Investigation of colistin heteroresistance in multidrug-resistant Acinetobacter baumannii clinical isolates

Colistin is considered a last-resort antibiotic against Acinetobacter baumannii strains with advanced drug resistance. Various mechanisms, such as those affecting the bacterial outer membrane and efflux pumps, mediate colistin resistance.

One hundred clinical A. baumannii isolates were collected from various infections and identified using standard tests. The minimum inhibitory concentrations (MICs) of nine antibiotics, as well as colistin, against the isolates were determined using the broth microdilution method. The population analysis profile (PAP) was conducted to determine colistin heteroresistance, and a time-kill assay was also performed. Using polymerase chain reaction (PCR) and sequencing, the presence of mcr1-3 genes, as well as mutations in the pmrCAB and lpx genes, were investigated.

All isolates were multidrug-resistant, and more than 85 % of the isolates were resistant to all antibiotics tested except colistin (colistin MIC ≤1 mg/L). Eleven colistin-heteroresistant isolates were found. All heteroresistant isolates were also resistant to imipenem. The time-kill assay indicated at 1 × MIC and 2 × MIC concentrations, the three heteroresistant isolates showed noteworthy regrowth at 6 and 12 h, respectively. All of the isolates were negative for mcr genes. Among 11 heteroresistant isolates, mutations in the pmrB, lpxC, and lpxD genes were found in four isolates, with one isolate having double mutations in the pmrB and lpxD genes.

Our research indicates that resistant subpopulations may develop during colistin monotherapy. These findings highlight the importance of continuous surveillance, the use of combination therapy, and the development of new strategies to combat A. baumannii infections.