Propionate restores susceptibility to colistin-resistant clinical isolates of Pseudomonas aeruginosa.

There are very few treatment options against infections caused by multi-resistant Pseudomonas aeruginosa , especially when even the activity of colistin is compromised. Here, the synergy between propionate and colistin is reported: at sub-inhibitory concentrations (0.2%), propionate diminished the colistin minimal inhibitory concentration of four colistin-resistant P. aeruginosa clinical isolates, from values indicative of resistance (4–8 µg/mL), down to 1 µg/mL, in checkerboard experiments. Propionate had no effect upon other resistance phenotypes (meropenem, ciprofloxacin, amikacin) present in these isolates. Propionate exposure had mixed effects upon several putative virulence phenotypes: biofilm formation, swarming and pyocyanin production, increasing them in some, diminishing or without effect in others. Although reaching plasma concentrations of propionate as high as 0.2% seems unlikely, the topical application of highly concentrated solutions or ointments has been used before, opening the possibility of propionate-colistin co-treatment of skin infections. While there is a risk of enhancing the virulence of surviving microorganisms, the lack of better options against pan-resistant P. aeruginosa infections may warrant the clinical testing of this co-treatment.