Antimicrobial Natural Products Research Articles

Physicochemical and metagenomic analysis of samples from the Roman Baths (Bath, UK) reveals high bacterial and archaeal diversity and a potential for antimicrobial discovery

Antimicrobial resistance of pathogenic bacteria to currently used antibiotics killed an estimated 1.27 million people in 2019 and it is believed that this toll will increase to 10 million per year by 2050. This remains one of the most urgent threats to human health and the hunt for novel antimicrobial natural products (NPs) from microorganisms is essential in efforts to combat this threat, but recent efforts have been unsuccessful. The investigation of extreme ecological niches, such as hot springs, has gained recent interest due to their unique ecosystems, increasing the chance for novel antimicrobial NP discovery. The Roman Baths (Bath) is the UK’s only hot spring and it remains unexplored in terms of microbial diversity. In this study, samples of water, biofilm and sediment were taken from the King’s Spring and the Great Bath, and 16 S rRNA metagenomic sequencing was performed for bacterial and archaeal community exploration. Additionally, samples were used for culture-dependent isolation of antimicrobial NP producing isolates. At the phylum level, data from water samples revealed a shift from an archaeal dominant community in the King’s Spring (Crenarchaeota, Aenigmarchaeota and Nanoarchaeota) to bacteria in the Great Bath (Cyanobacteria and Proteobacteria). Archaea were still present in the biofilm and sediment of the hot spring but consistently the bacterial phyla Cyanobacteria, Bacteroidota, Proteobacteria and Nitrospirota were dominant at both sites. The antimicrobial NP producing candidate phyla Actinobacteria and Myxococcota were found in all samples. From initial isolation experiments, 297 isolates were purified, of which 15 showed broad spectrum activity against human pathogens, though interestingly these were not from target genera in the Actinobacteria. From these data, there is clear potential for novel NP discovery from the Roman Baths, as has been demonstrated from other thermal hot springs globally.

Nisin as a food preservative: physicochemical, sensory properties and antimicrobial activity in Mexican tomato sauce

SummaryBiopreservation is a technique that utilises natural microbiota and antimicrobial products to extend the shelf life of food in a controlled manner. Nisin, bacteriocins, is used commonly in meat and dairy items and is acknowledged as safe by the FDA under the acronym GRAS. Tomato sauce is an essential component of Mexican cuisine but has a limited shelf life. Nisin (0.01%, 0.1% and 0.2%) was used as a preservative to preserve its physicochemical and microbiological properties. The results were compared with those of a control group (with no preservative) and a sample that used potassium sorbate (0.2%) as a preservative. The Mexican tomato sauces were stored at 4, 20 and 35 °C for 180 days to test their stability. The results indicated that the nisin‐preserved Mexican tomato sauces remained stable at all three temperatures for 180 days. In addition, the sauce preserved with 0.01% nisin was preferred by untrained panellists, as it maintained its desirable taste and texture. The sauces preserved with 0.1% and 0.2% nisin exhibited no significant changes in their physicochemical properties. However, after 150 days of preservation, there was a noticeable difference in the antimicrobial activity of the nisin‐containing sauces compared with the sauce with sorbate and the control sauce without any additives. Based on the product's unique characteristics and physicochemical, microbiological and sensory properties, nisin can serve as a natural preservative for Mexican tomato sauces.

Diverse RNA viruses in the venom-related microenvironment of different animal phyla.

Venom is known as the source of natural antimicrobial products. Previous studies have largely focused on the expression of venom-related genes and the biochemical components of venom. With the advent of metagenomic sequencing, many more microorganisms, especially viruses, have been identified in highly diverse environments. Herein, we investigated the RNA virome in the venom-related microenvironment through analysis of a large volume of venom-related RNA-sequencing data mined from public databases. From this, we identified viral sequences belonging to thirty-six different viruses, of which twenty-two were classified as 'novel' as they exhibited less than 90 per cent amino acid identity to known viruses in the RNA-dependent RNA polymerase. Most of these novel viruses possessed genome structures similar to their closest relatives, with specific alterations in some cases. Phylogenetic analyses revealed that these viruses belonged to at least twenty-two viral families or unclassified groups, some of which were highly divergent from known taxa. Although further analysis failed to find venom-specific viruses, some viruses seemingly had much higher abundance in the venom-related microenvironment than in other tissues. In sum, our study provides insights into the RNA virome of the venom-related microenvironment from diverse animal phyla.

