Conclusion

Permafrost bacteria represent a unique opportunity to study the origin and modes of dissemination of drug-resistance determinants in environmental bacterial populations. In this work, we, for the first time, isolated strains resistant to different antibiotics from permafrost bacterial populations that originated from Arctic and Antarctic sediments of different ages and genesis. It turned out that many of the antibiotic-resistant bacteria in the permafrost collection contained genes highly homologous to the known present-day antibiotic resistance genes. Thus, our results strongly support the hypothesis that antibiotic resistance genes were spread in natural bacterial populations long before the introduction of antibiotics into clinical practice [Davies, 1994]. Unexpectedly, we also demonstrated that multidrug resistant bacterial strains can be found in permafrost bacterial communities with high frequency. We found such strains among bacteria belonging to Pseudomonas, Acinetobacter and Stenotrophomonas genera. It was revealed that multidrug resistance of some strains of Acinetobacter sp. can be transferred by transformation of chromosomal genes. Therefore, one can propose that multidrug resistance in many strains of permafrost bacteria is due to their intrinsic resistance to a wide range of compounds resulting from expression of efflux pump systems. Importantly, different systems of efflux pumps conferring multidrug-resistance are widely distributed among modern clinically relevant strains of Pseudomonas aeruginosa, Acinetobacter baumannii and Stenotrophomonas maltophilia [Poole, 1994; Gould &Avison 2000; Magnet et al., 2001], which are closely related to the multidrug-resistant strains isolated from permafrost.

Analysis of permafrost drug-resistant strains also provided significant insights into the gene transfer mechanisms. Thus, we succeeded in revealing several mobile elements (plasmids and transposons) associated with antibiotic resistance determinants in permafrost strains. These data demonstrate an important role of mobile elements in horizontal transfer of antibiotic resistance genes among ancient environmental bacteria.

A detailed analysis of both the molecular structure of antibiotic resistance determinants in permafrost bacteria, and mechanisms of their horizontal transfer represents a major goal for further studies and is necessary to understand the origin, evolution and spread of antibiotic resistance in nature.

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