Background: Early and accurate diagnosis of rifampicin (RIF)-resistant Mycobacterium tuberculosis  (MTB) is essential for controlling community spread of drug-resistant tuberculosis (TB). In order to discover mutations residing outside the rifampicin resistance-determining region (RRDR) of the MTB rpoB  gene, we conducted this retrospective study.
Methods: We retrospectively screened patient records to obtain Xpert MTB/RIF assay results for patients who received care at the Beijing Chest Hospital from 2016 to 2019 in order to identify subjects who met study selection criteria. Stored frozen patient isolates were cultured, harvested, and then subjected to drug susceptibility testing. Concurrently, entire rpoB  gene DNA of each isolate was amplified and then sequenced to reveal rpoB  mutations.
Results: Overall, 104 RIF-susceptible tuberculosis patients who were tested using the Xpert MTB/RIF assay also had poor first-line regimen treatment responses. Isolates obtained from these cases included 101 MTB isolates that possessed wild-type rpoB  allelic profiles, as demonstrated using Sanger sequencing. However, sequences from the other three isolates confirmed that rpoB  of one isolate harbored a mutation encoding the amino acid substitution Ile491Phe and that rpoB  genes of two isolates contained a mutation encoding the amino acid substitution Ser450Leu.
Conclusion: Our data demonstrated that mutations found outside the RRDR of MTB rpoB  are rare in Beijing, China, indicating that World Health Organization-approved molecular diagnostics are generally suitable for diagnosing RIF resistance.
Keywords: tuberculosis, rifampicin-resistance, rpoB , diagnosis