Tuberculosis Phenotypic and Genotypic Drug Susceptibility Testing and Immunodiagnostics: A Review

Abstract

Tuberculosis (TB), considered an ancient disease, is still killing one person every 21 seconds. Diagnosis of Mycobacterium tuberculosis (M.tb) still has many challenges, especially in low and middle-income countries with high burden disease rates. Over the last two decades, the amount of drug-resistant (DR)-TB cases has been increasing, from mono-resistant (mainly for isoniazid or rifampicin resistance) to extremely drug resistant TB. DR-TB is problematic to diagnose and treat, and thus, needs more resources to manage it. Together with+ TB clinical symptoms, phenotypic and genotypic diagnosis of TB includes a series of tests that can be used on different specimens to determine if a person has TB, as well as if the M.tb strain+ causing the disease is drug susceptible or resistant. Here, we review and discuss advantages and disadvantages of phenotypic vs. genotypic drug susceptibility testing for DR-TB, advances in TB immunodiagnostics, and propose a call to improve deployable and low-cost TB diagnostic tests to control the DR-TB burden, especially in light of the increase of the global burden of bacterial antimicrobial resistance, and the potentially long term impact of the coronavirus disease 2019 (COVID-19) disruption on TB programs.

Keywords: TB diagnostics; active TB; anti-TB drug regimens; multi-drug resistance; point of care (POC).

Figure 1

Layout of the 1G and 2G test used to diagnose drug-resistance of M.tb infection. (A) The 1G test in which four different drugs are contained in each quadrant. A patient’s sputum is collected and de-contaminated. Following the arrows, (1) sputum is mixed with decontaminant, (2) the mixture of decontaminant plus sputum (2:1, v/v) is plated into the 1G test. (3) Showing a patient with DS-TB in which colonies are only present in the DS quadrant and not in any of the quadrants with drugs present. (4) Showing a patient with MDR-TB since M.tb colonies grow in three of the quadrants, which include no drug (clear quadrant), isoniazid (INH, green quadrant) and rifampicin (RIF, yellow quadrant). (B) The 2G test showing the expansion from a quadrant plate (1G test) to 12-wells plate. In this case, the 2G test has 11 different anti-TB drugs that can be used to i) diagnose patients with TB (2G DX test), and ii) track the treatment progression to determine if a subject is responding well or not to the treatment (2G TX test). Subjects with MDR-TB and XDR-TB are by definition INH and RIF resistant; thus the treatment tracking plate has two replacement drugs, in this case DLM and LEV. Abbreviations: DS (drug susceptible); INH (isoniazid), RIF (rifampicin), PZA (pyrazinamide), EMB (ethambutol), BDQ (bedaquiline), LNZ (linezolid), AMK (amikacin), PRO (prothionamide), CYL (cycloserine), MOX (moxifloxacin), and CLO (clofazimine), DLM (delamanid), LEV (levofloxacin).