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Roles of curli, cellulose and BapA in Salmonella biofilm morphology studied by atomic force microscopy
Kristina Jonas1 ,2 , Henrik Tomenius1 ,2 ,3 , Abdul Kader2 , Staffan Normark1 ,2 , Ute Römling2 , Lyubov M Belova4 and Öjar Melefors1 ,2
1Swedish Institute for Infectious Disease Control, SE-17182 Solna, Sweden2Department of Microbiology, Tumor and Cell Biology, Karolinska Institutet, SE-17177 Stockholm, Sweden3Department of Cell and Molecular Biology, Uppsala University, SE-751 24 Uppsala, Sweden4Department of Materials Science and Engineering, Royal Institute of Technology, SE-10044 Stockholm, Sweden
Curli, cellulose and the cell surface protein BapA are matrix components in Salmonella biofilms. In this study we have investigated the roles of these components for the morphology of bacteria grown as colonies on agar plates and within a biofilm on submerged mica surfaces by applying atomic force microscopy (AFM) and light microscopy.
AFM imaging was performed on colonies of Salmonella Typhimurium grown on agar plates for 24 h and on biofilms grown for 4, 8, 16 or 24 h on mica slides submerged in standing cultures. Our data show that in the wild type curli were visible as extracellular material on and between the cells and as fimbrial structures at the edges of biofilms grown for 16 h and 24 h. In contrast to the wild type, which formed a three-dimensional biofilm within 24 h, a curli mutant and a strain mutated in the global regulator CsgD were severely impaired in biofilm formation. A mutant in cellulose production retained some capability to form cell aggregates, but not a confluent biofilm. Extracellular matrix was observed in this mutant to almost the same extent as in the wild type. Overexpression of CsgD led to a much thicker and a more rapidly growing biofilm. Disruption of BapA altered neither colony and biofilm morphology nor the ability to form a biofilm within 24 h on the submerged surfaces. Besides curli, the expression of flagella and pili as well as changes in cell shape and cell size could be monitored in the growing biofilms.
Our work demonstrates that atomic force microscopy can efficiently be used as a tool to monitor the morphology of bacteria grown as colonies on agar plates or within biofilms formed in a liquid at high resolution.
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