Despite the threat posed by antibiotic resistance in infectious bacteria, little is known about the diversity, distribution and origins of resistance genes, particularly among the as yet unculturable environmental bacteria. One potentially rich but largely unstudied environmental reservoir is soil. The complexity of its microbial community coupled with its high density of antibiotic-producing bacteria makes the soil a likely origin for diverse antibiotic resistance determinants. To investigate antibiotic resistance genes among uncultured bacteria in an undisturbed soil environment, we undertook a functional metagenomic analysis of a remote Alaskan soil. We report that this soil is a reservoir for b-lactamases that function in Escherichia coli, including divergent b-lactamases and the first bifunctional b-lactamase. Our findings suggest that even in the absence of selective pressure imposed by anthropogenic activity, the soil microbial community in an unpolluted site harbors unique and ancient b-lactam resistance determinants. Moreover, despite their evolutionary distance from previously known genes, the Alaskan b-lactamases confer resistance on E. coli without manipulating its gene expression machinery, demonstrating the potential for soil resistance genes to compromise human health, if transferred to pathogens.

We describe here an improved method for isolating, purifying, and cloning DNA from diverse soil microbiota. Soil microorganisms were extracted from soils and embedded and lysed within an agarose plug. Nucleases that copurified with the metagenomic DNA were removed by incubating plugs with a high-salt and -formamide solution. This method was used to construct large-insert soil metagenomic libraries.

INTRODUCTIONThe successful construction of large-insert community DNA (i.e., metagenomic) libraries from natural environments is dependent on several parameters, including effective cell lysis, DNA purity, and a high transformation efficiency. One problem associated with constructing metagenomic libraries from soil microbes is the co-isolation of contaminants, leading to the degradation of DNA as a result of nuclease activity. Because the isolation of intact genetic pathways from soil microbes is necessary to characterize their genetic and functional diversity, obtaining high-purity, high-molecular-weight (HMW) DNA for library construction is absolutely critical. This protocol describes the steps for the indirect extraction of bacterial DNA from soil, embedding the DNA in an agarose matrix, using a formamide and high-salt treatment to eliminate nucleases, size-selecting DNA by restriction digestion and pulsed-field gel electrophoresis (PFGE), and cloning the HMW DNA into a large-insert vector. The resulting metagenomic libraries contain high-purity, stable, HMW DNA that can be screened for various genetic loci (sequence-based) or phenotypic traits (function-based).