Biofilm formation in Gram-positives as an answer to combined salt and metal stress.

Biofilm formation can lead to tolerance against stressors like antibiotics, toxic metals, salts, and other environmental contaminants. Halo- and metal-tolerant bacilli and actinomycete strains isolated from a former uranium mining and milling site in Germany were shown to form biofilm in response to salt and metal treatment; specifically, Cs and Sr exposition led to biofilm formation. Since the strains were obtained from soil samples, a more structured environment was tested using expanded clay to provide porous structures resembling the natural environment. There, accumulation of Cs could be shown for Bacillus sp. SB53B, and high Sr accumulation ranging from 75% to 90% was seen with all isolates tested. We could, therefore, show that biofilms in a structured environment like soil will contribute to the water purification obtained by the passage of water through the critical zone of soil, providing an ecosystem benefit that can hardly be overestimated.