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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 anomalus, 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 investigation, growth promoting role of indole-3-acetic acid (IAA) was assessed on yeasts representing three genera and four species viz Wickerhamomyces anomalus, Saccharomyces cerevisiae, Candida tropicalis and Candida glabrata isolated from traditional fermentative starter materials of Ahom, Rabha, Bodo, Karbi, Kachari and Mishing communities of Assam, India. Isolates were first assessed for their ability to synthesize IAA in presence and absence of external tryptophan and was examined for stimulatory effect of growth on the tested isolates. Tryptophan dependent IAA synthesis was observed in 92% of isolates while 72% of isolates could synthesize IAA in absence of exogenous tryptophan. Candida glabrata KC3X could synthesize maximum IAA while Saccharomyces cerevisiae KR5.6 did not synthesize IAA in presence of external tryptophan. 3 out of 14 Wickerhamomyces isolates and the sole Saccharomyces isolate KR4.10 were found to synthesize significant amount of IAA in absence of tryptophan. Treatment of exogenous IAA on the growth of tested yeasts revealed that Saccharomyces isolates were more pronounced than others. Wickerhamomyces anomalus, Candida tropicalis and Candida glabrata did not show any significant response of growth in presence of exogenous IAA. This study concludes that plant condiments present in starter materials may aid in accelerated growth of Saccharomyces yeasts compared to non-Saccharomyces ones. Yeasts capable of synthesizing IAA but unable to show appreciable growth in presence of IAA may presumably facilitate sustenance of Saccharomyces spp. in fermentative consortia.

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.

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. Saccharomyces yeasts are primarily involved in alcoholic fermentation, whereas non-Saccharomyces yeasts, 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.