Synergistic fungicidal activity of the lipopeptide bacillomycin D with amphotericin B against pathogenic Candida species

A promising antifungal lipopeptide from Bacillus subtilis: its characterization and insight into the mode of action

Emerging resistance of fungal pathogens and challenges faced in drug development have prompted renewed investigations into novel antifungal lipopeptides. The antifungal lipopeptide AF3 reported here is a natural lipopeptide isolated and purified from Bacillus subtilis. The AF3 lipopeptide's secondary structure, functional groups, and the presence of amino acid residues typical of lipopeptides were determined by circular dichroism, Fourier transform infrared spectroscopy, and nuclear magnetic resonance spectroscopy. The lipopeptide's low minimum inhibitory concentrations (MICs) of 4-8 mg/L against several fungal strains demonstrate its strong antifungal activity. Biocompatibility assays showed that ~ 80% of mammalian cells remained viable at a 2 × MIC concentration of AF3. The treated Candida albicans cells examined by scanning electron microscopy, transmission electron microscopy, and atomic force microscopy clearly showed ultrastructural alterations such as the loss of the cell shape and cell membrane integrity. The antifungal effect of AF3 resulted in membrane permeabilization facilitating the uptake of the fluorescent dyes-acridine orange (AO)/propidium iodide (PI) and FUN-1. Using 1,6-diphenyl-1,3,5-hexatriene (DPH) and 4-(2-[6-(dioctylamino)-2-naphthalenyl] ethenyl)-1-(3-sulfopropyl) pyridinium inner salt (di-8-ANEPPS), we observed that the binding of AF3 to the membrane bilayer results in membrane disruption and depolarization. Flow cytometry analyses revealed a direct correlation between lipopeptide activity, membrane permeabilization (~ 75% PI uptake), and reduced cell viability. An increase in 2',7'-dichlorodihydrofluorescein diacetate (DCFH-DA) fluorescence demonstrates endogenous reactive oxygen species production. Lipopeptide treatment appears to induce late-stage apoptosis and alterations to nuclear morphology, suggesting that AF3-induced membrane damage may lead to a cellular stress response. Taken together, this study illustrates antifungal lipopeptide's potential as an antifungal drug candidate. KEY POINTS: • The studied lipopeptide variant AF3 displayed potent antifungal activity against C. albicans • Its biological activity was stable to proteolysis • Analytical studies demonstrated that the lipopeptide is essentially membranotropic and able to cause membrane dysfunction, elevated ROS levels, apoptosis, and DNA damage.

The elucidation of the multimodal action of the investigational anti-Candida lipopeptide (AF4) lead from Bacillus subtilis

Background: Candida species are the main etiological agents for candidiasis, and Candida albicans are the most common infectious species. Candida species' growing resistance to conventional therapies necessitates more research into novel antifungal agents. Antifungal peptides isolated from microorganisms have potential applications as novel therapeutics. AF4 a Bacillus-derived lipopeptide demonstrating broad-spectrum antifungal activity has been investigated for its ability to cause cell death in Candida species via membrane damage and oxidative stress. Methods: Using biophysical techniques, the secondary structure of the AF4 lipopeptide was identified. Scanning electron microscopy and confocal microscopy with fluorescent dyes were performed to visualise the effect of the lipopeptide. The membrane disruption and permeabilization were assessed using the 1,6-diphenyl hexatriene (DPH) fluorescence assay and flow cytometric (FC) assessment of propidium iodide (PI) uptake, respectively. The reactive oxygen species levels were estimated using the FC assessment. The induction of apoptosis and DNA damage were studied using Annexin V-FITC/PI and DAPI. Results: Bacillus-derived antifungal variant AF4 was found to have structural features typical of lipopeptides. Microscopy imaging revealed that AF4 damages the surface of treated cells and results in membrane permeabilization, facilitating the uptake of the fluorescent dyes. A loss of membrane integrity was observed in cells treated with AF4 due to a decrease in DPH fluorescence and a dose-dependent increase in PI uptake. Cell damage was also determined from the log reduction of viable cells treated with AF4. AF4 treatment also caused elevated ROS levels, induced phosphatidylserine externalisation, late-stage apoptosis, and alterations to nuclear morphology revealed by DAPI fluorescence. Conclusion: Collectively, the mode of action studies revealed that AF4 acts primarily on the cell membrane of C. albicans and has the potential to act as an antifungal drug candidate.

Jeongeuplla avenae gen. nov., sp. nov., a novel β-carotene-producing bacterium that alleviates salinity stress in Arabidopsis

