Proc. Natl. Acad. Sci. USA Vol. 77, No. 9, pp. 5502-5506, September 1980 Microbiology Generation of an electrochemical proton gradient in Streptococcus cremoris by lactate efflux (fermentation/transport) ROEL OTTO, ANTON S. M. SONNENBERG, HANS VELDKAMP, AND WIL N. KONINGS* Department of Microbiology, Biological Centre, University of Groningen, Kerklaan 30, 9751 NN Haren, The Netherlands Communicated by Peter Mitchell, June 9, 1980 ABSTRACT Recently an energy-recycling model was pro- solute will occur when [(n - 1)AT - nZApH] < -Z posed that postulates the generation of an electrochemical log(A-/A-,) and the energy of the solute gradient then will be gradient in fermentative bacteria by carrier-mediated excretion into of the of metabolic end products in symport with protons. In this paper converted energy electrochemical proton gra- experimental support for this model is given. In batch cultures dient. of Streptococcus cremoris with glucose as the sole energy source During fermentation, excretion of metabolic end products the maximal specific growth rate decreased by 30% when the via a carrier in symport with proton(s) can occur only when the external lactate concentration was decreased from 50 to 90 mM. outwardly directed driving force supplied by the chemical end In the same range of external lactate concentrations the molar product gradient exceeds the inwardly directed driving force growth yield Y for glucose as measured in energy-limited che- supplied by the electrochemical gradient. The excretion of end mostat cultures also showed a 30% drop. From ynaxtose values of S. cremoris grown in the presence and absence of added lactate products will then lead to the generation of an electrochemical it was calculated that the net energy ain from the lactate efflux gradient. Michels et al. (1) calculated the generation of the system was at least 12%. Lactate effux from de-energized cells electrochemical gradient for a model cell that excreted lactate loaded with lactate could drive the uptake of leucine. This up- in symport with a variable number (1 or 2) of protons. It was take was sensitive to carbonylcyanide p-trifluoromethoxy- concluded that in these cells lactate efflux could account for an phenylhydrazone and was only partly inhibited by dicyclo- additional 30% of metabolic energy. hexylcarbodiimide (DCCD). The limited inhibition by DCCD of lactate-induced leucine uptake indicates that ATP hydrolysis In this paper experimental support is given for this energy- was not the driving force for transport of leucine. recycling model. The effects of external lactate on maximum Uptake studies with the lipophilic cation tetraphenylphos- specific growth rate, cell yield, and maintenance requirements phonium demonstrated that lactate efflux increased the elec- of Streptococcus cremoris were studied. The chemostat culture trical potential across the membrane by 51 mV. The generation technique was used in this study because it allowed a quanti- of an electrical potential by lactate efflux and the demonstration tation of the effects of lactate on energy metabolism under of a potassium efflux-induced uptake of lactate indicates that energy-limited conditions. In addition, we investigated whether lactate is translocated across the membrane by a symport system lactate efflux could generate an electrochemical gradient in with more than one proton. energy-depleted cells. The energy-recycling model recently proposed by Michels et al. (1) is an extension of the chemosmotic model given by MATERIALS AND METHODS Mitchell (2-4). The energy-recycling model postulates that Culture Conditions. S. cremoris Wg2 was obtained from the carrier-mediated excretion of metabolic end products can lead Dutch Institute of Dairy Research (Nederlands Instituut voor to the generation of an electrochemical gradient across the Zuivelouderzoek, Ecle, The Netherlands). The organism was cytoplasmic membrane, thus providing metabolic energy to routinely maintained in 10% (wt/vol) skimmed milk and stored the cell. The proposed model is based on the following consid- at -20'C. From the milk cultures S. cremoris was transferred erations. to a complex MRS medium (6) and subsequently to a chemically The driving force for translocation of solute A across the defined medium (7). Batch cultures were grown anaerobically cytoplasmic membrane by a solute-proton symport system (2) at 30'C in screw-capped tubes (diameter 10 mm, length 10 cm) is the sum of the electrochemical gradient and the solute gra- or in pH-controlled 3-liter erlenmeyer flasks. Chemostat cul- dient (5): tures were grown anaerobically under N2 atmosphere in glass chemostats with a working volume of 200 ml at 30'C and Z log(A-/A- t) + (n - 1)A' - nZApH, controlled pH of 6.3 as described by Laanbroek et al. (8). in which AiT is the electrical potential and ApH is the pH Maximal Specific Growth Rate in Batch