Pseudomonas aeruginosa is a common cause of chronic infections in people with cystic fibrosis. Detecting bacterial proteins in sputum—the best source of bacteria from patient airways—is difficult. Thus researchers don’t know whether P. aeruginosa produces different proteins in patient airways versus in a lab dish. By using two stages of centrifugation at different spin speeds, Xia Wu and coworkers at the University of Washington enriched bacteria from sputum to allow analysis of their proteins with liquid chromatography/tandem mass spectrometry (J. Proteome Res. 2019, DOI: 10.1021/acs.jproteome.9b00122). The researchers detected 1,304 P. aeruginosa proteins directly from sputum. Compared with the cultured bacteria, airway bacteria produced more of 67 proteins. Airway bacteria produced higher levels of proteins involved in iron and phosphate uptake and in the production and transport of alginate, a polysaccharide involved in the formation of bacterial biofilms. They produced less of 117 proteins, including many that are involved with carbon metabolism. This is likely because the bacteria grow more slowly in sputum. The findings highlight how lab culture methods can provide poor models of how bacteria persist in the body. The new bacterial enrichment method is allowing further studies of chronic infections in humans for which it has been difficult to develop useful animal models, the researchers note.