Comparison of the Mycobacterium tuberculosis molecular bacterial load assay, microscopy and GeneXpert versus liquid culture for viable bacterial load quantification before and after starting pulmonary tuberculosis treatment

Abstract

Molecular bacterial load assay (MBLA) rapidly quantifies viable Mycobacterium tuberculosis (Mtb) and may be useful for monitoring treatment response and treatment efficacy. We conducted a prospective study in 56 adults with pulmonary tuberculosis from whom 244 sputum samples were collected before and during the first month of treatment. We evaluated MBLA for early monitoring of bacterial burden and investigated bactericidal activities of first-line therapy in patients infected with drug susceptible and resistant isolates. Mtb loads measured by MBLA and culture were correlated after one-week (r = 0.56) and one-month (r = 0.73) of treatment. Correlations between culture and GeneXpert or microscopy were weaker during treatment. Mtb load by MBLA declined more rapidly than GeneXpert after one-week (2.73 Ct, P < 0.001; 0.95 Ct, P = 0.297, respectively) and one-month (8.94 Ct, P < 0.001; 6.78 Ct, P < 0.001). Mtb loads in multidrug resistant (MDR) infections were significantly greater than in both sensitive and poly/mono-resistance after one-week (P < 0.02) and one-month treatment (P = 0.001). MBLA performed better than GeneXpert and microscopy in comparison to culture for quantifying viable Mtb during treatment. It can be used for monitoring bacterial load during TB treatment, facilitating early detection of treatment failure thus improving outcomes.

Keywords: 16S rRNA; Bactericidal activity; Mycobacterium tuberculosis; Treatment; Viable bacterial load.

Fig. 1

Correlations of Mtb load measured by MGIT versus MBLA, GeneXpert and microscopy. Plots of MGIT TTP (days) against MBLA (Ct), GeneXpert (Ct) and microscopy (log₁₀ AFB) in (A) all samples (n = 168), (B) before treatment (n = 54), (C) after week 1 (n = 50), (D) after week 2 (n = 42) and (E) after week 4 of treatment (n = 22). TTP, Ct and log₁₀ AFB number were used to calculate correlations using the Spearman method. The analysis included all drug-sensitive and drug-resistant bacteria samples from patients. Only samples with detected values of bacterial load were included in analyses (n = 168). Spearman's correlation coefficients (r) and 95% confident interval are presented in the figure. Best-fit linear regression lines are shown with 95% confidence intervals. TTP: time to positivity; Ct: threshold cycle value.

Fig. 2

Mtb load quantified by MBLA in drug sensitive and resistant Mtb-infected patient groups before and during the first month of treatment. Bacteria loads were measured by MBLA in four Mtb drug-susceptibility phenotype groups: the sensitive group (black, n = 27), EMB-PZA R group (green, n = 10), INH R group (blue, n = 12) and MDR group (red, n = 5). Bacterial loads in the four groups were compared (A) before treatment, (B) after week 1 and (C) after week 4 of treatment. (D) Bacterial loads in the four groups were plotted against time during the first month of TB treatment. Error bars in plots represent medians and interquartile ranges. Statistical comparisons were made using Wilcoxon rank sum test. Only significant results are indicated with P values. Sensitive: sensitive group; EMB-PZA R: EMB and/or PZA resistant group, INH R: INH resistant group, MDR: multidrug resistant group. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)