Effect of enzymes on the diauxic lag of denitrifying bacteria switching between electron acceptors

The hypothesis that presence of the periplasmic nitrate reductase during aerobic growth minimizes or eliminates the diauxic lag was tested. Indeed, little or no lag is observed when bacteria known to express the periplasmic enzyme (Paracoccus pantotrophus) are subjected to an anoxic environment after a period of aerobic growth. Long lags were still observed when aerobically grown cultures of bacteria lacking the periplasmic enzyme (Pseudomonas denitrificans) were switched to anoxic conditions. There was some indication that more oxidized substrates led to longer diauxic lag lengths for Paracoccus pantotrophus, although no long lags were observed for any of the substrates with this species. Enzyme assays were performed to confirm activity of the enzyme.; The periplasmic nitrate reductase is not expressed in all denitrifying bacteria. However, all denitrifying bacteria have a membrane-bound nitrate reductase to catalyze the use of nitrate as electron acceptor. A simple model has been formulated to simulate diauxic growth of denitrifying bacteria that only have the membrane-bound enzyme. It successfully depicts the observed lags when a pure culture of Pseudomonas denitrificans switches from oxygen to nitrate as terminal electron acceptor. The model includes the effect of carbon substrate limitation and length of aerobic phase; and does not run into problems when switching from anoxic to aerobic conditions, unlike earlier models of diauxic growth. In order to fit growth data of bacteria that also have the periplasmic nitrate reductase, a preliminary model was devised that includes the contribution of both enzymes.