Validation of the AlamarBlue® Assay as a Fast Screening Method to Determine the Antimicrobial Activity of Botanical Extracts

Screening the NCI diversity set V for anti-MRSA activity: cefoxitin synergy and LC-MS/MS confirmation of folate/thymidine biosynthesis inhibition

IMPORTANCE

New antibacterial agents are urgently needed to counter increasingly resistant bacteria. One approach to this problem is library screening for new antibacterial agents. Library screening efforts can be improved by increasing the information content of the screening effort. In this study, we screened the National Cancer Institute diversity set V against methicillin-resistant Staphylococcus aureus (MRSA) with several enhancements. One of these is to screen the library before and after microsomal metabolism as means to identify potential active metabolites. A second enhancement is to screen the library in the absence and presence of sub-minimum inhibitory concentration levels of another antibiotic, such as cefoxitin in this study. This identified four agents with synergistic activity with cefoxitin out of 16 agents with good MRSA activity alone. Finally, active agents from this effort were counter-screened in the presence of thymidine, which quickly identified three folate/thymidine biosynthesis inhibitors, and also screened for bactericidal vs bacteriostatic activity.

Synergistic Combinations of FDA-Approved Drugs with Ceftobiprole against Methicillin-Resistant Staphylococcus aureus

New strategies are urgently needed to address the public health threat of antimicrobial resistance. Synergistic agent combinations provide one possible pathway toward addressing this need and are also of fundamental mechanistic interest. Effective methods for comprehensively identifying synergistic agent combinations are required for such efforts. In this study, an FDA-approved drug library was screened against methicillin-resistant Staphylococcus aureus (MRSA) (ATCC 43300) in the absence and presence of sub-MIC levels of ceftobiprole, a PBP2a-targeted anti-MRSA β-lactam. This screening identified numerous potential synergistic agent combinations, which were then confirmed and characterized for synergy using checkerboard analyses. The initial group of synergistic agents (sum of the minimum fractional inhibitory concentration ∑FIC_min ≤0.5) were all β-lactamase-resistant β-lactams (cloxacillin, dicloxacillin, flucloxacillin, oxacillin, nafcillin, and cefotaxime). Cloxacillin-the agent with the greatest synergy with ceftobiprole-is also highly synergistic with ceftaroline, another PBP2a-targeted β-lactam. Further follow-up studies revealed a range of ceftobiprole synergies with other β-lactams, including with imipenem, meropenem, piperacillin, tazobactam, and cefoxitin. Interestingly, given that essentially all other ceftobiprole-β-lactam combinations showed synergy, ceftaroline and ceftobiprole showed no synergy. Modest to no synergy (0.5 < ∑FIC_min ≤ 1.0) was observed for several non-β-lactam agents, including vancomycin, daptomycin, balofloxacin, and floxuridine. Mupirocin had antagonistic activity with ceftobiprole. Flucloxacillin appeared particularly promising, with both a low intrinsic MIC and good synergy with ceftobiprole. That so many β-lactam combinations with ceftobiprole show synergy suggests that β-lactam combinations can generally increase β-lactam effectiveness and may also be useful in reducing resistance emergence and spread in MRSA. IMPORTANCE Antimicrobial resistance represents a serious threat to public health. Antibacterial agent combinations provide a potential approach to combating this problem, and synergistic agent combinations-in which each agent enhances the antimicrobial activity of the other-are particularly valuable in this regard. Ceftobiprole is a late-generation β-lactam antibiotic developed for MRSA infections. Resistance has emerged to ceftobiprole, jeopardizing this agent's effectiveness. To identify synergistic agent combinations with ceftobiprole, an FDA-approved drug library was screened for potential synergistic combinations with ceftobiprole. This screening and follow-up studies identified numerous β-lactams with ceftobiprole synergy.

Assessment of innate immune response modulating impurities in glucagon for injection

Glucagon for Injection is a polypeptide hormone medication used to treat patients with severe hypoglycemia or low blood sugar. Only recently, was a generic version of glucagon for injection approved by the FDA. While the generic version was deemed equivalent to its brand-name counterpart, the two glucagon products were produced using different manufacturing processes. The brand-name glucagon is produced via recombinant DNA while the generic glucagon is produced by peptide synthesis. Different manufacturing processes can result in different levels of innate immune response modulating impurities (IIRMIs). This study utilized a cell-based assay method, which allows for detection of a broad spectrum of impurities, to investigate the IIRMI risks of the generic glucagon to make sure it has similar or less immunogenicity risks than the brand-name glucagon product. Three commercial cell lines (RAW-Blue™, HEK-Blue™-hNOD1 and HEK-Blue™-hNOD2) carrying a secreted embryonic alkaline phosphatase reporter construct were used to quantify the level of innate immune responses after being treated with the glucagon drugs. The study results showed that despite differences in manufacturing process, the innate immunogenicity risk in the synthetic (generic) glucagon was at negligible level and comparable to the recombinant (brand-name) glucagon product.

