Heteroresistance in Mycobacterium tuberculosis

SETTING: Drug resistance in A Mycobacterium tuberculosis is often linked to specific mutations in a limited number of resistance genes. Detection of t hese mutations in a cultured isolate can predict the resistant phenotype. G enotypic analysis of the mycobacteria directly in a clinical specimen would result in considerable time saving for resistance prediction. OBJECTIVE: To find out whether resistance-predicting genotypes of mycobacte ria found after cultivation always give a good reflection of those in the o riginal clinical sample. DESIGN: Restriction fragment length polymorphisms of repetitive polymerase chain reaction (PCR) amplification and cloning of PCR products were used as non-integrative methods to describe the composition of katG, rgsL and embB genotypes involved in resistance to isoniazid, streptomycin and ethambutol , respectively, in the original sample. This result was then compared to th e phenotypic resistance profile after cultivation. RESULTS: Using both methods, mixed, heteroresistant populations could be de tected in almost every fifth analyzed sample (katG: 5 of 16; rpsL: 3 of 17; embB: 1 of 21). Direct sequencing, a widely used integrative method, repea tedly failed to detect heteroresistance. CONCLUSION: Heteroresistance is a valid phenomenon in clinical tuberculosis . It is not rare and not restricted to a particular resistance gene, and is obscured by cultivation as well as by some, not all, culture-independent r esistance prediction tests.