causes acute and chronic infections, such as those that occur in the lungs of people with cystic fibrosis (CF). In infection environments, oxygen (O) concentrations are often low. The transcription factor Anr (naerobic regulation of arginine deiminase and itrate eduction) responds to low O by upregulating genes necessary for fitness in microoxic and anoxic conditions. Anr regulates Dnr (issimilative itrate respiration egulator), a gene encoding a transcriptional regulator that promotes the expression of genes required for using nitrate as an alternative electron acceptor during denitrification. In CF sputum, transcripts involved in denitrification are highly expressed. While Dnr is necessary for the anoxic growth of in CF sputum and artificial sputum media (ASMi), the contribution of denitrification to fitness in oxic conditions has not been well described. Here, we show that requires for fitness in ASMi, and the requirement for is abolished when nitrate is excluded from the media. Additionally, we show that consumes nitrate in lysogeny broth (LB) under microoxic conditions. Furthermore, strains without a functioning quorum sensing regulator LasR, which leads to elevated Anr activity, consume nitrate in LB even in normoxia. There was no growth advantage for when nitrate was present at concentrations from 100 to 1,600 µM. However, consumption of nitrate in oxic conditions created a requirement for Dnr and Dnr-regulated NorCB, likely due to the need to detoxify nitric oxide. These studies suggest that Anr- and Dnr-regulated processes may impact physiology in many common culture conditions.IMPORTANCE is an opportunistic pathogen commonly isolated from low-oxygen environments such as the lungs of people with cystic fibrosis. While the importance of energy generation by denitrification is clear in anoxic environments, the effects of denitrification in oxic cultures are not well understood. Here, we show that nitrate is consumed in microoxic environments and, in some strains, in normoxic environments. While nitrate does not appear to stimulate microoxic growth rate or yield, it does impact physiology. We show that the regulators Anr (naerobic regulation of arginine deiminase and itrate eduction) and Dnr (issimilative itrate respiration egulator), which are best known for their roles in anoxic conditions, contribute to fitness in common laboratory media in the presence of oxygen.