The Evaluation of Four In Vitro Susceptibility Testing Methods for Colistin on Carbapenenm-Resistant Acinetobacter baumannii

The carbapenem resistance in Acinetobacter baumannii (CRAB) is a global health problem because of the worldwide distribution of the bacteria and a few available therapeutic options. Colistin is considered as the last resort to treat the infection. At present, there are several methods to detect the colistin susceptibility, including broth microdilution with 0.002% polysorbate 80 (BMD-P80), the E-test, broth microdilution (BMD), and agar dilution (AD). However, the differences in efficacy between the methods are not well studied.
The current study aimed at evaluating the 4 available methods to test in vitro susceptibility to colistin and observing the degree of heteroresistance in CRAB species in China.
To evaluate the methods, a total of 202 CRAB species isolated from 12 hospitals in Zhejiang province, China, collected from January to December in 2010 were employed retrospectively. Colistin minimum inhibitory concentrations (MICs) were determined by the 4 different methods. Population analysis profiles (PAPs) were also conducted in 29 CRAB strains.
The proportions of colistin-sensitive isolates were 100%, 100%, 99.5%, and 90.6% by BMD-P80, E-test, BMD, and AD methods, respectively, according to the EUCAST breakpoints. The AD methods produced an excessive number of major errors (MEs) (9.4%), while E-test and BMD produced 0.5% MEs. Moreover, the AD method resulted in a minimum essential agreement (EA) at 9.4%, and 179 isolates obtained a higher (≥ 3 log 2) number of dilutions than the BMD-P80. Not very major errors (VMEs) were found by any of the tested methods. In 29 selected CRAB isolates, a total of 31% were heterogeneous and the rate of heteroresistance was also 31%.
The AD method was not a prior choice of in vitro colistin susceptibility testing because it led to false colistin resistance results. E-test, BMD, and BMD-P80 may be more reliable to test the susceptibility when colistin is considered as a potential therapeutic agent. A high rate of heterogeneous and heteroresistant species were found in CRAB in China; it highlighted the importance of MICs monitoring before and through the colistin therapy period.