The inhA mutation has previously been described in the literature [24] as being the most common variation in the inhA promoter region related to INH resistance. Mutations in ahpC have been found before, however to our knowledge not at this position. In one of the resistant strains no mutation was found in neither the complete katG gene nor in inhA or in ahpC. This result suggests a so far unknown resistance mechanism as being responsible for INH resistance of this strain. Mutations in rpoB at codons 526 and 531 occur most frequently in the RIF resistant strains analyzed. Those SNPs are located in the RRDR and are well known for mediating

resistance [27, 28]. The mutation at codon 481, which only occurs in one RIF resistant isolate, has to our knowledge not been described previously. Selonsertib purchase The mutations at codon 511 (Leu → Pro), 516 (Asp → Tyr) and 533 (Leu → Pro) conferred low-level resistance in agreement with previous studies [29, 30]. It has been shown that various substitutions in the same codon lead to different levels of resistance. For example mutations at codon 516 can confer either low- or high-level resistance depending on the amino acid change [30]. Furthermore, the phenomenon of RIF low-level resistance has only recently

been described in a work by Van Deun and colleagues [31], where mutations at codon 511, 516 and 533 have been found LCZ696 order in strains tested susceptible by the radiometric Bactec 460 TB and Bactec 960 MGIT methods. Our data confirm the existence of low-level RIF resistance mediated by specific mutations in rpoB that is not detected by standard drug susceptibility testing methods. However, MIC values, especially for the mutations at codon 516 and 533, are even lower (0.5-1.0 μg/ml) than have been described in the literature. This fact may be due to the presence of further mutations in the operon or in other regions of the genome. In a recent study [32] the therapeutic challenge of low-level RIF resistance has

been addressed and may, according to the authors, be overcome by the application of higher RIF doses (20 mg/kg) in treatment regimens. However, the clinical relevance and interpretation next of these data is still not fully LY3023414 understood and needs further investigation in animal treatment models or clinical trials. Despite these discordant findings, we found a good correlation between the results from molecular and phenotypic testing for INH and RIF, as has been observed in another study [33]. In fact, the strains analyzed in this study predominantly harbour well described mutations which allows for the application of standard sequencing protocols or commercial line probe assays. The analysis of SM resistance mechanisms revealed an interesting observation. None of the SM resistant strains carried a mutation in the rrs gene, although those mutations have been described as main resistance mechanisms that confer high-level SM resistance [12].