Disk diffusion testing with polymyxin and amikacin for differentiation of Mycobacterium fortuitum and Mycobacterium chelonei.

Disk diffusion is one method of susceptibility testing of the Mycobacterium fortuitum complex to antibacterial agents. We utilized disks of polymyxin B (300 U), amikacin, and kanamycin to determine whether they could also be used for species identification when compared with standard biochemical methods. With the polymyxin disk, 100% of 75 M. fortuitum strains produced zones of inhibition, whereas none (0%) of 58 Mycobacterium chelonei subspecies abscessus and chelonei strains had any zone of inhibition. With the amikacin disk, 99% of M. fortuitum biovariant fortuitum had zones of greater than or equal to 30 mm compared with 6% of M. chelonei. The rare M. chelonei-like organisms gave variable results, and 42% of the unnamed "third group" biovariant of M. fortuitum exhibited an unusual but diagnostic pattern of small zone sizes to amikacin and no zone to kanamycin. The kanamycin disk was otherwise not helpful, although it resulted in larger zone sizes for M. chelonei than did amikacin. Thus, disk diffusion susceptibilities which include these disks (especially polymyxin) will provide presumptive evidence of species as well as susceptibility data.