Abstract

We investigated the effect of cyanide 3-chlorophenylhydrazone (CCCP) and other efflux pump inhibitors (EPIs) on the colistin susceptibility in Acinetobacter baumannii. While minimum inhibitory concentrations (MICs) of colistin in all colistin-resistant strains decreased significantly with 25 μM of CCCP and 2,4-dinitrophenol (DNP), phenyl-arginine-β- naphthylamide (PAβN), and reserpine did not decrease the colistin MICs. However, CCCP and DNP as well as PAβN and reserpine did not have a significant effect on the MICs of the other agents. Efflux pump gene expressions in colistinresistant strains were not increased compared with those in colistin-susceptible strains. When only 5X MIC of colistin (5 mg/L) was provided to a colistin-susceptible A. baumannii strain, the bacterial cell number was reduced by 9 h after exposure to colistin, but regrowth was observed. When CCCP was added to colistin, bacterial cells were completely killed after 24 to 48 h of incubation, which was not due to the toxicity of CCCP itself. Colistin resistance in A. baumannii may not be due to efflux pumps. Our present study suggests that bacterial cells with reduced metabolic activity by CCCP are more susceptible to colistin in A. baumannii. It may show the possibility that combined therapy with colistin and other antimicrobial agents could effective against A. baumannii infections.

• Keywords: time-kill assay; colistin resistance; efflux pump