Plant compounds from Berberisholstii Engl. may block tuberculosis drug pumps

Tuberculosis, caused by Mycobacterium tuberculosis (Mtb), remains a leading infectious killer, with 10.8 million new cases and 1.25 million deaths reported in 2023. The burden in Sub-Saharan Africa is especially severe because of high HIV-AIDS prevalence and rising drug resistance, so new ways to strengthen TB treatment are urgently needed. In a study led by Ephantus G. Ndirangu, researchers turned to a plant used in traditional medicine, investigating compounds characterised from Berberisholstii Engl. for their ability to interfere with bacterial drug pumps. These pumps, called efflux pumps, can push antibiotics out of bacterial cells and contribute to treatment failure. The team tested extracts from roots, stem bark and leaves in the lab against a harmless mycobacterial model, Mycobacterium smegmatis (Msm), and against wild-type Mtb. They combined laboratory tests with computer-based molecular docking to see whether individual plant molecules might fit into known Mtb efflux proteins, and they used genetic tools to validate whether blocking those pumps would change drug activity. The work aimed to find plant-derived molecules that could be added to existing TB treatments to make them more effective.

The researchers evaluated MeOH extracts and aqueous extracts from different plant parts and measured growth inhibition using MIC99 values. Against wild-type Msm, MeOH extracts of the root and the stem bark, and aqueous extracts of the roots showed MIC99 values of 906.25 μg/ml, 4500 μg/ml and 1875 μg/ml, respectively. Against wild-type Mtb, MeOH extract of the leaves and aqueous extract of the stem bark had MIC99 values of 60.43 μg/ml and 1.74 μg/ml respectively. To explore mechanism, the team carried out molecular docking of chillanamine, isoboldine, berberrubine, reticuline, N-methylcoclaurine, thalifoline, apoglaziovine, and orientine onto Mtb efflux pumps Rv1258c, MmpS5-MmpL5 and Rv2333c, finding string binding affinities of <-5 kcal/mol. They then tested seven fractions containing these bioactive compounds in checkerboard assays combined with spectinomycin (SPEC) and calculated fractional inhibitory concentration index (FICI) values; six fractions were synergistic with SPEC (FICI <0.5). Two fractions lowered the MIC99 of SPEC by 8-fold against wild-type Mtb. Finally, validation of Mtb efflux pump inhibition was performed using CRISPRi knockdown strains of Rv1258c and Rv2333c.

The study identifies a set of phytochemicals from Berberisholstii Engl. that have the hallmarks of efflux pump inhibitors for Mtb. By docking into known Mtb pump proteins and showing synergy with spectinomycin (SPEC) in laboratory assays, these compounds could help antibiotics stay inside bacterial cells where they can act. The use of CRISPRi knockdown strains of Rv1258c and Rv2333c provides genetic support that the mechanism involves efflux inhibition rather than unrelated toxicity. Importantly, several fractions produced strong synergy (FICI <0.5) and two fractions produced an eightfold reduction in the MIC99 of SPEC, suggesting combination therapy might substantially lower the amount of antibiotic needed. While further work is required to isolate pure compounds, assess safety and test activity in animal models and humans, these findings point to a promising complementary strategy: pairing existing drugs with plant-derived efflux inhibitors to help counter drug resistance and improve TB treatment outcomes.