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[Cross-resistance between rifampicin and rifabutin among multi-drug resistant Mycobacterium tuberculosis strains].

OBJECTIVE
To compare the susceptibility of rifampicin (RFP) and rifabutin (RBT) against multi-drug resistant Mycobacterium tuberculosis (MDR-TB).


METHOD
A total of 44 confirmed MDR-TB strains collected by Ryoken consortium and 97 susceptible M. tuberculosis strains were tested for the susceptibility to RBT, following CLSI M24-A laboratory standard. The core 81bp region of rpoB gene was sequenced for MDR-TB strains, and the mutations were defined.


RESULTS
Among the 44 MDR-TB strains tested, 12 strains were susceptible to RBT. The genetic alterations were identified in 43 of 44 MDR-TB strains. The RBT susceptible strains had alterations mainly at codons 506-508, 511, 512, and 516. The mutation at codons 526 and 531 closely related to RBT resistance with two exceptions. The overall cross-resistance between RFP and RBT was 72.7%.


DISCUSSION
RBT will be a candidate drug for the treatment of MDR-TB. In this study, approximately 27% of MDR-TB in Japan had susceptibility to RBT. However, the drug susceptibility testing for RBT is not easy at general microbiology laboratory considering the preparation of medium and laboratory facilities. It is also difficult to transfer confirmed MDR-TB strains to one place to another for testing legally. The mutations at codons 526 and 531 in rpoB are closely related to RBT resistance and will be useful to predict its susceptibility.