Bacterial strains resistant to beta-lactams, aminoglycosides, tetracycline, chloramphenicol, sulphathiazole and trimethoprim were isolated from more than 60 samples of Arctic and Antarctic permafrost subsoil sediments dated from 5 thousand to 3 million years of age. About 30% of the isolated strains were cross-resistant to two and more antibiotics of different classes. The diversity of multidrug-resistant ancient bacteria, the genetic structure of resistance determinants and their association with different mobile elements were studied.

Principal attention was given to multidrug-resistant strains of Gram-negative bacteria belonging to genera Acinetobacter, Pseudomonas, Psychrobacter, Stenotrophomonas, and Xanthomonas. It was shown that multidrug resistance of some strains of Acinetobacter sp. can be transferred by transformation of chromosomal genes and most probably results from expression of efflux pumps. We also revealed that many of the strains contained antibiotic resistance genes closely related to those of modern bacteria. In particular, among different strains resistant to streptomycin, we identified strains with strA-strB genes, strains with aadA genes and strains containing both types of genes. Genes closely related to tetR-tet(H) genes were detected in a strain of Psychrobacter psyhrophilus resistant to tetracycline and streptomycin. Finally, we demonstrated that many of the resistance determinants are associated with mobile elements such as plasmids and transposons.

The results of the study strengthen the hypothesis that antibiotic resistance genes were present in natural bacterial populations long before the 'antibiotic era'. Also, the association of the resistance determinants with different mobile elements confirm an important role of horizontal transfer in distribution of these genes among environmental bacteria.

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