A small number of flavoprotein oxidase enzymes are responsible for the direct interaction of lactic acid bacteria (LAB) with oxygen; hydrogen peroxide or water are produced in these reactions. In some cultures exposed to oxygen, hydrogen peroxide accumulates to inhibitory levels.Through these oxidase enzymes and NADH peroxidase, 02 and H202 can accept electrons from sugar metabolism, and thus have a sparing effect on the use of metabolic intermediates, such as pyruvate or acetaldehyde, as electron acceptors. Consequently, sugar metabolism in aerated cultures of LAB can be substantially different from that in unaerated cultures. Energy and biomass yields, end-products of sugar metabolism and the range of substrates which can be metabolised are affected.Lactic acid bacteria exhibit an inducible oxidative stress response when exposed to sublethal levels of H202. This response protects them if they are subsequently exposed to lethal concentrations of H202. The effect appears to be related to other stress responses such as heat-shock and is similar, in some but not all respects, to that previously reported for enteric bacteria.