Pseudomonas aeruginosa-secreted respiratory toxin HQNO triggers fatty acid accumulation in respiring Staphylococcus aureus, decreasing SaeRS-dependent transcriptional regulation.

Staphylococcus aureus and Pseudomonas aeruginosa are the two pathogens that colonize the airway of cystic fibrosis patients. As patients age, P. aeruginosa outcompetes S. aureus to become the predominant organism in the airway, which overlaps with worsening symptoms. This inverse correlation is partly due to the ability of P. aeruginosa to secrete secondary metabolites and virulence factors that are antagonistic to the host cells and other bacteria present. Several of these secondary metabolites inhibit S. aureus respiration. SaeRS is a two-component regulatory system that promotes the transcription of numerous virulence genes in S. aureus. The transcription of SaeRS-regulated genes is decreased as a function of respiratory status. The accumulation of intracellular fatty acids also negatively impacts the activity of SaeRS. Incubation of S. aureus with P. aeruginosa cell-free conditioned culture medium decreased the transcriptional output of the SaeRS system. Further analyses using P. aeruginosa mutant strains and chemical genetics determined that 2-heptyl-4-quinolone N-oxide (HQNO) was responsible for the SaeRS-dependent changes in gene regulation. Treatment with HQNO increased the abundance of cell-associated fatty acids. HQNO inhibits cell respiration, and the SaeRS system did not respond to HQNO treatment in a respiration-impaired S. aureus strain, which accumulates fatty acids. The data presented are consistent with a working model wherein treatment of S. aureus with HQNO inhibits respiration, increasing free fatty acid accumulation, which negatively impacts SaeRS signaling. This results in decreased expression of the SaeRS regulon, which has significant roles in pathogenesis.IMPORTANCEPseudomonas aeruginosa and Staphylococcus aureus are often co-isolated from the airways of cystic fibrosis patients. P. aeruginosa secretes non-essential metabolites that alter S. aureus physiology, providing P. aeruginosa with a competitive advantage. S. aureus can adapt to the presence of these metabolites, but the genetic mechanisms used to sense these P. aeruginosa-produced metabolites and/or the induced physiological changes are largely unknown. The S. aureus SaeRS two-component regulatory system positively regulates the expression of various virulence factors, including toxins and proteases, that facilitate adaptation to and survival in hostile host environments. This study demonstrates that the P. aeruginosa-produced respiratory toxin 2-heptyl-4-quinolone N-oxide inhibits respiration, decreasing the transcription of SaeRS-regulated genes and thereby decreasing virulence factor production. These findings could be exploited to decrease the ability of S. aureus to express virulence factors in various infection settings.