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Abstract

Objectives

This study focused on the isolation, structural elucidation, and antimicrobial activities of compounds from the culture broth of the strain Penicillium sp. OM01 (isolated from the marine mollusk sample Spondylus squamosus (Schreibers, 1793)).

Methods

Various chromatographic methods were used to isolate compounds from the ethyl acetate and MeOH extracts of the strain Penicillium sp. OM01. Their structures were elucidated using HR-ESI-MS and NMR data. The compounds were evaluated for antimicrobial effects using the dilution turbidimetric broth method.

Results

One new alkaloid, penicritin (1), together with seven known compounds, including phenol A acid (2), topsentisteron D₃ (3), ergostanol (4), (22E,24R)-24-methylcholesta-5,22-diene-3β, 7β-diol (5), 5α,6α-epoxy-(22E,24R)-ergosta-8,22-diene-3β,7α-diol (6), staphylopeptide A (7), and 4-hydroxybenzoic acid (8) were isolated from the culture broth of the marine-derived fungus Penicillium sp. OM01. Compounds 2-4 and 6 exhibited antibacterial activity against Gram-(+) bacteria, Enterococcus faecalis, Staphylococcus aureus, Bacillus cereus, and yeast Candida albicans with MIC values ranging from 32 to 256 µg/mL.

Conclusions

This is the first report of the strain Penicillium sp. OM01 from the marine mollusk sample S. squamosus (Schreibers, 1793). One new and seven known compounds were isolated from the strain Penicillium sp. OM01 (isolated from the marine mollusk sample S. squamosus (Schreibers, 1793). Compound 4 is a potential antibacterial agent.

Keywords

penicritin citrinin skeleton cyclopeptide antibacterial antifungal

Introduction

In recent years, marine-derived fungi have attracted considerable attention as a new drug discovery and development source due to their ability to produce bioactive and unique structures.^1,2 Many new natural products have been characterized from marine fungi and possess pharmaceutically relevant bioactivities. Among them, Penicillium genus is a large fungal genus comprising more than 354 described species,³ many of which produce a diversified array of active secondary metabolites. So far, approximately 600 compounds have been isolated from the marine-derived Penicillium fungi, mainly including polyketides, alkaloids, terpenoids, sterols, and macrolides.⁴ These compounds displayed antimicrobial, anti-inflammatory, cytotoxic, and immunosuppressant activities with potential applications in new drug development.^5,6

As part of an ongoing project to investigate natural products from marine-derived fungi, the ethyl acetate extract of Penicillium sp. OM01 fermentation from the marine mollusk Spondylus squamosus (Schreibers, 1793) exhibited antimicrobial activity against all tested Gram (+) bacteria: (Enterococcus faecalis ATCC29212, Staphylococcus aureus ATCC25923, and Bacillus cereus ATCC 14579), and one yeast strain, Candida albicans ATCC10231 with MIC values of 16, 32, 32, 16 µg/mL, respectively, while the methanol extract of Penicillium sp. OM01 exhibited antimicrobial activity against E. faecalis ATCC29212 with MIC value of 256 µg/mL. Herein, we report the isolation, structural elucidation, and biological evaluation of one new alkaloid and seven known compounds (Figure 1) from the fermentation of the strain Penicillium sp. OM01.

Figure 1.

Chemical structures of compounds 1–8.

