Antifungal Natural Products Originating from Endophytic and Rhizospheric Microbes Isolated from Coastal Vegetation

Candida infections severely impact patients who are immunocompromised. Currently, there are limited options to treat fungal infections, especially drug-resistant-fungal infections. Therefore, investigating alternative or repurposed antifungals is paramount. Endophytic microbes (EMs) and rhizospheric microbes (RMs) emerge as promising reservoirs of bioactive natural compounds. Interestingly, plants that have adapted to various environmental conditions harbour a plethora of microbes producing a variety of bioactive natural products that can be assessed for potential antifungal activity. To date, EMs and RMs residing in coastal plants and their associated antifungals have not been extensively studied or reviewed. Therefore, this comprehensive review will focus on antifungal natural products, extracted from coastal-vegetation-associated microbiota to draw the attention of research in this field. A comprehensive literature search was conducted by examining both Scopus and Google Scholar databases during the period of 2013-2024 related to the following coastal vegetation: mangroves, sand dune plants, salt marsh plants, and seagrasses. To date, 65 novel antifungal compounds derived from coastal-plant EMs and RMs have been identified. Mangroves were found to be the most prominent host harbouring antifungal-producing EMs and RMs compared with other coastal plants. Coastal-plant-associated fungal partners were the most prominent producers of antifungals compared to their bacterial counterparts. Fifty-four fungal-EM/RM derived antifungals have been reported to demonstrate activities against plant pathogenic fungi as well as human fungal pathogens. Most of the bacterial-derived antifungals (11 antifungals) have previously been reported to have antifungal activity against Candida albicans.

Virulence-Related Genes Expression in Planktonic Mixed Cultures of Candida albicans and Non-Albicans Candida Species

INTRODUCTION

Candida albicans is the most common opportunistic pathogen causing fungal infections worldwide, especially in high-risk patients. Its pathogenicity is related to virulence factors gene expression, such as hyphal growth (HWP1), cell adhesion (ALS3), and protease secretion (SAP1) during infection spreading mechanisms. In recent years, an increase in non-albicans Candida infections has been reported, which may present coinfection or competitive interactions with C. albicans, potentially aggravating the patient's condition. This study aims to evaluate the expression of genes related to virulence factors of C. albicans and non-albicans Candida during planktonic stage.

METHODS

C. albicans (ATCC MYA-3573) as well as with three clinical strains (C. albicans DCA53, C. tropicalis DCT6, and C. parapsilosis DCP1) isolated from blood samples, were grown in 24-well plates at 37°C for 20 h, either in monocultures or mixed cultures. Quantitative real-time polymerase chain reaction was used to evaluate the expression levels of the genes HWP1, ALS3, and SAP1 in cells collected during the planktonic stage. In addition, hyphal filamentation was observed using a Scanning Electron Microscope.

RESULTS

The overexpression of HWP1 and ASL3 genes in mixed growth conditions between C. albicans and non-albicans Candida species suggests a synergistic relationship as well as an increased capacity for hyphal growth and adhesion. In contrast, C. parapsilosis versus C. tropicalis interaction shows an antagonistic relationship during mixed culture, suggesting a decreased virulence profile of C. parapsilosis during initial coinfection with C. tropicalis.

CONCLUSION

The expression of HWP1, ALS3, and SAP1 genes associated with virulence factors varies under competitive conditions among species of the genus Candida during planktonic stage.

Streptomyces: The biofactory of secondary metabolites

Natural products derived from microorganisms serve as a vital resource of valuable pharmaceuticals and therapeutic agents. Streptomyces is the most ubiquitous bacterial genus in the environments with prolific capability to produce diverse and valuable natural products with significant biological activities in medicine, environments, food industries, and agronomy sectors. However, many natural products remain unexplored among Streptomyces. It is exigent to develop novel antibiotics, agrochemicals, anticancer medicines, etc., due to the fast growth in resistance to antibiotics, cancer chemotherapeutics, and pesticides. This review article focused the natural products secreted by Streptomyces and their function and importance in curing diseases and agriculture. Moreover, it discussed genomic-driven drug discovery strategies and also gave a future perspective for drug development from the Streptomyces.

Fungichromin production by Streptomyces sp. WP-1, an endophyte from Pinus dabeshanensis, and its antifungal activity against Fusarium oxysporum

In this study, we isolated an endophytic Streptomyces sp. strain, WP-1, from surface-sterilized barks of Pinus dabeshanensis, an endangered Chinese plant. WP-1 showed strong antifungal activity against diverse pathogenic fungi, such as Fusarium oxysporum, Rhizoctonia solani, Phytophthora infestan, and Candida albicans. Based on phylogenetic analyses, preliminary identification suggested that the WP-1 strain belonged to the genus Streptomyces. Column chromatogram and HPLC were employed to isolate the primary antifungal component from the culture medium of WP-1, and it was identified as the methylpentaene macrolide antibiotic, fungichromin (FC). In this study, for the first time, using in vitro bioassay studies, we revealed that FC strongly inhibited mycelial growth and conidia germination in Fusarium oxysporum. The median inhibitory concentration of FC was found to be 3.80 mg/L. The fermentation conditions of the WP-1 strain were further investigated to improve FC production. We found that supplementation of the synthetic medium with oils (soybean oil, oleic acid, and so on), particularly during the initial stage of fermentation, significantly increased the FC yield. Ammonium-trapping agent (magnesium phosphate) was used as an additive to increase FC yield to 5741.7 mg/L. It was 2.9-fold more as compared to the highest FC yield reported so far where Streptomyces padanus PMS-702 was used for FC production. KEY POINTS: • Isolation and identification of a fungichromin-producing endophytic actinomycete WP-1 strain. • Fungichromin production was significantly improved via oils and ammonium-trapping agents addition. • Discovery of the antifungal activity of fungichromin against Fusarium oxysporum.

