RESUMO
Background: Serpentine cord formation in BACTEC MGIT 960 medium was evaluated as a rapid method for the presumptive identification of M. tuberculosis complex (MTBC). Material & Methods: Total 2527 samples were processed for AFB culture using MGIT 960 TB system over a period of three months. AFB smears were prepared from 1000 MGIT tubes flagged positive by the MGIT instrument and stained by ZN method to examine presence or absence of serpentine cording. The cord formation was compared with PNBA [pnitro benzoic acid] test on MGIT system and all controversial cases were further evaluated by NAP [p-nitro-a-acetylaminophydroxypropiophenone] test on BACTEC 460 TB system. Results & Discussion: Of the 1000 culture positives, 904 (90.4%) were identified as mycobacteria, of which 869 (96%) showed cording by smear microscopy. One (0.1%) was identified as nocardia. In the remaining 95 (9.5%) cases, primary smear made from MGIT vial was negative. Of 869 cultures showing serpentine cord formation, 842 were confirmed as MTBC and 27 as NTM by PNBA assay on MGIT 960 TB system. The sensitivity, specificity, positive and negative predictive values are found to be 99.6%, 54%, 96% and 91% respectively. An average detection time for PNBA assay was found to be eight days whereas cording results were available on the same day of culture positivity. Conclusion: Though highly sensitive it is not very specific and hence cannot be the only test for presumptive diagnosis of MTBC.
RESUMO
Objectives: To evaluate Pyrazinamide (PZA) susceptibility results obtained by phenotypic MGIT 960 TB system against enzymatic Pyrazinamidase assay and genotypic pncA gene sequencing. To find the prevalence of infections caused by M. bovis in PZA resistant M. tuberculosis complex isolates. Methods: 33 consecutive PZA resistant and 30 consecutive PZA susceptible isolates reported for PZA susceptibility testing by MGIT 960 TB system were included in this study. Presence of active pyrazinamidase enzyme was sought by using the Wayne assay. The pncA gene was amplified by PCR and then sequenced to screen mutations. All the PZA resistant isolates were further spoligotyped to identify M. bovis, if present. Results: Of 33 PZA resistant strains by MGIT 960, 31 were Wayne assay negative and two were positive. Of the 30 susceptible PZA strains six were Wayne assay negative reporting false resistance. PncA gene sequencing revealed that 32 of the 33 MGIT PZA resistant isolates had diverse nucleotide changes scattered throughout the pncA gene (one isolate did not show any mutation). Of the 30 phenotypically susceptible isolates, 21 were wild types whilst nine isolates showed the presence of a silent mutation C-T at codon 195. Fifteen mutations found in this study has not been described earlier. Not a single isolate of M. bovis was detected among PZA resistant M. tuberculosis complex isolates. Conclusion: MGIT 960 showed better concordance with sequencing results in comparison with Wayne assay. In present study, a high proportion (85%) of MDR-TB isolates from patients receiving anti-TB treatment were found to be resistant to PZA.