Evaluation of antibacterial and antibiofilm mechanisms by usnic acid against methicillin-resistant Staphylococcus aureus

Antibacterial mechanism of the methanol extract of Thamnolia subuliformis (Ehrh.) W. Culb against Staphylococcus aureus

Thamnolia subuliformis (Ehrh.) W. Culb is a species of lichen with edible and medicinal applications in China. Our previous studies demonstrated that the methanol extract of Thamnolia subuliformis (METS) exhibits broad antibacterial activity and stability against foodborne pathogens. This study aimed to investigate the antibacterial mechanism of METS against Staphylococcus aureus using nontargeted metabolomics, focusing on cell wall and membrane damage. The results revealed that the minimum inhibitory concentration (MIC) was 0.625 mg ml-1 and that METS had good biosafety at this concentration. METS caused significant damage to the cell wall and membrane integrity, based on both morphological observation by electron microscopy and the leakage of alkaline phosphatase, protein, and nucleic acid in the cell cultures. Treatment with METS at the MIC disrupted the lipid metabolism of S. aureus, causing a decrease in the metabolism of various phospholipids and sphingolipids in the cell membrane and an increase in the ratio of saturated fatty acids to unsaturated fatty acids. Moreover, it influenced intracellular amino acid and energy metabolism. These results shed light on the antibacterial mechanism of METS against S. aureus while also serving as a reference for the further development of natural antibacterial compounds derived from Thamnolia subuliformis.

Restoring Colistin Sensitivity and Combating Biofilm Formation: Synergistic Effects of Colistin and Usnic Acid against Colistin-Resistant Enterobacteriaceae

Colistin (COL), the last line of defense in clinical medicine, is an important therapeutic option against multidrug-resistant Gram-negative bacteria. In this context, the emergence of colistin-resistant (COL-R) bacteria mediated by broad-spectrum efflux pumps, mobile genetic elements, and biofilm formation poses a significant public health concern. In response to this challenge, a novel approach of combining COL with usnic acid (UA) has been proposed in this study. UA is a secondary metabolite derived from lichens and is well-known for its anti-inflammatory properties. This study aimed to investigate the synergistic effects of UA and COL against COL-R Enterobacteriaceae both in vitro and in vivo. The exceptional synergistic antibacterial activity exhibited by the combination of COL and UA was demonstrated by performing a comprehensive set of assays, including the checkerboard assay, time-dependent killing assay, and Live/Dead bacterial cell viability assay. Furthermore, crystal violet staining and scanning electron microscopy assays revealed the inhibitory effect of this combination on the biofilm formation. Mechanistically, the combination of UA and COL exacerbated cell membrane rupture, induced DNA damage, and generated a significant amount of reactive oxygen species, which ultimately resulted in bacterial cell death. In addition, erythrocyte hemolysis and cell viability tests confirmed the biocompatibility of the combination. The evaluation of the COL/UA combination in vivo using Galleria mellonella larvae and a mouse infection model showed a significant improvement in the survival rate of the infected larvae as well as a reduction in the bacterial load in the mouse thigh muscle. These findings, for the first time, provide strong evidence for the potential application of COL/UA as an effective alternative therapeutic option to combat infections caused by COL-R Enterobacteriaceae strains.

Effectiveness of a Self-Decontaminating Coating Containing Usnic Acid in Reducing Environmental Microbial Load in Tertiary-Care Hospitals

