Drug sensitivity of low growing non-­­tuberculous mycobactria

Abstract

Objective — analysis of drug resistance (DR) of some types of low growing non-­tuberculous mycobactria (NTMB) by the method of double serial microdilutions of drugs of different groups to determine the minimum inhibitory concentra­tions (MIC) in a liquid culture medium using the TREK Diagnostic Systems test system, Thermo Scientific SLOWMYCO for low growing mycobacteria.
 Materials and methods. Investigated 122 strains of low growing NTMB (M. avium, M. intracellulare, M. gordonae, M. kansasii, M. xenopi, M. malmoense and M. simiae), which were isolated in a liquid nutrient medium during sputum inoculation. DR was determined by the culture method using the TREK Diagnos­tic Systems kits (panel for low growing NTMB, which allows to determine the MIC of 13 drugs). The results were interpreted using an automatic bacteriological analyzer Sensititre Vizion System TREK Diagnostic Systems (USA).
 Results and discussion. Was found that for most M. avium strains the MIC of amikacin was 16.0—32.0 µg/ml, for ciprofloxacin — 16.0 µg/ml and higher, clarithromycin — 2.0—4.0 µg/ml, doxycycline — 16.0 µg/ml and above, ethambutol — 8.0—16.0 µg/ml. The MIC spectrum of ethionamide was distributed in the range from 1.2 to more than 20.0 µg/ml. MIC of isoniazid for most strains of M. avium was more than 8.0 µg/ml, linezolid — 16.0—32.0 µg/ml, moxifloxacin — 2.0—4.0 µg/ml, rifabutin — 0.25 µg/ml, rifampicin 4.0 µg/ml and above, streptomycin 64 µg/ml and above, and trimethoprim/sulfamethoxazole — more than 8.0/152 µg/ml. For M. intracellulare strains, in general, a similar situation with the M. avium strains with the MIC spectrum was observed. The work also provides MICs for the strains M. gordonae, M. kansasii, M. xenopi, M. simiae and M. malmoense. Based on the determination of the DR, the MIC50 and MIC90 values of each preparation of the SLOWMYCO panel were calculated for the studied species of low growing NTMB. Comparison of the MIC50 and MIC90 values with the limiting drug concentrations made it possible to determine the drugs effective against the studied types of NTMs.
 Conclusions. Determination of DR by the micromethod of serial dilutions in a liquid nutrient medium showed that most of the studied strains of low growing NTMPs are sensitive to clarithromycin and rifabutin. Amikacin, linezolid, and moxifloxacin were also quite effective. At the same time, drugs such as ethambutol, isoniazid, streptomycin, trimethoprim/sulfamethoxazole, suppressed the growth of the studied strains mainly in high concentrations, significantly exceeding the critical one.Until now, there is no single criterion for determining the DR NTM, and for the method used in this work, recommended by the Institute for Clinical and Laboratory Standards (USA), there are limitations in the interpretation of the results due to the untreated limit concentrations of drugs for various types of NTM. This is especially true for MAC, which play a major role in the development of mycobacteriosis. It was shown that the resistance profile of M. avium strains included the largest number of SLOWMYCO panel preparations compared to other types of NTMB, which dictates the need for studies aimed at comparing the results of in vitro DST with the effectiveness of therapy.