Rifabutin Cross-resistance and Associated rpoB Mutations in Rifampicin-resistant Mycobacterium tuberculosis Complex...

This study aimed to phenotypically evaluate rifabutin susceptibility in rifampicin-resistant Mycobacterium tuberculosis complex isolates and to identify, through DNA sequence analysis of the rifampicin resistance-determining region (RRDR), specific rpoB mutations associated with rifabutin susceptibility.

A total of 43 rifampicin-resistant and 23 rifampicin-susceptible (control) Mycobacterium tuberculosis complex isolates, collected between 2000 and 2020 at the Mycobacteriology Laboratories of Manisa Celal Bayar University and Ege University Faculty of Medicine, were included. Phenotypic drug susceptibility testing was performed using the MGIT 960 system. Mutations in a 682-bp fragment encompassing the 81-bp RRDR of the rpoB gene were investigated by Sanger sequencing.

Among the 43 rifampicin-resistant isolates, 39 (91%) were resistant to rifabutin, while four (9%) remained susceptible. All rifampicin-susceptible isolates were also susceptible to rifabutin. A total of 44 non-synonymous single nucleotide polymorphisms were detected in rifampicin-resistant isolates, whereas no mutations were observed in the control group. Seven distinct mutation patterns were identified, including one isolate with a double mutation. The most common mutations were S531L (76%), H526Y (12%), and D516Y (4%). The mutations D516Y, L511P, and L533P, as well as the M515I+D516Y double mutation, were associated with rifabutin susceptibility.

Early identification of rpoB mutations associated with rifabutin susceptibility may allow substitution of rifampicin with rifabutin in selected rifampicin-resistant cases, potentially improving treatment outcomes and limiting the emergence of resistance to second-line drugs. Further in vitro studies, including minimum inhibitory concentration testing, and additional clinical data on rifabutin-containing regimens are needed.