Evaluation of Antimicrobial Activity of Alocasia Cordfolia Rhizomes Extract

Objective: Discovering new antibacterial drugs with high efficacy and low toxicity from medicinal plants has received increasing attention worldwide. The aim of this study was to investigate the antimicrobial activity of ethanolic extract of Alocasia cordfolia rhizomes (ACRE). Methods: S.aureus, E.coli and C.albicans were selected as the tested microorganisms. The diameter of the inhibition zone was determined by the disc diffusion method, and MIC and MBC were measured using the broth microdilution method. Results: The diameter of the inhibition zone, MIC and MBC values of ACRE against S. aureus were 7.67±0.58 mm, 160 μg/μL and 320 μg/μL, and that of ACRE against C.albicans were 8.33±1.15 mm, 80 μg/μL and 160 μg/μL, respectively. However, ACRE did not show antimicrobial activity against E. coli. Conclusion: ACRE may be a potentially promising natural antimicrobial product, and further studies are needed to identify the specific active compounds present in this extract and to elucidate their antimicrobial mechanisms.

In Vitro and In Silico Studies of Antimicrobial Saponins: A Review

Antibiotics are important drugs for the treatment of microbial infections and related diseases. However, due to the abuse of antibiotics, drug resistance has become a serious and urgent problem. The development of new antibiotics is a crucial area of research, and natural products are one of the main sources of novel antibiotics. Among various potential natural antimicrobial products, saponins attracted much attention due to their excellent and broad-spectrum antimicrobial properties. Although there are several reviews on antibacterial saponins, this review is the first to highlight the potential antibacterial mechanisms of saponins from both experimental and molecular simulation perspectives to provide a comprehensive panorama of the field. This review presents the current progress in the development and repurposing of natural-product antibiotics. The focus is centered on antimicrobial saponins discovered in recent years as well as the synergistic effect of some saponins with traditional antibiotics. This review presents experimental and simulation studies in this field to provide a multiscale overview of the antimicrobial mechanisms of saponins and potential directions for future research.

Screening Amazon rainforest plant extracts for antimicrobial activity: a 15-year commitment to the Brazilian biodiversity

IntroductionThe need for new tools to treat infections is constantly growing due to the possibilities of emerging diseases related to environmental changes, climatic catastrophes, microorganism resistance, and human and animal aging, leading to an evident unbalance in the planet’s health. Brazil contains the most significant portion of world biodiversity, a potential source of new antimicrobial natural products. Nonetheless, its environment, particularly its forests, and rainforests, is under threat, meaning that rapidly conducted, comprehensive research into the potential of antimicrobial activity to address this threat is urgently needed.MethodsIn this study, plants from the Amazon rainforest and the Atlantic forests were collected and tested against several pathogenic microbes relevant to humans, animals, and the environment, and subjected to large-scale susceptibility assays, bioautography, and Artemia salina toxicity assays. From the plants, 2,280 organic and aqueous extracts were obtained from different organs, namely leaves, barks, flowers, fruits, and seeds, and subjected to a large-scale susceptibility screening assay against Staphylococcus aureus, Staphylococcus epidermidis, Enterococcus faecalis, Streptococcus mutans, Streptococcus sanguinis, Escherichia coli, Pseudomonas aeruginosa, Candida albicans, Malassezia pachydermatis, Malassezia furfur, and Listeria monocytogenes. Results and discussionThe selected extracts were subjected to antimicrobial susceptibility tests to determine their inhibition zone diameters and minimum bactericidal concentrations, to bioautography, and to an Artemia salina toxicity assay, which resulted in 154 active extracts. Moreover, 111 out of 154 extracts were ranked based on scores established by the p-values and the mean rank differences in each set of test results. The final ranking identified which extracts should be studied in further phytochemical research using thin-layer chromatography techniques as a priority. The extracts obtained from plants belonging to Combretaceae, Connaraceae, Convolvulaceae, Fabaceae, Malpighiaceae, Moraceae, Piperaceae, Polygonaceae, and Salicaceae were selected as the most promising ones and used to support the identification of plant-based antimicrobial active compounds from the immense biodiversity of Brazilian forests.

Enhancing the Production of Xenocoumacin 1 in Xenorhabdus nematophila CB6 by a Combinatorial Engineering Strategy.

