Background: A rising isolation trend of drug-resistant M. bovis from human clinical cases is documented in the literature. Here we assessed Mycobacterium tuberculosis complex isolates from cattle for drug susceptibility by the gold standard agar proportion method and a simplified resazurin microtitre assay (d-REMA). A total of 38 M. tuberculosis complex strains, including M. bovis (n = 36) and M. caprae (n = 2) isolates, from cattle in Tunisia were tested against isoniazid, rifampin, streptomycin, ethambutol, kanamycin and pyrazinamide.
Results: M. caprae isolates were found to be susceptible to all test drugs. All M. bovis strains were resistant to pyrazinamide, as expected. In addition, one M. bovis isolate showed high-level resistance to streptomycin (MIC > 500.0 μg/ml). Concordant results with the two methods were found. The most common target genes associated with streptomycin resistance, namely the rrs, rpsL and gidB genes, were DNA sequenced. A non-synonymous mutation at codon 43 (K43R) was found in the rpsL gene. To the best of our knowledge, this is the first report describing the isolation of a streptomycin-resistant M. bovis isolate from animal origin.
Conclusions: Antitubercular drug susceptibility testing of M. bovis isolates from animals should be performed in settings where bTB is endemic in order to estimate the magnitude of the risk of drug-resistant tuberculosis transmission to humans.
Keywords: Bovine tuberculosis; Mycobacterium tuberculosis complex; Resazurin microtitre assay; Streptomycin resistance; d-REMA.