization and permeabilization, as determined by loss of salt tolerance, flow cytometric analysis, propidium idodie and fluorescent microscopy. It reduces the intracellular invasion of HEK-293 cells by S. aureus and decreases the staphylococcal load in different organs of infected mice models. In addition to anti-staphylococcal activity, IMRG4 inhibits the multidrug efflux pump, NorA, which was determined by molecular docking and EtBr efflux assays. In combination, IMRG4 significantly reduces the MIC of norfloxacin for clinical strains of S. aureus including VISA. Development of resistance against IMRG4 alone and in combination with norfloxacin was low and IMRG4 prolongs the post-antibiotic effect of norfloxacin. These virtues combined with the low toxicity of IMRG4, assessed by MTT assay and haemolysis, makes it an ideal candidate to enter drug development pipeline against S. aureus.

Antimicrobial and anti-biofilm activity of hexadentated macrocyclic complex of copper (II) derived from thiosemicarbazide against Staphylococcus aureus

Multidrug-resistant pathogens causing nosocomial and community acquired infections delineate a significant threat to public health. It had urged to identify new antimicrobials and thus, generated interest in studying macrocyclic metal complex, which has been studied in the past for their antimicrobial activity. Hence, in the present study, we have evaluated the antimicrobial activity of the hexadentated macrocyclic complex of copper (II) (Cu Complex) derived from thiosemicarbazide against Gram-positive and Gram-negative bacteria. We observed increased susceptibility against standard isolates of Staphylococcus aureus with a minimum inhibitory concentration (MIC) range of 6.25 to 12.5 μg/mL. Similar activity was also observed towards methicillin resistant and sensitive clinical isolates of S. aureus from human (n = 20) and animal (n = 20) infections. The compound has rapid bactericidal activity, and we did not observe any resistant mutant of S. aureus. The compound also exhibited antibiofilm activity and was able to disrupt pre-formed biofilms. Cu complex showed increased susceptibility towards intracellular S. aureus and was able to reduce more than 95% of the bacterial load at 10 μg/mL. Overall, our results suggest that Cu complex with its potent anti-microbial and anti-biofilm activity can be used to treat MRSA infections and evaluated further clinically.

1-((2,4-Dichlorophenethyl)Amino)-3-Phenoxypropan-2-ol Kills Pseudomonas aeruginosa through Extensive Membrane Damage

The ever increasing multidrug-resistance of clinically important pathogens and the lack of novel antibiotics have resulted in a true antibiotic crisis where many antibiotics are no longer effective. Further complicating the treatment of bacterial infections are antibiotic-tolerant persister cells. Besides being responsible for the recalcitrant nature of chronic infections, persister cells greatly contribute to the observed antibiotic tolerance in biofilms and even facilitate the emergence of antibiotic resistance. Evidently, eradication of these persister cells could greatly improve patient outcomes and targeting persistence may provide an alternative approach in combatting chronic infections. We recently characterized 1-((2,4-dichlorophenethyl)amino)-3-phenoxypropan-2-ol (SPI009), a novel anti-persister molecule capable of directly killing persisters from both Gram-negative and Gram-positive pathogens. SPI009 potentiates antibiotic activity in several in vitro and in vivo infection models and possesses promising anti-biofilm activity. Strikingly, SPI009 restores antibiotic sensitivity even in resistant strains. In this study, we investigated the mode of action of this novel compound using several parallel approaches. Genetic analyses and a macromolecular synthesis assays suggest that SPI009 acts by causing extensive membrane damage. This hypothesis was confirmed by liposome leakage assay and membrane permeability studies, demonstrating that SPI009 rapidly impairs the bacterial outer and inner membranes. Evaluation of SPI009-resistant mutants, which only could be generated under severe selection pressure, suggested a possible role for the MexCD-OprJ efflux pump. Overall, our results demonstrate the extensive membrane-damaging activity of SPI009 and confirm its clinical potential in the development of novel anti-persister therapies.

