In vitro antimicrobial susceptibility testing of rapidly growing mycobacteria using the tablet diffusion method: resistance pattern of Norwegian Mycobacterium fortuitum and Mycobacterium chelonae isolates

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Thirty-one Norwegian clinical isolates of rapidly growing mycobacteria classified as Runyon's group IV, including 20 Mycobacterium fortuitum and 11 Mycobacterium chelonae strains, were found resistant to a majority of tuberculostatic agents. Minimal inhibitory concentration (MIC) was determined for twelve other antimicrobial agents: amikacin, tobramycin, streptomycin, cefoxitin, imipenem, norfloxacin, ciprofloxacin, doxycycline, erythromycin, fusidic acid, co-trimoxazole and capreomycin. The agar plate dilution method was employed and compared with the agar tablet diffusion method. Regression lines were established correlating MIC values and inhibition zones. The agar tablet diffusion method was found to be a simple and useful method for testing antimicrobial susceptibilities of M. fortuitum and M. chelonae, and a good correlation between MIC values and zone sizes with twelve antimicrobial agents was revealed. Correlation coefficients for most of these antimicrobial agents were around -0.90. M. chelonae was generally more resistant than M. fortuitum. Four antimicrobial agents, capreomycin, norfloxacin, ciprofloxacin and amikacin, showed differences between M. fortuitum and M. chelonae large enough to allow the zone diameter to be used diagnostically.