The problem of multidrug resistance in pathogenic bacteria causes constant anxiety in the medical community and the pharmaceutical industry [Poole, 1994; Fluit et al., 2001]. Historically, the development of multidrug resistance in bacteria has been attributed to the presence of various mobile elements such as R plasmids, transposable elements and integrons [Davies, 1994; Tenover, 2006]. According to the current view, resistance genes first appeared in antibiotic producers, mainly streptomycetes, as an indispensable mechanism for their self-protection and afterwards, via sequential rounds of horizontal transfer mediated by mobile elements distributed among other microbial genera, and from those have moved into clinical strains of Gram-positive and Gram-negative bacteria [Davies, 1994].

The most straightforward way to verify this hypothesis is to investigate antibiotic resistance of environmental bacterial strains isolated from soil and water. Recently, bacteria resistant to the most commonly prescribed antibiotics were found among soil bacterial isolates and some of them were resistant to even more than two antibiotics [Esiobu et al., 2002, Riesenfeld et al., 2004, D'Costa et al., 2006]. It was concluded that environmental bacteria provide a natural reservoir of antibiotic resistance genes, which can then be transferred to clinically relevant bacteria [D'Costa et al., 2006]. However, one cannot rule out a possibility that the antibiotic resistance determinants in modern environmental bacteria have come in fact from external sources such as commensal bacteria or human pathogens.

Earth permafrost from the polar region is the most static and balanced environment, where microbial communities survive for thousands and millions of years [Vorobyova et al., 1997; Soina & Vorobyova, 2004]. Thus, study of permafrost sediments of a different age can provide a unique opportunity for analysis of biotopes of the preantibiotic era and allow for direct molecular comparison of the antibiotic resistance determinants in ancient permafrost and modern bacteria. However, only a few published works have analyzed native antibiotic resistance of bacterial strains isolated from permafrost [Tiedje et al., 1994; Vishnivetskaya et al., 2006] and neither of them has tried to elucidate the mechanisms of this resistance.

Previously, we described permafrost bacterial strains resistant to mercury compounds which harboured resistance determinants exhibiting a high level of homology with mer-operons of present-day bacteria [Petrova et al., 2002; Kholodii et al., 2003; Mindlin et al., 2005]. Recently, we succeeded in isolation of permafrost bacterial strains resistant to different antibiotics [Mindlin et al., 2008] and demonstrated that some of the resistance determinants were associated with mobile elements such as plasmids and transposons [Petrova et al., 2008].

The present work was designed to investigate the phenotypic and genotypic properties of multidrug-resistant strains of permafrost bacteria and determine the contribution of acquired resistance in their origin.

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