DfrA-thyA double deletion in para-aminosalicylic acid resistant Mycobacterium tuberculosis Beijing strains

Abstract

Para-aminosalicylic acid (PAS) is a group 4 anti-tuberculosis agent (1). It targets folate metabolism as shown in Fig. S1, which also summarises the known resistance mechanisms to this pro-drug (2). Recently, we reported a multidrug-resistant (MDR) Mycobacterium tuberculosis Beijing strain harbouring a deletion of both dfrA and thyA from Australia (Fig. 1A and Table S1) (3). Since then, we have found deletions affecting both genes in five further MDR Beijing strains (two isolated in Australia and three from Peru) and one extensively drug-resistant (XDR) Beijing strain from China. The Australian MDR strains were recovered from three patients with no apparent epidemiological links and were likely infected in their country of origin (Table S1). The three Peruvian isolates were closely related and consequently shared the same deletion, whereas the remaining strains were distantly related and had deletions that differed in size (Fig. 1A). Consequently, these five distinct deletions were acquired independently, which can be a signal for positive selection of resistance mechanisms. In line with this hypothesis, the strains from Australia and China were PAS resistant when tested with the BACTEC MGIT 960 system and on Löwenstein-Jensen medium, respectively (Supplementary Methods). Two out of the three Peruvian deletion mutants were also PAS resistant on 7H10 medium at 8 μg/mL, whereas the two closely related ancestral wild-type strains were susceptible (Fig. 1B). We were unable to retest the strains at 2 μg/mL, the recommended critical concentration by the Clinical and Laboratory Standards Institute and World Health Organization, which would have clarified whether the susceptible result for the third deletion mutant was an artefact (1, 4).