Proton-dependent kinetics of citrate uptake in growing cells ofLactococcus lactissubsp.lactisbv.diacetylactis

Citrate, low pH and amino acid limitation induce citrate utilization in Lactococcus lactis biovar diacetylactis

In Lactococcus lactis subsp. lactis biovar diacetylactis, citrate transport is facilitated by the plasmid-encoded citrate permease (CitP). In this work, we analysed regulation of citrate utilization by pH, nutrient limitation and the presence of citrate at four different levels: (i) plasmid copy number, (ii) citP transcription, (iii) citP mRNA processing and (iv) citrate utilization capacity. Citrate was supplied as cosubstrate together with lactose. The citP gene is known to be induced in cells grown at low pH. However, we demonstrated that transcription of citP was even higher in the presence of citrate (3.8-fold compared with 2.0-fold). The effect of citrate has been overlooked by other researchers because they determined the effect of citrate using M17 medium, which already contains 0.80 ± 0.07 mM citrate. The plasmid copy number increased in cells grown under amino acid limitation (1.6-fold) and/or at low pH (1.4-fold). No significant differences in citP mRNA processing were found. Citrate utilization rates increased from approximately 1 to 65 μmol min-1  gDW-1 from lowest to highest citP expression. Acetoin formation increased during growth in an acidic environment due to induction of the acetoin pathway. Quantification of the relative contributions allowed us to construct a model for regulation of citrate utilization in L. lactis biovar diacetylactis. This knowledge will help to select conditions to improve flavour formation from citrate.

Natural-abundance isotope ratio mass spectrometry as a means of evaluating carbon redistribution during glucose-citrate cofermentation by Lactococcus lactis

The cometabolism of citrate and glucose by growing Lactococcus lactis ssp. lactis bv. diacetylactis was studied using a natural-abundance stable isotope technique. By a judicious choice of substrates differing slightly in their 13C/12C ratios, the simultaneous metabolism of citrate and glucose to a range of compounds was analysed. These end-products include lactate, acetate, formate, diacetyl and acetoin. All these products have pyruvate as a common intermediate. With the objective of estimating the degree to which glucose and citrate metabolism through pyruvate may be differentially regulated, the delta13C values of the products accumulated over a wide range of concentrations of citrate and glucose were compared. It was found that, whereas the relative accumulation of different products responds to both the substrate concentration and the ratio between the substrates, the delta13C values of the products primarily reflect the availability of the two substrates over the entire range examined. It can be concluded that in actively growing L. lactis the maintenance of pyruvate homeostasis takes precedence over the redox status of the cells as a regulatory factor.

The citrate transport system of Lactococcus lactis subsp. lactis biovar diacetylactis is induced by acid stress

Citrate transport in Lactococcus lactis subsp. lactis biovar diacetylactis is catalyzed by citrate permease P (CitP), which is encoded by the plasmidic citP gene. We have shown previously that citP is included in the citQRP operon, which is mainly transcribed from the P1 promoter in L. lactis subsp. lactis biovar diacetylactis. furthermore, transcription of citQRP and citrate transport are not induced by the presence of citrate in the growth medium. In this work, we analyzed the influence of the extracellular pH on the expression of citP. The citrate transport system is induced by natural acidification of the medium during cell growth and by a shift to media buffered at acidic pHs. This inducible response to acid stress takes place at the transcriptional level and seems to be due to increased utilization of the P1 promoter. Increased transcription correlates with increased synthesis of CitP and results in higher citrate transport activity catalyzed by the cells. Finally, this acid stress response seems to provide L. lactis subsp. lactis biovar diacetylactis with a selective advantage resulting from cometabolism of glucose and citrate at low pHs.