Antimicrobial activity of natural compounds from sponge – derived fungus Aspergillus flocculosus 01NT.1.1.5

Abstract

The Aspergillus fungi have been an important source of natural products that are useful for exploration in medicine, agriculture and industry. In our continuous investigation to search for new antimicrobial agents from marine-derived fungi, one new phomaligol A2 (1), together with three known compounds, wasabidienone E (2), aspertetranone D (3) and mactanamide (4), were obtained from the EtOAc extract of the culture medium of the marine-derived fungus Aspergillus flocculosus (A. flocculosus) 01NT.1.1.5 isolated from the sponge Stylissa sp. at Nhatrang Bay, Vietnam. Their chemical structures were elucidated by analysis of 1D and 2D NMR and mass spectroscopic data, as well as by comparison of the corresponding data to those previously reported in the literature. Furthermore, the aim of this study was also to evaluate the antimicrobial activity of these compounds against pathogenic microbes including Escherichia coli (E. coli) ATCC 25922, Pseudomonas aeruginosa (P. aeruginosa) ATCC 27853, Staphylococcus aureus (S. aureus) ATCC 25923, Bacillus cereus (B. cereus) ATCC 11778, Streptococcus faecalis (S. faecalis) ATCC 19433, Listeria monocytogenes (L. monocytogenes) ATCC 19111, and Candida albicans (C. albicans) ATCC 10231. Among the compounds, 1-3 were inhibitory on the growth of the yeast C. albicans with minimum inhibitory concentration (MIC) value of 16 μg/mL, which was more potent than amoxicillin and cefotaxime (MIC > 256 μg/mL), antimicrobial drugs as positive references. Moreover, compounds 1-4 were also found to be active against other pathogens including P. aeruginosa and S. faecalis with MIC values of 16 μg/mL and 32 μg/mL, respectively. Compound 4 had no inhibitory activity against L. monocytogenes, whereas compounds 1-3 had ability to against this strain with MICs of 32 to 64 μg/mL. Four of tested compounds exhibited antibacterial activity against B. cereus and E. coli with MIC values of 64-128 μg/mL. This is the first report about these compounds with antimicrobial activity obtained from marine fungus A. flocculosus isolated in Vietnam.

Reisolation and Structure Revision of Asperspiropene A

“…The revised structure of ring B is closely similar to that of sarcosone derivatives, which are common fungal polyketides containing a 3-methoxycyclohexenone ring as a substructure, − and we have isolated several 3-methoxycyclohexenone analogues from the same strain. , Therefore, we compared the chemical shifts of the revised structure of ring B and 3-methoxycyclohexenone derivatives and found that they are well matched to each other (Figure S19). Moreover, the UV spectrum of 1 (Figure S27) displayed a strong absorption peak at 256 nm, which was similar to that of other 3-methoxycyclohexenones, and the IR spectrum (Figure S28) showed an absorption band at 1643 cm –1 corresponding to an α,β-conjugated carbonyl as in the revised structure (unconjugated carbonyl as in the proposed structure absorbs at >1700 cm –1 ). , Additionally, an extensive literature review revealed that the chemical shift of the methyl group in methyl esters of glucuronic acid (similar to the proposed structure) is smaller than 53 ppm (δ C 51–53) (Table S2).…”

“…During the course of our ongoing research for novel marine natural products, we have isolated a number of oxygenated cyclohexenone derivatives from two strains of Aspergillus flocculosus , 01NT-1.1.5 and 168ST-16.1. − Among them, compound 1 was isolated as a pure compound with identical 1D and 2D NMR data to those of asperspiropene A. However, the HRESIMS data of 1 were different from the previously published data for asperspiropene A. Additionally, we found that some 2D NMR data of asperspiropene A (COSY and HMBC) were incorrectly interpreted .…”

Polyketides and Meroterpenes from the Marine-Derived Fungi Aspergillus unguis 158SC-067 and A. flocculosus 01NT-1.1.5 and Their Cytotoxic and Antioxidant Activities

“…01NT-1.1.5 grown on rice medium led to the isolation of fungal metabolites having antimicrobial properties and the suppression of RANKL-induced osteoclastogenesis activities [4,5]. To further study the secondary metabolites from marine-derived fungi, the "one strain many compounds" (OSMAC) strategy was applied by changing the culture medium from rice medium to Bennett's broth medium.…”

“…As part of our ongoing program to investigate marine-derived fungi as an underexplored source of new natural products, we focused our attention on Aspergillus unguis 158SC-067 and A. flocculosus 01NT-1.1.5 strains, which showed good antimicrobial activity in the preliminary screening. Our previous studies on the EtOAc extract of A. flocculosus 01NT-1.1.5 grown on rice medium led to the isolation of fungal metabolites having antimicrobial properties and the suppression of RANKL-induced osteoclastogenesis activities [ 4 , 5 ]. To further study the secondary metabolites from marine-derived fungi, the “one strain many compounds” (OSMAC) strategy was applied by changing the culture medium from rice medium to Bennett’s broth medium.…”

Diversity of prokaryotic communities associated with sponges in Vietnam and their secondary metabolite biosynthetic potential

“…Contrary to studies on sponge taxonomy and sponge-derived natural products, Vietnamese sponge-associated microbiota and their natural products remain largely unexplored and have only been studied in the last few years. These studies focused on the isolation of spongeassociated microbes that produce bioactive compounds (Trinh et al, 2016;Cao et al, 2019;Cao et al, 2020;Quang et al, 2020;Trinh et al, 2020;Chau et al, 2021;Quang et al, 2021a;Kim et al, 2022;Phan et al, 2022;Trinh et al, 2022). Interestingly, several new antimicrobial and cytotoxic compounds have been isolated successfully from Vietnamese sponge-associated microbes (Cao et al, 2019;Cao et al, 2020;Trinh et al, 2020;Quang et al, 2021a;Kim et al, 2022;Trinh et al, 2022), revealing that microbes associated with Vietnamese sponges may be a promising source of new bioactive compounds.…”