Deciphering colistin heteroresistance in Acinetobacter baumannii clinical isolates from Wenzhou, China

This study aimed to investigate the characteristics and mechanisms responsible for heteroresistance to colistin in Acinetobactor baumannii clinical isolates from a Chinese teaching hospital. Five hundred and seventy-six nonduplicate A. baumannii clinical isolates isolated from 2014 to 2015 were tested. Colistin heteroresistance was determined using population analysis profiles (PAPs). Susceptibility testing was conducted using the broth microdilution method (BMD). The ability to form biofilm formation was determined using 96-well flat bottom microtiter plates. Time-kill assays were also conducted. PCR and sequencing were used to detect the presence of resistant genes. Expression levels of efflux pump genes were determined by qRT-PCR. LPS analysis was conducted by SDS-PAGE. Lipid A characteristic were determined via MALDI-TOF MS. Nine colistin heteroresistant A. baumannii clinical isolates which were selected by PAPs, exhibited multidrug-resistant phenotypes. The microplate biofilm assay revealed that colistin heteroresistant A. baumannii clinical isolates had weaker biofilm formation capacity than A. baumannii ATCC19606. Colistin-heteroresistant A. baumannii isolates exhibited regrowth after 12 h at 0.5 x MIC, 1 x MIC, and 2 x MIC. The results of PCR and sequencing revealed mutations of lpxACD in some colistin heteroresistant A. baumannii isolates. qRT-PCR also showed that the expressions of efflux pump genes were upregulated in some of the heteroresistant isolates. Our study was the first report of colistin heteroresistant A. baumannii clinical isolates in China. The transition of colistin heteroresistance to resistance should be of concern in future clinical surveillance.