A novel endophytic bacterium, designated DY-R2A-6T, was isolated from oat (Avena sativa L.) seeds and found to produces β-carotene. Phylogenetic analysis based on 16S rRNA gene sequences revealed that strain DY-R2A-6T had 96.3% similarity with Jiella aquimaris LZB041T, 96.0% similarity with Aurantimonas aggregate R14M6T and Aureimonas frigidaquae JCM 14755T, and less than 95.8% similarity with other genera in the family Aurantimonadaceae. The complete genome of strain DY-R2A-6T comprised 5,929,370 base pairs, consisting of one full chromosome (5,909,198 bp) and one plasmid (20,172 bp), with a G + C content was 69.1%. The overall genome-related index (OGRI), including digital DNA-DNA hybridization (<20.5%), ANI (<79.2%), and AAI (<64.2%) values, all fell below the thresholds set for novel genera. The major cellular fatty acids (>10%) of strain DY-R2A-6T were C16:0, C19:0 cyclo ω8c, and summed feature 8 (C18:1ω7c and/or C18:1ω6c). Ubiquinone-10 was the main respiratory quinone. We identified the gene cluster responsible for carotenoid biosynthesis in the genome and found that the pink-pigment produced by strain DY-R2A-6T is β-carotene. In experiment with Arabidopsis seedlings, co-cultivation with strain DY-R2A-6T led to a 1.4-fold increase in plant biomass and chlorophyll content under salt stress conditions, demonstrating its capacity to enhance salt stress tolerance in plants. Moreover, external application of β-carotene to Arabidopsis seedlings under salt stress conditions also mitigated the stress significantly. Based on these findings, strain DY-R2A-6T is proposed to represent a novel genus and species in the family Aurantimonadaceae, named Jeongeuplla avenae gen. nov., sp. nov. The type strain is DY-R2A-6T (= KCTC 82985T = GDMCC 1.3014T). This study not only identified a new taxon but also utilized genome analysis to predict and confirm the production of β-carotene by strain DY-R2A-6T. It also demonstrated the ability of this strain to enhance salt stress tolerance in plants, suggesting potential application in agriculture to mitigate environmental stress in crops.

Antimicrobial Peptide Synergies for Fighting Infectious Diseases

Antimicrobial peptides (AMPs) are essential elements of thehost defense system. Characterized by heterogenous structures and broad-spectrumaction, they are promising candidates for combating multidrug resistance. Thecombined use of AMPs with other antimicrobial agents provides a new arsenal ofdrugs with synergistic action, thereby overcoming the drawback of monotherapiesduring infections. AMPs kill microbes via pore formation, thus inhibitingintracellular functions. This mechanism of action by AMPs is an advantage overantibiotics as it hinders the development of drug resistance. The synergisticeffect of AMPs will allow the repurposing of conventional antimicrobials andenhance their clinical outcomes, reduce toxicity, and, most significantly,prevent the development of resistance. In this review, various synergies ofAMPs with antimicrobials and miscellaneous agents are discussed. The effect ofstructural diversity and chemical modification on AMP properties is firstaddressed and then different combinations that can lead to synergistic action,whether this combination is between AMPs and antimicrobials, or AMPs andmiscellaneous compounds, are attended. This review can serve as guidance whenredesigning and repurposing the use of AMPs in combination with other antimicrobialagents for enhanced clinical outcomes.

Activity of Novel Ultrashort Cyclic Lipopeptides against Biofilm of Candida albicans Isolated from VVC in the Ex Vivo Animal Vaginal Model and BioFlux Biofilm Model-A Pilot Study

In recent years, clinicians and doctors have become increasingly interested in fungal infections, including those affecting the mucous membranes. Vulvovaginal candidiasis (VVC) is no exception. The etiology of this infection remains unexplained to this day, as well as the role and significance of asymptomatic vaginal Candida colonization. There are also indications that in the case of VVC, standard methods of determining drug susceptibility to antifungal drugs may not have a real impact on their clinical effectiveness-which would explain, among other things, treatment failures and relapse rates. The aim of the study was to verify the promising results obtained previously in vitro using standard methods, in a newly developed ex vivo model, using tissue fragments of the mouse vagina. The main goal of the study was to determine whether the selected ultrashort cyclic lipopeptides (USCLs) and their combinations with fluconazole at specific concentrations are equally effective against Candida forming a biofilm directly on the surface of the vaginal epithelium. In addition, the verification was also performed with the use of another model for the study of microorganisms (biofilms) in vitro-the BioFlux system, under microfluidic conditions. The obtained results indicate the ineffectiveness of the tested substances ex vivo at concentrations eradicating biofilm in vitro. Nevertheless, the relatively most favorable and promising results were still obtained in the case of combination therapy-a combination of low concentrations of lipopeptides (mainly linear analogs) with mycostatic fluconazole. Additionally, using BioFlux, it was not possible to confirm the previously obtained results. However, an inhibiting effect of the tested lipopeptides on the development of biofilm under microfluidic conditions was demonstrated. There is an incompatibility between the classic in vitro methods, the newer BioFlux method of biofilm testing, offering many advantages postulated elsewhere, and the ex vivo method. This incompatibility is another argument for the need, on the one hand, to intensify research on the pathomechanism of VVC, and, on the other hand, to verify and maybe modify the standard methods used in the determination of Candida susceptibility.

Insights From the Lactobacillus johnsonii Genome Suggest the Production of Metabolites With Antibiofilm Activity Against the Pathobiont Candida albicans

Lactobacillus johnsonii is a probiotic bacterial species with broad antimicrobial properties; however, its antimicrobial activities against the pathobiont Candida albicans are underexplored. The aim of this study was to study the interactions of L. johnsonii with C. albicans and explore mechanisms of bacterial anti-fungal activities based on bacterial genomic characterization coupled with experimental data. We isolated an L. johnsonii strain (MT4) from the oral cavity of mice and characterized its effect on C. albicans growth in the planktonic and biofilm states. We also identified key genetic and phenotypic traits that may be associated with a growth inhibitory activity exhibited against C. albicans. We found that L. johnsonii MT4 displays pH-dependent and pH-independent antagonistic interactions against C. albicans, resulting in inhibition of C. albicans planktonic growth and biofilm formation. This antagonism is influenced by nutrient availability and the production of soluble metabolites with anticandidal activity.