Surfaces have been implicated in the transmission of pathogens in hospitals. This study aimed to assess the effectiveness of an usnic-acid-containing self-decontaminating coating in reducing microbial surface contamination in tertiary-care hospitals. Samples were collected from surfaces 9 days before coating application, and 3, 10, and 21 days after its application (phases 1, 2, 3, and 4, respectively). Samples were tested for bacteria, fungi, and SARS-CoV2. In phase 1, 53/69 (76.8%) samples tested positive for bacteria, 9/69 (13.0%) for fungi, and 10/139 (7.2%) for SARS-CoV-2. In phase 2, 4/69 (5.8%) samples tested positive for bacteria, while 69 and 139 samples were negative for fungi and SARS-CoV-2, respectively. In phase 3, 3/69 (4.3%) samples were positive for bacteria, 1/139 (0.7%) samples tested positive for SARS-CoV-2, while 69 samples were negative for fungi. In phase 4, 1/69 (1.4%) tested positive for bacteria, while no fungus or SARS-CoV-2 were detected. After the coating was applied, the bacterial load was reduced by 87% in phase 2 (RR = 0.132; 95% CI: 0.108-0.162); 99% in phase 3 (RR = 0.006; 95% CI: 0.003-0.015); and 100% in phase 4 (RR = 0.001; 95% CI: 0.000-0.009). These data indicate that the usnic-acid-containing coating was effective in eliminating bacterial, fungal, and SARS-CoV-2 contamination on surfaces in hospitals.Our findings support the benefit ofan usnic-acid-containing coating in reducing the microbial load on healthcare surfaces.

Safety in Rats of a Novel Nasal Spray Formulation for the Prevention of Airborne Viral Infections

Hexedra+^® is a nasal spray containing hydroxypropyl methylcellulose, beta-cyclodextrin, and usnic acid. It has been developed with the aim of reducing the risk of transmission of airborne viral infections, with particular reference to influenza and COVID-19. As part of the preclinical development of the product, we carried out a study on thirty male Wistar rats divided into three study groups and treated with Hexedra+, an alternative formulation containing a double concentration of usnic acid (0.015% instead of 0.0075%) or saline solution. Products were administered at the dose of 30 μL into each nostril, three times a day for seven consecutive days by means of a micropipette. By the end of the treatment period, no significant changes were observed in body weight. Histological examination of nasal mucosa and soft organs did not show any significant difference in the three study groups. Serum transaminase level remained in the normal limit in all the animals treated. The serum level of usnic acid was measured in order to assess the absorption of the molecule through the nasal mucosa. By the end of the study period, the usnic acid serum level was negligible in all the animals treated. In conclusion, the safety profile of Hexedra+ appears favorable in the animal model studied.

Exploring Possible Ways to Enhance the Potential and Use of Natural Products through Nanotechnology in the Battle against Biofilms of Foodborne Bacterial Pathogens

Biofilms enable pathogenic bacteria to survive in unfavorable environments. As biofilm-forming pathogens can cause rapid food spoilage and recurrent infections in humans, especially their presence in the food industry is problematic. Using chemical disinfectants in the food industry to prevent biofilm formation raises serious health concerns. Further, the ability of biofilm-forming bacterial pathogens to tolerate disinfection procedures questions the traditional treatment methods. Thus, there is a dire need for alternative treatment options targeting bacterial pathogens, especially biofilms. As clean-label products without carcinogenic and hazardous potential, natural compounds with growth and biofilm-inhibiting and biofilm-eradicating potentials have gained popularity as natural preservatives in the food industry. However, the use of these natural preservatives in the food industry is restricted by their poor availability, stability during food processing and storage. Also there is a lack of standardization, and unattractive organoleptic qualities. Nanotechnology is one way to get around these limitations and as well as the use of underutilized bioactives. The use of nanotechnology has several advantages including traversing the biofilm matrix, targeted drug delivery, controlled release, and enhanced bioavailability, bioactivity, and stability. The nanoparticles used in fabricating or encapsulating natural products are considered as an appealing antibiofilm strategy since the nanoparticles enhance the activity of the natural products against biofilms of foodborne bacterial pathogens. Hence, this literature review is intended to provide a comprehensive analysis of the current methods in nanotechnology used for natural products delivery (biofabrication, encapsulation, and nanoemulsion) and also discuss the different promising strategies employed in the recent and past to enhance the inhibition and eradication of foodborne bacterial biofilms.

