Etest for routine clinical antimicrobial susceptibility testing of rapid-growing mycobacteria isolates

The Etest has become a widely accepted alternative susceptibility-testing method for difficult-to-assess organisms, including rapid-growing Mycobacterium spp. Following an internal validation and literature reviews, the Etest was applied as the routine method for testing Mycobacterium chelonae and Mycobacterium fortuitum isolates. Results from testing 31 strains confirmed the utility of the Etest and the simplicity of the procedure. Mycobacterium chelonae were generally more resistant to all drugs except amikacin (MIC90, 32 micrograms/ml), compared with M. fortuitum strains that were inhibited (MIC50 in the susceptible range) by amikacin (1 microgram/ml), ciprofloxacin (0.032 microgram/ml), doxycycline (0.125 microgram/ml), and trimethoprim-sulfamethoxazole (0.032 microgram/ml). The polymyxin-B disk used as an identification method was confirmed (> or = 10 mm = M. fortuitum). The Etest provides a simple and accurate method for selecting appropriate therapy for infections caused by rapid-growing mycobacteria (a typical case report is presented).