The Antimicrobial Peptide Esc(1-21) Synergizes with Colistin in Inhibiting the Growth and in Killing Multidrug Resistant Acinetobacter baumannii Strains

Multidrug-resistant microbial infections and the scarce availability of new antibiotics capable of eradicating them are posing a serious problem to global health security. Among the microorganisms that easily acquire resistance to antibiotics and that are the etiological cause of severe infections, there is Acinetobacter baumannii. Carbapenems are the principal agents used to treat A. baumannii infections. However, when strains develop resistance to this class of antibiotics, colistin is considered one of the last-resort drugs. However, the appearance of resistance to colistin also makes treatment of the Acinetobacter infections very difficult. Antimicrobial peptides (AMP) from the innate immunity hold promise as new alternative antibiotics due to their multiple biological properties. In this study, we characterized the activity and the membrane-perturbing mechanism of bactericidal action of a derivative of a frog-skin AMP, namely Esc(1-21), when used alone or in combination with colistin against multidrug-resistant A. baumannii clinical isolates. We found that the mixture of the two compounds had a synergistic effect in inhibiting the growth and killing of all of the tested strains. When combined at dosages below the minimal inhibitory concentration, the two drugs were also able to slow down the microbial growth and to potentiate the membrane-perturbing effect. To the best of our knowledge, this is the first report showing a synergistic effect between AMPs, i.e., Esc(1-21), and colistin against colistin-resistant A. baumannii clinical isolates, highlighting the potential clinical application of such combinational therapy.

The combination of two Bacillus strains suppresses Meloidogyne incognita and fungal pathogens, but does not enhance plant growth

BACKGROUND

The combination of biocontrol agents is a desirable strategy to improve control efficacy against the root-knot nematode (RKN) Meloidogyne incognita under field conditions. However, strains compatibility is generally tested in vitro and incompatible combinations are normally not further examined in experiments in planta. Therefore, there is virtually no information on the performance of incompatible strains. In this study, we evaluated two Bacillus strains previously described as incompatible in vitro for effects on plant growth and suppression of M. incognita, pathogenic fungi and nematophagous fungi.

RESULTS

Strains BMH and INV were shown to be closely related to Bacillus velezensis. These strains, when applied individually, reduced the number of galls and eggs of M. incognita by more than 90% in tomato roots. When BMH and INV were combined (BMH + INV), RKN suppression and tomato shoot weight were lower compared to single-strain applications. Additionally, metabolites in cell-free supernatants and volatile organic compounds (VOCs) from strains BMH and INV had strong effects against the plant pathogens M. incognita, Fusarium oxysporum, Rhizoctonia solani and Sclerotium rolfsiii, but not against three species of nematophagous fungi. Although strain INV and the combination BMH + INV emitted fewer VOCs than strain BMH, they were still capable of killing second-stage juveniles of M. incognita.

CONCLUSIONS

Bacillus strains BMH and INV inhibited M. incognita and fungal pathogens, and promoted tomato growth. However, strain INV emitted fewer VOCs and the combination BMH + INV did not enhance the activity of the biocontrol strains against the RKN or their capacity to promote plant growth. © 2021 Society of Chemical Industry.

Antifungal Activity of Linear and Disulfide-Cyclized Ultrashort Cationic Lipopeptides Alone and in Combination with Fluconazole against Vulvovaginal Candida spp

Vulvovaginal candidiasis (VVC) occurs in over 75% of women at least once during their lifetime and is an infection that significantly affects their health. Candida strains resistant to standard azole antifungal therapy and relapses of VVC are more and more common. Hypothetically, biofilm is one of the main reasons of relapses and failure of the therapy. Ultrashort cationic lipopeptides (USCLs) exhibit high antimicrobial activities. Our previous study on USCLs revealed that disulfide cyclization can result in selective antifungal compounds. Therefore, four USCL were selected and their antifungal activity were studied on 62 clinical strains isolated from VVC. The results confirmed previous premises that cyclic analogs have increased selectivity between fungal cells and keratinocytes and improved anticandidal activity compared to their linear analogs against both planktonic and biofilm cultures. On the other hand, linear lipopeptides in combination with fluconazole showed a synergistic effect. It was found that the minimum inhibitory concentrations of the tested compounds in combination with fluconazole were at least four times lower than when used separately. Our results indicate that combination therapy of VVC with USCLs and fluconazole at low non-toxic concentrations can be beneficial owing to the synergistic effect. However, further in vivo studies are needed to confirm this hypothesis.

Effect of Lipopeptide-Loaded Chitosan Nanoparticles on Candida albicans Adhesion and on the Growth of Leishmania major

Cyclic lipopeptides produced by Bacillus species exhibit interesting therapeutic potential. However, their clinical use remains limited due to their low stability, undesirable interactions with host macromolecules, and their potential toxicity to mammalian cells. The present work aims to develop suitable lipopeptide-loaded chitosan nanoparticles with improved biological properties and reduced toxicity. Surfactin and bacillomycin D lipopeptides produced by Bacillus amyloliquefaciens B84 strain were loaded onto chitosan nanoparticles by ionotropic gelation process. Nanoformulated lipopeptides exhibit an average size of 569 nm, a zeta potential range of 38.8 mV, and encapsulation efficiency (EE) of 85.58%. Treatment of Candida (C.) albicans cells with encapsulated lipopeptides induced anti-adhesive activity of 81.17% and decreased cell surface hydrophobicity (CSH) by 25.53% at 2000 µg/mL. Nanoformulated lipopeptides also induced antileishmanial activity against Leishmania (L.) major promastigote and amastigote forms at respective IC₅₀ values of 14.37 µg/mL and 22.45 µg/mL. Nanoencapsulated lipopeptides exerted low cytotoxicity towards human erythrocytes and Raw 264.7 macrophage cell line with respective HC₅₀ and LC₅₀ values of 770 µg/mL and 234.56 µg/mL. Nanoencapsulated lipopeptides could be used as a potential delivery system of lipopeptides to improve their anti-adhesive effect against C. albicans cells colonizing medical devices and their anti-infectious activity against leishmania.

