A Nusrath Unissa - National Institute for Research in Tuberculosisº Chetpet, Chennai-۶۰۰۰۳۱, India.
Sujatha Narayanan - National Institute for Research in Tuberculosisº Chetpet, Chennai-۶۰۰۰۳۱, India.
N Selvakumar - National Institute for Research in Tuberculosisº Chetpet, Chennai-۶۰۰۰۳۱, India.

Isoniazid, is the only antituberculous drug for which the relation between lack of virulence and acquisition of resistance was associated. INH-resistant mutants were shown to contain defective katG gene. Classical studies showed that INH-resistant south Indian isolates have lower virulence in guinea pigs and higher susceptibility to H۲O۲. It is of interest to assess the virulence in south Indian clinical mutants of KatG (catalase-peroxidase enzyme) associated with INH resistance. Five INH-resistant clinical isolates were selected on the basis of mutation in katG gene. Mutant isolates were used for infecting macrophage cell line (THP-۱) and for the assessment of enzyme activity.  In comparison to control H۳۷Rv, Ser۳۱۵Thr mutant exhibits similar virulence and reduction (۴% and ۱۷%) in catalase (C) and peroxidase (P) activity. For the mutants, Ser۳۱۵Iso and Ser۳۱۵Arg, the virulent nature was slightly reduced with ۳% and ۲%, and significant reduction was observed in the C (۵۰%; ۴۷%) and P (۳۶%; ۴۳%) activity respectively. Ser۳۱۵Asn indicates ۴% reduction in virulence with significant reduction in C (۵۶%) and P (۶۴%) activity. Asn۱۳۸Ser mutant displayed low-level virulence, CP activity showed ۴۷% and ۵۶% reduction. The results indicate that similar level of virulence in all the mutants except Asn۱۳۸Ser which showed relatively low-level whereas a significant decrease in C and P activity was observed in all mutants except Ser۳۱۵Thr. The study suggests despite the fact that all mutants do not compromise survival or virulence yet provides INH resistance.

Mycobacterium tuberculosis, Virulence, INH resistance, KatG mutants