Pseudomonas aeruginosa is an important pathogen that infects the lungs of cystic fibrosis patients. A paper published in the October issue of Developmental Cell (Yoon et al., 2002) proves that the bacterial cells in the lung live in anaerobic biofilms and adopt a metabolic pattern and a phenotype that differ significantly from those of test tube-grown cells, and even from those of aerobic biofilms.