Bioactive compounds: a goldmine for defining new strategies against pathogenic bacterial biofilms?

Most human infectious diseases are caused by microorganisms growing as biofilms. These three-dimensional self-organized communities are embedded in a dense matrix allowing microorganisms to persistently inhabit abiotic and biotic surfaces due to increased resistance to both antibiotics and effectors of the immune system. Consequently, there is an urgent need for novel strategies to control biofilm-associated infections. Natural products offer a vast array of chemical structures and possess a wide variety of biological properties; therefore, they have been and continue to be exploited in the search for potential biofilm inhibitors with a specific or multi-locus mechanism of action. This review provides an updated discussion of the major bioactive compounds isolated from several natural sources - such as plants, lichens, algae, microorganisms, animals, and humans - with the potential to inhibit biofilm formation and/or to disperse established biofilms by bacterial pathogens. Despite the very large number of bioactive products, their exact mechanism of action often remains to be clarified and, in some cases, the identity of the active molecule is still unknown. This knowledge gap should be filled thus allowing development of these products not only as novel drugs to combat bacterial biofilms, but also as antibiotic adjuvants to restore the therapeutic efficacy of current antibiotics.

Biofilm Formation by Staphylococcus aureus in the Specific Context of Cystic Fibrosis

Staphylococcus aureus is a major human pathogen whose characteristics support its success in various clinical settings including Cystic Fibrosis (CF). In CF, S. aureus is indeed the most commonly identified opportunistic pathogen in children and the overall population. S. aureus colonization/infection, either by methicillin-susceptible or methicillin-resistant strains, will become chronic in about one third of CF patients. The persistence of S. aureus in CF patients' lungs, despite various eradication strategies, is favored by several traits in both host and pathogen. Among the latter, living in biofilm is a highly protective way to survive despite deleterious environmental conditions, and is a common characteristic shared by the main pathogens identified in CF. This is why CF has earned the status of a biofilm-associated disease for several years now. Biofilm formation by S. aureus, and the molecular mechanisms governing and regulating it, have been extensively studied but have received less attention in the specific context of CF lungs. Here, we review the current knowledge on S. aureus biofilm in this very context, i.e., the importance, study methods, molecular data published on mono- and multi-species biofilm and anti-biofilm strategies. This focus on studies including clinical isolates from CF patients shows that they are still under-represented in the literature compared with studies based on reference strains, and underlines the need for such studies. Indeed, CF clinical strains display specific characteristics that may not be extrapolated from results obtained on laboratory strains.

Phytochemical Analysis, Antioxidant, Antimicrobial, and Cytotoxic Activity of Different Extracts of Xanthoparmelia stenophylla Lichen from Stara Planina, Serbia

The aim of this study was to identify some of the secondary metabolites present in acetonic, methanolic, and hexanic extracts of lichen Xanthoparmelia stenophylla and to examine their antioxidant, antimicrobial, and cytotoxic activity. Compounds of the depsid structure of lecanoric acid, obtusic acid, and atranorin as well as usnic acid with a dibenzofuran structure were identified in the extracts by HPLC. The acetone extract was shown to have the highest total phenolic (167.03 ± 1.12 mg GAE/g) and total flavonoid content (178.84 ± 0.93 mg QE/g) as well as the best antioxidant activity (DPPH IC50 = 81.22 ± 0.54). However, the antimicrobial and antibiofilm tests showed the best activity of hexanic extract, especially against strains of B. cereus, B. subtilis, and S. aureus (MIC < 0.08, and 0.3125 mg/mL, respectively). Additionally, by using the MTT method, the acetonic extract was reported to exhibit a strong cytotoxic effect on the HeLa and HCT-116 cell lines, especially after 72 h (IC50 = 21.17 ± 1.85 and IC50 = 21.48 ± 3.55, respectively). The promising antioxidant, antimicrobial, and cytotoxic effects of Xanthoparmelia stenophylla extracts shown in the current study should be further investigated in vivo and under clinical conditions.

