Citrate metabolism by Enterococcus faecium FAIR-E 198, an isolate from Greek Feta cheese, was studied in modified MRS (mMRS) medium under different pH conditions and glucose and citrate concentrations. In the absence of glucose, this strain was able to metabolize citrate in a pH range from constant pH 5.0 to 7.0. At a constant pH 8.0, no citrate was metabolized, although growth took place. The main end products of citrate metabolism were acetate, formate, acetoin, and carbon dioxide, whereas ethanol and diacetyl were present in smaller amounts. In the presence of glucose, citrate was cometabolized, but it did not contribute to growth. Also, more acetate and less acetoin were formed compared to growth in mMRS medium and in the absence of glucose. Most of the citrate was consumed during the stationary phase, indicating that energy generated by citrate metabolism was used for maintenance. Experiments with cell-free fermented mMRS medium indicated that E. faecium FAIR-E 198 was able to metabolize another energy source present in the medium.Citrate metabolism is widespread among lactic acid bacteria (LAB), e.g., Lactococcus lactis, Leuconostoc spp., Lactobacillus casei, Lactobacillus plantarum, Lactobacillus rhamnosus, Enterococcus faecalis, Enterococcus faecium, and Oenococcus oeni (8,11,17,26,28,31,32,34,35,36,37). The breakdown of citrate results in the production of acetate, formate, ethanol, acetaldehyde, acetoin, 2,3-butanediol, diacetyl, and carbon dioxide. Some of these compounds contribute to the flavor development in fermented foods. For instance, diacetyl production displays a distinct effect on the quality of dairy products, such as butter, buttermilk, and certain cheeses (8,24). Also, the formation of eyes due to the production of carbon dioxide contributes to the texture development in some cheeses (25).The ability to metabolize citrate is dependent on the presence of the enzyme citrate permease (CitP), which has been characterized in strains belonging to the genera Leuconostoc, Oenococcus, and Lactococcus (3,13,30,47). Citrate can be used as the sole energy source or is cometabolized (17). When citrate is present as the sole energy source, uptake occurs via symport of divalent citrate and one proton or via uniport of monovalent citrate (21, 30). Also, it has been suggested that divalent citrate can be taken up via an antiport mechanism, which releases intermediates (e.g., pyruvate) or end products (e.g., acetate) of citrate metabolism in the medium (18). During cometabolism of citrate and a sugar (citrolactic fermentation), CitP catalyzes the exchange (antiport) of divalent anionic citrate and monovalent lactate. Citrate uptake is an electrogenic process, which together with the formation of a pH gradient across the cell membrane, results in the formation of a proton motive force and hence generation of metabolic energy (3, 21).Among different LAB genera, optimal citrate uptake occurs at low pH values, ranging from pH 4.0 to pH 5.5 (23,28,31). In the presence of sugar, citrate consumption has been show...