Elucidation of antimicrobial activity and mechanism of action by N-substituted carbazole derivatives

Compounds belonging to a carbazole series have been identified as potent fungal plasma membrane proton adenosine triphophatase (H+-ATPase) inhibitors with a broad spectrum of antifungal activity. The carbazole compounds inhibit the adenosine triphosphate (ATP) hydrolysis activity of the essential fungal H+-ATPase, thereby functionally inhibiting the extrusion of protons and extracellular acidification, processes that are responsible for maintaining high plasma membrane potential. The compound class binds to and inhibits the H+-ATPase within minutes, leading to fungal death after 1-3h of compound exposure in vitro. The tested compounds are not selective for the fungal H+-ATPase, exhibiting an overlap of inhibitory activity with the mammalian protein family of P-type ATPases; the sarco(endo)plasmic reticulum calcium ATPase (Ca2+-ATPase) and the sodium potassium ATPase (Na+,K+-ATPase). The ion transport in the P-type ATPases is energized by the conversion of ATP to adenosine diphosphate (ADP) and phosphate and a general inhibitory mechanism mediated by the carbazole derivative could therefore be blocking of the active site. However, biochemical studies show that increased concentrations of ATP do not change the inhibitory activity of the carbazoles suggesting they act as allosteric inhibitors. Furthermore decreased levels of intracellular ATP would suggest that the compounds inhibit the H+-ATPase indirectly, but Candida albicans cells exposed to potent H+-ATPase-inhibitory carbazoles result in increased levels of intracellular ATP, indicating direct inhibition of H+-ATPase.

The Effect of Citrus Essential Oils and Their Constituents on Growth of Xanthomonas citri subsp. citri

Citrus bacterial canker (CBC) caused by Xanthomonas citri subsp. citri (Xcc), is the most devastating of the citrus diseases worldwide. During our study, we found that Essential oils (EOs) of some citrus cultivars are effective on Xcc. Therefore, it prompted us to determine the plant metabolites responsible for the antibacterial properties. We obtained EOs from some locally cultivated citrus by using a Clevenger apparatus and their major constituents were identified by gas chromatography/mass spectrometry (GC-MS). The effect of Citrus aurantium, C. aurantifolia, Fortunella sp. EOs and their major constituents were evaluated against Xcc-KVXCC1 using a disk diffusion assay. Minimal inhibitory and bactericidal concentration of the EOs and their constituents were determined using the broth microdilution method. C. aurantium, C. aurantifolia Eos, and their major constituents including citral, linalool, citronellal, geraniol, α-terpineol, and linalyl acetate indicated antibacterial effects against Xcc. The C. aurantifolia EO and citral showed the highest antibacterial activity among the tested EOs and constituents with inhibition zones of 15 ± 0.33 mm and 16.67 ± 0.88 mm, respectively. Synergistic effects of the constituents were observed between α-terpineol-citral, citral-citronellal, citral-geraniol, and citronellal-geraniol by using a microdilution checkerboard assay. Transmission electron microscopy revealed that exposure of Xcc cells to citral caused cell wall damage and altered cytoplasmic density. We introduced C. aurantifolia and C. aurantium EOs, and their constituents citral, α-terpineol, citronellal, geraniol, and linalool as possible control agents for CBC.

Repurposing AM404 for the treatment of oral infections by Porphyromonas gingivalis

Porphyromonas gingivalis is a major pathogen involved in oral diseases such as periodontitis and peri‐implantitis. Management of these diseases typically includes mechanical debridement of the colonized surfaces followed by application of antiseptics or antibiotics. Disadvantages associated with the use of antiseptics and the growing worldwide problem of antibiotic resistance have necessitated the search for alternative agents. In this study, the antibacterial and antibiofilm properties of AM404, an active metabolite of paracetamol, were tested against P. gingivalis and other bacterial pathogens. The activity of AM404 was tested against 10 bacteria, including both oral and nonoral human pathogens. The minimal inhibitory concentration (MIC) of AM404 was determined by measuring optical density (OD) values. The minimum biofilm inhibitory concentration (MBIC) was detected by crystal violet staining. The activity of structural analogs of AM404 was tested by MIC determinations. The effect of AM404 on P. gingivalis biofilms formed on titanium disks as a model for dental implants was evaluated by colony forming unit counting. Potential cytotoxicity of AM404 towards HEK‐293 (human embryonic kidney cells), HepG2 (human hepatoma cells), IEC‐6 (rat intestinal cells), and Panc‐1 cells (pancreatic cancer cells) was assessed by 3‐(4,5‐dimethylthiazol‐2‐yl)‐2,5‐diphenyltetrazolium bromide assays. To get more insight in the mode of action of AM404, we used the fluorescent dyes N‐phenyl‐1‐napthylamine and SYTOX green to investigate outer and inner membrane damage of P. gingivalis induced by AM404, respectively. Of all tested pathogens, AM404 only inhibited growth and biofilm formation of P. gingivalis. Moreover, it showed potent activity against P. gingivalis biofilms formed on titanium surfaces. A structure–activity analysis demonstrated that the unsaturated carbon chain is essential for its antibacterial activity. Importantly, AM404 was not toxic towards the tested mammalian cells up to concentrations approaching 4× the MIC. Membrane damage assays using fluorescent probes N‐phenyl‐1‐napthylamine and SYTOX green revealed that membrane permeabilization presumably is the primary antibacterial mode of action of AM404. Collectively, our results suggest that AM404 has the potential to be used for the development of new drugs specifically targeting P. gingivalis‐related infections.

