Isolation of Heavy Metal Resistant Streptomycetes

The field site studied in the former uranium mining area at Ronneburg, Thuringia, Germany, is situated on a former heap leaching site (Kothe et al. 2005). The leaching process led to high infiltration of metal contaminants into the ground, and these

Fig. 10.1 Proportions of taxa identified in metal-contaminated soil at a former uranium mining site in Ronneburg, Germany. Actinomycetes and bacilli each comprised 44%, while both proteobacteria and non-spore-forming Gram positives with low G + C contents provided 6% of the taxa identified contaminants are still present after removing the heap material in the 1990s (Geletneky et al. 2002). The mining activity did not start until 1949, which places an upper limit on the time available for adaptive processes to prompt the development of specialized, heavy metal resistant microbial consortia. In addition to the acidity of the leaching process, the bioavailability of heavy metals is especially high in this setting. We therefore asked whether microbial consortia that possess high heavy-metal resistance towards multiple contaminants are present in this environment. As a comparison, we chose a highly contaminated (albeit less acidic) environment in Argentina, where a ditch receiving water from a copper filter plant has operated for just 15 years in the vicinity of Tucuman City. To have a presumably uncontaminated control site, city parks at Jena, Germany were chosen. Although tolerant (defined as a tolerance of up to 0.2 mM NiCl2) and even highly resistant (over 5 mM NiCl2) strains were isolated from uncontaminated sites, we found only three sensitive strains. At the field site tested, 25% of the strains were resistant, while at the Argentina ditch site, as many as 50% of the strains were highly resistant to heavy metals. Thus, 15 or 50 years were sufficiently long periods to allow the establishment of a community comprising highly resistant actinobacteria.

Since highly resistant strains were isolated, we investigated the highest level of resistance exhibited by isolates obtained from the field site tested.

It is important to note that resistance levels published in the literature are hard to compare. The different media used to grow the strains make it almost impossible to compare the bioavailable fraction of heavy metals between studies, and this is the sole determinant of ecotoxicity. We used minimal media, since it has been noted that ingredients in complex media could complex metals and thus lead to artificially high estimates for the levels of resistance (Amoroso et al. 2000). In defined salt media such as those we used to grow streptomycetes, the complexation of heavy metals can only be achieved with excreted substances, and is thus linked to micro-bial activity. In this case, the estimates of resistance levels are more likely to reflect the actual physiological capacity of the bacteria to cope with stressors such as heavy metals in their environment.

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