Cultures. Growth gradient across the cytoplasmic membrane, n is the number of rate was determined from the increase of OD660 during expo- protons transported in symport with A-; Z is 2.3RT/F (R, gas nential growth. OD6co was followed by placing the screw- constant; T, absolute temperature; F, Faraday constant); and capped tubes in special adaptors in a Vitatron UC 200 spec- A- and A- t are the concentrations of A- in the cell and the trophotometer (Vitatron Scientific Instruments, Dieren, The external medium, respectively. A steady-state level of accu- Netherlands). mulation is reached when this driving force is zero, thus when Cell Suspensions for Transport Studies. Suspensions were (n - 1)AT - nZApH =-Z log(Aj-/A- t). According to this obtained from 3-liter batch cultures (OD6W1 = 0.8). The cells equation accumulation of solute A- will occur when [(n - were washed twice with 2 liters of 40mM potassium phosphate, 1)AT - nZApH] > -Z log(Aj-/A- t). However, excretion of pH 7.0, at room temperature, resuspended in this buffer to a The publication costs of this article were defrayed inpart by page Abbreviations: MeSGal, methyl 1-thio-3-D-galactopyranoside; FCCP, charge payment. This article must therefore be hereby marked "ad- carbonylcyanide p-trifluoromethoxyphenylhydrazone; DCCD, di- vertisement" in accordance with 18 U. S. C. §1734 solely to indicate cyclohexylcarbodiimide; Ph4P+, tetraphenylphosphonium. this fact. * To whom reprint requests should be addressed. 5502 Downloaded by guest on October 2, 2021 Microbiology: Otto et al. Proc. Natl. Acad. Sci. USA 77 (1980) 5503 density of 100 mg/ml (dry wt) and stored as 0.25-ml samples described by Bakker et al. (17). For batch-grown cells the in- in liquid nitrogen. tracellular volume was 3.79 ,l/mg of cell protein. De-energization of S. cremoris. A 0.25-ml cell suspension Materials. Radioactive labeled leucine and L-lactate were was thawed quickly and washed twice at room temperature obtained from the Radiochemical Centre (Amersham). [3H]- with 10 ml of choline/Hepes/KCI buffer (10 mM choline/ Ph4P+ was generously supplied by H. R. Kaback (Roche Insti- Hepes, pH 7.0, and 2 mM KCI). Methyl I-thio-f3-D-galacto- tute of Molecular Biology, Nutley, NJ). Carbonylcyanide p- pyranoside (MeSGal) was added to the washed cell suspension trifluoromethoxyphenylhydrazone (FCCP) and dicyclohex- [2.5 mg/ml (dry wt)] to a final concentration of 1 mM (9). The ylcarbodiimide (DCCD) were dissolved in absolute ethanol. cell suspension was incubated for 1 hr at room temperature. The Additions to incubation mixtures were made to maximal eth- cells were washed twice with 10 ml of choline/Hepes/KCI anol concentrations of 1% (vol/vol). buffer and finally resuspended in this buffer to a density of 100 mg/ml (dry wt). RESULTS Loading of De-energized Cells with Lactate or Chloride. Effects of External L-Lactate on Growing Cells of S. cre- De-energized cells (0.25 ml) [100 mg/ml (dry wt)] were washed moris. According to the energy-recycling model, efflux of twice with 10 ml of choline/Hepes/KCI buffer supplemented lactate from S. cremoris, growing anaerobically on glucose, with 50 mM choline L-lactate. The suspension was acidified to should result in the generation of an electrochemical proton pH 4.3 with 0.2 M L-lactic acid and incubated at room tem- gradient that contributes to the energy metabolism of the cell. perature-for 10 min. Subsequently the suspension was neu- A decrease of the lactate gradient across the cytoplasmic tralized with 0.2 M choline hydroxide and incubated for an membrane will reduce the energy yield and consequently will additional 30 min at room temperature. Finally the cells were lower the molar growth yield as well as the maximal specific concentrated to a cell density of 100 mg/ml (dry wt). Loading growth rate, if it is assumed that this is determined by the rate of de-energized cells with choline chloride was performed in of ATP supply. the same way except that choline lactate and L-lactic acid were The effect of increasing extracellular L-lactate concentrations replaced by choline chloride and HCI, respectively. on the maximal specific growth rate of S. cremoris, grown Uptake of L-Leucine and Ph4P+. A sample (1 Ml) of un- anaerobically in a synthetic medium in batch culture, is shown loaded or loaded cells [100 mg/ml (dry wt)] was diluted into in Fig. 1. This effect is clearly different from that of other 100 Ml of 10 mM choline/Hepes/KCI buffer containing 2.85 compounds, such as sodium propionate and NaCI (Fig. 1). The AM ['4C]leucine (351 mCi/mmol; 1 Ci = 3.7 X 1010 becquerels) latter compounds exhibited a significant inhibitory effect at or 40 MM [3H]tetraphenylphosphonium bromide (Ph4P+) (54 concentrations as low as 10 mM. Inhibition patterns with KC1, mCi/mmol) and 0-50mM choline L-lactate or 0-50mM cho- choline chloride, and potassium propionate did not differ sig- line chloride.