Effective Strategies to Deal With Infection in Burn Patient

Treatment of bacterial infection is difficult. Treatment protocol of burned patient is hard. Furthermore, treatment in burned patients is accompanied with problems such as complexity in diagnosis of infection's agent, multiple infections, being painful, and involving with different organelles. There are different infections of Gram-positive and Gram-negative bacteria in burned patients. From important bacteria can be noted to Pseudomonas aeruginosa, Acinetobacter baumannii, and Staphylococcus aureus that have high range of morbidity and mortality. Treatment of those bacterial infections is extremely important. Hence, many studies about methods of treatment of bacterial infections have published. Herein, we have suggested practical methods for example ant virulence therapies, nanotechnology, vaccine, and photodynamic therapy in treatment of bacterial infections. Those methods have been done in many researches and had good effect.

Antimicrobial and antioxidant activity of Evernia prunastri extracts and their isolates

Lichens are symbiotic organisms formed by a fungus and one or more photosynthetic partners which are usually alga or cyanobacterium. Their diverse and scarcely studied metabolites facilitate adaptability to extreme living conditions. We investigated Evernia prunastri (L.) Ach., a widely distributed lichen, for its antimicrobial and antioxidant potential. E. prunastri was sequentially extracted by hexane (Hex), dichloromethane (DCM) and acetonitrile (ACN) that were screened for their antioxidant and antimicrobial (against Staphylococcus aureus, Pseudomonas aeruginosa, Escherichia coli and Candida albicans) activities. The Hex extract possessed the highest antioxidant capacity (87 mg ascorbic acid/g extract) corresponding to the highest content of phenols (73 mg gallic acid/g extract). The DCM and Hex extracts were both active against S. aureus (MICs of 4 and 21 µg/ml, respectively) but were less active against Gram-negative bacteria and yeast. The ACN extract exhibited activity on both S. aureus (MIC 14 µg/ml) and C. albicans (MIC 38 µg/ml) and was therefore further fractionated by silica gel column chromatography. The active compound of the most potent fraction was subsequently characterized by 1H and 13C-NMR spectroscopy and identified as evernic acid. Structural similarity analyses were performed between compounds from E. prunastri and known antibiotics from different classes. The structural similarity was not present. Antioxidant and antimicrobial activities of E. prunastri extracts originate from multiple chemical compounds; besides usnic acid, most notably evernic acid and derivatives thereof. Evernic acid and its derivatives represent possible candidates for a new class of antibiotics.

Synergistic Effect between Usnic Acid and Polymyxin B against Resistant Clinical Isolates of Pseudomonas aeruginosa

The present study aimed to characterize the susceptibility profile of Pseudomonas aeruginosa and Acinetobacter spp. clinical isolates to polymyxin B in a public hospital in Recife-PE, Brazil, between the years of 2018 and 2019, as well as to search for the presence of the mcr-1 gene and evaluate the interaction between polymyxin B and usnic acid against these isolates. The strains were identified using the BD Phoenix™ automated system and the agar-spot test was used to determine the susceptibility profile to polymyxin B. The minimum inhibitory concentrations (MICs) of usnic acid and polymyxin B were determined through the broth microdilution method according to the Clinical and Laboratory Standards Institute (CLSI). Subsequently, Polymerase Chain Reaction (PCR) was performed to detect the mcr-1 gene in the isolates. The interaction between usnic acid and polymyxin B was evaluated by the Checkerboard assay. Among 34 isolates of P. aeruginosa, 26.5% (9/34) were positive for the polymyxin B agar-spot test, and 11.8% (4/34) presented an intermediate susceptibility (MIC = 4 μg/mL), while 14.7% (5/34) presented antimicrobial resistance with MIC values ranging from 8 to 32 μg/mL. Among 38 isolates of Acinetobacter spp., 13.2% (5/38) were positive for the polymyxin B agar-spot test and all of them were resistant to polymyxin B with a MIC value > 32 μg/mL. The mcr-1 gene was not detected in the clinical isolates. Regarding usnic acid, it presented a moderate antibacterial activity against two P. aeruginosa isolates (MIC = 250 μg/mL) and no activity was detected against the others. A synergistic effect between usnic acid and polymyxin B was observed against three clinical isolates of P. aeruginosa which were resistant to polymyxin B (FICI ≤ 0.5). Therefore, it was possible to observe that usnic acid is a promising candidate to be used in combination with polymyxin B against infections caused by resistant P. aeruginosa.

