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The survival of probiotic microorganisms during their exposure to harsh environments plays a critical role in the fulfillment of their functional properties. In particular, transit through the human gastrointestinal tract (GIT) is considered one of the most challenging habitats that probiotics must endure, because of the particularly stressful conditions (e.g., oxygen level, pH variations, nutrient limitations, high osmolarity, oxidation, peristalsis) prevailing in the different sections of the GIT, which in turn can affect the growth, viability, physiological status, and functionality of microbial cells. Consequently, probiotics have developed a series of strategies, called "mechanisms of stress response," to protect themselves from these adverse conditions. Such mechanisms may include but are not limited to the induction of new metabolic pathways, formation/production of particular metabolites, and changes of transcription rates. It should be highlighted that some of such mechanisms can be conserved across several different strains or can be unique for specific genera. Hence, this review attempts to review the state-of-the-art knowledge of mechanisms of stress response displayed by potential probiotic strains during their transit through the GIT. In addition, evidence whether stress responses can compromise the biosafety of such strains is also discussed., (© 2022. The Author(s), under exclusive licence to Springer Science+Business Media, LLC, part of Springer Nature.)

Artisanal fermented beverages have been associated with beneficial effects for a long time. In Mexico, there are a wide variety of artisanal fermented beverages such as Tepache, where the fermentation is initiated by the addition of grains of a polysaccharide-containing biofilm matrix formed by a symbiotic association of bacteria and yeasts known as "Tibicos." These microorganisms can be responsible for the beneficial effect associated with fermented beverages inoculated with Tibicos. The probiotic potential of microorganisms has been widely studied, mainly in lactic acid bacteria, while despite the wide distribution of yeasts, these have not been so studied. Therefore, the aim of this study was to evaluate in vitro the probiotic potential properties of a yeast isolated from Tibicos. For this, the yeast was identified by molecular techniques as Saccharomyces cerevisiae, which showed a good resistance to pH 2.0, bile salts and in vitro digestion. The results also showed a good ability to form cellular aggregates as a result of having a hydrophobic surface. In addition, it can be considered as safe since it does not show hemolytic activity and is sensitive to nystatin. Additionally, the yeast presented an excellent antioxidant capacity to reduce the DPPH radical. The S. cerevisiae strain C41 isolated from Tibicos was successfully compared by means of in vitro tests with the only recognized probiotic yeast, S. boulardii. These findings point Saccharomyces cerevisiae C41 as a potentially probiotic yeast; nevertheless, it is necessary to consider further in vitro and in vivo studies that establish the benefits that this yeast could provide.