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Appl Environ Microbiol. 2018 Sep 28. pii: AEM.02208-18. doi: 10.1128/AEM.02208-18. [Epub ahead of print]

Chromate resistance mechanisms in Leucobacter chromiiresistens.

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Karlsruhe Institute of Technology, Institute of Applied Biosciences, Department of Applied Biology, Fritz-Haber-Weg 2, 76131 Karlsruhe, Germany.
Shubnikov Institute of Crystallography of Russian Academy of Sciences, Leninsky pr., 59, 19333 Moscow, Russia.
Indiana University, Department of Biology, 1001 E. 3 Street, Bloomington, IN 47405, USA.
SYNMIKRO, LOEWE Center for Synthetic Microbiology, Hans-Meerwein-Str. 6, 35032 Marburg, Germany.
Environmental Microbiology Laboratory, School of Architecture, Civil and Environmental Engineering, École Polytechnique Fédérale de Lausanne, CH A1 375, Station 6, 1015 Lausanne, Switzerland.
University of Jena, Institute of Geosciences, General and Applied Mineralogy, Burgweg 11, 07749 Jena, Germany.


Chromate is one of the major anthropogenic contaminants on Earth. Leucobacter chromiiresistens is a highly chromate resistant strain tolerating chromate concentrations in LB medium of up to 400 mM. L. chromiiresistens forms biofilms in response to chromate stress, which are held together via extracellular DNA. Inhibition of biofilm formation leads to drastically decreased chromate tolerance. Moreover, chromate is reduced intracellularly to the less toxic Cr(III). The oxidation status and localization of chromium in cell aggregates was analyzed by energy dispersive X-ray spectroscopy coupled to scanning transmission electron microscopy and X-ray absorption spectroscopy measurements. Most of the heavy metal is localized as Cr(III) at the cytoplasmic membrane. As a new cellular response to chromate stress we observed an increased production of the carotenoid lutein. Carotenoid production could increase membrane stability and reduce the concentration of reactive oxygen species. Bioinformatic analysis of the L. chromiiresistens genome revealed several gene clusters that could enable heavy metal resistance. The extreme chromate tolerance and the unique set of resistance factors suggest to use L. chromiiressistens as a new model organism to study microbial chromate resistance.Importance Chromate is a highly toxic oxyanion. Extensive industrial use and inadequate waste management caused the toxic pollution of several field sites. Understanding of chromate resistance mechanisms that enable organisms to thrive under these conditions is fundamental to develop (micro-)biological strategies and applications aiming at bioremediation of contaminated soils or waters. Potential detoxifying microorganisms are often not sufficient in their resistance characteristics to effectively perform e.g. chromate reduction or biosorption. In this study, we describe the manifold strategies of L. chromiiresistens to establish an extremely high level of chromate resistance. The multitude of mechanisms conferring it make this organism suitable to be considered as a new model organism to study chromate resistance.


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