The recent findings of Ojoo et al¹ are of considerable interest. However, one confounding factor that appears to have been overlooked in recent studies of airway pH and exhaled breath nitric oxide (eNO) levels in cystic fibrosis (CF) is that of bacterial respiration. Pseudomonas aeruginosa adopts an anaerobic and biofilm mode of existence within the CF lung and, under such environmental conditions, it uses NO rather than oxygen as an electron donor to generate energy via oxidative phosphorylation. This bacterial denitrification results in the stepwise reduction of NO to nitrite (NO₂), nitrate (NO₃), and ultimately ammonium.^2,3 It is surprising that such an important metabolic process has been ignored as the high energy …