Discrepancies in Drug Susceptibility Test for Tuberculosis Patients Resulted from the Mixed Infection and the Testing System

Abstract

To find the potential reasons for the discrepancies in the drug susceptibility test (DST) of M. tuberculosis isolates, twenty paired isolates with disputed drug susceptibilities to isoniazid (INH) were selected according to the MGIT960 testing and Löwenstein-Jensen (L-J) proportion methods. Their MICs were confirmed again by broth microdilution method and by L-J proportion method. The spoligotyping results showed that, of all the 20 paired strains, 11 paired isolates belonged to the Beijing genotype and 6 paired isolates belonged to SIT1634, and that each of the remaining 3 paired isolates had two genotypes, namely, SIT1 and SIT1634. Those 3 paired isolates with different intrapair spoligotypes were further confirmed as mixed infection by the results that those three pairs of isolates with different 12 locus MIRU intrapair types and one pair carried different base pair at codon 315 (AGC versus AAC). Totally mutations in the katG gene were identified in 13 paired isolates. No mutations were found in the regulatory sequences and open reading frames (ORF) of the inhA and ahpC genes in any of the tested isolates. Those results showed that the different test systems and the mixed infection with particular genotypes of M. tuberculosis strains contributed to the drug susceptibility discrepancies.

Figure 1

Strains selected in this experiment. A total of 1014 culture positive isolates were included in this study which were isolated in 2006. We focus on the INH as it is a very important antibiotic in curing tuberculosis. In this study of all the total 1412 culture positive isolates 1216 were positive on the L-J medium, of which 339 were resistant by L-J method to INH and 1303 isolates were positive by the MGIT960, of which 357 were resistant to INH by the MGIT960 system. Total 330 were DST positive to INH by both MGIT960 and agar proportion methods, of which 20 isolates with positive both by MGIT960 system but negative by agar proportion method were examined in this study.