<i>P</i>. <i>aeruginosa</i> secondary metabolites inhibit <i>S</i>. <i>aureus</i> aerobic respiration resulting in a drop in intracellular ATP and protection from ciprofloxacin killing.

<p>(A) <i>S</i>. <i>aureus</i> strain HG003 harboring plasmid P<i>pflB</i>∷<i>gfp</i> was grown to mid-exponential phase and treated with supernatant from <i>P</i>. <i>aeruginosa</i> PAO1, PA14, CF isolates (blue) or burn isolates (green), for 30 min. OD₆₀₀ and <i>gfp</i> expression levels were determined after 16 h using a Biotek Synergy H1 microplate reader. (B) Intracellular ATP was measured after 1.5 h incubation with supernatant. ***<i>p</i> < 0.0005 (one-way ANOVA with Tukey’s multiple comparison post-test). (C) <i>S</i>. <i>aureus</i> strain HG003 was grown to mid-exponential phase in MHB media and pre-treated with sterile supernatants from <i>P</i>. <i>aeruginosa</i> strains PA14 wild-type or its isogenic mutants or (D) physiologically-relevant concentrations of HQNO, PYO, or NaCN for 30 min prior to antibiotic challenge [<a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.2003981#pbio.2003981.ref026" target="_blank">26</a>,<a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.2003981#pbio.2003981.ref027" target="_blank">27</a>]. At indicated times, an aliquot was washed and plated to enumerate survivors. All experiments were performed in biological triplicate. Underlying data can be found in <a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.2003981#pbio.2003981.s003" target="_blank">S1 Data</a>. Error bars represent mean ± sd. CF, cystic fibrosis; CFU, colony-forming units; GFP, green fluorescent protein; HQNO, 4-hydroxyquinoline <i>N</i>-oxide; MHB, Mueller-Hinton broth; NaCN, sodium cyanide; OD, optical density; PYO, pyocyanin.