Results and Discussion

Compound 1 was obtained as a white solid and was optically active ( $[α]_{D}^{25}$ -61.0 (c 0.08, MeOH)). The HR-ESI MS of 1 showed a pseudomolecular ion peak [M + H]⁺ at m/z 497.2666, corresponding with the molecular formula of C₂₈H₃₆N₂O₆ (Calcd. for [C₂₈H₃₇N₂O₆]⁺ 497.2652), together with the ¹³C-NMR data. The IR spectrum of 1 suggested the presence of carbonyl groups (1700 and 1638 cm⁻¹) and hydroxyl groups (3403 cm⁻¹). Analyses of the ¹³C-NMR, DEPT, and HSQC spectra of 1 showed the presence of twenty-eight carbons, including five methyl groups at δ_C 9.5, 10.4, 17.2, 17.6, and 20.1, six methylene groups at δ_C 19.7, 27.1, 28.5, 31.9, 34.0, and 46.2, six methine groups at δ_C 34.7, 45.6, 69.9, 71.7, 124.5, and 131.1, and eleven quaternary carbons at δ_C 101.8, 108.5, 110.0, 115.2, 140.4, 156.3, 158.8, 161.3, 162.2, 162.7, and 175.6 (Table 1). In addition, the ¹H-NMR spectrum of 1 exhibited signals of two olefinic protons at δ_H 5.40 (2H, m), four sp³ methines at δ_H 5.14 (1H, d, J = 2.4 Hz), 4.54 (1H, m), 3.82 (1H, m), and 2.52 (1H, m), five methyl groups at δ_H 0.96 (3H, d, J = 7.2 Hz), 1.26 (3H, d, J = 7.2 Hz), and 1.59 (3H, d, J = 7.2 Hz), 1.95 (3H, s), and 1.96 (3H, s). The chemical shifts of methylene group (δ_H 3.80 and 3.86, δ_C 46.2) suggested its linkage to a nitrogen atom. Detailed analyses of ¹H- and ¹³C-NMR data and COSY and HMBC experiments of 1 revealed three partial structures, designated units I (citrinin skeleton),⁷ II, and III (long chain)⁸ (Figure 2). Unit II has not been reported yet. The unit I: The HMBC correlations from H-9 (δ_H 0.96) to C-3 (δ_C 71.7)/C-4 (δ_C 34.7), from H-10 (δ_H 1.26) to C-3 (δ_C 71.7)/C-4 (δ_C 34.7)/C-4a (δ_C 140.4), from H-11 (δ_H 1.95) to C-4a (δ_C 140.4)/C-5 (δ_C 110.0)/C-6 (δ_C 158.8), from 6-OH (δ_H 14.71) to C-5 (δ_C 110.0)/C-6 (δ_C 158.8)/C-7 (δ_C 101.8)/C-12 (δ_C 175.6), from 8-OH (δ_H 15.50) to C-7 (δ_C 101.8)/C-8 (δ_C 156.3)/C-8a (δ_C 108.5)/C-12 (δ_C 175.6) and from H-1 (δ_H 5.14) to C-4a (δ_C 140.4) /C-8 (δ_C 156.3)/C-8a (δ_C 108.5) were observed (Figure 2). These suggested positions of methyl groups at C-3/C-4/C-5, hydroxy groups at C-6/C-8, and the carboxylic group at C-7. Next, the unit II: The COSY correlations of H-6′ (δ_H 3.80 and 3.86)/H-7′ (δ_H 1.86 and 1.90)/H-8′ (δ_H 4.54) and the HMBC correlations from H-6′ (δ_H 3.80 and 3.86)/H-7′ (δ_H 1.86 and 1.90) to C-9′ (δ_C 162.2) confirmed the presence of tetrahydropyrol ring. The HMBC correlations from H-10′ (δ_H 1.96) to C-2′ (δ_C 162.7)/C-3′ (δ_C 115.2)/C-4′ (δ_C 161.3) suggested the presence of pyrimidine ring. These rings were connected at [1,2-a]. Units I and II were connected at C-1 and C-8′ confirmed by HMBC correlations from H-1 (δ_H 5.14) to C-7′ (δ_C 19.7)/C-8′ (δ_C 45.6)/C-9′ (δ_C 162.2). The remaining signals belong to unit III. The double bond at C-15/C-16 of long chain was confirmed by HMBC correlations from H-17′ (δ_H 1.59) to C-15′ (δ_C 131.1)/C-16′ (δ_H 124.5) and COSY correlation of H-11′ (δ_H 2.54 and 2.56)/H-12′ (δ_H 1.57 and 1.62)/H-13′ (δ_H 1.38)/H-14′ (δ_H 1.59). The E configuration of the double bond at C-15′ and C-16′ was proven by the chemical shifts of C-17′ (δ_C 17.6) and C-14′ (δ_C 31.9), comparing to the calculated values, E double bond [δ_C 33.0 (C-14′) and 17.0 (C-17′)] and Z double bond [δ_C 27.0 (C-14′) and 11.0 (C-17′)].^8–10 The relative conﬁguration of 1 was further determined from the NOESY experiment. The NOE correlation from H-1 to H₃-9 and the absence of correlation between H-9 and H-10 indicated that the configuration at C-3 and C-4 was trans. The absolute configurations of 1 were also proved by the comparison of their experimental ECD spectra with those calculated spectra. The TDDFT calculated ECD spectra of two pair enantiomers (1R,3R,4S,8′R)-1a and (1S,3S,4R,8′S)-1c; (1R,3R,4S,8′S)-1b and (1S,3S,4R,8′R)-1d were shown in Figure 3 (See Supplemental Material).¹¹ The experimental ECD spectrum of 1 was well matched with TDDFT calculated ECD spectrum of (1R,3R,4S,8′S)-1b. Thus, the absolute configurations of 1 were assigned as (1R,3R,4S,8′S). Based on the above evidence, the chemical structure of 1 was determined and named as penicritin.