Xenocoumacin 1 (Xcn1) is an excellent antimicrobial natural product against Phytophthora capsici. However, the commercial development of Xcn1 is hindered by the low yield, which results in high application costs. In this study, multiple metabolic strategies, including blocking the degradation pathway, promoter engineering, and deletion of competing biosynthetic gene clusters, were employed to improve the production of Xcn1, which was increased from 0.07 to 0.91 g/L. The formation of Xcn1 reached 1.94 g/L in the TB medium with the final strain T3 in a shake flask and further reached 3.52 g/L in a 5 L bioreactor, which is the highest yield ever reported. The engineered strain provides a valuable platform for production of Xcn1, and the possible commercial development of the biofungicide. We anticipate that the metabolic engineering strategies utilized in this study and the constructed constitutive promoter library can be widely applied to other bacteria of the genera Xenorhabdus and Photorhabdus.

Screen for New Antimicrobial Natural Products from the NCI Program for Natural Product Discovery Prefractionated Extract Library.

The continuing emergence of antibiotic-resistant microbes highlights the need for the identification of new chemotypes with antimicrobial activity. One of the most prolific sources of antimicrobial molecules has been the systematic screening of natural product samples. The National Institute of Allergy and Infectious Diseases and the National Cancer Institute here report a large screen of 326,656 partially purified natural product fractions against a panel of four microbial pathogens, resulting in the identification of >3000 fractions with antifungal and/or antibacterial activity. A small sample of these active fractions was further purified and the chemical structures responsible for the antimicrobial activity were elucidated. The proof-of-concept study identified many different chemotypes, several of which have not previously been reported to have antimicrobial activity. The results show that there remain many unidentified antibiotic compounds from nature.

Antimicrobial activity and chemical profile of wood vinegar from eucalyptus (Eucalyptus urophylla x Eucalyptusgrandis -clone I144) and bamboo (Bambusa vulgaris).

Microbial resistance to drugs is a public health problem; therefore, there is a search for alternatives to replace conventional products with natural agents. One of the potential antimicrobial agents is wood vinegar derived from the carbonization of lignocellulosic raw materials. The objectives of the present work were to evaluate the antibacterial and antifungal action of two kinds of wood vinegar (WV), one of Eucalyptus urograndis wood and another of Bambusa vulgaris biomass, and determine their chemical profile. The antimicrobial effect was assessed against Staphylococcus aureus, Pseudomonas aeruginosa, Salmonella enteritidis, Escherichia coli, Streptococcus agalactiae, and Candida albicans. The minimum inhibitory concentration and the minimum bactericidal and fungicidal concentrations were determined. Micrographs of the microorganisms before and after exposure to both kinds of wood vinegar were obtained by scanning electron microscopy. The chemical profile of the eucalyptus and bamboo vinegar was carried out by gas chromatography and mass spectrometry (GC/MS). Both types of WV presented significant antimicrobial activity, with the bamboo one having a higher efficiency. Both studied pyroligneous extracts seem promising for developing natural antimicrobials due to their efficiency against pathogens. GC/MS analyses demonstrated that the chemical profiles of both kinds of WV were similar but with some significant differences. The major component of the eucalyptus vinegar was furfural (17.2%), while the bamboo WV was phenol (15.3%). Several compounds in both WVs have proven antimicrobial activity, such as acetic acid, furfural, phenol, cresols, guaiacol, and xylenols. Together, they are the major in the chemical composition of the organic fraction of both WVs. Bamboo vinegar had a more expressive content of organic acids. Micrographs of microorganisms taken after exposure to both kinds of wood vinegar displayed several cell modifications. The potential of both types of wood vinegar as a basis for natural antimicrobial products seems feasible due to their proven effect on inhibiting the microorganisms' growth assessed in this experiment.

Essential Oils Infused Poly-ε-Caprolactone/Gelatin Electrospun Nanofibrous Mats: Biocompatibility and Antibacterial Study.

Infections caused by antibiotic-resistant pathogens result in a delayed wound-healing process. As an approach to prevent infections, alternatives in the form of natural antimicrobial products have become public interest. Essential oils derived from plants are used as antimicrobials owing to their broad-spectrum activity against pathogenic organisms. In this study, essential oil from seeds of watermelon, jackfruit, and papaya was incorporated into poly-ε-caprolactone/gelatin nanofibers using an electrospinning technique. The synthesized nanofibers were smooth, continuous, and bead-free. The nanofibers were found to be mechanically competent as confirmed by the universal tensile tester. The antibacterial activity of the various essential oil-loaded nanofibrous mats was determined by disc diffusion assay. Furthermore, they were found to be non-toxic and biocompatible by MTT and CMFDA assays on fibroblast cells. The obtained results have demonstrated that essential oil-loaded nanofiber mats are promising alternatives to conventional antibacterial wound dressings.

