DRUG SUSCEPTIBILITY TESTING OF MYCOBACTERIUM INTRACELLULARE BY RING DIFFUSION METHOD

Abstract

The susceptibility testing of 86 strains of Mycobacterium intracellulare, comprising 49 strains of human isolate, 21 of swine isolate and 16 of environmental isolate, to 10 kinds of anti-tuberculous drugs was performed, using ring diffusion method (HIRAMINE, S.: J. Med. Technol., 15: 271-273, 1971). M. tuberculosis H 37 Rv strain served as the control.
Susceptibility to the first choice anti-tuberculous drugs: The minimal inhibitory concentration (MIC) of SM, PAS and INH against 86 strains of M. intracellulare were: SM, 20μg/ml to 45 strains, 200 μg/ml to 38 strains, namely 200μg/ml or less to 83 strains (97%) in total; PAS, 10μg/ml or less to 2 strains (2%): INH, 5μg/ml or less to 19 strains (22%). The SM, PAS and INH completely inhibited the growth of H 37 Rv strain of M. tuberculosis at MIC of 20, 1 and 0.1 μg/ml, respectively.
Susceptibility to the second choice anti-tuberculous drugs: All the drugs tested completely inhibited the growth of H 37 Rv strain of M. tuberculosis at MIC: KM, 25μg/ml; CS, 20μg/ml; TH, 25μg/ml; EB, 2.5μg/ml; VM, 25μg/ml; CPM, 25μg/ml; and RFP, 10μg/ml. The M. intracellulare strains inhibited to grow completely at the concentration noted above were few as follows, with the exception of 66 strains (77%) to RFP: KM, 24 strains; CS, 3 strains; TH, 2 strains; EB, 3 strains; VM, 5 strains; and CPM, 1 strain. A consideradle number of strains were barely inhibited to grow at the high concentration: KM, 100 μg/ml to 53 strains (62%), 1000μg/ml to 8 strains (9%), namely 61 strains (71%) in total; CS, 40μg/ml to 59 strains (69%); VM, 100μg/ml to 51 strains (59%); CPM, 100μg/ml to 61 strains (71%). In the case of TH and EB, however, susceptible strains were limited as follows: TH, 50 μg/ml to 17 strains (20%); EB, 5μg/ml to 13 strains (15%). The cumulative numbers of susceptible strains at the maximal inhibitory concentrations tested were as follows: KM, 1, 000 μg/ml or less, 85 strains (99%); Cs, 40 genii or less, 62 strains (72%); TH, 50μg/ml or less, 19 strains (22%); EB, 5μg/ml or less, 16 strains (19%); VM, 100μg/ml or less, 56 strains (65%); CPM, 100 μg/ml or less, 62 strains (72%); and RFP, 50μg/ml or less, 83 strains (97%).
It is concluded that many strains of M. intracelluare were sensitive to RFP, whereas few strains were susceptible to SM, KM, CS, VM, and CPM. However, the susceptibility of this bacterium to anti-tuberculous drugs was generally lower than that of M. tuberculosis. These findings nearly coincided with those of previous investigators who used the routine dilution method.