Production of Ramoplanin and Ramoplanin Analogs by Actinomycetes

Ramoplanin is a glycolipodepsipeptide antibiotic obtained from fermentation of Actinoplanes sp. ATCC 33076 that exhibits activity against clinically important multi-drug-resistant, Gram-positive pathogens including vancomycin-resistant Enterococcus (VRE), methicillin-resistant Staphylococcus aureus (MRSA), and vancomycin-intermediate resistant Clostridium difficile. It disrupts bacterial cell wall through a unique mechanism of action by sequestering the peptidoglycan intermediate Lipid II and therefore does not show cross-resistance with other antibiotics. However, while demonstrating excellent antimicrobial activity in systemic use in animal models of infection, ramoplanin presents low local tolerability when injected intravenously. As a consequence of this limitation, new derivatives are desirable to overcome this issue. During a natural product screening program developed to discover compounds that disrupt bacterial cell wall synthesis by inhibiting peptidoglycan transglycosylation through binding to the intermediate Lipid II, 49 actinomycete strains were identified by HR-LCMS as producers of ramoplanin-related compounds. The producing strains were isolated from environmental samples collected worldwide comprising both tropical and temperate areas. To assess the diversity of this microbial population, the 49 isolates were initially identified to the genus level on the basis of their micromorphology, and 16S sequencing confirmed the initial identification of the strains. These analyses resulted in the identification of members of genus Streptomyces, as well as representatives of the families Micromonosporaceae, Nocardiaceae, Thermomonosporaceae, and Pseudonocardiaceae, suggesting that the production of ramoplanins is relatively widespread among Actinomycetes. In addition, all of these isolates were tested against a panel of Gram-positive and Gram-negative bacteria, filamentous fungi, and yeast in order to further characterize their antimicrobial properties. This work describes the diversity of actinomycete strains that produced ramoplanin-related compounds, and the analysis of the antimicrobial activity exhibited by these isolates. Our results strongly suggest the presence of new ramoplanin-analogs among these actinomycete producers.

Repurposing Toremifene for Treatment of Oral Bacterial Infections

The spread of antibiotic resistance and the challenges associated with antiseptics such as chlorhexidine have necessitated a search for new antibacterial agents against oral bacterial pathogens. As a result of failing traditional approaches, drug repurposing has emerged as a novel paradigm to find new antibacterial agents. In this study, we examined the effects of the FDA-approved anticancer agent toremifene against the oral bacteria Porphyromonas gingivalis and Streptococcus mutans We found that the drug was able to inhibit the growth of both pathogens, as well as prevent biofilm formation, at concentrations ranging from 12.5 to 25 μM. Moreover, toremifene was shown to eradicate preformed biofilms at concentrations ranging from 25 to 50 μM. In addition, we found that toremifene prevents P. gingivalis and S. mutans biofilm formation on titanium surfaces. A time-kill study indicated that toremifene is bactericidal against S. mutans Macromolecular synthesis assays revealed that treatment with toremifene does not cause preferential inhibition of DNA, RNA, or protein synthesis pathways, indicating membrane-damaging activity. Biophysical studies using fluorescent probes and fluorescence microscopy further confirmed the membrane-damaging mode of action. Taken together, our results suggest that the anticancer agent toremifene is a suitable candidate for further investigation for the development of new treatment strategies for oral bacterial infections.

Application of Omics Technologies for Evaluation of Antibacterial Mechanisms of Action of Plant-Derived Products

In the face of increasing bacterial resistance to antibiotics currently in use, the search for new antimicrobial agents has received a boost in recent years, with natural products playing an important role in this field. In fact, several methods have been proposed to investigate the antibacterial activities of natural products. However, given that the ultimate aim is future therapeutic use as novel drugs, it is extremely necessary to elucidate their modes of action, stating the molecular effects in detail, and identifying their targets in the bacterial cell. This review analyzes the application of “omics technologies” to understand the antibacterial mechanisms of bioactive natural products, to stimulate research interest in this area and promote scientific collaborations. Some studies have been specifically highlighted herein by examining their procedures and results (targeted proteins and metabolic pathways). These approaches have the potential to provide new insights into our comprehension of antimicrobial resistance/susceptibility, creating new perspectives for the struggle against bacteria, and leading to the development of novel products in the future.