Library Screening for Synergistic Combinations of FDA-Approved Drugs and Metabolites with Vancomycin against VanA-Type Vancomycin-Resistant Enterococcus faecium

Antimicrobial resistance is a major public health threat, and there is an urgent need for new strategies to address this issue. In a recent study, a library screening strategy was developed in which an FDA-approved drug library was screened against methicillin-resistant Staphylococcus aureus (MRSA) in both its original (unmetabolized [UM]) and its human liver microsome metabolized (postmetabolized [PM]) forms and in the absence and presence of a resistant-to antibiotic. This allows the identification of agents with active metabolites and agents that can act synergistically with the resistant-to antibiotic. In this study, this strategy is applied to VanA-type vancomycin-resistant Enterococcus faecium (VREfm) in the absence and presence of vancomycin. Thirteen drugs with minimum MICs that were ≤12.5 μM under any tested condition (UM/PM vs. -/+vancomycin) were identified. Seven of these appeared to act synergistically with vancomycin, and follow-up checkerboard analyses confirmed synergy (∑FICmin ≤0.5) for six of these. Ultimately four rifamycins, two pleuromutilins, mupirocin, and linezolid were confirmed as synergistic. The most synergistic agent was rifabutin (∑FICmin = 0.19). Linezolid, a protein biosynthesis inhibitor, demonstrated relatively weak synergy (∑FICmin = 0.5). Only mupirocin showed significantly improved activity after microsomal metabolism, indicative of a more active metabolite, but efforts to identify an active metabolite were unsuccessful. Spectra of activity of several hits and related agents were also determined. Gemcitabine showed activity against a number vancomycin-resistant E. faecium and E. faecalis strains, but this activity was substantially weaker than previously observed in MRSA. IMPORTANCE Resistance to currently used antibiotics poses a serious threat to public health. This study reports a complete screen of 1,000 FDA-approved drugs and their metabolites against vancomycin-resistant Enterococcus faecium (VREfm) in both the absence and presence of vancomycin. This identified potentially synergistic combinations of FDA-approved drugs with vancomycin, and a number of these were confirmed in follow-up checkerboard assays. Among intrinsically active FDA-approved drugs, gemcitabine was identified as having activity against a panel of VRE strains.

Identification of quinoline derivatives as growth inhibitors of MDR pathogen Klebsiella pneumoniae

Aims: This study was aimed to identify compounds with significant inhibitory potential against multidrug-resistant (MDR), multidrug-sensitive, and clinical isolates of Klebsiella pneumoniae. Materials & methods: Antibacterial activity of the nitroquinoline derivatives was assessed by micro-plate Alamar Blue assay. Results: Nitroquinoline derivatives 9, 11 and 14 showed inhibitory activity against MDR K. pneumoniae. Docking studies of these compounds with topoisomerase IV of K. pneumonia indicated the interactions of these compounds at the active site residues of enzyme near to cofactor (Mg⁺²). Furthermore, compound 11 was identified as a reactive oxygen species (ROS) inducer. None of the compounds showed hemolytic effect. Conclusion: This study was designed to identify compounds active against MDR K. pneumoniae which causes infections, such as pneumonia and urinary tract infections.

Resazurin-based high-throughput screening method for the discovery of dietary phytochemicals to target microbial transformation of L-carnitine into trimethylamine, a gut metabolite associated with cardiovascular disease

Nowadays, there is great interest in the discovery of food compounds that might inhibit gut microbial TMA production from its methylamine precursors. In this work, an innovative novel screening strategy capable of rapidly determining the differences in the metabolic response of Klebsiella pneumoniae, a bacteria producing TMA under aerobic conditions, to a library of extracts obtained from food and natural sources was developed. The proposed high-throughput screening (HTS) method combines resazurin reduction assay in 384-well plates and Gaussian Processes as a machine learning tool for data processing, allowing for a fast, cheap and highly standardized evaluation of any interfering effect of a given compound or extract on the microbial metabolism sustained by L-carnitine utilization. As a proof-of-concept of this strategy, a pilot screening of 39 extracts and 6 pure compounds was performed to search for potential candidates that could inhibit in vitro TMA formation from L-carnitine. Among all the extracts tested, three of them were selected as candidates to interfere with TMA formation. Subsequent in vitro assays confirmed the potential of oregano and red thyme hexane extracts (at 1 mg mL^-1) to inhibit TMA formation in bacterial lysates. In such in vitro assay, the red thyme extract exerted comparable effects on TMA reduction (∼40%) as 7.5 mM meldonium (∼50% TMA decrease), a reported L-carnitine analogue. Our results show that metabolic activity could be used as a proxy of the capacity to produce TMA under controlled culture conditions using L-carnitine to sustain metabolism.

