Utility of Phenotypic and Genotypic Testing in the Study of Mycobacterium tuberculosis Resistance to First-Line Anti-Tuberculosis drugs

Abstract

Objective: To determine the utility of molecular techniques in the diagnosis of resistance and the extent of resistance to first-line drugs in our region.

Material and method: From 2004 to 2013, 1,889 strains of Mycobacterium tuberculosis complex isolated in Asturias, Spain, were studied using phenotypic (Clinical and Laboratory Standards Institute guidelines) and molecular (INNOLiPA RIF-TB^©; GenotypeMDRplus^©; GenotypeMDRsl^©) sensitivity tests.

Results: 1,759 strains (94.52%) were sensitive to all first-line drugs, and 102 strains (5.48%) showed some resistance: 81 strains (4.35%) were resistant to 1 single drug, 14 (0.75%) were polyresistant, and 7 (0.37%) were multiresistant (resistant to rifampicin and isoniazid). In total, 137 resistances were identified: 60 to isoniazid (3.22%), 7 to rifampicin (0.37%), 9 to pyrazinamide (0.48%), 11 to ethambutol (0.59%), and 50 to streptomycin (2.68%). Of the mutations detected, 75.9% (63/83) correlated with resistance, while 24.09% of mutations detected (20/83) were not associated with resistance; 16 of these involved a silent mutation at codon 514 of the rpoB gene. Between 0 and 90% of strains, depending on the drug under consideration, were resistant even when no gene mutations were detected using marketed systems.

Conclusions: Molecular techniques are very useful, particularly for obtaining rapid results, but these must be confirmed with standard phenotypic sensitivity testing. The rate of resistance in our region is low and multi-drug resistantcases (0.37%) are sporadic.

Keywords: Epidemiology; Epidemiología; Molecular methods.; Métodos fenotípicos; Métodos moleculares; Phenotypic methods; Resistance; Resistencias; Tuberculosis.