Contribution of exofacial thiol groups in the reducing activity of Lactococcus lactis

Lactococcus lactis can decrease the redox potential at pH 7 (E(h7)) from 200 to -200 mV in oxygen free Man-Rogosa-Sharpe media. Neither the consumption of oxidizing compounds or the release of reducing compounds during lactic acid fermentation were involved in the decrease in E(h7) by the bacteria. Thiol groups located on the bacterial cell surface appear to be the main components that are able to establish a greater exchange current between the Pt electrode and the bacteria. After the final E(h7) (-200 mV) was reached, only thiol-reactive reagents could restore the initial E(h7) value. Inhibition of the proton motive force showed no effect on maintaining the final E(h7) value. These results suggest that maintaining the exofacial thiol (-SH) groups in a reduced state does not depend on an active mechanism. Thiol groups appear to be displayed by membrane proteins or cell wall-bound proteins and may participate in protecting cells against oxidative stress.