Figure 2.

Key HMBC and COSY correlations of 1 and its partial structures.

Figure 3.

Experimental CD spectrum of compound 1 and calculated ECD spectra of structures (1a-1d) (calculated spectra are shifted by +4 nm).

Table 1.

¹H- and ¹³C-NMR Data for Compound 1 in DMSO-d₆.

Pos.	δ _C	Group	δ_H mult. (J in Hz)	HMBC (H→C)
1	69.9	CH	5.14 d (2.4)	C-8a, C-4a, C-8, C-9′, C-7′, C-8′
3	71.7	CH	3.82 m	C-4a, C-1, C-4, C-10
4	34.7	CH	2.52 m	C-4a, C-8a, C-5, C-10
4a	140.4	C	-
5	110.0	C	-
6	158.8	C	-
7	101.8	C	-
8	156.3	C	-
8a	108.5	C	-
9	17.2	CH₃	0.96 d (7.2)	C-3, C-4
10	20.1	CH₃	1.26 d (7.2)	C-4a, C-4, C-3
11	10.4	CH₃	1.95 s	C-5, C-6, C-4a
12	175.6	C = O	-
2′	162.7	C	-
3′	115.2	C	-
4′	161.3	C = O	-
6′	46.2	CH₂	3.80 m	C-9′, C-8′, C-7′
6′	46.2	CH₂	3.86 m	C-9′, C-8′, C-7′
7′	19.7	CH₂	1.86 m
7′	19.7	CH₂	1.90 m
8′	45.6	CH	4.54 m
9′	162.2	C	-
10′	9.5	CH₃	1.96 s	C-2′, C-3′, C-4′
11′	34.0	CH₂	2.54 m	C-2′, C-12′
11′	34.0	CH₂	2.56 m	C-2′, C-12′
12′	27.1	CH₂	1.57 m
12′	27.1	CH₂	1.62 m
13′	28.5	CH₂	1.38 m
14′	31.9	CH₂	1.98 m	C-15′, C-16′
15′	131.1	CH	5.40 m	C-14′
16′	124.5	CH	5.40 m	C-17′
17′	17.6	CH₃	1.59 d (7.2)	C-15′, C-16′
6-OH			14.71 s	C-7, C-5, C-6, C-12
8-OH			15.50 s	C-7, C-8a, C-8, C-12

The known compounds were elucidated as phenol A acid (2),¹² topsentisteron D₃ (3),¹³ ergostanol (4),¹⁴ (22E,24R)-24-methylcholesta-5,22-diene-3β,7β-diol (5),¹⁵ 5α,6α-epoxy-(22E,24R)-ergosta-8,22-diene-3β,7α-diol (6),¹⁶ staphylopeptide A (7),¹⁷ and 4-hydroxybenzoic acid (8) (Figure 1).¹⁸ Their chemical structures were determined by their spectroscopic data and comparing with reported NMR data in the literature.

