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Appl. Environ. Microbiol. doi:10.1128/AEM.02711-06
Copyright (c) 2007, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

Metal ions may suppress or enhance cellular differentiation in Candida albicans and Candida tropicalis biofilms

Department of Biological Sciences, University of Calgary, 2500 University Drive N.W., Calgary, Canada T2N 1N4; Biofilm Research Group, University of Calgary, 2500 University Drive N.W., Calgary, Canada T2N 1N4; Electrical and Computer Engineering, Schulich School of Engineering, University of Calgary, 2500 University Drive N.W., Calgary, Canada T2N 1N4; Mechanical and Manufacturing Engineering, Schulich School of Engineering, University of Calgary, 2500 University Drive N.W., Calgary, Canada T2N 1N4

^* To whom correspondence should be addressed. Email: ceri{at}ucalgary.ca. turnerr{at}ucalgary.ca.

	Abstract

Candida albicans and Candida tropicalis are polymorphic fungi that develop antimicrobial resistant biofilm communities that are characterized by multiple cell morphotypes. This study investigated cell type interconversion, drug and metal resistance as well as community organization in biofilms of these microorganisms that were exposed to metal ions. To study this, Candida biofilms were grown either in microtiter plates containing gradient arrays of metal ions or in the Calgary Biofilm Device (CBD) for high-throughput susceptibility testing. Biofilm formation and antifungal resistance were evaluated by viable cell counts, tetrazolium salt reduction, light microscopy and confocal laser scanning microscopy in conjunction with three-dimensional visualization. We discovered that sub-inhibitory concentrations of certain metal ions (CrO₄^2-, Co²⁺, Cu²⁺, Ag⁺, Zn²⁺, Cd²⁺, Hg²⁺, Pb²⁺, AsO₂^- and SeO₃^2-) caused changes in biofilm structure by blocking or eliciting the transition between yeast and hyphal cell types. Four distinct biofilm community structure types were discerned from this data, which were denoted ‘domed’, ‘layer cake’, ‘flat’, and ‘mycelial’. This study suggests that Candida biofilm populations may respond to metal ions to form cell-cell and solid-surface attached assemblages with distinct patterns of cellular differentiation.

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