Comparison of MGIT 960 & Pyrazinamidase Activity Assay for Pyrazinamide Susceptibility Testing of Mycobacterium Tuberculosis (Report)
Pyrazinamide (PZA) is an important front line drug used in conjunction with isoniazid and rifampicin in short course chemotherapy to treat tuberculosis. It is a synthetic pyrazine analogue of nicotinamide and a prodrug, which is converted to the active form pyrazinoic acid by an enzyme pyrazinamidase (PZase) (1). Rapid detection of drug susceptibility profile of Mycobacterium tuberculosis is important for patient management and in prevention of development of resistance. Susceptibility testing of PZA is technically difficult and not well standardized. Among the methods used, proportion method on solid medium (agar base or egg base) is universally accepted as gold standard (2,3). However, it has limitations as PZA has maximum in vitro activity at pH 5.5 at which many isolates of M. tuberculosis fail to grow (4) and it takes 21 to 42 days to report the results. Moreover, the result depends on inoculum size and the most common problem is false resistance when heavy inoculum is used which can lead to alkalization of medium (5). To conquer these problems various semiautomated and automated methods have been developed which use liquid based medium at pH 5.9. These methods are BACTEC 460 TB system (Becton Dickinson Microbiology Systems, Sparks, Md.), MB/BacT system (Organon-Teknika, Durham, N.C.), ESP culture system II (AccuMed International,Westlake, Ohio), MGIT 960 (Becton Dickinson Microbiology Systems, Sparks, Md.) and BacT/ALERT 3D system (Bio-Merieux, Durham, NC.). These systems have shortened the turn around time. Another method used for detection of PZA susceptibility is pyrazinamidase (PZase) activity test. It detects the presence of enzyme PZase required for the activation of the drug. In sensitive strains the enzyme is present and can be used as a marker of sensitivity while in resistant strains the enzyme is absent. PZase activity can be tested by Wayne’s method (6,7) or thin layer chromatography method (8). Wayne’s method is rapid but there is subjective error as a very faint band is formed that is difficult to interpret. A modified method by Singh et al (9) has also been reported recently.