Time to detection of Mycobacterium tuberculosis in BACTEC systems as a viable alternative to colony counting

Abstract

Objective: To investigate whether time to detection (TTD) of Mycobacterium tuberculosis in BACTEC Mycobacteria Growth Indicator Tube (MGIT) 960 and BACTEC 460 TB systems can be used as an alternative to colony-forming unit (cfu) counting.

Design: A single sputum sample recovered from each of 22 patients with tuberculosis (TB) was cultured on Middlebrook 7H11 agar and in BACTEC MGIT 960 and BACTEC 460 to investigate the relationship between cfu/ml and TTD. The relationship between TTD and treatment response was investigated by culturing a single sputum sample from each of 125 patients with TB in the BACTEC 460 system and comparing TTD values with their treatment response.

Results: An inverse correlation between TTD and bacterial number, as assessed by inoculum size and cfu/ml, was observed. For the 125 patients followed up during treatment, TTD values at diagnosis correlated with smear conversion rates at 2 months and treatment outcomes. Drug resistance of the infecting strain was associated with decreased killing, as indicated by the delayed increase in TTD during the first few days of treatment.

Conclusion: The TTD of M. tuberculosis in BACTEC MGIT 960 and BACTEC 460 TB systems is a viable alternative to colony counting. TTD in liquid culture will facilitate mycobacterial quantification, especially in the evaluation of early bactericidal activity.