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

<i>Staphylococcus aureus</i> and <i>Pseudomonas aeruginosa</i> are the two pathogens that colonize the airway of cystic fibrosis patients. As patients age, <i>P. aeruginosa</i> outcompetes <i>S. aureus</i> to become the predominant organism in the airway, which overlaps with worsening symptoms. This inverse correlation is partly due to the ability of <i>P. aeruginosa</i> to secrete secondary metabolites and virulence factors that are antagonistic to the host cells and other bacteria present. Several of these secondary metabolites inhibit <i>S. aureus</i> respiration. SaeRS is a two-component regulatory system that promotes the transcription of numerous virulence genes in <i>S. aureus</i>. 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 <i>S. aureus</i> with <i>P. aeruginosa</i> cell-free conditioned culture medium decreased the transcriptional output of the SaeRS system. Further analyses using <i>P. aeruginosa</i> 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 <i>S. aureus</i> strain, which accumulates fatty acids. The data presented are consistent with a working model wherein treatment of <i>S. aureus</i> 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.IMPORTANCE<i>Pseudomonas aeruginosa</i> and <i>Staphylococcus aureus</i> are often co-isolated from the airways of cystic fibrosis patients. <i>P. aeruginosa</i> secretes non-essential metabolites that alter <i>S. aureus</i> physiology, providing <i>P. aeruginosa</i> with a competitive advantage. <i>S. aureus</i> can adapt to the presence of these metabolites, but the genetic mechanisms used to sense these <i>P. aeruginosa</i>-produced metabolites and/or the induced physiological changes are largely unknown. The <i>S. aureus</i> 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 <i>P. aeruginosa</i>-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 <i>S. aureus</i> to express virulence factors in various infection settings.