Improvement of chlorination and sterilization of pathogenic bacteria by natural products

There are many highly resistant pathogenic bacteria in the environment that cannot be removed by conventional chlorination disinfection. This study demonstrated a combined sterilization strategy for combating the extension of pathogenic bacteria by enhancing the bactericidal ability of sodium hypochlorite at subinhibitory concentrations with the utilization of the natural antimicrobial molecule cinnamaldehyde. The results indicated that treatment of cinnamaldehyde enhances the effect of chlorination disinfection, leading to an increase in concentration-dependent inactivation efficiency from 33.5 to 86.1%. Our results also proved that the combined sterilization strategy showed an inhibitory and destructive effect on the hazardous biofilm formed by Pseudomonas aeruginosa. Additionally, the cell membrane permeability and reactive oxygen species (ROS) levels were 1.8-fold and 2.3-fold increased compared with the controls, respectively, which revealed the synergistic effect and hypothetical mechanisms of natural antimicrobial products treatment to chlorination disinfection. This study provides further research for the control of drug-resistant bacteria in water treatment systems and contributes to combating the growing threat of antibiotic resistance.

Fusarium-Derived Secondary Metabolites with Antimicrobial Effects.

Fungal microbes are important in the creation of new drugs, given their unique genetic and metabolic diversity. As one of the most commonly found fungi in nature, Fusarium spp. has been well regarded as a prolific source of secondary metabolites (SMs) with diverse chemical structures and a broad spectrum of biological properties. However, little information is available concerning their derived SMs with antimicrobial effects. By extensive literature search and data analysis, as many as 185 antimicrobial natural products as SMs had been discovered from Fusarium strains by the end of 2022. This review first provides a comprehensive analysis of these substances in terms of various antimicrobial effects, including antibacterial, antifungal, antiviral, and antiparasitic. Future prospects for the efficient discovery of new bioactive SMs from Fusarium strains are also proposed.

Vaginal Sheets with Thymbra capitata Essential Oil for the Treatment of Bacterial Vaginosis: Design, Characterization and In Vitro Evaluation of Efficacy and Safety.

We aimed to incorporate Thymbra capitata essential oil (TCEO), a potent antimicrobial natural product against bacterial vaginosis (BV)-related bacteria, in a suitable drug delivery system. We used vaginal sheets as dosage form to promote immediate relief of the typical abundant vaginal discharge with unpleasant odour. Excipients were selected to promote the healthy vaginal environment reestablishment and bioadhesion of formulations, while the TCEO acts directly on BV pathogens. We characterized vaginal sheets with TCEO in regard to technological characterization, predictable in vivo performance, in vitro efficacy and safety. Vaginal sheet D.O (acid lactic buffer, gelatine, glycerine, chitosan coated with TCEO 1% w/w) presented a higher buffer capacity and ability to absorb vaginal fluid simulant (VFS) among all vaginal sheets with EO, showing one of the most promising bioadhesive profiles, an excellent flexibility and structure that allow it to be easily rolled for application. Vaginal sheet D.O with 0.32 µL/mL TCEO was able to significantly reduce the bacterial load of all in vitro tested Gardnerella species. Although vaginal sheet D.O presented toxicity at some concentrations, this product was developed for a short time period of treatment, so this toxicity can probably be limited or even reversed when the treatment ends.

Antimicrobial Activity of Bioactive Compounds Isolated from Plant Endophytes

Abstract: To combat the escalating antimicrobial resistance, we need to develop new medications constantly. Recently, scientists more focused on endophytes to search for novel antimicrobial natural products. Endophytes are explored as 'bio-factories' of natural bioactive therapeutic compounds. These are microbes exhibited in plant living tissues without causing any harmful effect.Many therapeutic compounds isolated from endophytes are produced from biosynthetic pathways and belong to various structural groups like terpenoids, steroids, quinines, and phenols. Therapeutic compounds produced by endophytes exhibit different pharmacological properties. Fungal genera more commonly reside to plant tissue as compared to bacterial genera. The current review emphasizes comprehensive data on the antibacterial and antifungal activity and bio-therapeutic compounds produced by fungal and bacterial endophytes of some medicinal plants.

Identification and Analysis of Antimicrobial Activities from a Model Moss Ceratodon purpureus.

