Detection and quantification of differentially culturable tubercle bacteria in sputum from patients with tuberculosis

Rationale: Recent studies suggest that baseline tuberculous sputum comprises a mixture of routinely culturable and differentially culturable tubercle bacteria (DCTB). The latter seems to be drug tolerant and dependent on resuscitation-promoting factors (Rpfs). Objectives: To further explore this, we assessed sputum from patients with tuberculosis for DCTB and studied the impact of exogenous culture filtrate (CF) supplementation ex vivo. Methods: Sputum samples from adults with tuberculosis and HIV-1 and adults with no HIV-1 were used for most probable number (MPN) assays supplemented with CF and Rpf-deficient CF, to detect CF-dependent and Rpf-independent DCTB, respectively. Measurements and Main Results: In 110 individuals, 19.1% harbored CF-dependent DCTB and no Rpf-independent DCTB. Furthermore, 11.8% yielded Rpf-independent DCTB with no CFdependent DCTB. In addition, 53.6% displayed both CF-dependent and Rpf-independent DCTB, 1.8% carried CF-independent DCTB, and 13.6% had no DCTB. Sputum from individuals without HIV-1 yielded higher CF-supplemented MPN counts compared with counterparts with HIV-1. Furthermore, individuals with HIV-1 with CD4 counts greater than 200 cells/mm³ displayed higher CFsupplemented MPN counts compared with participants with HIV-1 with CD4 counts less than 200 cells/mm³. CF supplementation allowed for detection of mycobacteria in 34 patients with no culturable bacteria on solid media. Additionally, the use of CF enhanced detection of sputum smear-negative individuals. Conclusions: These observations demonstrate a novel Rpfindependent DCTB population in sputum and reveal that reduced host immunity is associated with lower prevalence of CF-responsive bacteria. Quantification of DCTB in standard TB diagnosis would be beneficial because these organisms provide a putative biomarker to monitor treatment response and risk of disease recurrence.