Polymorphonuclear leukocytes (PMNs) direct adaptation by shaping the chemical microenvironment of infections . PMNs accelerate O2 depletion and Pseudomonas aeruginosa adapts by employing anaerobic respiration in lung secretions from cystic fibrosis patients with chronic P. aeruginosa pneumonia. Therefore, I focus on the importance of O2 depletion for microbial pathogenicity, activity of antibiotics, and host response in inflammations involving PMNs.