The emergence of bacterial drug resistance is often viewed as the next great health crisis of our time. While more antimicrobial agents are urgently needed, very few new antibiotics are currently in the production pipeline. Here, we aim to identify and characterize novel antimicrobial natural products from a model dioicous moss, Ceratodon purpureus. We collected secreted moss exudate fractions from two C. purpureus strains, male R40 and female GG1. Exudates from the female C. purpureus strain GG1 did not exhibit inhibitory activity against any tested bacteria. However, exudates from the male moss strain R40 exhibited strong inhibitory properties against several species of Gram-positive bacteria, including Staphylococcus aureus and Enterococcus faecium, though they did not inhibit the growth of Gram-negative bacteria. Antibacterial activity levels in C. purpureus R40 exudates significantly increased over four weeks of moss cultivation in liquid culture. Size fractionation experiments indicated that the secreted bioactive compounds have a relatively low molecular weight of less than 1 kDa. Additionally, the R40 exudate compounds are thermostable and not sensitive to proteinase K treatment. Overall, our results suggest that the bioactive compounds present in C. purpureus R40 exudates can potentially add new options for treating infections caused by antibiotic-resistant Gram-positive bacteria.

Bioassay-Guided Isolation of Antimicrobial Components and LC/QToF Profile of Plumeria obtusa: Potential for the Treatment of Antimicrobial Resistance.

The methanolic fraction (M-F) of the total extract (TE) of Plumeria obtusa L. aerial parts showed promising antibacterial effects against the MDR (multidrug-resistant) gram-negative pathogens Klebsiella pneumoniae and Escherichia coli O157:H7 [Shiga toxin-producing E. coli (STEC)]. In addition, M-F had a synergistic effect (in combination with vancomycin) against the MDR gram-positive strains MRSA (methicillin-resistant Staphylococcus aureus) and Bacillus cereus. After treating the K. pneumoniae- and STEC-infected mice with M-F (25 mg/kg, i.p.), the level of IgM and TNF-α was decreased and the severity of pathological lesions were reduced better than that observed after administration of gentamycin (33 mg/kg, i.p.). Thirty-seven compounds including 10 plumeria-type iridoids and 18 phenolics, 7 quinoline derivatives, 1 amino acid, and 1 fatty acid were identified in TE using LC/ESI-QToF. Furthermore, five compounds; kaempferol 3-O-rutinoside (M1), quercetin 3-O-rutinoside (M2), glochiflavanoside B (M3), plumieride (M4), and 13-O-caffeoylplumieride (M5) were isolated from M-F. M5 was active against K. pneumoniae (MIC of 64 μg/mL) and STEC (MIC of 32 μg/mL). These findings suggested that M-F and M5 are promising antimicrobial natural products for combating MDR K. pneumoniae and STEC nosocomial infections.

Antimicrobial Natural Products Derived from Microorganisms Inhabiting the MENA Region

Objective/Background Natural products (NPs) derived from microorganisms are the basis of a plethora of clinically utilized medications, namely, antimicrobial remedies. Although these secondary metabolites have been extensively explored all over the planet, they remain understudied in the Middle East and North Africa (MENA) region. Methods A literature search was conducted to first find NPs that were isolated from environmental fungi and bacteria that inhabit the soils and seawater of the MENA region. Then, purified molecules with biological activity against pathogenic bacteria, biofilms, fungi, and parasites were described in terms of structure, function, and location. Moreover, the methods that could be used to ameliorate the discovery of novel NPs from this region were investigated. Results A multitude of antimicrobial molecules from various chemical classes were found to be derived from the environmental microbes of MENA. Although many were rediscovered, some represented novel structural scaffolds for novel families of antimicrobial agents. Additionally, the geographical distribution showed a high number of these NPs were unraveled in a restricted area leaving much of MENA untapped. Furthermore, as relatively traditional and low-efficiency methods were typically used in the discovery process, advanced high-throughput techniques were suggested to enhance this practice at the regional level. Conclusion MENA represents a fairly unexploited region where antimicrobial drug discovery could be performed comprehensively through the concomitant exploration of untouched geographical locations and advanced molecular techniques.