All isolated compounds were evaluated for their antibacterial activity against Escherichia coli, Pseudomonas aeruginosa, Salmonella enterica, E. faecalis, S. aureus, and B. cereus, and anti-yeast activity against C. albicans (Table 2). Streptomycin and cyclohexamide were used as positive controls. In previous studies, compound 2 was inactive in B. cereus and C. albicans.¹⁹ Compounds 4 and 7 did not show antimicrobial activity.^17,20 Compound 8 showed weak antimicrobial activity.¹⁷ Compounds 3 and 5 have not been tested antimicrobial yet. As our results, compounds 2-4 and 6 exhibited the antibacterial properties against three Gram-(+) bacteria E. faecalis, Staphylococcus aureus, B. cereus, and yeast C. albicans with MIC values in the range of 32-256 µg/mL. Both compounds 1 and 4 showed the highest activity against B. cereus with the MIC value of 32 µg/mL. Remaining compounds 5, 7, and 8 did not inhibit all tested microorganisms. The results suggested compound 4 is a potential antibacterial agent. Additional testing against a broader range of clinically relevant bacteria and yeast would provide a more comprehensive understanding of the compounds’ antimicrobial potential. Further studies should be focused on the mechanisms of action and application of 4.

Table 2.

Antimicrobial Activities of Compounds 1–8 and the Extract.

	MIC (µg/mL)
Compound	Gram (+) bacteria			Gram (−) bacteria			Yeast
Compound	E. faecalis	S. aureus	B. cereus	E. coli	P. aeruginosa	S. enterica	C. albicans
1	-	64	32	-	-	-	256
2	64	-	-	-	-	-	128
3	64	256	-	-	-	-	128
4	64	64	32	64	-	256	128
5	-	-	-	-	-	-	-
6	64	-	256	-	-	-	128
7	-	-	-	-	-	-	-
8	-	-	-	-	-	-	-
EtOAc extract	16	32	32	-	-	-	16
MeOH	256	-	-	-	-	-	-
Streptomycin	256	256	128	32	256	128	-
Cyclohexamide	-	-	-	-	-	-	32

(-) MIC >256 µg/mL. All experiments were done in three times.

Material and Methods

General

See Supplemental Material.

Isolation and Taxonomic Identification

The fungus strain OM01 was isolated from the marine mollusk sample S. squamosus (Schreibers, 1793) collected at a depth of 6 m, from the sea area of Quangninh, Vietnam, in May 2021. A voucher specimen was deposited at the Institute of Marine Environment and Resources, VAST.

The taxonomy of the strain OM01 was identified by using 18S rRNA gene sequence analysis and compared with fungal 18S rRNA sequences in GenBank database by NCBI Blast program. The results showed that strain OM01 belonged to the genus Penicillium. Its 18S rRNA gene sequence was registered on GenBank database with the accession number of OR758795.

Fermentation

The strain OM01was activated and inoculated into 1 L of PDB broth medium pH 7, comprising potato extract (30 g/L), dextrose (20 g/L), soluble starch (5 g/L), and instant ocean (30 g/L). After 7 days of incubation at 28 °C, while shaking at 100 rpm, the culture broth was used to spread on the agar surface of 50 flasks containing 1L of PDA solid medium, including potato extract (30 g), dextrose (20 g), soluble starch (5 g), instant ocean (30 g), and agar (15 g). The fermentation was incubated in an incubator at 28°C and harvested on the twenty-five day.

