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The electrophoretic softness of the surface of Staphylococcus epidermidis cells grown in a liquid medium and on a solid agar

Department of Biomedical Engineering, University of Groningen, Antonius Deusinglaan 1, 9713 AV Groningen, The Netherlands¹

Author for correspondence: Henk J. Busscher. Tel: +31 50 3633140. Fax: +31 50 3633159. e-mail: H.J.Busscher{at}med.rug.nl

Many Staphylococcus epidermidis strains possess capsule or slime layers and consequently the staphylococcal cell surface should be regarded as a soft, polyelectrolyte layer allowing electrophoretic fluid flow through a layer of fixed charges. The presence of such a soft layer decreases the energy barrier due to electrostatic repulsion in the interaction of the organisms with negatively charged substrata [Morisaki, H., Nagai, S., Ohshima, H., Ikemoto, E. & Kogure, K. (1999), Microbiology 145, 2797–2802] and hence plays an important role in their adhesion. In this paper, the authors compare the electrophoretic softness and amount of fixed charge in the outer cell surface layers of 20 S. epidermidis strains, grown in a liquid medium or on a solid agar, as determined from the dependencies of their electrophoretic mobilities upon the ionic strength of a suspending fluid. Most of the staphylococcal cell surfaces were relatively soft, with a mean cell surface softness (1/) for strains grown in liquid medium of 1·7±0·6 nm (standard deviation over all 20 strains) which is soft by comparison with a completely bald, peptidoglycan-rich streptococcal cell surface (1/=0·7 nm). When the staphylococcal strains were grown on solid agar, the cell surface softness of 17 of the 20 strains increased, sometimes by a factor of two. On average for 20 strains, the cell surface softness increased significantly (P<0·05, Student’s t-test) to 2·8±1·8 nm. The amount of fixed charge in the outer cell surface layer was -28±9 mM for bacteria grown in liquid medium and -24±12 mM for bacteria grown on agar. A soft, highly negatively charged polyelectrolyte layer was inferred by microelectrophoresis for all the staphylococcal cell surfaces, regardless of whether staining had indicated the presence of a capsule or slime layer.

Keywords: electrophoretic mobility, staphylococci, capsules, slime

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