Evaluating amikacin minimum inhibitory concentration in trailing growth for Mycobacterium avium complex

Amikacin is a first-line drug that must be evaluated when performing an antimycobacterial susceptibility test (AST) for Mycobacterium avium complex (MAC). However, the presence of sporadic trailing growth in MAC makes determining the precise point for reading its minimal inhibitory concentration (MIC) challenging.

Susceptibility was re-tested for 134 MAC clinical isolates using the Sensititre SLOMYCOI panel, the rrs gene was sequenced, and amikacin exposure history was investigated. The MIC₅₀, MIC₉₀, and the epidemiological cut-off value (ECOFF) were calculated using the EUCAST method.

After re-testing and ignoring trailing growth, of the 22 M. intracellulare isolates originally classified as resistant to amikacin according to the CLSI guideline, 10 strains were reclassified as intermediate and four as susceptible. Similarly, from the seven resistant M. avium strains, one was reclassified as intermediate and four as susceptible. No rrs gene mutations were detected in any isolates, including resistant strains. When ignoring trailing growth, the calculated MIC₅₀, MIC₉₀, and ECOFF values closely aligned with the EUCAST MIC distribution.

To maintain the current CLSI breakpoint, trailing growth should be ignored when reading the amikacin MIC of MAC. To read the MIC at complete bacterial inhibition, the CLSI breakpoint needs to be raised.