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.