Abstract <p>The use of acid whey as a medium is an innovative approach to bacterial cellulose (BC) biosynthesis in co-cultures of acetic acid bacteria with lactic acid bacteria. The aim of this study was to evaluate the possibility of obtaining BC in acid whey by co-culturing <i>K. xylinus</i> with selected strains of lactic acid bacteria and comparing the properties of this biopolymer with BC obtained in <i>K. xylinus</i> monoculture. The <i>K. xylinus</i> + <i>Lb. acidophilus</i> co-culture yielded 2.19&#xa0;g·L⁻¹ of BC, which was 125% more than the <i>K. xylinus</i> monoculture. Additionally, <i>K. xylinus</i> in co-culture with <i>Lb. acidophilus</i> increased the degradation temperature of BC to 361&#xa0;°C compared to 303&#xa0;°C for BC obtained in monoculture. The BC obtained in the co-cultures showed better mechanical properties. BC obtained in co-culture with <i>Lb. delbrueckii</i> showed more than twice the Young’s modulus than BC from monoculture. Moreover, strain at break BC from co-culture with <i>Lb. acidophilus</i> and stress at break BC from co-culture with <i>Lb. helveticus</i> were 72% and 54% higher, respectively, than BC obtained from monoculture <i>K. xylinus.</i> In this study, it was shown that conducting acetic-lactic co-cultures increased the efficiency of BC biosynthesis and improved its properties. Moreover, this study has shown that acid whey is a sufficient and complete substrate for obtaining BC. Results presented in this paper indicate new possibilities for the management of this side product.</p> Key points <p>• <i>The K. xylinus + Lb. acidophilus co-culture produced 125% more cellulose than the monoculture.</i></p> <p>• <i>High lactic acid content and low pH of acid whey enhance cellulose biosynthesis.</i></p> <p>• <i>Acetic acid-lactic acid co-cultures improved the mechanical properties of cellulose.</i></p>