CITRATE METABOLISM IN LACTOBACILLUS-CASEI AND LACTOBACILLUS-PLANTARUM

Information on the factors influencing citrate metabolism in lactobaci lli is limited and could be useful in understanding the growth of lact obacilli in ripening cheese. Citrate was not used as an energy source by either Lactobacillus casei ATCC 393 or Lact. plantarum 1919 and did -not affect the growth rate when co-metabolized with glucose or galact ose. In growing cells, metabolism of citrate was minimal at pH6 but si gnificant at pH4.5 and was greater in cells co-metabolizing galactose than in those co-metabolizing glucose or lactose. In non-growing cells , optimum utilization of citrate also occurred at pH 4.5 and was not i ncreased substantially by the presence of fermentable sugars. In both growing and non-growing cells, acetate and acetoin were the major prod ucts of citrate metabolism; pyruvate was also produced by non-growing cells and was transformed to acetoin once the citrate was exhausted. C itrate was metabolized more rapidly than sugar by non-growing cells; t he reverse was true of growing cells. Citrate metabolism by Lact. plan tarum 1919 and Lact, casei ATCC 393 increased six- and 22-fold, respec tively, when the cells were pre-grown on galactose plus citrate than w hen pre-grown on galactose only. This was probably due to induction of citrate lyase by growth on citrate plus sugar. These results imply th at lactobacilli, if present in large enough numbers, can metabolize ci trate in ripening cheese in the absence of an energy source.