Synthesis of pH responsive, photocrosslinked gelatin-based hydrogel system for control release of ceftriaxone

Stimuli-responsive or smart nanocarriers are emerging class of materials used for drug delivery and tissue engineering applications. The objective of this study was to develop environment friendly, economic and smart gelatin based hydrogel for controlled delivery of ceftriaxone. Gelatin was modified with furfuryl amine and converted to hydrogel via photo oxidation reaction, initiated with visible light in the presence of crosslinking agent (eosin Y and riboflavin) to enhance the mechanical properties and increase structural integrity of the hydrogel. The conjugated gelatin-furfuryl amine (GFA) was characterized by ¹H NMR and FTIR techniques. An antimicrobial drug (ceftriaxone) was encapsulated in hydrogels systems (HG-E and HG-R) and its encapsulation efficiency was found as 88 % and 74 %, respectively. The in-vitro swelling property and release profile of the drug reveals that the behavior of hydrogel was pH dependent for both the hydrogels systems. Antibacterial study performed on HG-R hydrogel system via disc diffusion method revealed a greater zone of inhibition for Staphylococcus aureus. Findings of the presented work revealed the formation of an excellent, smart hydrogel system using Eosin-Y as cross-linker having greater encapsulation efficiency that can potentially be used for drug delivery application of ceftriaxone to enhance its therapeutic effects.

The antimicrobial capacity of Cistus salviifolius and Punica granatum plant extracts against clinical pathogens is related to their polyphenolic composition

Antimicrobial resistance poses a serious threat to human health worldwide. Plant compounds may help to overcome antibiotic resistance due to their potential resistance modifying capacity. Several botanical extracts and pure polyphenolic compounds were screened against a panel of eleven bacterial isolates with clinical relevance. The two best performing agents, Cistus salviifolius (CS) and Punica granatum (GP) extracts, were tested against 100 Staphylococcus aureus clinical isolates, which resulted in average MIC50 values ranging between 50-80 µg/mL. CS extract, containing hydrolyzable tannins and flavonoids such as myricetin and quercetin derivatives, demonstrated higher activity against methicillin-resistant S. aureus isolates. GP extract, which contained mostly hydrolyzable tannins, such as punicalin and punicalagin, was more effective against methicillin-sensitive S. aureus isolates. Generalized linear model regression and multiple correspondence statistical analysis revealed a correlation between a higher susceptibility to CS extract with bacterial resistance to beta-lactam antibiotics and quinolones. On the contrary, susceptibility to GP extract was related with bacteria sensitive to quinolones and oxacillin. Bacterial susceptibility to GP and CS extracts was linked to a resistance profile based on cell wall disruption mechanism. In conclusion, a differential antibacterial activity against S. aureus isolates was observed depending on antibiotic resistance profile of isolates and extract polyphenolic composition, which may lead to development of combinatorial therapies including antibiotics and botanical extracts.

Antimicrobial Capacity of Plant Polyphenols against Gram-positive Bacteria: A Comprehensive Review

BACKGROUND

Multi-drug-resistant bacteria such as Methicillin-Resistant Staphylococcus aureus (MRSA) disseminate rapidly amongst patients in healthcare facilities and suppose an increasingly important cause of community-associated infections and associated mortality. The development of effective therapeutic options against resistant bacteria is a public health priority. Plant polyphenols are structurally diverse compounds that have been used for centuries for medicinal purposes, including infections treatment and possess, not only antimicrobial activity, but also antioxidant, anti-inflammatory and anticancer activities among others. Based on the existing evidence on the polyphenols' antibacterial capacity, polyphenols may be postulated as an alternative or complementary therapy for infectious diseases.

OBJECTIVE

To review the antimicrobial activity of plant polyphenols against Gram-positive bacteria, especially against S. aureus and its resistant strains. Determine the main bacterial molecular targets of polyphenols and their potential mechanism of action.

