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Journal of Bacteriology, March 1999, p. 1451-1457, Vol. 181, No. 5
0021-9193/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.

Mechanism of Citrate Metabolism in Lactococcus lactis: Resistance against Lactate Toxicity at Low pH

Christian Magni,1,2 Diego de Mendoza,2 Wil N. Konings,1 and Juke S. Lolkema1,*

Department of Microbiology, Groningen Biotechnology and Biomolecular Sciences Institute, University of Groningen, 9751 NN Haren, The Netherlands,1 and Programa Multidisciplinario de Biología Experimental (PROMUBIE-CONICET) and Departamento de Microbiología, Facultad de Ciencias Bioquímicas y Farmaceuticas, Universidad Nacional de Rosario, 2000 Rosario, Argentina2

Received 13 July 1998/Accepted 20 December 1998

Measurement of the flux through the citrate fermentation pathway in resting cells of Lactococcus lactis CRL264 grown in a pH-controlled fermentor at different pH values showed that the pathway was constitutively expressed, but its activity was significantly enhanced at low pH. The flux through the citrate-degrading pathway correlated with the magnitude of the membrane potential and pH gradient that were generated when citrate was added to the cells. The citrate degradation rate and proton motive force were significantly higher when glucose was metabolized at the same time, a phenomenon that could be mimicked by the addition of lactate, the end product of glucose metabolism. The results clearly demonstrate that citrate metabolism in L. lactis is a secondary proton motive force-generating pathway. Although the proton motive force generated by citrate in cells grown at low pH was of the same magnitude as that generated by glucose fermentation, citrate metabolism did not affect the growth rate of L. lactis in rich media. However, inhibition of growth by lactate was relieved when citrate also was present in the growth medium. Citrate did not relieve the inhibition by other weak acids, suggesting a specific role of the citrate transporter CitP in the relief of inhibition. The mechanism of citrate metabolism presented here provides an explanation for the resistance to lactate toxicity. It is suggested that the citrate metabolic pathway is induced under the acidic conditions of the late exponential growth phase to make the cells (more) resistant to the inhibitory effects of the fermentation product, lactate, that accumulates under these conditions.

* Corresponding author. Mailing address: Department of Microbiology, Biological Centre, Kerklaan 30, 9751 NN Haren, The Netherlands. Phone: 31-50-3632155. Fax: 31-50-3632154. E-mail: j.s.lolkema{at}biol.rug.nl.

Journal of Bacteriology, March 1999, p. 1451-1457, Vol. 181, No. 5
0021-9193/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.


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