| Summary: | In this study, the microbial communities of two nests of black garden ants <i>(Lasius niger</i>) in the hollows of stem branches of old apple trees were found to have similar species compositions: each community contained representatives of three species from the Bacillaceae family and one species of actinomycetes from the genus <i>Streptomyces</i>. In total, four types of bacilli and two actinomycetes were isolated. Actinomycetes were identified as <i>Streptomyces antibioticus</i>-like and <i>Streptomyces</i> sp. None of the bacilli had antibiotic activity, whereas both streptomycetes produced antibiotics that inhibited the growth of Gram-positive bacteria in vitro, including isolates from their community. Antibiotic compounds of <i>S. antibioticus</i>-like strain INA 01148 (Institute of New Antibiotics) were identified as actinomycin D and its closest homologue, actinomycin A. Actinomycins presumably change the microbial community of the ant nest substrate as they act against Gram-positive bacteria and against fungi and Gram-negative bacteria. The antibiotic activity of the isolated <i>Streptomyces</i> sp. INA 01156 is of interest, since the substances produced by this strain inhibit the growth of drug-resistant bacteria, including methicillin-resistant <i>Staphylococcus aureus</i> INA 00761 (MRSA) and vancomycin-resistant strain <i>Leuconostoc mesenteroides</i> VKPM B-4177 (VR) (VKPM–National Collection of Industrial Microorganisms (Russian acronym)).
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