Abstract
BackgroundDrug resistance displays a problem for the therapy of Mycobacterium tuberculosis infections. For molecular resistance testing, it is essential to have precise knowledge on genomic variations involved in resistance development. However, data from high-incidence settings are only sparely available. Therefore we performed a systematic approach and analyzed a total of 97 M. tuberculosis strains from previously treated patients in Sierra Leone for mutations in katG, rpoB, rrs, rpsL, gidB, embB, pncA and where applicable in inhA and ahpC. Of the strains investigated 50 were either mono- or poly-resistant to isoniazid, rifampin, streptomycin, ethambutol and pyrazinamide or MDR and 47 fully susceptible strains served as controls.ResultsThe majority of isoniazid and rifampin resistant strains had mutations in katG315 (71.9%) and rpoB531 (50%). However, rpoB mutations in codons 511, 516 and 533 were also detected in five rifampin susceptible strains. MIC determinations revealed low-level rifampin resistance for those strains. Thus, the sensitivity and specificity of sequencing of katG for detection of drug resistance were 86.7% and 100% and for sequencing of rpoB 100% and 93.8%, respectively.Strikingly, none of the streptomycin resistant strains had mutations in rrs, but 47.5% harboured mutations in rpsL. Further changes were detected in gidB. Among ethambutol resistant strains 46.7% had mutations at embB306. Pyrazinamide resistant strains displayed a variety of mutations throughout pncA. The specificities of sequencing of rpsL, embB and pncA for resistance detection were high (96-100%), whereas sensitivities were lower (48.8%, 73.3%, 70%).ConclusionsOur study reveals a good correlation between data from molecular and phenotypic resistance testing in this high-incidence setting. However, the fact that particular mutations in rpoB are not linked to high-level resistance is challenging and demonstrates that careful interpretation of molecular resistance assays is mandatory. In addition, certain variations, especially in gidB, appear to be phylogenetically informative polymorphisms rather than markers for drug resistance.
Highlights
Drug resistance displays a problem for the therapy of Mycobacterium tuberculosis infections
Mutations responsible for RIF resistance are primarily located in the so-called rifampin resistance determining region (RRDR; codon 507–533 according to E. coli numbering system) of the rpoB gene which encodes the beta subunit of the RNA polymerase [11]
Drug susceptibility testing to PZA (100 μg/ml) was performed by using the BACTECTM Pyrazinamide (PZA) Drug Kit in the BACTEC 460 TB system according to the manufacturers instructions
Summary
Drug resistance displays a problem for the therapy of Mycobacterium tuberculosis infections. We performed a systematic approach and analyzed a total of 97 M. tuberculosis strains from previously treated patients in Sierra Leone for mutations in katG, rpoB, rrs, rpsL, gidB, embB, pncA and where applicable in inhA and ahpC. Molecular assays that detect the genetic variants that mediate resistance constitute a rapid alternative to conventional drug susceptibility testing (DST) and may even be performed directly on clinical specimens without culture [5,6]. Mutations responsible for RIF resistance are primarily located in the so-called rifampin resistance determining region (RRDR; codon 507–533 according to E. coli numbering system) of the rpoB gene which encodes the beta subunit of the RNA polymerase [11]. Resistance to ethambutol (EMB) is primarily mediated by mutations in the embB gene, coding for an arabinosyltransferase participating in mycobacterial cell wall synthesis, with codon 306 being most frequently affected [14]. Resistant strains lack pyrazinamidase activity which is essential for pro drug activation
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