Killing Effect of Bacillus Velezensis FZB42 on a Xanthomonas Campestris pv. Campestris (Xcc) Strain Newly Isolated from Cabbage Brassica Oleracea Convar. Capitata (L.): A Metabolomic Study

The potential use of Bacillus velezensis FZB42 for biological control of various phytopathogens has been documented over the past few years, but its antagonistic interactions with xanthomonads has not been studied in detail. Novel aspects in this study consist of close observation of the death of Xanthomonas campestris pv. campestris cells in a co-culture with B. velezensis FZB42, and quantification of lipopeptides and a siderophore, bacillibactin, involved in the killing process. A new robust Xcc-SU isolate tolerating high concentrations of ferric ions was used. In a co-culture with the antagonist, the population of Xcc-SU was entirely destroyed within 24–48 h, depending on the number of antagonist cells used for inoculation. No inhibitory effect of Xcc-SU on B. velezensis was observed. Bacillibactin and lipopeptides (surfactin, fengycin, and bacillomycin) were present in the co-culture and the monoculture of B. velezensis. Except for bacillibactin, the maximum contents of lipopeptides were higher in the antagonist monoculture compared with the co-culture. Scanning electron microscopy showed that the death of Xcc-SU bacteria in co-culture was caused by cell lysis, leading to an enhanced occurrence of distorted cells and cell ghosts. Analysis by mass spectrometry showed four significant compounds, bacillibactin, surfactin, fengycin, and bacillomycin D amongst a total of 24 different forms detected in the co-culture supernatant: Different forms of surfactin and fengycin with variations in their side-chain length were also detected. These results demonstrate the ability of B. velezensis FZB42 to act as a potent antagonistic strain against Xcc.

Pyrogallol and Fluconazole Interact Synergistically In Vitro against Candida glabrata through an Efflux-Associated Mechanism

Candida glabrata is currently the first or second most commonly encountered non-albicans Candida species worldwide. The potential severity of Candida resistance mandates the discovery of novel antifungal agents, including those that can be used in combination therapies. In this study, we evaluated the in vitro interactions of pyrogallol (PG) and azole drugs against 22 clinical C. glabrata isolates. The potential mechanism underlying the synergism between PG and fluconazole (FLC) was investigated by the rhodamine 6G efflux method and quantitative reverse transcription (qRT)-PCR analysis. In susceptibility tests, PG showed strong synergism with FLC, itraconazole (ITC), and voriconazole (VRC), with fractional inhibitory concentration index values of 0.18 to 0.375 for PG+FLC, 0.250 to 0.750 for PG+ITC, and 0.141 to 0.750 for PG+VRC. Cells grown in the presence of PG+FLC exhibited reduced rhodamine 6G extrusion and significantly downregulated expression of the efflux-related genes CgCDR1, CgCDR2, and CgPDR1 compared with cells grown in the presence of PG or FLC alone. PG did not potentiate FLC when tested against a ΔCgpdr1 strain. Restoration of a functional CgPDR1 allele also restored the synergism. These results indicate that PG is an antifungal agent that synergistically potentiates the activity of azoles. Furthermore, PG appears to exert its effects by inhibiting efflux pumps and downregulating CgCDR1, CgCDR2, and CgPDR1, with CgPDR1 probably playing a crucial role in this process.

Antagonistic Activity of Antimicrobial Metabolites Produced from Seaweed-Associated Bacillus amyloliquefaciens MTCC 10456 Against Malassezia spp

Members of the genus Malassezia are known to be opportunistic pathogens responsible for causing skin disorders such as seborrheic dermatitis or dandruff, pityriasis versicolor, folliculitis, atopic dermatitis, and psoriasis. Due to the side effects caused by prolonged use of current topical antifungal agents, development of an alternative treatment is necessary. Fermentative production of antimicrobial metabolites from Bacillus amyloliquefaciens MTCC 10456 was carried out, and their antagonistic activity against Malassezia furfur and Malassezia globosa was evaluated. The antifungal metabolites were isolated by acid precipitation, and bioassay-guided simultaneous separation of the antimicrobial compounds was done by reversed-phase high-performance liquid chromatography (RP-HPLC). The fraction which demonstrated antifungal activity consisted of bacilysin, homologues of bacillomycin D, and members of the macrolactin family. The presence of bacilysin was detected using specific inhibitor assays and homologues of bacillomycin D, and macrolactins were identified using liquid chromatography/high-resolution electrospray ionization-mass spectrometry (LC/HRESI-MS/MS) analysis. Synergism among the identified compounds was observed which enhanced the antagonistic activity against Malassezia spp. To our knowledge, this is the first study to report the co-production and separation of members of macrolactin antibiotics, lipopeptides such as bacillomycin D and dipeptide antibiotic bacilysin, by any Bacillus strain from marine environment. Activity of individual compounds against Malassezia has been reported which may facilitate their application in the field of dermatology and in cosmetic products.