METHODOLOGY

The most relevant reports on plant polyphenols' antibacterial activity and their putative molecular targets were studied. We also performed virtual screening of thousand different polyphenols against proteins involved in the peptidoglycan biosynthesis to find potential valuable bioactive compounds. The bibliographic information used in this review was obtained from MEDLINE via PubMed.

RESULTS

Several polyphenols: phenolic acids, flavonoids (especially flavonols), tannins, lignans, stilbenes and combinations of these in botanical mixtures, have exhibited significant antibacterial activity against resistant and non-resistant Gram-positive bacteria at low μg/mL range MIC values. Their mechanism of action is quite diverse, targeting cell wall, lipid membrane, membrane receptors and ion channels, bacteria metabolites and biofilm formation. Synergic effects were also demonstrated for some combinations of polyphenols and antibiotics.

CONCLUSION

Plant polyphenols mean a promising source of antibacterial agents, either alone or in combination with existing antibiotics, for the development of new antibiotic therapies.

Perspective on Antibacterial Lead Identification Challenges and the Role of Hypothesis-Driven Strategies

For the past three decades, the pharmaceutical industry has undertaken many diverse approaches to discover novel antibiotics, with limited success. We have witnessed and personally experienced many mistakes, hurdles, and dead ends that have derailed projects and discouraged scientists and business leaders. Of the many factors that affect the outcomes of screening campaigns, a lack of understanding of the properties that drive efflux and permeability requirements across species has been a major barrier for advancing hits to leads. Hits that possess bacterial spectrum have seldom also possessed druglike properties required for developability and safety. Persistence in solving these two key barriers is necessary for the reinvestment into discovering antibacterial agents. This perspective narrates our experience in antibacterial discovery-our lessons learned about antibacterial challenges as well as best practices for screening strategies. One of the tenets that guides us is that drug discovery is a hypothesis-driven science. Application of this principle, at all steps in the antibacterial discovery process, should improve decision making and possibly the odds of what has become, in recent decades, an increasingly challenging endeavor with dwindling success rates.

Dimensionally Enhanced Antibacterial Library Screening

The emergence and spread of antimicrobial resistance is a major public health threat, and there is an urgent need to develop new strategies to address the issue. In this study, the possibility of enhancing a whole cell based antibacterial library screen by increasing the dimensionality of the screening effort is explored using methicillin-resistant Staphylococcus aureus (MRSA) as the target organism. One dimension involved generating and screening a human liver microsome metabolized FDA approved drug library. Comparative screening of the un-metabolized (UM) and pre-metabolized (PM) libraries allows identification of intrinsically active agents from the UM library screen and of agents with active metabolites from the PM library screen. To further enhance this screening effort, it was combined with a -/+ resistant to antibiotic screen (-/+ cefoxitin; Cef). This allows the identification of agents that can act synergistically with the resistant to antibiotic. This approach revealed five compounds with substantially improved activity after metabolism and four compounds with substantial synergistic activity with cefoxitin. Capecitabine in particular only had significant antibacterial activity after metabolism. Its metabolites were isolated, identified, and characterized for spectrum of activity along with several other anticancer drugs with anti-MRSA activity. Floxuridine, gemcitabine, novobiocin, and rifaximin were identified as having substantial synergy with cefoxitin from the -/+Cef screens. Checkerboard assays verified synergy for these agents. Floxuridine demonstrated a particularly high degree of synergy with cefoxitin (FIC = 0.14). This study demonstrates how a dimensionally enhanced comparative screening effort can identify new antibacterial agents and strategies for countering antibacterial agent resistance.

Preclinical Evaluation of the Antimicrobial-Immunomodulatory Dual Action of Xenohormetic Molecules against Haemophilus influenzae Respiratory Infection

Chronic obstructive pulmonary disease (COPD) is characterized by abnormal inflammation and impaired airway immunity, providing an opportunistic platform for nontypeable Haemophilus influenzae (NTHi) infection. In this context, therapies targeting not only overactive inflammation without significant adverse effects, but also infection are of interest. Increasing evidence suggests that polyphenols, plant secondary metabolites with anti-inflammatory and antimicrobial properties, may be protective. Here, a Cistus salviifolius plant extract containing quercetin, myricetin, and punicalagin was shown to reduce NTHi viability. Analysis of these polyphenols revealed that quercetin has a bactericidal effect on NTHi, does not display synergies, and that bacteria do not seem to develop resistance. Moreover, quercetin lowered NTHi airway epithelial invasion through a mechanism likely involving inhibition of Akt phosphorylation, and reduced the expression of bacterially-induced proinflammatory markers il-8, cxcl-1, il-6, pde4b, and tnfα. We further tested quercetin’s effect on NTHi murine pulmonary infection, showing a moderate reduction in bacterial counts and significantly reduced expression of proinflammatory genes, compared to untreated mice. Quercetin administration during NTHi infection on a zebrafish septicemia infection model system showed a bacterial clearing effect without signs of host toxicity. In conclusion, this study highlights the therapeutic potential of the xenohormetic molecule quercetin against NTHi infection.

