Actinobacteria are often known to be great producers of antibiotics. The rapid increase in the global burden of antibiotic-resistance with the concurrent decline in the discovery of new antimicrobial molecules necessitates the search for novel and effective antimicrobial metabolites from unexplored ecological niches. The present study investigated the antimicrobial producing actinobacterial strains isolated from the soils of two microbiologically unexplored forest ecosystems, viz. Nameri National Park (NNP) and Panidehing Wildlife Sanctuary (PWS), located in the Eastern Himalayan Biodiversity hotspot region. A total of 172 putative isolates of actinobacteria were isolated, of which 24 isolates showed strong antimicrobial bioactivity. Evaluation of the ethyl acetate extracts of culture supernatants against test microbial strains revealed that isolates PWS22, PWS41, PWS12, PWS52, PWS11, NNPR15, NNPR38, and NNPR69 were the potent producers of antimicrobial metabolites. The antimicrobial isolates dominantly belonged to Streptomyces, followed by Nocardia and Streptosporangium. Some of these isolates could be putative novel taxa. Analysis of the antimicrobial biosynthetic genes (type II polyketide synthase and nonribosomal peptide synthetase genes) showed that the antimicrobial metabolites were associated with pigment production and belonged to known families of bioactive secondary metabolites. Characterization of the antimicrobial metabolites of Streptomyces sp. PWS52, which showed lowest taxonomic identity among the studied potent antimicrobial metabolite producers, and their interaction with the test strains using GC-MS, UHPLC-MS, and scanning electron microscopy revealed that the potential bioactivity of PWS52 was due to the production of active antifungal and antibacterial metabolites like 2,5-bis(1,1-dimethylethyl) phenol, benzeneacetic acid and nalidixic acid. Our findings suggest that the unexplored soil habitats of NNP and PWS forest ecosystems of Northeast India harbor previously undescribed actinobacteria with the capability to produce diverse antimicrobial metabolites that may be explored to overcome the rapidly rising global concern about antibiotic-resistance.

Aim Tyrosol, a quorum sensing molecule in yeasts, was reported to reduce lag phase and induces hyphae formation during cell proliferation. However, evidence of any enhancing effect of tyrosol in cellular proliferation within fermentative environment is unclear. In this investigation, selected yeast cells were assessed for their ability to synthesize tyrosol followed by examining the role of the molecule during fermentation. Methods and results Tyrosols were characterized in four fermentative yeasts viz., Saccharomyces cerevisiae, Wickerhamomyces anomalous, Candida glabrata and Candida tropicalis isolated from traditional fermentative cakes of northeast India. All the isolates synthesized tyrosol while C. tropicalis exhibited filamentous growth in response to tyrosols retrieved from other isolates. Purified tyrosols showed protective behaviour in C. tropicalis and S. cerevisiae under ethanol mediated oxidative stress. During fermentation, tyrosol significantly enhanced growth of W. anomalus in starch medium while C. tropicalis exhibited growth enhancement in starch and glucose sources. The chief fermentative yeast S. cerevisiae showed notable enhancement in fermentative capacity in starch medium under the influence of tyrosol con‐commitment of ethanol production. Conclusion The study concludes that tyrosol exerts unusual effect in cellular growth and fermentative ability of both Saccharomyces and non‐Saccharomyces yeasts. Significance and impact of the study This is the first report of expression of tyrosol by non‐conventional yeasts, where the molecule was found to exert enhancing effect during fermentation thereby augmenting the process of metabolite production during traditional fermentation.

