Date Published: April 26, 2017
Author(s): Sharana Mahomed, Nomonde R. Dlamini-Mvelase, Moses Dlamini, Koleka Mlisana.
For the optimal recovery of Mycobacterium tuberculosis from the BACTEC™ Mycobacterium Growth Indicator Tube 960™ system, an incubation period of 42–56 days is recommended by the manufacturer. Due to logistical reasons, it is common practice to follow an incubation period of 42 days. We undertook a retrospective study to document positive Mycobacterium Growth Indicator Tube cultures beyond the 42-day incubation period. In total, 98/110 (89%) were positive for M. tuberculosis complex. This alerted us to M. tuberculosis growth detection failure at 42 days.
Tuberculosis remains an important cause of morbidity and mortality, especially in developing countries. Recent molecular methods, in particular the Cepheid GeneXpert™, have revolutionised the rapid diagnosis of tuberculosis due to their high sensitivity, even in smear-negative sputum samples. Although culture-based methods take longer, they remain an integral part of the laboratory diagnosis of tuberculosis. The Clinical and Laboratory Standards Institute recommends that both solid- and liquid-based culture methods be used to maximise the recovery of Mycobacterium tuberculosis.1
A total of 20 914 MGIT tubes flagged positive during this period. Of these, 159 flagged positive after 42 days (0.8%). A total of 49/159 (31%) were negative on ZN staining and were regarded as late contaminants, and 110/159 (69%) were positive on ZN staining. A total of 12/110 AFB-positive isolates (11%) exhibited no cording, underwent MPT antigen testing, were found to be negative, and were thus identified as mycobacteria other than tuberculosis. A total of 98/110 (89%) exhibited cording, were MPT-positive, and were therefore identified as positive for M. tuberculosis complex. Only 78/98 isolates were submitted for line probe assay testing, which further identified all 78 isolates as M. tuberculosis complex. Of these 78 isolates, 37/78 (47.4%) were sensitive to both isoniazid and rifampicin, 7/78 (9%) were resistant only to rifampicin, 33/78 (42.3%) were resistant to both isoniazid and rifampicin (i.e., multi-drug resistant) and 1/78 (1.3%) was identified as extensively drug resistant.
A number of studies have reported that the BACTEC MGIT 960 system is efficient, rapid, reliable, safe and fully automated, with the added advantage of continuous monitoring of the culture tubes.3,4,5,6,7,8,9 Although most studies have confirmed that the BACTEC MGIT 960 has a high sensitivity for recovery of mycobacteria, it has been reported that the instrument detection system may occasionally fail to detect mycobacterial growth at the end of a 42-day incubation period.4,5,10 This has been observed with mycobacteria other than tuberculosis isolates, with M. xenopi being highlighted specifically.4,10,11 This failure to detect growth at 42 days was attributed to the granular growth pattern of the organism, which created less surface contact, keeping oxygen consumption below the detection threshold. The small bacterial load, slower metabolism, and biochemical and thermophilic characteristics of these isolates were also considered as possible contributing factors.10 Failure of the BACTEC MGIT 960 to specifically detect M. tuberculosis complex isolates within a 42-day incubation period has not previously been reported.