ANTAGONISTIC PROPERTIES OF THE BLACK SEA STREPTOMYCETES ISOLATED FROM THE FOULING OF SHELL ROCK AND MUSSELS

SummaryThe rapid emergence of resistance by bacterial and fungal pathogens is a serious problem in the health care system, which causes the search for new promising producers of antimicrobial natural products in various ecological niches. Aim. To determine the antagonistic activity of streptomycetes isolated from the biological fouling of natural shell rock and mussels of the Odesa gulf of the Black Sea. Methods. The antagonistic activity of 19 and 14 strains of streptomycetes isolated from the fouling of shell rock and mussels of the Odesa gulf, respectively, were investigated. Streptomycetes were pre-cultivated on agar media Gause 1, Gause 2 and oat agar with sea salt (2%) at a temperature of 30 °C for 10 days. Antagonistic activity against 12 test cultures was determined by the block method. Results. All isolated marine streptomycetes are antagonists of at least one strain of the indicator microorganism. Antibiotic activity depended on the source of the streptomycetes isolation, culture medium and properties of specific strains of both producers and test cultures. The best activity of streptomycetes strains from shell rock was shown after cultivation on Gause 1 medium, and streptomycetes from mussels – after cultivation on Gause 2 medium. The zones of no growth of sensitive indicators ranged from 12,4±0,3 mm to 20,6±0,2 mm (under the influence of streptomycetes from shell rock) and from 12,4±0,2 mm to 39,7±0,2 mm (under the influence of streptomycetes from mussels). Streptomyces sp. Lim 2.2 (strain from a shell rock) inhibited the growth of 8 test cultures, and strains from mussels Streptomyces sp. Myt 4b and Myt 7ch – 10 test cultures. Indicator strains of gram-positive bacteria were the most sensitive to all streptomycetes, in particular, strain Staphylococcus aureus ATCC 25923 was most inhibited by metabolites of Streptomyces spp. Myt 12a and Myt 12b. Conclusions. Antagonistic activity of streptomycetes isolated from the Black Sea depended on the source of isolation, pre-cultivation medium and properties of both producer strains and indicator microorganisms. The greatest activity of streptomycete strains from shell rock and mussels was shown after preliminary cultivation, respectively, on Gause 1 and Gause 2 media against gram-positive bacteria. The best antibiotic potential was found in strains of Streptomyces sp. Lim 2.2, Lim 4, Lim 5.1 and Lim 7.2, isolated from the fouling of shell rock, and strains of Streptomyces sp. Myt 7b, Myt 7ch, Myt 12a and Myt 12b isolated from mussels.

Antimicrobial Natural Products from Plant Pathogenic Fungi.

Isolates of a variety of fungal plant pathogens (Alternaria radicina ICMP 5619, Cercospora beticola ICMP 15907, Dactylonectria macrodidyma ICMP 16789, D. torresensis ICMP 20542, Ilyonectria europaea ICMP 16794, and I. liriodendra ICMP 16795) were screened for antimicrobial activity against the human pathogenic bacteria Acinetobacter baumannii, Pseudomonas aeruginosa, Escherichia coli, Mycobacterium abscessus, and M. marinum and were found to have some activity. Investigation of the secondary metabolites of these fungal isolates led to the isolation of ten natural products (1-10) of which one was novel, (E)-4,7-dihydroxyoct-2-enoic acid (1). Structure elucidation of all natural products was achieved by a combination of NMR spectroscopy and mass spectrometry. We also investigated the antimicrobial activity of a number of the isolated natural products. While we did not find (E)-4,7-dihydroxyoct-2-enoic acid (1) to have any activity against the bacteria and fungi in our assays, we did find that cercosporin (7) exhibited potent activity against Methicillin resistant Staphylococcus aureus (MRSA), dehydro-curvularin (6) and radicicol (10) exhibited antimycobacterial activity against M. marinum, and brefeldin A (8) and radicicol (10) exhibited antifungal activity against Candida albicans. Investigation of the cytotoxicity and haemolytic activities of these natural products (6-8 and 10) found that only one of the four active compounds, radicicol (10), was non-cytotoxic and non-haemolytic.

A Permissive Amide N-Methyltransferase for Dithiolopyrrolones.

Amide N-methylation is important for the activity and permeability of bioactive compounds but can be challenging to perform selectively. The broad-spectrum antimicrobial natural products thiolutin and holomycin differ only by an N-methyl group at the endocyclic amide of thiolutin, but only thiolutin exhibits antifungal activity. The enzyme responsible for amide N--methylation in thiolutin biosynthesis has remained elusive. Here, we identified and characterized the amide N-methyltransferase DtpM that is encoded >400 kb outside of the thiolutin gene cluster. DtpM catalyzes efficient conversion of holomycin to thiolutin, exhibits broad substrate scope toward dithiolopyrrolones, and has high thermal stability. In addition, sequence similarity network analysis suggests DtpM is more closely related to phenol O-methyltransferases than some amide methyltransferases. This study expands the limited examples of amide N-methyltransferases and may facilitate chemoenzymatic synthesis of diverse dithiolopyrrolone compounds as potential therapeutics.