## 1 Introduction  
The most important brewer's yeasts are mostly part of the genus _Saccharomyces_. Thus, ale and lager yeasts belong to _Saccharomyces cerevisiae (Sc)_ and _Saccharomyces pastoriamus_, respectively [1]. _S. pastoriamus_ is a hybrid of _Sc_ and _Saccharomyces eubyannus_. _Sc_ is one of the most important brewing yeast species utilised for ale beer production due to its tolerance to alcohol [2], growth and fermentation rate on an industrial scale [3]. _Saccharomyces cerevisiae var. boulardii_ (_Scb_) is another popular yeast species that was isolated from fruit in Indochina and has been utilised as a remedy for diarrhea since 1950 [4; 5]. _Scb_ has shown a capacity to prevent travellers' diarrhea usually caused by bacteria such as _Escherichia coli_ and _Salmonella sp_. [6; 7]. Probiotics are microorganisms used as food supplements that in a correct dosage are potentially beneficial for human health, especially for the intestinal microbial balance [8]. Gastrointestinal microbiote imbalances can develop into several disorders, such as diarrhea under antibiotic treatment, ulcers, inflammatory bowel diseases or irritable bowel syndrome [9]. _Scb_ is the only yeast species with probiotic properties able to keep gastrointestinal conditions at a suitable health level. _Scb_, in particular, has been very effective against _Clostridium difficile_ in the prevention and treatment of antibioticassociated diarrhea [10; 11] and may help to eradicate _Helicobacter pylori_[12]. In fact, the \(Sc_{b}\) yeast is now commercially available in most countries globally. \(Sc_{b}\) shows enhanced ability for pseudohyphal switching upon nitrogen limitation and increased resistance to acidic pH, which are not correlated with increased adherence to epithelial cells or transit through mouse gut [13]. However, a recent genetic comparative study of 5 \(Sc_{b}\) against 145 \(Sc\) yeast strains has revealed the presence of two genes, _HXT11_ and _HXT9_, in \(Sc\), which are not present in \(Sc_{b}\)[14]. The absence of these genes has been related to greater resistance against several antifungals, such as cycloheximide, sulfometuron methyl and 4-nitroquinoline-N-oxide [15]. Other genetic particularities that have been reported for \(Sc_{b}\) include the absence of ASP3 genes involved in asparagine degradation [16] and the abundant repetition of Flocculaton (FLO) genes, which could explain the ability of \(Sc_{b}\) to adhere to the intestinal epithelium [17; 18]. The optimal growth temperature of \(Sc_{b}\) is 37 \({}^{\circ}\)C and thus it can reach very high gastrointestinal tract (GT) concentrations in a very short period [19]. In addition, it possesses the capacity to inhibit the growth, adherence and invasion of multiple pathogens such _C. difficile_[20], _E. coli_[21], and _Candida albicans_[22], both in in vitro and in vivo conditions to the epithelial layer of the GT. In recent years, there has been a worldwide increase in the consumer interest in craft beers, which are unpasteurized, unfiltered and without additional nitrogen or carbon dioxide pressure [23]. Even though there are many controversial studies about the beneficial effects of beer consumption on human health, epidemiological studies have shown that beer has positive effects on cardiovascular protection inhibiting atherosclerosis, decreasing the content of serum low-density lipoprotein cholesterol and triglycerides, by acting as an in vivo free radical scavenger [24; 25]. This alcoholic beverage is usually mainly composed of four basic ingredients: water, hops, malt, and brewer's yeast, as well as non-nutrient components such as phenolic compounds [26]. Brewer's yeast strains are facultative anaerobes and this ability supports their property as an important industrial microorganism [27]. Phenolic compounds are present in vegetable foods such as malt and hops [28]. The use of \(Sc_{b}\) as a mixed starter with \(Sc\) for craft beer [29] or as a single starter for alcohol-free beer production [30] has been proposed recently.