Generation of an electrochemical proton gradient in Streptococcus cremoris by lactate efflux

Recently an energy-recycling model was proposed that postulates the generation of an electrochemical gradient in fermentative bacteria by carrier-mediated excretion of metabolic end products in symport with protons. In this paper experimental support for this model is given. In batch cultures of Streptococcus cremoris with glucose as the sole energy source the maximal specific growth rate decreased by 30% when the external lactate concentration was decreased from 50 to 90 mM. In the same range of external lactate concentrations the molar growth yield Y for glucose as measured in energy-limited chemostat cultures also showed a 30% drop. From Y max lactose values of S. cremoris grown in the presence and absence of added lactate it was calculated that the net energy gain from the lactate efflux system was at least 12%. Lactate efflux from de-energized cells loaded with lactate could drive the uptake of leucine. This uptake was sensitive to carbonylcyanide p-trifluoromethoxyphenylhydrazone and was only partly inhibited by dicyclohexylcarbodiimide (DCCD). The limited inhibition by DCCD of lactate-induced leucine uptake indicates that ATP hydrolysis was not the driving force for transport of leucine. Uptake studies with the lipophilic cation tetraphenylphosphonium demonstrated that lactate efflux increased the electrical potential across the membrane by 51 mV. The generation of an electrical potential by lactate efflux and the demonstration of a potassium efflux-induced uptake of lactate indicates that lactate is translocated across the membrane by a symport system with more than one proton.