Wild-type distributions of seven oral second-line drugs against Mycobacterium tuberculosis
METHODS: We determined wild-type MIC distributions of M. tuberculosis using a 96-stick replicator in Middlebrook 7H10 (7H10) medium for ethionamide (ETH), prothionamide, thiacetazone, cycloserine, rifabutin (RFB), clofazimine and linezolid in consecutive susceptible clinical isolates (n = 78).
RESULTS: Tentative epidemiological wild-type cut-offs (ECOFF) were determined for all investigated drugs where World Health Organization recommended critical concentrations for 7H10 are lacking, except for ETH. As the ECOFF was closely related to the non-wild-type strains for ETH and thiacetazone, the use of an intermediary (I) category in drug susceptibility testing could increase reproducibility. The cross-resistance between ETH and isoniazid was 21%. Applying 0.5 mg/l as a breakpoint for RFB classified two non-wild type and rpoB mutated isolates as susceptible for RFB and resistant against rifampicin.
CONCLUSIONS: We propose that wild-type MIC distributions should be used as a tool to define clinical breakpoints against second-line drugs. This is increasingly important considering the rapid emergence of drug resistance.
Document Type: Regular Paper
Affiliations: 1: Department of Clinical Microbiology, Kalmar County Hospital, Kalmar, Sweden 2: Department of Bacteriology, Swedish Institute of Disease Control (SMI), Stockholm, Sweden 3: Department of Clinical Microbiology, Department of Microbiology, Tumor and Cell Biology, Karolinska Institutet, Karolinska University Hospital, Stockholm, Sweden 4: Medical Microbiology, Department of Laboratory Medicine, Lund University, Malmö, Sweden 5: Department of Clinical Microbiology, Växjö Hospital, Växjö, Sweden; and Department of Medical Sciences, Uppsala University, Uppsala, Sweden
Publication date: April 1, 2011
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