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Opportunistic infections are often polymicrobial. Two of the most important bacterial opportunistic pathogens of humans, Pseudomonas aeruginosa and Staphylococcus aureus, frequently are coisolated from infections of catheters, endotracheal tubes, skin, eyes, and the respiratory tract, including the airways of people with cystic fibrosis (CF). Here, we show that suppression of S. aureus respiration by a P. aeruginosa exoproduct, 4-hydroxy-2-heptylquinoline-N-oxide (HQNO), protects S. aureus during coculture from killing by commonly used aminoglycoside antibiotics such as tobramycin. Furthermore, prolonged growth of S. aureus with either P. aeruginosa or with physiological concentrations of pure HQNO selects for typical S. aureus small-colony variants (SCVs), well known for stable aminoglycoside resistance and persistence in chronic infections, including those found in CF. We detected HQNO in the sputum of CF patients infected with P. aeruginosa, but not in uninfected patients, suggesting that this HQNO-mediated interspecies interaction occurs in CF airways. Thus, in all coinfections with P. aeruginosa, S. aureus may be underappreciated as a pathogen because of the formation of antibiotic-resistant and difficult to detect small-colony variants. Interspecies microbial interactions, analogous to those mediated by HQNO, commonly may alter not only the course of disease and the response to therapy, but also the population structure of bacterial communities that promote the health of host animals, plants, and ecosystems.

In many human infections, hosts and pathogens coexist for years or decades. Important examples include HIV, herpes viruses, tuberculosis, leprosy, and malaria. With the exception of intensively studied viral infections such as HIV/AIDs, little is known about the extent to which the clonal expansion that occurs during long-term infection by pathogens involves important genetic adaptations. We report here a detailed, whole-genome analysis of one such infection, that of a cystic fibrosis (CF) patient by the opportunistic bacterial pathogen Pseudomonas aeruginosa. The bacteria underwent numerous genetic adaptations during 8 years of infection, as evidenced by a positive-selection signal across the genome and an overwhelming signal in specific genes, several of which are mutated during the course of most CF infections. Of particular interest is our finding that virulence factors that are required for the initiation of acute infections are often selected against during chronic infections. It is apparent that the genotypes of the P. aeruginosa strains present in advanced CF infections differ systematically from those of "wild-type" P. aeruginosa and that these differences may offer new opportunities for treatment of this chronic disease.