Bactericidal Activity of Usnic Acid-Chitosan Nanoparticles against Persister Cells of Biofilm-Forming Pathogenic Bacteria

The present study aimed to prepare usnic acid (UA)-loaded chitosan (CS) nanoparticles (UA-CS NPs) and evaluate its antibacterial activity against biofilm-forming pathogenic bacteria. UA-CS NPs were prepared through simple ionic gelification of UA with CS, and further characterized using Fourier transform infrared spectroscopy, X-ray diffraction, and field-emission transmission electron microscopy. The UA-CS NPs presented a loading capacity (LC) of 5.2%, encapsulation efficiency (EE) of 24%, and a spherical shape and rough surface. The maximum release of UA was higher in pH 1.2 buffer solution as compared to that in pH 6.8 and 7.4 buffer solution. The average size and zeta potential of the UA-CS NPs was 311.5 ± 49.9 nm in diameter and +27.3 ± 0.8 mV, respectively. The newly prepared UA-CS NPs exhibited antibacterial activity against persister cells obtained from the stationary phase in batch culture, mature biofilms, and antibiotic-induced gram-positive and gram-negative pathogenic bacteria. Exposure of sub-inhibitory concentrations of UA-CS NPs to the bacterial cells resulted in a change in morphology. The present study suggests an alternative method for the application of UA into nanoparticles. Furthermore, the anti-persister activity of UA-CS NPs may be another possible strategy for the treatment of infections caused by biofilm-forming pathogenic bacteria.

A facile in vivo procedure to analyze metabolic pathways in intact lichens

Lichen secondary metabolites show important biological activities as well as pharmaceutical and chemotaxonomic potential. In order to utilize such substances of interest, detailed knowledge of their biosynthetic pathways is essential. ¹³ CO₂ -pulse/chase experiments using intact thalli of the lichen Usnea dasopoga resulted in multiple ¹³ C-labeled isotopologs in amino acids, but not in the dibenzofuran derivative usnic acid - one of the best-studied lichen metabolites, with considerable and renewed interest for pharmaceutical and lifestyle applications. Spraying an aqueous solution of [U-¹³ C₆ ]glucose onto the thalli of U. dasopoga afforded a specific mixture of multiple ¹³ C-labeled isotopologs in usnic acid. One- and two-dimensional NMR analysis of the crude lichen extract corroborated the polyketide biosynthetic pathway via methylphloroacetophenone but not via phloroacetophenone. With usnic acid as an exemplar, we provide proof-of-principle experiments that can be used in general to study metabolic pathways and fluxes in intact lichens.

Usnic acid modifies MRSA drug resistance through down-regulation of proteins involved in peptidoglycan and fatty acid biosynthesis

Multidrug‐resistant Staphylococcus aureus infections place a huge burden on the healthcare sector and the wider community. An increasing rate of infections caused by methicillin‐resistant Staphylococcus aureus (MRSA) has necessitated the development of alternative agents. We previously reported that usnic acid (UA) has activity against MRSA; here, we report the effect of UA in combination with norfloxacin on the drug resistance of MRSA clinical isolates. We observed that the combination of UA–norfloxacin significantly reduces the bacterial burden in mouse models infected with S. aureus, without causing any detectable associated toxicity. Proteomic analysis indicated that UA–norfloxacin induces oxidative stress within cells, which leads to membrane damage and inhibits metabolic activity and biosynthesis of peptidoglycan and fatty acids. Collectively, this study provides evidence that UA in combination with norfloxacin may be a potential candidate for development into a resistance‐modifying agent for the treatment of invasive MRSA infections.

