Evaluation of eravacycline susceptibility testing by MIC test strip and disk diffusion against broth microdilution in...

Eravacycline is a novel fluorocycline with potent activity against carbapenem-resistant Enterobacterales (CRE). However, comparative validation of gradient diffusion (MIC Test Strip, MTS) and disk diffusion (DD) against reference broth microdilution (BMD) specifically in CRE isolates remains limited, particularly under region-specific breakpoint systems.

This study evaluated the performance of the MTS and DD against BMD for eravacycline susceptibility testing in 358 CRE clinical isolates (174 carbapenem-resistant Klebsiella pneumoniae, CRKP and 144 carbapenem-resistant Escherichia coli, CREC), using both U.S. Food and Drug Administration (FDA) and Expert Committee of the National Health Commission on Antimicrobial Susceptibility Testing and Standard Research (ChinaCAST) breakpoints. Categorical agreement (CA), essential agreement (EA), major error (ME), and very major error (VME) were assessed according to CLSI M52 criteria, and Bland-Altman analysis was performed for quantitative agreement between MTS and BMD.

BMD showed MIC₅₀/MIC₉₀ of 0.25/1 mg/L, with overall susceptibility rate of 81.2% by FDA breakpoints. For MTS, overall CA and EA were 92.8% and 92.5%, but VME was 33.8% under FDA breakpoints, concentrated in CRKP (35.5%). Under ChinaCAST breakpoints, CRKP CA increased to 94.8%, yet VME decreased to 18.2% and ME increased to 4.3%. For DD, overall CA was 89.9% with VME of 32.3%; only CREC under FDA breakpoints achieved acceptable CA (97.9%) and error rates, whereas ChinaCAST breakpoints yielded unacceptable CA (77.0-77.8%) and elevated ME (22.4-24.5%) for both species.

Eravacycline retains potent in vitro activity against CRE. MTS is suitable for routine testing of CREC, and DD is reliable for CREC under FDA breakpoints. For CRKP, neither method satisfied CLSI M52 criteria, and reference BMD remains indispensable.