In this study, yeasts inherent in traditional starter materials of four indigenous communities from northeast India were characterized. These included Saccharomyces and non-Saccharomyces types representing several genera and species such as Saccharomyces cerevisiae, Wickerhamomyces anomalus, Candida glabrata and Candida tropicalis. The yeasts were cultured in synthetic minimal media (2% glucose and 2% starch), both individually and in combinations. The propensity of growth of individual isolates in glucose appeared as W. anomalus > C. tropicalis > C. glabrata > S. cerevisiae. In starch, the propensity appeared as C. tropicalis > W. anomalus > C. glabrata > S. cerevisiae. The findings were incongruent when isolates were co-cultured in dual combinations in glucose and starch. In glucose, W. anomalus could not prosper with C. tropicalis, which otherwise was reversed in starch. C. tropicalis dominated all the co-cultures in starch followed by W. anomalus, S. cerevisiae and C. glabrata. In glucose, the order appeared as W. anomalus > C. tropicalis > S. cerevisiae > C. glabrata. S. cerevisiae could not thrive in competition with C. tropicalis and appeared dominant over C. glabrata in both glucose and starch. W. anomalus was dominant over S. cerevisiae and C. glabrata, while C. tropicalis outnumbered C. glabrata in both carbon sources. The fermentation efficiency was highest when S. cerevisiae and C. glabrata were co-cultured together, in both starch and glucose. This study suggests an advantage in co-culturing selective indigenous yeasts as consortia to yield a productive fermentation output that could have commercial benefit but additional strains of each species still needed to be investigated.

In this review, the relevance of diversity of yeasts and their interactive association in household ethnic fermentation are discussed. The longstanding traditional household fermentation practice involves preparation of fermented product such as alcoholic beverages from various indigenous agricultural products with the help of microorganisms cultivated from local environment and perpetuated for hundreds of years through generations indoctrinating an indigenous knowledge system. Northeast India is known for its rich physiographic and geo-demographic diversity and is home to several ethnicities who follow unique practices of household traditional fermentation. The diversity of yeasts present within the microbial inoculum used for fermentation by different indigenous communities has been keenly studied and reported to be unique in spite of their common source for starter substrates.Saccharomycesyeasts are primarily involved in alcoholic fermentation, whereas non-Saccharomycesyeasts, which are reportedly confined to a particular geographical region, have been reported to contribute toward the final outcome of fermentation produce. During fermentation, interaction among these large microbial communities and their resulting physiological expression within the fermentation micro-environment is believed to affect the final quality of the product. Mechanism of quorum sensing plays an important role in these interactions in order to maintain proportionality of different yeast populations wherein the quorum sensing molecules not only regulate population density but also effectively aid in enhancement of alcoholic fermentation. Additionally, various secondary metabolites, which are secreted as a result of inter-species interactions, have been found to affect the quality of beverages produced. This review concludes that diverse species of yeasts and their interaction within the fermentation micro-environment influence the sustainability and productivity of household ethnic fermentation.

Heavy metals act as co-factors for several microbial enzymes and are required in low concentrations for proper biological functioning of yeasts. Since concentrations beyond permitted threshold can damage cell's functionality and viability, metal tolerance in yeasts towards such heavy metals is therefore desirable during fermentation. Tyrosol, a quorum sensing molecule in yeasts, protects yeasts from oxidative stress induced by various factors, but the performance of the molecule under heavy metal induced stress is not known. In this investigation, metal tolerance of four species of endemic yeasts from northeast India, viz Wickerhamomyces anomalus, Candida tropicalis, Saccharomyces cerevisiae and Candida glabrata, isolated from traditional starter culture cakes were tested towards zinc (Zn+2), manganese (Mn+2), cobalt (Co+2) and copper (Cu+2) in presence and absence of tyrosols retrieved from these isolates. The decreasing order of tolerance of isolates were found to be Mn+2 > Zn+2 > Co+2 > Cu+2. Under the influence of tyrosols, isolates showed enhanced growth in their upper metal tolerance limit. C. tropicalis showed enhanced growth (2−48 fold, p<0.0001) in all the tested metal consisting medium (2 mM Zn+2, 5 mM Mn+2, 2 mM Co+2 and 1 mM Cu+2) whilst W. anomalus, C. glabrata, S. cerevisiae showed increased growth (3−17 fold, p<0.0001) in Zn+2 (2 mM), Mn+2 (5 mM) and Cu+2 (1 mM) augmented medium. Overall result suggests that tyrosol exert protective effect under heavy metal induced stress, which could be useful in enhancing the quality of fermented products.