Evaluation of thin-layered agar for Mycobacterium tuberculosis isolation and drug susceptibility testing

Tuberculosis (TB) is India's major public health problem. According to the WHO, India harbors the largest number of cases, and TB control remains a challenge in diagnosis, drug resistance, and treatment. We undertook this study to compare the isolation rates of Mycobacterium tuberculosis (MTB) in agar, egg-based media, incidence of multidrug-resistant (MDR), and extended drug resistance (XDR) in MTB. This study aimed to compare and evaluate thin-layered agar (TLA) for the cultivation and drug susceptibility pattern of MTB with the conventional egg-based Lowenstein-Jensen's (LJ) medium and to differentiate atypical Mycobacterium by incorporating para-nitrobenzoic acid (PNB) in the TLA medium. This cross-sectional study was conducted in Sri Ramachandra Medical College and Research Institute, Porur, Chennai.

A total of 68 smear-positive samples were inoculated into TLA (Middle Brook 7H 11) and LJ media with and without antibiotics (rifampicin, isoniazid, and ofloxacin) simultaneously after decontamination by the modified Petroff's method. TLA with PNB was also used to differentiate the growth of nontuberculous mycobacterium (NTM). Incubation was done at 37°C, and reading was taken every 3^rd day for 6 weeks in case of TLA and for 8 weeks in case of LJ medium.

Out of the 68 samples, 64 (94.1%) grew in LJ, and the growth observed at the end of the 1^st, 2^nd, 3^rd, 4^th, 5^th, and 6-10^th weeks was 0, 12 (18.8%), 10 (15.6%), 14 (21.9%), 15 (23.4%), and 13 (20.3%), respectively. Similarly, in TLA, 65 (95.5%) samples were grown, among which 22 (33.8%) grew in the 1^st week and the rest (43 [66.2%]) in the 2^nd week. MDR and XDR were observed in 4 (5.8%) and 3 (4.4%) samples, respectively. Seven of them were NTM.

TLA is a better medium, with time to positivity ranging from 1 to 2 weeks with drug susceptibility and the pattern is also comparable with LJ medium. Incorporation of PNB in TLA helps in differentiating NTM.