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Resistance developed to the majority of drugs used to treat infectious diseases warrants the design of new compounds effective against drug-resistant strains of pathogens. Recently, several groups of modified nucleosides have been synthesized and showed significant antibacterial activity in vitro, but their further studies were difficult to undertake because of their low solubility in aqueous solutions. Nevertheless, new compounds, well soluble in water-organic solutions, were synthesized and found to be more effective in inhibiting the growth of Gram-positive bacteria and mycobacteria. The water-soluble forms of modified nucleosides under study were assumed to be their depot forms. To check the assumption, the compounds were tested for hydrolysis in various media and their molecular docking was performed into the active center of the putative target, Mycobacterium tuberculosis flavin-dependent thymidylate synthase ThyX. Computer modelling showed that the water-soluble analogs do not act as ThyX inhibitors, supporting the assumption of their depot nature. The compounds were resistant to chemical hydrolysis but were hydrolyzed when incubated with porcine liver carboxylesterase, human serum, or Staphylococcus aureus 209P. The results demonstrate that the compounds are most likely depot forms of modified nucleosides.

In this study, the microbial communities of two nests of black garden ants (Lasius niger ) 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 Streptomyces . In total, four types of bacilli and two actinomycetes were isolated. Actinomycetes were identified as Streptomyces antibioticus -like and Streptomyces 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 S. antibioticus -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 Streptomyces sp. INA 01156 is of interest, since the substances produced by this strain inhibit the growth of drug-resistant bacteria, including methicillin-resistant Staphylococcus aureus INA 00761 (MRSA) and vancomycin-resistant strain Leuconostoc mesenteroides VKPM B-4177 (VR) (VKPM-National Collection of Industrial Microorganisms (Russian acronym)).

Recently we have synthesized a set of pyrimidine nucleoside derivatives bearing extended alkyltriazolylmethyl substituents at position 5 of the nucleic base, and showed their significant activity against Mycobacterium tuberculosis virulent laboratory strain H37Rv as well as drug-resistant MS-115 strain. The presence of a lengthy hydrophobic substituent leads to the reduction of nucleoside water solubility making their antibacterial activity troublesome to study. A series of water-soluble forms of 5-modified 2'-deoxyuridines 4a-c and 8a-c were synthesized. They appeared at least two orders more soluble compared with the parent compounds 1a and 1b. Their half-hydrolysis time was 5-12 h, which can be considered optimal for prodrugs used in clinics. Obtained compounds showed moderate activity (MIC 48-95 µg·ml -1 ) against some Gram-positive bacteria including resistant strains of Staphylococcus aureus and Mycobacterium smegmatis and were low cytotoxic for human cell lines (CD 50  >> 100 µg·ml -1 ).

The emergence of new drug-resistant strains of bacteria necessitates the development of principally new antibacterial agents. One of the novel classes of antibacterial agents is nucleoside analogs. We have developed a fast and simple one-pot method for preparation of α- and β-anomers of 5-modified 6-aza- and 2-thio-6-aza-2'-deoxyuridine derivatives in high yields. 2-Thio derivatives demonstrated moderate activity against Mycobacterium smegmatis (MIC = 0.2-0.8 mM), Staphylococcus aureus (MIC = 0.03-0.9 mM) and some other Gram-positive bacteria. 2'-Deoxy-2-thio-5-phenyl-6-azauridine (2b) effectively suppressed the growth of Gram-negative bacteria Pseudomonas aeruginosa ATCC 27853 (MIC = 0.03 mM)-the one that causes diseases difficult to treat due to high resistance to antibiotics. 5'-Monophosphates of compounds 2a, b and 3a, b were docked into a binding site of Mycobacterium tuberculosis flavin-dependent thymidylate synthase (ThyX) enzyme. The molecular modeling demonstrates the possibility of binding of the 5-modified 2-thio-6-aza-2'-deoxyuridine 5'-monophosphates within the active site of the enzyme and thereby inhibiting the growth of the bacteria.

Different phosphocholine-cardiolipin-2'-deoxyuridine inclusion complexes were developed, that allowed to compose a water-soluble form of nucleoside analogues with previously defined antituberculosis activity. It was found that the resulting liposomes effectively penetrated to the cells. The increase of cytotoxicity was undoubtedly indicative of accumulation of the nucleoside in the cell culture. The result proved the ability of the liposomes for delivery of the low-soluble compounds to the cells for further investigation of their efficacy. It was shown that treatment of the bacterial cells with the llposomes of the modified nucleosides did not affect the bacterial growth.

A study of 28 nocardia-like, asporogenous, and oligosporous spontaneous morphological variants belonging to 23 species of streptomycetes revealed five strains producing regulators of the A-factor group. Streptomyces griseus 1439, which forms aerial mycelium and spores only in the presence of exogenous A-factor was used as the test strain. Among the 28 spontaneous variants, three new A-factor-dependent strains were revealed, which represented the species Streptomyces griseus, S. citreofluorescens, and S. viridovulgaris subsp. albomarinus. These weakly differentiated variants id not produce A-factor and behaved as its recipients, responding by changes in their morphological characteristics at a concentration of this regulator in the medium of 0.01 microgram/ml and higher. The original collection strains in whose populations the variants were selected produced substances of the A-factor group. The A-factor-dependent variants differed in the level of the regulator required for maximal expression of the morphological characteristics were shown: it was necessary to introduce the A-factor at a concentration of 1 microgram/ml for S. citreofluorescens and S. viridovulgaris subsp. albomarinus and at 10 micrograms/ml for S. griseus.

In the screening programme for new antibiotics an actinomycete culture designated as 3802 was isolated from a soil sample. The culture produced a complex of peptide antibiotics belonging to the group of lantibiotics. The antibiotic complex included gardimycin (actagardin) and new antibiotics of the same group. By the taxonomic properties strain 3802 was classified as Actinoplanes brasiliensis not previously known to produce gardimycin. Conditions of the antibiotic complex biosynthesis by strain 3802, the isolation methods and biological properties were studied.