Antimicrobial Peptides and their Multiple Effects at Sub-Inhibitory Concentrations

The frequent occurrence of multidrug-resistant strains to conventional antimicrobials has led to a clear decline in antibiotic therapies. Therefore, new molecules with different mechanisms of action are extremely necessary. Due to their unique properties, antimicrobial peptides (AMPs) represent a valid alternative to conventional antibiotics and many of them have been characterized for their activity and cytotoxicity. However, the effects that these peptides cause at concentrations below the minimum growth inhibitory concentration (MIC) have yet to be fully analyzed along with the underlying molecular mechanism. In this mini-review, the ability of AMPs to synergize with different antibiotic classes or different natural compounds is examined. Furthermore, data on microbial resistance induction are reported to highlight the importance of antibiotic resistance in the fight against infections. Finally, the effects that sub-MIC levels of AMPs can have on the bacterial pathogenicity are summarized while showing how signaling pathways can be valid therapeutic targets for the treatment of infectious diseases. All these aspects support the high potential of AMPs as lead compounds for the development of new drugs with antibacterial and immunomodulatory activities.

Synergistic potential of Citrus aurantium L. essential oil with antibiotics against Candida albicans

ETHNOPHARMACOLOGICAL RELEVANCE

Citrus aurantium L. is used in traditional medicine in India for treating stomach ache, vomiting, blood pressure, dysentery, diarrhea, cardiovascular analeptic, sedative, boils and urinary tract infections. Its essential oil from fruit peels has antioxidant, antimicrobial, antifungal, antiparasitic, and anti-inflammatory activities.

AIM OF THE STUDY

The aim of the study was to characterize the antifungal activity and synergistic potential of essential oil extracted from leaves of Citrus aurantium L. of North-Western Himalayas against Candida albicans.

MATERIALS AND METHODS

Citrus aurantium essential oil (CAEO) was extracted from leaves and characterized by GC-MS. The antifungal activity and synergistic potential of CAEO against C. albicans was studied by agar well diffusion, and broth microdilution assay. The anti-fungal potential of the phytoconstituents of CAEO was studied by in silico interaction with two fungal drug targets, N-myristoyl transferase (NMT) and Cytochrome P450 14 Alpha-sterol Demethylase (CYP51).

RESULTS

CAEO exhibited strong antifungal activity against two strains of C. albicans, with fungicidal effect. The MIC of CAEO against C. albicans strains was 0.15 - 0.31% (v/v). CAEO exhibited synergistic potential with fluconazole and amphotericin B against C. albicans and enhanced the antifungal efficacy of the clinical drugs by 8.3 to 34.4 folds. The GC-MS analysis of CAEO identified at least ten compounds, with 2-β pinene, δ-3 Carene and D-limonene as the major compounds. In silico molecular docking of the three major phytocompounds of CAEO with NMT and CYP51 revealed their potential to interact with both targets. δ-3 Carene showed best binding (E_total of -131.13 kcal/mol) with NMT, while D-limonene exhibited highest binding energy (E_total of -175.23 kcal/mol) with CYP51. ADME/T analysis showed that 2-β pinene, δ-3 Carene and D-limonene exhibit drug likeliness and ideal toxicity profiles for their use as drug candidates.

CONCLUSIONS

Thus, the essential oil from leaves of C. aurantium and its phytocomponents can be used as sustainable and natural therapeutic to treat candidiasis as well as a resource to enhance the potency of clinical antibiotics, which have lost efficacy due to emergence of drug resistance in C. albicans.

Bacillus subtilis TR47II as a source of bioactive lipopeptides against Gram-negative pathogens causing nosocomial infections

This study aimed to investigate the antimicrobial, antibiofilm, and cytotoxic effects of biosurfactant lipopeptides synthesized by Bacillus subtilis TR47II. For this purpose, the lipopeptides were partially purified using a three-step process and characterized. In the first step, the crude extract obtained from acid precipitation exhibited strong antibacterial activity against the Gram-negative opportunistic pathogens Alcaligenes faecalis ATCC 8750, Achromobacter xylosoxidans ATCC 13138, Pseudomonas alcaligenes ATCC 14909, and Pseudomonas putida ATCC 15175. Moreover, partial inhibition was observed against Klebsiella aerogenes ATCC 13048 (42%), Escherichia coli ATCC 25922 (16%), and Pseudomonas aeruginosa ATCC 27853 (47%). The lipopeptides in the crude extract were extracted with methanol and fractioned on a silica gel chromatography column, rendering four TLC-pooled chromatographic fractions, named F1, F2, F3, and F4. The chromatographic fraction F4 was the most bioactive, with MIC values between 300 and 600 µg mL^-1. Besides, F4 at sub-MIC doses dislodged the biofilms of A. faecalis, A. xylosoxidans, and P. alcaligenes by about 100, 85, and 81%, respectively. No cytotoxic effect was observed in mammalian cells at MIC. MALDI-TOF-MS analysis revealed that F4 contained cyclic lipopeptides belonging to two families: iturins (m/z 1004 to 1087) and fengycins (m/z 1424 to 1545). The dual effect of F4 on planktonic and sessile growth could suggest that the synergistic application of these biosurfactants could be efficient in the control of these opportunistic pathogens.

