Single and double drug susceptibility testing of Mycobacterium avium complex and mycobacteria other than the tubercle (MOTT) bacilli by a micro-dilution broth minimum inhibitory concentration (MIC) method

Abstract Setting: The incidence of Mycobacterioses is increasing annually, especially in patients with AIDS. There is no clear correlation between in vitro drug susceptibility testing of mycobacteria other than the tubercle (MOTT) bacilli and the in vivo response. Although in vitro, MOTT bacilli appear resistant, some patients respond to treatment possibly as a result of a synergistic action between the drugs being used. Objective: To produce a simple method to determine which individual drugs or combinations of drugs will be effective in killing the causative organism. Design: A broth microdilution method, using microtitre plates, was used to determine the minimum inhibitory concentration (MIC) of the drugs alone and then in combination. Results: It was found that the MIC values of the test drugs varied between but were constant within each of the species. Mycobacterium xenopi, M. malmoense and M. kansasii showed a large amount of susceptibility while M. avium complex, M. fortuitum and M. chelonae were limited in their response. These results were reproducible. The test was also easy to perform. The drug combination studies showed that each strain of the M. avium complex tested exhibited synergy between different combinations of drugs. Conclusion: This method, therefore, can be used to indicate individual or combinations of drugs that will or will not act upon the Mycobacterium species isolated. The method was also very rapid, giving a result within 7 days.