Grape Pomace as a Promising Antimicrobial Alternative in Feed: A Critical Review

Antimicrobial resistance is among the most urgent global challenges facing sustainable animal production systems. The use of antibiotics as growth promoters and for infectious disease prevention in intensive animal-farming practices has translated into the selection and spread of antimicrobial resistance genes in an unprecedented fashion. Several multi-resistant bacterial strains have been isolated from food-producing animals, thus constituting an alarming food-safety issue. Many industrial byproducts with potential antimicrobial properties are currently being investigated to identify empirical and affordable solutions/alternatives that can potentially be used in feed for animals. Grape pomace is among such byproducts that gained the attention as a result of its low cost, abundance, and, most importantly, its bioactive and antibacterial properties. This review discusses the recently reported studies with regard to exploring the use of grape pomace (and its extracts) in animal production to control pathogens, along with the promotion of beneficial bacterial species in the gut to ultimately alleviate antibacterial resistance. The review further summarizes realistic expectations connected with grape pomace usage and lists the still-to-be-addressed concerns about its application in animal agriculture.

Modeling photocatalytic degradation of diazinon from aqueous solutions and effluent toxicity risk assessment using Escherichia coli LMG 15862

In this study, modeling and degradation of diazinon from contaminated water by advanced oxidation process together with a new test for effluent bioassay using E. coli were investigated. The experiments were designed based on response surface methodology. Nanoparticles (NPs) were synthesized using the sol–gel method. The shape characteristics and specifications of elements in the nanoparticles were characterized using scanning electron microscope and energy dispersive X-ray, respectively. Diazinon was measured using high performance liquid chromatography device and by-products due to its decomposition were identified by gas chromatography-mass (GC–MS). In the present study, effluent bioassay tests were conducted by defining the rate of dehydrogenase enzyme reducing alamar blue method. According to statistical analyses (R² = 0.986), the optimized values for pH, dose of NPs, and contact time were found to be 6.75, 775 mg/L, and 65 min, respectively. At these conditions, 96.06% of the diazinon was removed. Four main by-products, diazoxon, 7-methyl-3-octyne, 2-isopropyl-6-methyl-4pyrimidinol and diethyl phosphonate were detected. According to the alamar blue reducing (ABR) test, 50% effective concentration, no observed effect concentration, and 100% effective concentration (EC₁₀₀) for the mortality rate of E. coli were obtained as 2.275, 0.839, and 4.430 mg/L, respectively. Based on the results obtained, it was found that mentioned process was high efficiency in removing diazinon, and also a significant relationship between toxicity assessment tests were obtained (P < 0.05).

Clove Bud Oil Modulates Pathogenicity Phenotypes of the Opportunistic Human Pathogen Pseudomonas aeruginosa

Earlier studies from our laboratory have demonstrated that clove bud oil (CBO) attenuates expression of certain virulence factors of Pseudomonas aeruginosa PAO1. Here, we probe more deeply into the effect of CBO on four pseudomonal proteases - elastase A, elastase B, protease IV and alkaline protease - each known to play key roles in disease pathogenesis. CBO inhibited the activity of these proteases present in the bacterial culture supernatant. Zymography studies indicated that these proteases can activate host matrix metalloproteases (MMPs) to establish infection, through conversion of pro-MMP-2 to active MMP-2. PAO1 is a predominant pathogen in burn wound infections and we show the modulatory effect of CBO on MMPs in an in vitro model of burn injury. Furthermore, CBO induced dose-dependent neutrophil extracellular trap formation in human neutrophils. CBO also increased the survival of C. elegans infected with PAO1, establishing an anti-infective role in a whole animal model of pathogenesis. LC-MS/MS analysis indicated that CBO treatment elicited a significant reduction of signalling molecules (Acyl-Homoserine-Lactone) involved in quorum sensing regulation. Our observations demonstrate that CBO attenuates key virulence mechanisms of this important human pathogen, while concomitantly enhancing host innate immunomodulatory functions, with potential implications for topical therapy against antibiotic-resistant infections.