Extraction and Isolation of Compounds

The agar culture (50 L) of the strain Penicillium sp. OM01 was minced and extracted with ethyl acetate (EtOAc) (10 L × 5 times) and methanol (10 L × 5 times), using a sonicator to afford corresponding extracts. These extracts were concentrated in vacuo to give the EtOAc extract (58 g) and the MeOH extract (240 g). The EtOAc extract (58 g) was subjected to column chromatography (CC) on silica gel eluting with CH₂Cl₂/MeOH (from 100/1, 50/1, 25/1, 10/1, 5/1, v/v) to yield 7 fractions, FE1-FE7. The fraction FE3 (0.4 g) was chromatographed on silica gel CC, eluted with n-hexane/EtOAc (from 0% to 100% EtOAc in n-hexane) to give 10 fractions, FE3.1-FE3.10. The fraction FE3.8 (65 mg) was further purified by CC on a Sephadex LH-20 CC eluting with MeOH to give compounds 3 (3.8 mg) and 4 (16 mg). The fraction FE4 (3.33 g) was separated on a silica gel CC eluting with CH₂Cl₂/MeOH (from 0% to 100% MeOH in CH₂Cl₂) to give compounds 5 (1.9 mg) and 6 (2.0 mg). The fraction FE6 (3.5 g) was chromatographed on a silica gel CC eluting with CH₂Cl₂/MeOH (from 0% to 100% MeOH in CH₂Cl₂) to give eight fractions, FE6.1-E6.8. The fraction FE6.5 (0.37 g) was separated on a Sephadex LH-20 CC, eluting with MeOH to yield compound 1 (2.0 mg). The fraction FE6.7 (0.72 g) was chromatographed on a Sephadex LH-20 CC eluting with MeOH followed by recrystallization in a mixture of MeOH/CH₂Cl₂ (4/6, v/v) to yield compound 2 (10 mg).

The MeOH extract (90 g) was subjected to a silica gel CC eluting with CH₂Cl₂/MeOH (100/1, 50/1, 20/1, 10/1) to yield 4 fractions, FM1-FM4. The fraction FM1 (2.5 g) was chromatographed on a silica gel CC eluting with CH₂Cl₂/acetone (2/1) to give 4 fractions, FM2.1-FM2.4. The fraction FM2.3 (0.41 g) was separated on a Sephadex LH-20 CC, eluting with MeOH to yield compound 8 (3.0 mg). The fraction FM3 (0.98 g) was chromatographed on a Sephadex LH-20 CC, eluting with MeOH to yield 7 (11.3 mg).

Penicritin (1)

White solid; R_f: 0.5 (CH₂Cl₂/MeOH 6/1, v/v); $[α]_{D}^{25}$ : −61.0 (c 0.08, MeOH); HR-ESI-MS m/z at 497.2666 [M + H]⁺ (Calcd. for C₂₈H₃₇N₂O₆, 497.2652); IR (KBr) ν_max 3403, 2927, 1638, 1587, 1544, 1260, 1124, 968, 608 cm⁻¹; CD: λ_max (Δε) 226 (+1.0), 245 (−0.46), 320 (+0.18) nm; ¹H- and ¹³C-NMR data (DMSO-d₆) (see Table 1).

Antimicrobial Assays

Antimicrobial assays were carried out using E. coli (ATCC25922), P. aeruginosa (ATCC27853), S. enterica (ATCC13076), E. faecalis (ATCC299212), S. aureus (ATCC25923), B. cereus (ATCC14579), and C. albicans (ATCC10231). Stock solutions of samples were prepared in DMSO, and the antimicrobial assays were carried out in 96-well microtiter plates against the microbial strains (5 × 10⁵ CFU/mL) using a modification of the published method.^21,22 After incubation for 24 h at 37°C, the absorbance at 650 nm was measured using a microplate reader. The minimal inhibitory concentration (MIC) is defined as the lowest concentration of an antimicrobial that inhibits visible growth of a microbial culture. This is accomplished by spreading cultures out on an agar plate. Streptomycin and cycloheximide were used as positive controls.

