Additive-induced pH determines bacterial community composition and metabolome in traditional mustard seed fermented products

Introduction

Kahudi and Kharoli are unique naturally fermented mustard seed products prepared and consumed in the northeastern region of India. The pre-fermentation processing of mustard seeds (soaking, pan-frying, mixing with alkaline or acidic additives, airtight packaging) renders a stringent fermentation environment. The metabolic activities of fermenting bacterial populations yield a myriad of glucosinolate-derived bioactive components which have not been described earlier.

Methods

This present study employed integrated 16S rRNA amplicon sequencing and LC-MS-based metabolomics to elucidate the bacterial diversity and metabolome of the two fermented mustard seed food products.

Results and Discussion

Univariate and multivariate analyses of metabolomics data revealed differential abundances of a few therapeutically-important metabolites viz., sinapine, indole-3-carbinol, γ-linolenic acid in Kahudi, and metabolites viz., β-sitosterol acetate, 3-butylene glucosinolate, erucic acid in Kharoli. A metagenomic investigation involving the 16S rRNA (V3–V4) amplicon sequencing showed the dominance of Firmicutes (99.1 ± 0.18%) in Kahudi, and Firmicutes (79.6 ± 1.92%) and Proteobacteria (20.37 ± 1.94%) in Kharoli. The most abundant genera were Bacillus (88.7 ± 1.67% in Kahudi; 12.5 ± 1.75% in Kharoli) followed by Lysinibacillus (67.1 ± 2.37% in Kharoli; 10.4 ± 1.74% in Kahudi). Members of both these genera are well known for proteolytic and endospore-forming abilities which could have helped in colonizing and thriving in the stringent fermentation environments.

Cinnabarinic acid from Trametes coccinea fruiting bodies exhibits antibacterial activity through inhibiting the biofilm formation

Wild mushrooms are rich sources of natural compounds with potent bioactive properties. Several important metabolites have been reported from mushrooms, which possess clinically important bioactive properties like antibacterial, anticancer, antidiabetic, and neuroprotective activity. In this study, we have evaluated the antimicrobial activity of Trametes coccinea fruiting body extracts against different bacterial isolates, viz., Bacillus subtilis, Bacillus cereus, and Escherichia coli. Fruiting bodies of three T. coccinea samples, of which two were collected from Santipur, Arunachal Pradesh and one collected from Jorhat, Assam, were used for extraction using methanol. The extracts showed significant antimicrobial activity against all the test bacteria. Minimum Inhibitory Concentration (MIC) of the extracts against Bacillus subtilis, Bacillus cereus, and Escherichia coli was recorded as 400 µg/ml, 400 µg/ml, and 300 µg/ml, respectively. Furthermore, the bioactive compounds of the extract were separated and detected using Thin Layer Chromatography (TLC). Presence of cinnabarinic acid (CBA)-a potent antimicrobial compound- was detected in TLC, which was further confirmed through High Performance Liquid Chromatography (HPLC) and Electrospray Ionization-Mass Spectrometry (ESI-MS). Cinnabarinic acid was able to inhibit the formation of biofilms in Bacillus subtilis and B. cereus, suggesting that the compound can be beneficial in the management of biofilm-based antimicrobial resistance.

Fungal interactions induce changes in hyphal morphology and enzyme production

In nature, species interacts/competes with one other within their surrounding for food and space and the type of interactions are unique to each species. The interacting partners secrete different metabolites, which may have high importance in human welfare. Fungal-fungal interactions are complex mechanisms that need better understanding. Here, 14 fungal isolates were facilitated in 105 possible combinations to interact on potato dextrose agar. Morphologically, no changes were observed when the same fungal isolates were allowed to interact within them. However, 10 interactions between different fungal isolates showed mutual replacement with each fungus; capturing territory from the other. Contrastingly, 35 interactions resulted into complete replacement as one of the fungi was inhibited by rapid growth of the other fungus. In 46 interactions, formation of barrage was observed leading to deadlock type of interaction wherein both fungi have restricted growth. To study in details about the barrage formation, two fungal interactions were taken (i) T. coccinea vs. L. lactinea and (ii) T. coccinea vs. T. versicolor. Microscopic changes in the hyphal growth during interaction were observed. There was significant increase in the enzymatic activities including cellulase, xylanase and chitinase during in-vitro fungal-fungal interaction, suggesting the importance of such interactions for commercial enzyme production.

Melanin production and laccase mediated oxidative stress alleviation during fungal-fungal interaction among basidiomycete fungi

Fungal-fungal interaction often leads to the change in metabolite profile of both the interacting fungus which may have potential implication in industry or agriculture. In the present study, we performed two sets of fungal-fungal interaction-Trametes coccinea (F3) with Leiotrametes lactinea (F9) and T. coccinea (F3) with T. versicolor (F1) to understand the changes in the metabolite profile during the interaction process and how this process impacts the hyphal/mycelial morphology of the participating fungi. The metabolites produced during interaction of T. coccinea (F3) with L. lactinea (F9) and T. coccinea (F3) with T. versicolor (F1) was analysed through liquid chromatography coupled to mass spectroscopy (LC-MS). Most of the metabolites secreted or produced during interaction are associated with defensive response. Further, visualization with scanning electron microscopy revealed that interaction between the tested fungi led to the changes in the hyphal morphology. The bipartite fungal interaction resulted in the production of a dark brown colour pigment-melanin as confirmed by the LC-MS, FTIR and NMR analysis. Moreover, the fungal-fungal interaction also led to increase in the production of laccase, a group of multicopper oxidases involved in detoxification of toxic compounds. Further, increased activity of superoxide dismutase, an enzyme that catalyzes the dismutation of the superoxide anion to hydrogen peroxide was also recorded during fungal-fungal interaction. Quantitative real-time PCR revealed upregulation of lcc1 (encoding a laccase enzyme) and few other stress related genes of T. versicolor during its hyphal interaction with T. coccinea, suggesting a direct correlation between laccase production and melanin production.