Detection of Tigecycline Resistance in <i>Acinetobacter baumannii</i>: The Discrepancy between the Minimal Inhibitory Concentration Method and the Disk Diffusion Test

Background:The emergence of tigecycline-resistant Acinetobacter baumannii has been reported, and the need for tigecycline susceptibility testing in this strain is increasing.However, neither the Clinical & Laboratory Standards Institute, nor the European Commission on Antimicrobial Susceptibility Testing have provided definitive criteria for tigecycline susceptibility testing of A. baumannii .In this study, the disk diffusion method and the minimal inhibitory concentration (MIC) method were compared to verify conventionally used Food and Drug Administration-identified interpretive criteria to detect tigecycline resistance of A. baumannii.Methods: Forty-four strains of A. baumannii with tigecycline resistance were collected through the Kor-GLASS (Korean Global Antimicrobial Resistance Surveillance System) study in 2022 using the disk diffusion test (DDT).This strain was retested with the MIC method using a Sensititre Gram Negative GN6F AST plate (Thermo Fisher Scientific, USA) to confirm tigecycline resistance.The confirmed strain was subjected to whole genome analysis to elucidate the tigecycline resistance mechanism.Results: Only one of the 44 isolates identified as resistant to tigecycline by the DDT showed resistance with the MIC method, thus the concordance rate of the two methods was 2.3% (1/44).Sequence type 195 strain, carrying bla OXA23 was identified.This strain had no resistance genes of the tetracycline family but had resistance genes to other antimicrobial families.Conclusions: Discrepancy of the tigecycline susceptibility test of A. baumannii was identified.To detect tigecycline resistance of A. baumannii, more reliable methods are required.