## 1 Introduction  
Exopolysaccharides (EPS) are biopolymers which are associated with the cell envelope or can be released into the external environment [1]. Bacterial EPS biosynthesis seems to be related to cell protection and abiotic/biotic stress [2; 3; 4]. They have strong potential to act as an immunoregulatory, antioxidant, anti-hypertensive, anti-bofilm, cholesterol-lowering, or prebiotic agent [5; 6; 7]. However, reported low yields of EPS limit their production on a larger scale and there is a lack of clinical studies on the effective bio functionality of these polymers [8]. EPS have physicochemical properties that can enhance the sensorial and textural attributes of a variety of food and beverage products. Traditionally, lactic acid bacteria exopolysaccharides have an important role in fermented dairy products and more recently are being applied for the improvement of bakery products or gluten free products. Several studies reported that the use of EPS led to increased water absorption of the dough, better dough rheology and machinability, increased loaf volume, and decreased bread staling rate. The application of LAB EPS, when produced in situ, does not require labeling and therefore is seen as positive in an industry where consumers continually demand products containing less additives [5; 7]. Obtaining bioingredients rich in microbial EPS is still a challenge. Alternatives to improve EPS production on a larger scale require more in-depth knowledge regarding EPS regulation and biosynthesis, optimizing fermentation conditions and formulating bacterial culture medium such as whey permeate [4] to valorize residual co-products of the agri-food industry, which can represent environmental and economic advantages [9]. Among other LAB, _Lactobacillus rhamnosus_ strains are able to produce EPS. In particular, _Lactobacillus rhamnosus_ R0011, _Lactobacillus rhamnus_ ATCC 9595, and _Lactobacillus rhamnus_ RW-9595M produce a range of quantities of the same excopolsaccharide [10]. This EPS is composed of a repetitive structure: a heptasaccharide containing glucose, galactose, and rhamnose in a 2:1:4 ratio and a pyruvate group [11]. Strain RW-9595M is the most productive of these three strains, with 543 mg/L of EPS without pH control and 1611-2775 mg/L in fermenters using whey permeate as the culture medium at controlled pH 6 [12; 13]. In biotechnology, co-culture has been shown to be more robust to environmental fluctuations and can perform more complex activities through the combination of metabolic capacities of different strains [14; 15]. It is also an important tool to better understand microorganism physiology, provide interspecific or interkingdom (eukaryotes - prokaryotes) communication, and to discover new substances [16]. Mixed populations of yeast _Saccharomyces cerevisiae_ and LAB are found together in numerous fermented products (kefir, kimchi, sourdough, wine, and cocoa), but the understanding of their interactions is still unclear [17; 18]. In the production of useful metabolites by LAB, the lactic acid must be removed to increase bacterial productivity and to enhance the economic feasibility of processes on an industrial scale [19; 20]. A beneficial aspect of LAB-yeast association is the improvement of EPS production (kefiran) or bacteriocins (nisin), probably due to lactate consumption by the yeast in pH-controlled culture [21; 22; 23]. Co-culture with _S. cerevisiae_ supplied alanine to _L. delbruveckii_ subspecies _bulgaricus_, enhanced the viability of _L. rhamnus_ HN001 under acidic conditions, and enhanced the tolerance of _L. paracasei_ H9 to simulated gastrointestinal juices as well as adhesion to intestinal mucosa cells [24; 25; 26]. However, until now, no studies on LAB gene expression related to EPS biosynthesis in co-culture with _S. cerevisiae_ have been reported under acidic conditions. The main objective of the present study was to determine the influence of co-culture of _S. cerevisiae_ and three _L.