Statistical Analysis

All the experiments were conducted in triplicate and statistical analysis of the data was performed by ANOVA using STARGRAPHICS plus 5.1. Differences of P values < .05 were considered statistically significant.

Conclusions

One new alkaloid, penicritin (1), together with seven known compounds: phenol A acid (2), topsentisteron D₃ (3), ergostanol (4), (22E,24R)-24-methylcholesta-5,22-diene-3β,7β-diol (5), 5α,6α-epoxy-(22E,24R)-ergosta-8,22-diene-3β,7α-diol (6), staphylopeptide A (7), and 4-hydroxybenzoic acid (8) were isolated from the culture broth of the marine-derived fungus Penicillium sp. OM01. All compounds were evaluated for their antibacterial activity against E. coli, P. aeruginosa, S. enterica, E. faecalis, S. aureus, and B. cereus, and anti-yeast activity against C. albicans. Compounds 2-4 and 6 exhibited antibacterial activity against Gram-(+) bacteria, E. faecalis with MIC value of 64 µg/mL. Compounds 1, 3, and 4 inhibited B. cereus MIC values of 64, 256, and 64 µg/mL, respectively. Only compounds 4 antibacterial activity against Gram-(−) bacteria, E. coli and S. enterica with a MIC value of 64 and 256 µg/mL, respectively. The results suggested compound 4 is a potential antibacterial agent. Further studies should be focused on the mechanisms action, and application of compound 4.

Supplemental Material

sj-docx-1-npx-10.1177_1934578X241250204 - Supplemental material for Antimicrobial Activities of a New Alkaloid from Marine-Derived Fungus, Penicillium sp. OM01

Supplemental material, sj-docx-1-npx-10.1177_1934578X241250204 for Antimicrobial Activities of a New Alkaloid from Marine-Derived Fungus, Penicillium sp. OM01 by Van Cuong Pham, Thi Hue Nguyen, Thuy Linh Nguyen, Mai Anh Nguyen, Thi Quyen Vu, Thi Thu Huyen Vu, Thi Hong Minh Le and Thi Mai Huong Doan in Natural Product Communications

Footnotes

Acknowledgments

The authors would like to thank Mr Dang Vu Luong, Institute of Chemistry, VAST for recording NMR and Korea Basic Science Institute (KBSI, Chuncheon Center) for recording HR-ESI-MS spectra.

Declaration of Conflicting Interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This research is funded by Vietnam Academy of Science and Technology (Grant No: QTRU02.10/21-22 and NVCC38.03/22-23).

Ethical Approval

Ethical approval is not applicable to this article.

Statement of Human and Animal Rights

This article does not contain any studies with human or animal subjects.

Informed Consent

There are no human subjects in this article and informed consent is not applicable.

ORCID iDs

Van Cuong Pham

Thi Mai Huong Doan

Supplemental Material

Supplemental material for this article is available online.

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Supplementary Material

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Antimicrobial Activities of a New Alkaloid from Marine-Derived Fungus,Penicillium sp. OM01

Abstract

Objectives

Methods

Results

Conclusions

Keywords

Introduction

Results and Discussion

Material and Methods

General

Isolation and Taxonomic Identification

Fermentation

Extraction and Isolation of Compounds

Penicritin (1)

Antimicrobial Assays

Statistical Analysis

Conclusions

Supplemental Material

sj-docx-1-npx-10.1177_1934578X241250204 - Supplemental material for Antimicrobial Activities of a New Alkaloid from Marine-Derived Fungus, Penicillium sp. OM01

Footnotes

Acknowledgments

Declaration of Conflicting Interests

Funding

Ethical Approval

Statement of Human and Animal Rights

Informed Consent

ORCID iDs

Supplemental Material

References

Supplementary Material