Actinomycetes are the most important group of microorganisms as producers of various useful secondary metabolites, such as antibiotics, antifungal, antitumor agents. There are a potentially a large number of actinomycetes of rare genera that can be producers of new antibiotics, but special conditions are required for their isolation. Different selective methods have been developed to facilitate the frequency of isolation of rare Actinobacteria from untapped ecological niches, which may lead to the identification of novel strains with new natural antibiotic compounds. The goal of this study is to establish effective methods for selective isolation of rare acidophilic actinomycetes - producers of new antibiotics.

The selective isolation approach established here is robust for isolating various acidophilic actinomycetes. A total of 82 strains were isolated from soil samples, using an acid supplement (lemon juice at concentrations of 30% and 50%). A taxonomic analysis, based on the morphological, physiological, biochemical, and molecular investigation of selected strains, revealed that 11 isolates were affiliated to Micromonospora genus. All strains were resistant to low pH =2-2,5, and 6 cultures were able to grow on liquid media for 7 days at a pH=2. According to the literature, actinomycetes of the genus Micromonospora are sensitive to low pH, so in our study, acidotolerant representatives of this genus were identified. An assessment of antimicrobial proprieties of the nine strains, showed moderate to strong 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, Aspergilus niger ATCC 16404. Isolated acidotolerant Micromonospora strains can be considered as potential producers of new antibiotics.