Ethambutol resistance testing by mutation detection.
OBJECTIVE
To identify chromosomal mutations that confer resistance to ethambutol (EMB) in Mycobacterium tuberculosis.


METHODS
Drug-resistant (n = 235) and drug-susceptible (n = 117) M. tuberculosis isolates collected from the Western Cape in South Africa were subjected to embB gene analysis and the results were compared to phenotypic EMB testing.


RESULTS
Genotypic analysis identified mutations at codon 306 of the embB gene in 20% (47/235) of the resistant isolates in comparison to only 1.7% (4/235) of those that were phenotypically resistant to EMB by the agar diffusion method. No gene mutations were detected in susceptible isolates. Phenotypic retesting in BACTEC demonstrated that the 47 genotypically resistant isolates were phenotypically resistant to EMB. This implies that 91.4% (43/47) of EMB resistance had been phenotypically missed by routine laboratory procedures. EMB resistance was closely linked to multidrug resistance (MDR); 87.2% (41/47) of the EMB-resistant isolates were resistant to both isoniazid and rifampicin. A newly developed one-step amplification refractory mutation system polymerase chain reaction (ARMS-PCR) method correctly detected the EMB-resistant genotype.


CONCLUSIONS
Implementation of more accurate diagnosis of EMB resistance may enhance patient management in South Africa, as standardised treatment of MDR-TB with second-line drugs is currently dependent on the outcome of the EMB resistance test.
Johnson
R
R
Medical Research Council Centre for Molecular and Cellular Biology Medical Biochemistry and Faculty of Health Sciences, Stellenbosch University, Tygerberg, South Africa.
Jordaan
A M
AM
Pretorius
L
L
Engelke
E
E
van der Spuy
G
G
Kewley
C
C
Bosman
M
M
van Helden
P D
PD
Warren
R
R
Victor
T C
TC
D000995:Antitubercular Agents / Q000494:pharmacology*; D024881:Drug Resistance, Bacterial / Q000235:genetics; D018432:Drug Resistance, Multiple / Q000187:drug effects / Q000235:genetics; D004977:Ethambutol / Q000494:pharmacology*; D005838:Genotype; D008826:Microbial Sensitivity Tests; D009154:Mutation; D009169:Mycobacterium tuberculosis / Q000187:drug effects* / Q000235:genetics*; D010641:Phenotype; D016133:Polymerase Chain Reaction
D000995:Antitubercular Agents; D004977:Ethambutol