According to the "energy recycling model" (Michels et al., 1979; Otto et al., 1980; Ten Brink and Konings, 1980) carrier-mediated effiux of metabolic end-products can generate an electrochemical proton gradient across the cytoplasmic membrane and therefore contributes to the energy generation of bacteria. The energy production by lactate excretion from Streptococcus crernoris grown on lactose equals 4 (n-1)/p ATP equivalents per lactose, where n is the H+/lactate stoichiometry and p the number of protons translocated per ATP synthesized or hydrolysed. During growth in a batch culture without pH-control n, which can be calculated from data on A~, A pH and the lactate concentration gradient, decreased from 1.8 to 0.9 (Ten Brink and Konings, 1982). From further experiments in pH-regulated batch and continuous cultures we concluded that this decrease ofn was caused by the decrease in external pH and the increase in the external lactate concentration.
These effects were studied in more detail in glycolysing S. cremoris cells. At pH 7.0 n equalled 1.8 at 2 mM external lactate and decreased to 1.1 at 60 mM external lactate. At pH 5.5 a decrease ofn from 1.4 to 0.7 was observed as the external lactate increased from 2 to 60 raM.
In conclusion, under conditions where n is higher than I, lactate excretion results in energy gain for S. cremoris (up to 50~ for n = 2 and p = 2); n decreases with a decrease in pH and/or an increase in external lactate; and n-values lower than t are observed, indicating that part of the lactate is excreted as the lactate anion.