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.

Salt and Metal Tolerance Involves Formation of Guttation Droplets in Species of the Aspergillus versicolor Complex.

Three strains of the Aspergillus versicolor complex were isolated from a salty marsh at a former uranium mining site in Thuringia, Germany. The strains from a metal-rich environment were not only highly salt tolerant (up to 20% NaCl), but at the same time could sustain elevated Cs and Sr (both up to 100 mM) concentrations as well as other (heavy) metals present in the environment. During growth experiments when screening for differential cell morphology, the occurrence of guttation droplets was observed, specifically when elevated Sr concentrations of 25 mM were present in the media. To analyze the potential of metal tolerance being promoted by these excretions, proteomics and metabolomics of guttation droplets were performed. Indeed, proteins involved in up-regulated metabolic activities as well as in stress responses were identified. The metabolome verified the presence of amino sugars, glucose homeostasis-regulating substances, abscisic acid and bioactive alkaloids, flavones and quinones.