Iturin A Extracted From Bacillus subtilis WL-2 Affects Phytophthora infestans via Cell Structure Disruption, Oxidative Stress, and Energy Supply Dysfunction

Potato late blight, caused by Phytophthora infestans (Mont.) de Bary, represents a great food security threat worldwide and is difficult to control. Recently, Bacillus spp. have been considered biocontrol agents to control many plant diseases. Here, Bacillus subtilis WL-2 was selected as a potent strain against P. infestans mycelium growth, and its functional metabolite was identified as Iturin A via electrospray ionization mass spectrometry (ESI-MS). Analyses using scanning electron microscopy (SEM) and transmission electron microscopy (TEM) revealed that Iturin A caused cell membrane disruption and an irregular internal cell structure. In addition, Iturin A triggered oxidative stress reactions similarly to reactive oxygen species (ROS) in P. infestans cells and caused mitochondrial damage, including mitochondrial membrane potential (MMP), mitochondrial respiratory chain complex activity (MRCCA), and ATP production decline. These results highlight that the cell structure disruption, oxidative stress, and energy supply dysfunction induced by Iturin A play an important role in inhibiting P. infestans. Additionally, B. subtilis WL-2 and Iturin A have great potential for inhibiting P. infestans mycelium growth and controlling potato late blight in the future.

Bacillus megaterium WL-3 Lipopeptides Collaborate Against Phytophthora infestans to Control Potato Late Blight and Promote Potato Plant Growth

Oomycete Phytophthora infestans [(Mont.) de Bary] is the cause of potato late blight, a plant disease which poses a serious threat to our global food security and is responsible for huge economic losses worldwide. Lipopeptides produced by Bacillus species are known to be potent antibacterial compounds against many plant pathogens. In this study, we show that Bacillus megaterium WL-3 has an antagonistic effect against potato late blight. Electrospray ionization mass spectrometry (ESI-MS) revealed that lipopeptides derived from the WL-3 strain contained three subfamilies, surfactin (C₁₃ - C₁₅), Iturin A (C₁₄ - C₁₆), and Fengycin A (C₁₅ - C₁₉). The Iturin A and Fengycin A lipopeptide families were each confirmed to have anti-oomycete effects against P. infestans mycelium growth as well as obvious controlling effects against potato late blight in greenhouse experiments and field assays. Furthermore, Iturin A and Fengycin A were able to promote plant photosynthetic efficiency, plant growth, and potato yield. Most importantly, the combination of Iturin A and Fengycin A (I + F) was superior to individual lipopeptides in controlling potato late blight and in the promotion of plant growth. The results of this study indicate that B. megaterium WL-3 and its lipopeptides are potential candidates for the control of late blight and the promotion of potato plant growth.

Antifungal activity of Punicalagin-nystatin combinations against Candida albicans

OBJECTIVES

Oral candidiasis is the most common opportunistic fungal infection of oral mucosa and results from an overgrowth of Candida, especially Candida albicans. The potential anti-C. albicans and cytotoxicity of punicalagin (PCG), isolated from Punica granatum, alone or with nystatin (NYS) were evaluated.

METHODS

Activity of compounds alone or in combinations was determined against two C. albicans strains (ATCC 90028 and SC5314). Minimal inhibitory concentration (MIC)-50 and Minimum Fungicidal Concentration (MFC) were assessed by XTT assay and CFU counts, respectively. For combinations, determination of fractional inhibitory concentration index was performed. Ergosterol pathway was investigated as a possible PCG antifungal mechanism. Cytotoxicity assays were undertaken on human primary oral keratinocytes and gingival fibroblasts incubated with antifungal concentrations of PCG and/or NYS for 24 hr.

RESULTS

Combination of NYS and PCG increased antifungal efficacy, compared with compounds tested alone. Combinations 4 (PCG-6.25 μg/ml; NYS-3.9 μg/ml) and 5 (PCG-12.5 μg/ml; NYS-1.95 μg/ml) were more effective since they reduced the MIC-50 of PCG (50 μg/ml) by 8 and 4 times, respectively, increased the candidal inhibition and nullified the PCG cytotoxicity for keratinocytes. PCG antifungal mechanism did not involve ergosterol biosynthesis pathway.

CONCLUSIONS

The favorable outcomes for combination of PCG and NYS encourage further testing this therapeutic strategy against C. albicans.

Eucalyptal D Enhances the Antifungal Effect of Fluconazole on Fluconazole-Resistant Candida albicans by Competitively Inhibiting Efflux Pump

The frequent emergence of azole-resistant strains has increasingly led azoles to fail in treating candidiasis. Combination with other drugs is a good option to effectively reduce or retard its incidence of resistance. Natural products are a promising synergist source to assist azoles in treating resistant candidiasis. Eucalyptal D (ED), a formyl-phloroglucinol meroterpenoid, is one of the natural synergists, which could significantly enhance the anticandidal activity of fluconazole (FLC) in treating FLC resistant C. albicans. The checkerboard microdilution assay showed their synergistic effect. The agar disk diffusion test illustrated the key role of ED in synergy. The rhodamine 6G (R6G) efflux assay reflected ED could reduce drug efflux, but quantitative reverse transcription PCR analysis revealed the upregulation of CDR1 and CDR2 genes in ED treating group. Efflux pump-deficient strains were hyper-susceptible to ED, thus ED was speculated to be the substrate of efflux pump Cdr1p and Cdr2p to competitively inhibit the excretion of FLC or R6G, which mainly contributed to its synergistic effect.

