Molecular and Growth-Based Drug Susceptibility Testing ofMycobacterium tuberculosisComplex for Ethambutol Resistance in the United States

Abstract

Ethambutol (EMB) is used as a part of drug regimens for treatment of tuberculosis (TB). Susceptibility of Mycobacterium tuberculosis complex (MTBC) isolates to EMB can be discerned by DNA sequencing to detect mutations in the embB gene associated with resistance. US Public Health Laboratories (PHL) primarily use growth-based drug susceptibility test (DST) methods to determine EMB resistance. The Centers for Disease Control and Prevention (CDC) provides a service for molecular detection of drug resistance (MDDR) by DNA sequencing and concurrent growth-based DST using agar proportion. PHL and CDC test results were compared for 211 MTBC samples submitted to CDC from September 2009 through February 2011. Concordance between growth-based DST results from PHL and CDC was 88.2%. A growth-based comparison of 39 samples, where an embB mutation associated with EMB resistance was detected, revealed a higher percentage of EMB resistance by CDC (84.6%) than by PHL (59.0%) which was significant (P value = 0.002). Discordance between all growth-based test results from PHL and CDC was also significant (P value = 0.003). Most discordance was linked to false susceptibility using the BACTEC™ MGIT™ 960 (MGIT) growth-based system. Our analysis supports coalescing growth-based and molecular results for an informed interpretation of potential EMB resistance.

What Is Resistance? Impact of Phenotypic versus Molecular Drug Resistance Testing on Therapy for Multi- and Extensively Drug-Resistant Tuberculosis

“…In other cases, problems with pDST played a role. The false-susceptible pDST results for ethambutol were likely due to the fact that some resistance mutations only result in slight MIC increases, which means that it can be difficult to distinguish the gWT strains from gNWT strains using pDST, unless secondary mutations increase the MICs even further ( 14 , 40 – 42 ). The lack of reproducibility of pDST was also apparent for isolate 3758-14, which initially tested susceptible to prothionamide but became resistant upon retesting (Table S2).…”

Mycobacterium tuberculosis resistance prediction and lineage classification from genome sequencing: comparison of automated analysis tools

“…By contrast, the remaining 15 cases of false-susceptibility were likely mostly due to problems with phenotype rather than the genotypic results ( Supplementary Table S6 ). For example, mutations in embB are known to result in only slight MIC increases to EMB, which means that the MIC distributions of wild-type and mutated strains overlap, unless secondary mutations increase the MICs even further 27 , 28 , 29 , 30 . Similarly, several rpoB mutations are known to test susceptible in MGIT, but resistant on Löwenstein-Jensen medium 31 , 32 , 33 .…”

Magnitude of Phenotypic and MTBDRplus Line Probe Assay First-Line Anti-Tuberculosis Drug Resistance Among Tuberculosis Patients; Northwest Ethiopia

“…18 Consistent DST for isolates of Mtb complex is critical for selecting effective treatment regimens, interruption of transmission, and prevention of further expansion of resistant forms of TB. 21 Ethiopia is one of the 30 high-TB-burden countries with a current estimated incidence of 140 new TB cases per 100,000 population. 10 As in other developing countries, TB/HIV co-infection and the emergence of MDR-TB are becoming pressing challenges in the efforts to control TB in Ethiopia.…”