Natural product drug discovery has regained interest due to low production costs, structural diversity, and multiple uses of active compounds to treat various diseases. Attention has been directed towards medicinal plants as these plants have been traditionally used for generations to treat symptoms of numerous diseases. It is established that plants harbour microorganisms, collectively known as endophytes. Exploring the as-yet untapped natural products from the endophytes increases the chances of finding novel compounds. The concept of natural products targeting microbial pathogens has been applied to isolate novel antimycobacterial compounds, and the rapid development of drug-resistant Mycobacterium tuberculosis has significantly increased the need for new treatments against this pathogen. It remains important to continuously screen for novel compounds from natural sources, particularly from rarely encountered microorganisms, such as the endophytes. This review focuses on bioprospecting for polyketides and small peptides exhibiting antituberculosis activity, although current treatments against tuberculosis are described. It is established that natural products from these structure classes are often biosynthesised by microorganisms. Therefore it is hypothesised that some bioactive polyketides and peptides originally isolated from plants are in fact produced by their endophytes. This is of interest for further endophyte natural product investigations.
Traditional Chinese medicine encompasses a rich empirical knowledge of the use of plants for the treatment of disease. In addition, the microorganisms associated with medicinal plants are also of interest as the producers of the compounds responsible for the observed plant bioactivity. The present study has pioneered the use of genetic screening to assess the potential of endophytes to synthesize bioactive compounds, as indicated by the presence of non-ribosomal peptide synthetase (NRPS) and polyketide synthase (PKS) genes. The total DNA extracts of 30 traditional Chinese herbs, were screened for functional genes involved in the biosynthesis of bioactive compounds. The four PCR screens were successful in targeting four bacterial PKS, six bacterial NRPS, ten fungal PKS and three fungal NRPS gene fragments. Analysis of the detected endophyte gene fragments afforded consideration of the possible bioactivity of the natural products produced by endophytes in medicinal herbs. This investigation describes a rapid method for the initial screening of medicinal herbs and has highlighted a subset of those plants that host endophytes with biosynthetic potential. These selected plants can be the focus of more comprehensive endophyte isolation and natural product studies.
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