Respiration capacity of the fermenting bacterium Lactococcus lactis and its positive effects on growth and survival

Oxygen is a major determinant of both survival and mortality of aerobic org anisms. For the facultative anaerobe Lactococcus lactis, oxygen has negativ e effects on both growth and survival. We show here that oxygen can be bene ficial to L. lactis if heme is present during aerated growth. The growth pe riod is extended and long-term survival is markedly improved compared to re sults obtained under the usual fermentation conditions. We considered that improved growth and survival could be due to the capacity of L. lactis to u ndergo respiration. To test this idea, we confirmed that the metabolic beha vior of lactococci in the presence of oxygen and hemin is consistent with r espiration and is most pronounced late in growth. We then used a genetic ap proach to show the following. (i) The cydA gene, encoding cytochrome d oxid ase, is required for respiration and plays a direct role in oxygen utilizat ion, cydA expression is induced Late in growth under respiration conditions , (ii) The hemZ gene, encoding ferrochelatase, which converts protoporphyri n IX to heme, is needed for respiration if the precursor, rather than the f inal heme product, is present in the medium. Surprisingly, survival improve d by respiration is observed in a superoxide dismutase-deficient strain, a result which emphasizes the physiological differences between fermenting an d respiring lactococci. These studies confirm respiratory metabolism in L. lactis and suggest that this organism may be better adapted to respiration than to traditional fermentative metabolism.