Mutations in katG and inhA genes of isoniazid-resistant and -sensitive clinical isolates of Mycobacterium tuberculosis from cases of pulmonary tuberculosis and their association with minimum inhibitory concentration of isoniazid

The evolution of resistant TB strain against isoniazid (INH) drug is alarming in the global population. The molecular mechanism of resistance for this compound is found to be associated with mutations in katG and inhA genes. However, types and frequency of mutations vary with geographical region. Therefore, this study aimed to explore new targets of katG and inhA gene mutations and their correlation with isoniazid (INH) minimal inhibitory concentration (MIC) in clinical isolates of Mycobacterium tuberculosis from Uttar Pradesh, India.Materials and methodsThe level of INH resistance was measured by absolute concentration method. DNA sequencing was performed to determine mutations in INH-resistant and -sensitive isolates.ResultsTotally, 44/50 INH-resistant isolates had the following mutations in katG gene at codon 315: Ser-Thr (41/44), Ser-Ile (1/44) and Ser-Asn (1/44), with INH MIC ranges 2 to >16 μg/ml. Only one isolate showed the following mutation at codon 299: Gly-Ser with MIC of 0.2 μg/ml. Two isolates showed the following novel mutation in inhA gene at codon 17: Pro-Gln. One isolate had the following mutation at codon 94: Ser-Ala in inhA gene. These 3 isolates also had mutations in katG gene. The following mutation was seen in katG gene at codon 234: Ala-Gly. There were also mutations at codon 265; synonymous mutations were seen in both INH-sensitive and -resistant isolates, and hence have no association with INH resistance.ConclusionThe katG 315 (Ser-Thr) mutation is most frequent and associated with high level of INH resistance. However, extremely low frequency of mutation at codon 94 (Ser-Ala) in structural region of inhA gene decreases their apparent contribution in INH drug resistance.