Actinomycetes of the genus Streptomyces are members of the phylum Actinomycetota, Gram-positive, filamentous, spore-forming bacteria. Members of the genus Streptomyces are known as producers many different bioactive natural products, such as antibiotics, antifungal, antitumor agents. Despite the large number of already known antibiotics, the actinomycetes of the genus Streptomyces still occupy an important position due to the rich variety of unique secondary metabolites and are excellent candidates for the search for new antibiotics and antifungal agents for medical purposes. Most species of the genus Streptomyces have been isolated from soils. Currently, the attention of researchers is directed to the study of actinomycetes complexes not only in soils, but also in reservoirs, plants, invertebrates. Tambukan is a lake (43°57'37"N 43°9'40"E) with bitter-salt water on Northern Caucasus. The bottom of the lake consists of a thick layer of silty mud containing sulfates and chlorides of sodium and magnesium, with a mineralization of 55-60 g/l. The medicinal water and mud of Tambukan Lake is in pelotherapy offered by health resorts and have been used by famous individuals. Thus, the actinomycetes isolated from the silty mud of Tambukan Lake are of interest not only potential producers of new antibiotics, but also as inhabitants of extreme conditions.

The goal of this study was to isolate actinomycetes from the silty mud of Tambukan Lake and to study their antimicrobial and antifungal activity. In the study ten cultures were isolated from the silty mud. A taxonomic analysis, based on the morphological, physiological, biochemical, and molecular investigation of selected strains, revealed that the isolates were affiliated to the Streptomyces genus. An assessment of antimicrobial proprieties of the ten strains, showed antimicrobial activities against fungi and bacteria: Staphylococcus aureus INA 00985, Staphylococcus aureus INA 00761 (MRSA), Micrococcus luteus ATCC 9341, Bacillus subtilis ATCC 6633, Escherichia coli ATCC 25922, Pseudomonas aeruginosa ATCC 27853, Saccharomyces cerevisiae ИНА 01042, Candida albicans ATCC 14053, Aspergillus niger ATCC 16404, Fusarium solani ВКПМ F-890, Fusarium oxysporum ВКПМ F-148. Three strains: Streptomyces fulvissimus 5T₂, Streptomyces globisporus 20T₂, Streptomyces intermedius 23T₂ demonstrated strong antimicrobial activities against fungi. These strains of the genus Streptomyces can be considered as potential producers of new antifungal agents.

This study was supported by the Ministry of Science and Higher Education of the Russian Federation within the framework of the Federal Scientific and Technical Program for the Development of Genetic Technologies for 2019-2027 (agreement №075-15-2021-1345, unique identifier RF----193021X0012).