Comparative study of three culture systems for optimal recovery of mycobacteria from different clinical specimens

Abstract

A new broth-based nonradioactive culture system, MB-Redox (Heipha Diagnostika Biotest, Germany), was compared with the liquid radiometric Bactec 460 TB system and the solid Lowenstein-Jensen (L-J) medium for recovery rate and time to detection of mycobacteria. Of the 605 clinical specimens studied, 100 grew acid-fast bacilli (AFB). The isolation rate for all AFB was 84% for Bactec, 69% for MB-Redox, and 48% for L-J. Eighteen percent of the 100 isolates grew only in Bactec, 10% only in MB-Redox, and 5% only in L-J. The average times to detection of the 100 isolates were 13.2, 17.8, and 28.3 days, respectively, and the mean growth times for the 34 AFB detected by all three media were 13.7, 15.2, and 26.8 days, respectively. Mycobacterium tuberculosis was isolated from 15 clinical samples, with detection times of 16.8 days for Bactec, 18.3 for MB-Redox, and 22.8 days for L-J medium; 10 of these isolates grew in all media, with detection times of 14.2, 16.4, and 18.7 days, respectively. Seven were also positive on direct smear, with detection times of 12.4, 13.7, and 17.7 days. Two of the Mycobacterium tuberculosis isolates were recovered only in Bactec and another one only in L-J medium. Mycobacterium haemophilum grew only in the liquid systems, which provided the special growth factors this bacterium requires. Only the combination of all three systems yielded optimal recovery. MB-Redox gave reliable results, offering the advantages of ready-to-use tubes in which the antibiotic supplement is already incorporated and easy and immediate reading of the results. Since this system does not contain any radioactive substance, results can be confirmed with acid-fast staining and conventional and molecular tests.