Mechanism of citrate metabolism in Lactococcus lactis: Resistance against lactate toxicity at low pH

Measurement of the flux through the citrate fermentation pathway in resting cells of Lactococcus lactis CRL264 grown in a pH-controlled fermenter at d ifferent pH values showed that the pathway was constitutively expressed, bu t its activity was significantly enhanced at low pH. The flux through the c itrate-degrading pathway correlated with the magnitude of the membrane pote ntial and pH gradient that were generated when citrate was added to the cel ls. The citrate degradation rate and proton motive force were significantly higher when glucose was metabolized at the same time, a phenomenon that co uld be mimicked by the addition of lactate, the end product of glucose meta bolism. The results clearly demonstrate that citrate metabolism in L. lacti s is a secondary proton motive force-generating pathway. Although the proto n motive force generated by citrate in cells grown at low pH was of the sam e magnitude as that generated by glucose fermentation, citrate metabolism d id not affect the growth rate of L. lactis in rich media. However, inhibiti on of growth by lactate was relieved when citrate also was present in the g rowth medium. Citrate did not relieve the inhibition by other weak acids, s uggesting 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 th e citrate metabolic pathway is induced under the acidic conditions of the l ate exponential growth phase to make the cells (more) resistant to the inhi bitory effects of the fermentation product, lactate, that accumulates under these conditions.