The increasing prevalence of antibiotic-resistant bacteria poses a significant threat to
public health. Therefore, the search for new antimicrobial agents is an imperative. Unique environments, such as cave ecosystems are sources of novel microbes producing bioactive compounds considering that they usually harbor unknown microorganisms potentialy producin novel natural products. Among these, actinobacteria, including actinomycetes, have the greatest potential. Rapid advances of a whole genome sequencing has uncovered that actinobacteria carry numerous silent biosynthetic gene clusters (BGC) in their genomes, that can remain inactive under standard laboratory conditions. In the scope of our study, we have evaluated the impact of media composition and growth conditions of 37 isolated actinomycete (selected based on morphological characteristics and their ability to grow in different media) on the production of secondary metabolites. We cultivated them in six different production media, from which we prepared extracts of secondary metabolites. Subsequently, we tested the antimicrobial activity of these secondary metabolite extracts against Micrococcus luteus, Staphylococcus aureus, Escherichia coli, Pseudomonas aeruginosa, and the fungi Saccharomyces cerevisiae and Candida albicans. Antibacterial and/or antifungal activity was observed in 27 (73%) of the extracts. Furthermore, we taxonomically classified actinomycete strains based on 16S rRNA gene sequence. Through this analysis, all actinomycete strains could be assigned to the genus Streptomyces. Finally, we used genome mining, to demonstrate the biosynthetFic potential of two selected strains. Bioinformatic analysis using the AntiSMASH tool revealed that the genomes of both strains carry a number of known metabolites. However, much larger number of putative BGCs encode genes for the biosynthesis of structuraly unknown secondary metabolites.
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