Susceptibility of -Lactam Antibiotics and Genetic Mutation of Drug-Resistant Mycobacterium tuberculosis Isolates in Korea

Background: Mycobacterium tuberculosis (Mtb) is resistant to the beta-lactam antibiotics due to a non-classical transpeptidase in the cell wall with beta-lactamase activity. A recent study showed that meropenem combined with clavulanate, a beta-lactamase inhibitor, was effective in multidrug-resistant (MDR) and extensively drug-resistant (XDR) tuberculosis (TB). However, in Korea, clavulanate can only be used as drugs containing amoxicillin. In this study, we investigated the susceptibility and genetic mutations of drug-resistant Mtb isolates to amoxicillin-clavulanate and meropenem-clavulanate to improve the diagnosis and treatment of drug-resistant TB patients. Methods: The minimum inhibitory concentration (MIC) of amoxicillin-clavulanate and meropenem-clavulanate was examined by resazurin microtiter assay. We used 82 MDR and 40 XDR strains isolated in Korea and two reference laboratory strains. Mutations of drug targets blaC, blal, ldtA, IdtB, dacB2, and crfA were analyzed by polymerase chain reaction and DNA sequencing. Results: The MIC(90 )values of amoxicillin/clavulanate and meropenem/clavulanate in drug-resistant Mtb isolates were 64/2.5 and 16/2.5 mg/L, respectively. Gene mutations related to amoxicillin/clavulanate and meropenem/clavulanate resistance could not be identified, but T448G mutation was found in the blaC gene related to beta-lactam antibiotics' high susceptibility. Conclusion: Our results provide clinical consideration of beta-lactams in treating drug-resistant TB and potential molecular markers of amoxicillin-clavulanate and meropenem-clavulanate susceptibility.