Testing a Human Antimicrobial RNase Chimera Against Bacterial Resistance

The emergence of bacterial resistance to the most commonly used antibiotics encourages the design of novel antimicrobial drugs. Antimicrobial proteins and peptides (AMPs) are the key players in host innate immunity. They exert a rapid and multifaceted action that reduces the development of bacterial adaptation mechanisms. Human antimicrobial RNases belonging to the vertebrate specific RNase A superfamily participate in the maintenance of tissue and body fluid sterility. Among the eight human canonical RNases, RNase 3 stands out as the most cationic and effective bactericidal protein against Gram-negative species. Its enhanced ability to disrupt the bacterial cell wall has evolved in detriment of its catalytic activity. Based on structure-functional studies we have designed an RNase 3/1 hybrid construct that combines the high catalytic activity of RNase 1 with RNase 3 bactericidal properties. Next, we have explored the ability of this hybrid RNase to target the development of bacterial resistance on an Acinetobacter baumannii cell culture. Synergy assays were performed in combination with colistin, a standard antimicrobial peptide used as an antibiotic to treat severe infections. Positive synergism was observed between colistin and the RNase 3/1 hybrid protein. Subsequently, using an in vitro experimental evolution assay, by exposure of a bacterial culture to colistin at incremental doses, we demonstrated the ability of the RNase 3/1 construct to reduce the emergence of bacterial antimicrobial resistance. The results advance the potential applicability of RNase-based drugs as antibiotic adjuvants.

Capability of iturin from Bacillus subtilis to inhibit Candida albicans in vitro and in vivo

Candida albicans is a fungal pathogen that is difficult to cure clinically. The current clinic C. albicans-inhibiting drugs are very harmful to humans. This study revealed the potential of iturin fractions from Bacillus subtilis to inhibit C. albicans in free status (MIC = 32 μg/mL) and natural biofilm in vitro. The inhibition mechanism was identified as an apoptosis pathway via the decrease of mitochondrial membrane potential, the increase of the reactive oxygen species (ROS) accumulation, and the induction of nuclear condensation. For in vivo experiments, the C. albicans infection model was constructed via intraperitoneal injection of 1 × 10⁸C. albicans cells into mice. One day after the infection, iturin was used to treat infected mice at different concentrations alone and in combination with amphotericin B (AmB) by intraperitoneal injection. The treatment with AmB alone could cause the death of infected mice, whereas treatment with 15 mg/kg iturin per day alone led to the survival of all infected mice throughout the study. After continuously treated for 6 days, all mice were sacrificed and analyzed. As results, the combination of 15 mg/kg iturin and AmB at a ratio of 2:1 had the most efficient effect to remove the fungal burden in the kidney and cure the infected mice by reversing the symptoms caused by C. albicans infection, such as the loss of body weight, change of immunology cells in blood and cytokines in serum, and damage of organ structure and functions. Overall, iturin had potential in the development of efficient and safe drugs to cure C. albicans infection.

Antifungal activity of analogues of antimicrobial peptides isolated from bee venoms against vulvovaginal Candida spp

Candida albicans is the main causative agent of vulvovaginal candidiasis (VVC), a common mycosis in women, relapses of which are difficult to manage due to biofilm formation. This study aimed at developing novel non-toxic compounds active against Candida spp. biofilms. We synthesised analogues of natural antifungal peptides LL-III (LL-III/43) and HAL-2 (peptide VIII) originally isolated from bee venoms and elucidated their structures by nuclear magnetic resonance spectroscopy. The haemolytic, cytotoxic, antifungal and anti-biofilm activities of LL-III/43 and peptide VIII were then tested. LL-III/43 and VIII showed moderate cytotoxicity to HUVEC-2 cells and had comparable inhibitory activity against C. albicans and non-albicans spp. The lowest minimum inhibitory concentration (MIC90) of LL-III/43 was observed towards Candida tropicalis (0.8 µM). That was 8-fold lower than that of antimycotic amphotericin B. Both peptides can be used to inhibit Candida spp. bio film f ormation. Biofilm inhibitory concentrations (BIC50) ranged from 0.9 to 58.6 µM and biofilm eradication concentrations (BEC50) for almost all tested Candida spp. strains ranged from 12.8 to 200 µM. Als o pro ven were the peptides’ abilities to reduce the area colonised by biofilms , inhibit hyphae formation and permeabilise cell membranes in biofil ms . LL-III/43 and VIII are promising candidates for further development as therapeutics against VVC.

Evaluation of Antifungal Efficacy of Three New Cyclic Lipopeptides of the Class Bacillomycin from Bacillus subtilis RLID 12.1

