Exploring Actinobacteria from Karst Cave on Sumba Island: A New Source of Antimycobacterial Compounds

Multidrug-resistant (MDR) mycobacteria are considered a major challenge in tuberculosis treatment, creating an urgent need for novel antimycobacterial drugs. Actinobacteria, known for their ability to produce bioactive compounds, are considered promising sources for new drug discovery. In this study, 87 actinobacteria isolates were successfully obtained from five samples collected in a karst cave on Sumba Island, Indonesia. The isolates were screened for antimycobacterial activity against Mycobacterium smegmatis wild-type (WT-M.smeg), rifampicin-resistant (RIFR-M.smeg), isoniazid-resistant (INHR-M.smeg), and multidrug-resistant (MDR-M.smeg) strains. Sixteen extracts were found to inhibit WT-M.smeg, with three extracts from isolates KRST 02-20, KRST 03-10, and KRST 05-08 showing potent activity against all resistant strains (≥95% growth inhibition). The extract of isolate KRST 03-10 was observed to exhibit the most significant inhibition, with IC50 values of 9.63 µg/mL (WT-M.smeg), 29.64 µg/mL (RIFR-M.smeg), 10.89 µg/mL (INHR-M.smeg), and 27.76 µg/mL (MDR-M.smeg). Molecular identification showed that this isolate has the highest similarity (98.91%) with Streptomyces cinereoruber NBRC 12756. Gas chromatography-mass spectrometry (GC-MS) analysis identified 2,4-di-tert-butylphenol as a compound with potential antituberculosis activity, while liquid chromatography-high-resolution mass spectrometry (LC-HRMS) detected nocardamine, L-α-palmitin, erucamide, and 2-anisic acid, all known for their antimicrobial activity. An unidentified compound, NP-011220 (C11H18O2), was also detected in high relative abundance. Further research is needed to evaluate the activity of the most promising isolate, KRST 03-10, against Mycobacterium tuberculosis and to purify its active compounds.