This is the first report on the drug resistance‐modifying potential of usnic acid (UA) through inhibition of the multidrug resistance (MDR) efflux pump and down‐regulation of proteins involved in peptidoglycan and fatty acid biosynthesis. This compound may be helpful in the management of infection caused by MRSA through (a) lowering the prescribed amount of antibiotics, (b) decreasing MDR generation, and (c) intensifying the efficacy of antibiotics against MRSA/VRSA under both in vitro and in vivo conditions. These results may be helpful in the development of anti‐MRSA drug combinations from economical and non‐toxic natural products.

Biosynthesis of Ag and Cu NPs by secondary metabolites of usnic acid and thymol with biological macromolecules aggregation and antibacterial activities against multi drug resistant (MDR) bacteria

Thymol and usnic acid as the important secondary metabolites of respectively Artemisia haussknechtii and Protoparmeliopsis muralis were used for reduction and stabilizing of AgNO₃ and CuSO₄ in metal nanoparticles (MNPs) biosynthesis process. Antibacterial effects of prepared Ag-thymol (ATNPs), Ag-usnic acid (AUNPs), Cu-thymol (CTNPs), and Cu-usnic acid (CUNPs) on multi drug resistant (MDR) bacteria including methicillin-resistant Staphylococcus aureus (MRSA) (gram positive), Acinetobacter baumannii (A52), and Klebsiella pneumonia (K38) (gram negative) were compared with thymol, usnic acid, AgNO₃, CuSO₄, and tetracycline. Results of this study showed higher antibacterial activities of usnic acid, CUNPs, and CTNPs with MIC/MBC values (20, 40, and 40 μg/mL, respectively) than ATNPs and AUNPs against MRSA bacteria. Leakage of macromolecules involving nucleic acids and proteins from bacteria under stress of MNPs, thymol, and usnic acid proved significant antibacterial activities of usnic acid, and Cu NPs. In addition, SEM images illustrated different patterns of aggregation in biofilms resulted from interactions of these antibacterial agents with bacterial macromolecules. Totally, this investigation illustrated new green method of Ag and Cu NPs biosynthesis with suitable antibacterial properties.

Antifungal and Anti-Biofilm Activities of Acetone Lichen Extracts against Candida albicans

Candida albicans is a commensal coloniser of the human gastrointestinal tract and an opportunistic pathogen, especially thanks to its capacity to form biofilms. This lifestyle is frequently involved in infections and increases the yeast resistance to antimicrobials and immune defenses. In this context, 38 lichen acetone extracts have been prepared and evaluated for their activity against C. albicans planktonic and sessile cells. Minimum inhibitory concentrations of extracts (MICs) were determined using the broth microdilution method. Anti-biofilm activity was evaluated using tetrazolium salt (XTT) assay as the ability to inhibit the maturation phase (anti-maturation) or to eradicate a preformed 24 h old biofilm (anti-biofilm). While none of the extracts were active against planktonic cells, biofilm maturation was limited by 11 of the tested extracts. Seven extracts displayed both anti-maturation and anti-biofilm activities (half maximal inhibitory concentrations IC_{50_mat} and IC_{50_biof} ≤ 100 µg/mL); Evernia prunastri and Ramalina fastigiata were the most promising lichens (IC_{50_mat} < 4 µg/mL and IC_{50_biof} < 10 µg/mL). Chemical profiles of the active extracts performed by thin layer chromatography (TLC) and high performance liquid chromatography (HPLC) have been analyzed. Depsides, which were present in large amounts in the most active extracts, could be involved in anti-biofilm activities. This work confirmed that lichens represent a reservoir of compounds with anti-biofilm potential.