Antifungal Agent 4-AN Changes the Genome-Wide Expression Profile, Downregulates Virulence-Associated Genes and Induces Necrosis in Candida albicans Cells

In the light of the increasing occurrence of antifungal resistance, there is an urgent need to search for new therapeutic strategies to overcome this phenomenon. One of the applied approaches is the synthesis of small-molecule compounds showing antifungal properties. Here we present a continuation of the research on the recently discovered anti-Candida albicans agent 4-AN. Using next generation sequencing and transcriptional analysis, we revealed that the treatment of C. albicans with 4-AN can change the expression profile of a large number of genes. The highest upregulation was observed in the case of genes involved in cell stress, while the highest downregulation was shown for genes coding sugar transporters. Real-time PCR analysis revealed 4-AN mediated reduction of the relative expression of genes engaged in fungal virulence (ALS1, ALS3, BCR1, CPH1, ECE1, EFG1, HWP1, HYR1 and SAP1). The determination of the fractional inhibitory concentration index (FICI) showed that the combination of 4-AN with amphotericin B is synergistic. Finally, flow cytometry analysis revealed that the compound induces mainly necrosis in C. albicans cells.

Zerumbone inhibits Candida albicans biofilm formation and hyphal growth

Candida albicans biofilm formation is considered an important matter because it can lead to strong resistance to conventional antifungal agents. Hyphae formed by C. albicans can also act as an important virulence factor related to its biofilm. The objective of this study was to determine the effect of zerumbone, a monocyclic sesquiterpene extracted from Zingiber zerumbet (L.) Smith, against C. albicans biofilm formation. Our results suggest that zerumbone possesses antifungal and antibiofilm activity that inhibits biofilm formation and eradicates preformed biofilm. Notably, zerumbone considerably reduced carbohydrate and DNA contents of biofilm matrix. In addition, zerumbone showed antivirulence effects by decreasing the growth of hyphae and inhibiting morphologic changes of C. albicans. Furthermore, zerumbone significantly downregulated expression levels of biofilm-related and hyphae-specific genes, including HWP1 and ALS3. Since zerumbone suppresses biofilm formation and hyphae growth, these results indicate that zerumbone could be used as a potential candidate to treat and prevent C. albicans biofilm-related infections.

In vitro and in vivo effects of suloctidil on growth and biofilm formation of the opportunistic fungus Candida albicans

As the most frequent fungal pathogen in humans, Candida albicans can develop serious drug resistance because its biofilms are resistant to most antifungal agents; this leads to an urgent need to develop novel antifungals. Here, we evaluated the efficacy of an antithrombotic drug, suloctidil, against C. albicans biofilms in vitro and in vivo. We found that suloctidil is effective to inhibit C. albicans biofilm, with a minimum inhibitory concentration (MIC₈₀) of 4 μg/mL, a biofilm inhibiting concentration (BIC₈₀) of 16 μg/mL and a biofilm eradicating concentration (BEC₈₀) of 64 μg/mL. Furthermore, the concentration-dependent characteristics of suloctidil were shown by its time-kill curves. Scanning electron microscopy images clearly revealed the morphological effects of suloctidil on biofilm. Yeast-to-hyphal form switching is a key virulence factor of C. albicans; therefore, we performed hyphal growth tests and observed that suloctidil inhibited yeast-to-hyphal form switching. This result was consistent with the down-regulation of hypha-specific gene (HWP1, ALS3, and ECE1) expression levels after suloctidil treatment. In vivo, 256 μg/mL of suloctidil significantly reduced fungal counts (P<0.01) compared to that in groups without treatment; the treatment group induced a slight histological reaction, especially when the treatment lasted for 5 days (P<0.01). Taken together, our data suggest that suloctidil is a potential antifungal agent.

The Structure-Activity Relationship of Pterostilbene Against Candida albicans Biofilms

Candida albicans biofilms contribute to invasive infections and dramatic drug resistance, and anti-biofilm agents are urgently needed in the clinic. Pterostilbene (PTE) is a natural plant product with potentials to be developed as an anti-biofilm agent. In this study, we evaluated the structure-activity relationship (SAR) of PTE analogues against C. albicans biofilms. XTT (Sodium 2,3-bis(2-methoxy-4-nitro-5-sulfophenyl)-2H-tetrazolium-5-carboxanilide inner salt) reduction assay was used to evaluate the activity of the analogues against C. albicans biofilms. Knowing that hyphal formation is essential for C. albicans biofilms, anti-hyphal assay was further carried out. By comparing a series of compounds tested in this study, we found that compounds with para-hydroxy (–OH) in partition A exhibited better activity than those with other substituents in the para position, and the double bond in partition B and meta-dimethoxy (–OCH₃) in partition C both contributed to the best activity. Consistent results were obtained by anti-hyphal assay. Collectively, para-hydroxy (–OH), double bond and meta-dimethoxy (–OCH₃) are all needed for the best activity of PTE against C. albicans biofilms.