New insights into the citrate metabolism of Enterococcus faecium FAIR-E 198 and its possible impact on the production of fermented dairy products

Citrate metabolism by Enterococcus faecium FAIR-E 198, an isolate from Greek Feta cheese, was studied in milk-based media compared with modified de Man–Rogosa–Sharpe (mMRS) medium. Co-metabolism of lactose and citrate was shown. The presence of fermentable lactose and glucose in mMRS medium under microaerophilic conditions increased the yield of acetate from citrate conversion but decreased the yield of acetoin compared with a fermentation in the presence of non-fermentable xylose. Under anaerobic conditions in mMRS medium, the yield of ethanol based on citrate increased and diacetyl was no longer found. In milk medium, additional nitrogen sources (e.g., casitone) were necessary to sustain co-metabolism of citrate and lactose. Addition of extra citrate to milk-based medium did not affect citrate conversion, although a lower yield of acetate was found. As citrate conversion was associated with the stationary growth phase, E. faecium FAIR-E 198 is an interesting adjunct starter culture for cheese ripening.