Bactericidal Effect of Tomatidine-Tobramycin Combination against Methicillin-Resistant Staphylococcus aureus and Pseudomonas aeruginosa Is Enhanced by Interspecific Small-Molecule Interactions

Abstract

ABSTRACTThis study investigated the antibacterial activity of the plant alkaloid tomatidine (TO) againstStaphylococcus aureusgrown in the presence ofPseudomonas aeruginosa. Since theP. aeruginosaexoproduct 4-hydroxy-2-heptylquinoline-N-oxide (HQNO) is known to cause a respiratory deficiency inS. aureusand respiratory-deficientS. aureusare known to be hypersensitive to TO, we assessed kill kinetics of TO (8 μg/ml) againstS. aureusin coculture withP. aeruginosa. Kill kinetics were also assessed usingP. aeruginosamutants deficient in the production of different exoproducts and quorum sensing-related compounds. After 24 h in coculture, TO increased the killing ofS. aureusby 3.4 log10CFU/ml in comparison to that observed in a coculture without TO. The effect of TO was abolished whenS. aureuswas in coculture with thelasRrhlR,pqsA,pqsL, orlasAmutant ofP. aeruginosa. The bactericidal effect of TO againstS. aureusin coculture with thepqsLmutant was restored by supplemental HQNO. In anS. aureusmonoculture, the combination of HQNO and TO was bacteriostatic, indicating that thepqsLmutant produced an additional factor required for the bactericidal effect. The bactericidal activity of TO was also observed against a tobramycin-resistant methicillin-resistantS. aureus(MRSA) in coculture withP. aeruginosa, and the addition of tobramycin significantly suppressed the growth of both microorganisms. TO shows a strong bactericidal effect againstS. aureuswhen cocultured withP. aeruginosa. The combination of TO and tobramycin may represent a new treatment approach for cystic fibrosis patients frequently cocolonized by MRSA andP. aeruginosa.