Introduction: Sputum specimen decontamination steps are essential due to the presence of other saprophytic and infectious organisms. However, they negatively affect the mycobacterial recovery. In addition, little is known about the Mycobacterium tuberculosis killing efficacy of the PANTA (polymyxin-B, amphotericin-B, nalidixic acid, trimethoprim, azilocillin) antibiotics. Moreover, M. tuberculosis can be present in more than one metabolic population, but the effect of different growth characteristics on the mycobacterial growth indicator tube (MGIT) based time-to-positive (TTP) is not well studied.
Methods: We performed-(1) experiments using the solid agar and MGIT method to determine the effect of the NALC-NaOH decontamination method, (2) concentration-response studies with each individual antibiotic in the PANTA, and (3) the effect of the M. tuberculosis metabolic population on the TTP. TTP was recorded using the Epicenter software and exponential growth equation was used to calculate the doubling time of the bacteria, whereas, CFU/mL was analyzed using the Inhibitory Sigmoid Emax model for each antibiotic.
Results: Decontamination resulted in 4.36+0.13 log10 CFU/mL difference in cultures treated with NALC-NaOH versus no decontamination process and the limit of detection decreased from 1.47 log10 CFU/mL to the 0.42 log10 CFU/mL following NALC-NaOH treatment. PANTA at currently used antibiotic concentrations, did not had negative effect on mycobacterial recovery. Exponential growth model estimated doubling time for the log-phase growth M. tuberculosis as 2.04 days, for the semi-dormant bacilli as 2.80 days, and 6.37 days for the anaerobic cultures.
Conclusion: Specimen decontamination method negatively affect the laboratory diagnosis of M. tuberculosis, polymyxin-B and nalidixic acid have anti-tuberculosis efficacy at high concentrations, and the doubling time of different metabolic population should be considered when deciding the time-in-protocol for the MGIT system.