Evaluation of a semi-automated reporter phage assay for susceptibility testing of Mycobacterium tuberculosis isolates in South Africa

Abstract

In a prospective study conducted by laboratory technologists in a diagnostic laboratory in Cape Town, South Africa, a semi-automated phage-based antibiotic susceptibility assay was implemented and the performance of the luciferase reporter mycobacteriophage (LRP) system for susceptibility testing of clinical Mycobacterium tuberculosis complex (MTC) isolates against rifampin and isoniazid was evaluated. Two hundred consecutive clinical MGIT cultures of MTC species were included in this study. Antibiotic susceptibility assays were set up manually for the LRP and BACTEC radiometric systems (BACTEC) and read in a plate luminometer and the BACTEC 460 instrument, respectively. Discrepant susceptibility results were resolved by the conventional agar proportion method. Of the 200 secondary cultures prepared for this study, 9 (4.5%) were lost to contamination (LRP 4, BACTEC 1, both 4). All of the remaining 191 cultures underwent susceptibility testing by both methods and the overall agreement between the LRP and BACTEC was 98.4% (rifampin 100%; isoniazid 96.9%). Of the 6 discrepant cultures tested by the agar proportion method, 2 gave results in agreement with the LRP. The sensitivity of the LRP for detection of drug-resistant isolates was 100% for both rifampin (n=9) and isoniazid (n=12). The median turnaround time for susceptibility testing was 2 days with the LRP and 9 days with BACTEC. In conclusion, the semi-automated LRP-based assay offers a rapid and practical approach for accurate susceptibility testing of M. tuberculosis cultures in diagnostic laboratories with limited financial resources, but with competent technologists.

Figure 1

Inoculum size for cultures with true and false isoniazid results. The size of inoculum used in the LRP-based susceptibility assay, as a measure of luciferase (Lux) activity, is plotted for cultures with true and false isoniazid results. Cultures were tested in duplicates. The mean rlu value for each group is shown with a horizontal bar.