Detection of inhibitors of phenotypically drug-tolerant Mycobacterium tuberculosis using an in vitro bactericidal screen.

Many whole cell screens of chemical libraries currently in use are based on inhibition of bacterial growth. The goal of this study was to develop a chemical library screening model that enabled detection of compounds that are active against drug-tolerant non-growing cultures of Mycobacterium tuberculosis. An in vitro model of low metabolically active mycobacteria was established with 8 and 30 day old cultures of M. smegmatis and M. tuberculosis, respectively. Reduction of resazurin was used as a measure of viability and the assay was applied in screens of chemical libraries for bactericidal compounds. The model provided cells that were phenotypically-resilient to killing by first and second-line clinical drugs including rifampicin. Screening against chemical libraries identified proteasome inhibitors, NSC310551 and NSC321206, and a structurally-related series of thiosemicarbazones, as having potent killing activity towards aged cultures. The inhibitors were confirmed as active against virulent M. tuberculosis strains including multi- and extensively-drug resistant clinical isolates. Our library screen enabled detection of compounds with a potent level of bactericidal activity towards phenotypically drug-tolerant cultures of M. tuberculosis.

Native New Zealand plants with inhibitory activity towards Mycobacterium tuberculosis.

BACKGROUND: Plants have long been investigated as a source of antibiotics and other bioactives for the treatment of human disease. New Zealand contains a diverse and unique flora, however, few of its endemic plants have been used to treat tuberculosis. One plant, Laurelia novae-zelandiae, was reportedly used by indigenous Maori for the treatment of tubercular lesions. METHODS: Laurelia novae-zelandiae and 44 other native plants were tested for direct anti-bacterial activity. Plants were extracted with different solvents and extracts screened for inhibition of the surrogate species, Mycobacterium smegmatis. Active plant samples were then tested for bacteriostatic activity towards M. tuberculosis and other clinically-important species. RESULTS: Extracts of six native plants were active against M. smegmatis. Many of these were also inhibitory towards M. tuberculosis including Laurelia novae-zelandiae (Pukatea). M. excelsa (Pohutukawa) was the only plant extract tested that was active against Staphylococcus aureus. CONCLUSIONS: Our data provide support for the traditional use of Pukatea in treating tuberculosis. In addition, our analyses indicate that other native plant species possess antibiotic activity.