New lipopeptide homologues (AF₃, AF₄, and AF₅) with antifungal activities against Candida and Cryptococcus spp. were purified from a cell-free supernatant of Bacillus subtilis RLID 12.1. The lipopeptides AF₃, AF₄, and AF₅ were identified with the same peptide sequence Asn-Pro-Tyr-Asn-Gln-Thr-Ser with variations in the fatty acid branching type and chain length (anteiso-C₁₇, iso-C₁₇, and iso-C₁₈, respectively). Upon comparing the three homologues for MICs against 81 Candida (n = 64) and Cryptococcus (n = 17) clinical isolates and their cytotoxicities, we found that AF₄ was the most promising antifungal lipopeptide, since it demonstrated 100% inhibition at geometric mean MICs of 3.31, 3.41, 3.48, and 2.83 μg/ml against Candida albicans, Candida tropicalis, Candida auris, and Cryptococcus neoformans, respectively, with low hemolysis values (<6%) and 50% inhibitory concentrations (13.31 μg/ml). The additive effects among the homologues AF₃, AF₄, and AF₅ were evaluated against three Candida species, along with the cytotoxicity studies. Five combinations exhibited good additive interaction effects: AF₃/AF₄ (at corresponding concentrations of 4 and 4 μg/ml [4/4 μg/ml]), AF₃/AF₅ (4/4 μg/ml), AF₃/AF₅ (2/4 μg/ml), AF₄/AF₅ (4/4 μg/ml), and AF₄/AF₅ (2/4 μg/ml) in planktonic cell inhibition and AF₃/AF₄ (4/4 μg/ml), AF₃/AF₅ (4/4 μg/ml), and AF₃/AF₅ (2/4 μg/ml) in the inhibition of biofilm formation. However, combinations AF₃/AF₄ and AF₃/AF₅, which showed >70% cell survival with low hemolysis (<5%), were found to be comparatively effective. We describe here the additive effects of lipopeptide homologues showing reduced cytotoxicity against mammalian cells; these combinations might serve as a potent antibiofilm-forming substitute.

Bacillomycin D Produced by Bacillus amyloliquefaciens Is Involved in the Antagonistic Interaction with the Plant-Pathogenic Fungus Fusarium graminearum

Fusarium graminearum (teleomorph: Ascomycota, Hypocreales, Gibberella, Gibberella zeae) is a destructive fungal pathogen that threatens the production and quality of wheat and barley worldwide. Controlling this toxin-producing pathogen is a significant challenge. In the present study, the commercially available strain Bacillus amyloliquefaciens (Bacteria, Firmicutes, Bacillales, Bacillus) FZB42 showed strong activity against F. graminearum The lipopeptide bacillomycin D, produced by FZB42, was shown to contribute to the antifungal activity. Purified bacillomycin D showed strong activity against F. graminearum, and its 50% effective concentration was determined to be approximately 30 μg/ml. Analyses using scanning and transmission electron microscopy revealed that bacillomycin D caused morphological changes in the plasma membranes and cell walls of F. graminearum hyphae and conidia. Fluorescence microscopy combined with different dyes showed that bacillomycin D induced the accumulation of reactive oxygen species and caused cell death in F. graminearum hyphae and conidia. F. graminearum secondary metabolism also responded to bacillomycin D challenge, by increasing the production of deoxynivalenol. Biological control experiments demonstrated that bacillomycin D exerted good control of F. graminearum on corn silks, wheat seedlings, and wheat heads. In response to bacillomycin D, F. graminearum genes involved in scavenging reactive oxygen species were downregulated, whereas genes involved in the synthesis of deoxynivalenol were upregulated. Phosphorylation of MGV1 and HOG1, the mitogen-activated protein kinases of F. graminearum, was increased in response to bacillomycin D. Taken together, these findings reveal the mechanism of the antifungal action of bacillomycin D.IMPORTANCE Biological control of plant disease caused by Fusarium graminearum is desirable. Bacillus amyloliquefaciens FZB42 is a representative of the biocontrol bacterial strains. In this work, the lipopeptide bacillomycin D, produced by FZB42, showed strong fungicidal activity against F. graminearum Bacillomycin D caused morphological changes in the plasma membrane and cell wall of F. graminearum, induced accumulation of reactive oxygen species, and ultimately caused cell death in F. graminearum Interestingly, when F. graminearum was challenged with bacillomycin D, the deoxynivalenol production, gene expression, mitogen-activated protein kinase phosphorylation, and pathogenicity of F. graminearum were significantly altered. These findings clarified the mechanisms of the activity of bacillomycin D against F. graminearum and highlighted the potential of B. amyloliquefaciens FZB42 as a biocontrol agent against F. graminearum.

Bacillomycin D and its combination with amphotericin B: promising antifungal compounds with powerful antibiofilm activity and wound-healing potency

AIMS

In this study, we evaluated the ability of the lipopeptide bacillomycin D and the antifungal drug amphotericin B as well as their combination, to inhibit Candida albicans biofilm formation and to accelerate keratinocyte cell migration.

METHODS AND RESULTS

The antibiofilm activity of bacillomycin D and its combination with amphotericin B was carried out by crystal violet colorimetric method. Our results have shown that, when combined together at low concentrations nontoxic to mammalian cells, corresponding to 1/32 MIC (0·39 μg ml(-1) ) and 1/4 MIC (0·06 μg ml(-1) ) for bacillomycin D and amphotericin B, respectively, a clear antibiofilm activity is manifested (95% inhibition of biofilm formation) along with a clear inhibition of germ tube formation. Moreover, the effect of both drugs on preformed biofilm of C. albicans strain was determined using 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) reduction assay. The combination of the two antifungal compounds at 0·39 and 1 μg ml(-1) for bacillomycin D and amphotericin B, respectively, resulted in a clear enhancement of biofilm eradication compared to the results obtained with each drug alone. Furthermore, this combination was found to promote the closure of a gap produced in a monolayer of human keratinocytes.

CONCLUSIONS

Bacillomycin D and its combination with amphotericin B display impressive anti-biofilm and wound-healing activities.

SIGNIFICANCE AND IMPACT OF THE STUDY

Application of the lipopeptide bacillomycin D and the antifungal drug amphotericin B in medical devices may offer a promising alternative for topical treatment of Candida-associated infections in the setting of a wound.