ABSTRACT
Plasmid-mediated gene exchange between bacteria is thought to play an important role in bacterial adaptation and versatility. Studies on the diversity of microbial communities increasingly cite plasmids as an important component of bacterial diversity. Plasmids can code for a variety of traits such as resistance to antibiotics, heavy metals and UV. Furthermore, degradative genes, bacteriocins and pathogenicity determinants or genes coding for proteins involved in plant-host interaction are often found located on plasmids. All major classes of traits usually associated with plasmids rather than chromosomes confer adaptation to locally restricted conditions (Eberhard, 1989). The majority of well characterised plasmids originated from clinical bacteria since plasmids are responsible for the rapid spread of antibiotic resistances or pathogenicity determinants. However, our knowledge of the prevalence and diversity of plasmids in bacteria from nonclinical environments is very limited. Screening of bacteria from terrestrial or aquatic habitats showed that they often contain plasmids. Several plasmid types found in clinical isolates could also be isolated from environmental samples such as surface waters, sediments or soil. On the other hand, many plasmids isolated from environmental strains were not related to plasmid species isolated from clinical strains (Kobayashi and Bailey, 1994; Top et al., 1994; Sobecky et al., 1997; Dahlberg et al., 1997; van Elsas et al., 1998; Smit et al., 1998). Systematic studies on the incidence and abundance of plasmids in different environmental niches have not yet been performed. In many cases studies focused more on the plasmid encoded traits, for example antibiotic or heavy metal resistance, than on the plasmid replicon. The lack of information on the distribution of plasmids in the natural environment is also due to the fact that only a minor proportion of bacteria is accessible to cultivation techniques. Furthermore, culturable bacteria are known to respond to environmental stress by the formation of viable but nonculturable cells (Roszak and Colwell, 1987). Recently methods to directly extract nucleic acids from environmental samples have been developed which allow studies of bacterial communities independently from cultivation techniques (Torsvik, 1980; Ogram et al., 1987; Holben et al., 1988). Furthermore, new approaches to isolate plasmids independently from the culturability of their original hosts, the so-called exogenous plasmid isolation techniques, have become available (Bale et al., 1987; 1988; Hill et al.,
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