PHYSIOLOGY AND INTERACTION OF NITRATE AND NITRITE REDUCTION IN STAPHYLOCOCCUS-CARNOSUS

Staphylococcus carnosus reduces nitrate to ammonia in two steps. (i) N itrate was taken up and reduced to nitrite, and nitrite was subsequent ly excreted. (ii) After depletion of nitrate, the accumulated nitrite was imported and reduced to ammonia, which again accumulated in the me dium. The localization, energy gain, and induction of the nitrate and nitrite reductases in S. carnosus were characterized. Nitrate reductas e seems to be a membrane-bound enzyme involved in respiratory energy c onservation, whereas nitrite reductase seems to be a cytosolic enzyme involved in NADH reoxidation. Syntheses of both enzymes are inhibited by oxygen and induced to greater or lesser degrees by nitrate or nitri te, respectively. In whole cells, nitrite reduction is inhibited by ni trate and also by high concentrations of nitrite (greater than or equa l to 10 mM). Nitrite did not influence nitrate reduction. Two possible mechanisms for the inhibition of nitrite reduction by nitrate that ar e not mutually exclusive are discussed. (i) Competition for NADH nitra te reductase is expected to oxidize the bulk of the NADH because of it s higher specific activity. (ii) The high rate of nitrate reduction co uld lead to an internal accumulation of nitrite, possibly the result o f a less efficient nitrite reduction or export. So far, we have no evi dence for the presence of other dissimilatory or assimilatory nitrate or nitrite reductases in S. carnosus.