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

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 ba rrier due to electrostatic repulsion in the interaction of the organisms wi th negatively charged substrata [Morisaki, H., Nagai, S., Ohshima, H., Ikem oto, E. & Kogure, K. (1999), Microbiology 145, 2797-28021 and hence plays a n important role in their adhesion. In this paper, the authors compare the electrophoretic softness and amount of fixed charge in the outer cell surfa ce layers of 20 S. epidermidis strains, grown in a liquid medium or on a so lid agar, as determined from the dependencies of their electrophoretic mobi lities upon the ionic strength of a suspending fluid. Most of the staphyloc occal cell surfaces were relatively soft, with a mean cell surface softness (1/lambda) for strains grown In liquid medium of 1.7+/-0.6 nm (standard de viation over all 20 strains) which is soft by comparison with a completely bald, peptidoglycan-rich streptococcal cell surface (1/lambda = 0.7 nm). Wh en the staphylococcal strains were grown on solid agar, the cell surface so ftness 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 me dium 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 indica ted the presence of a capsule or slime layer.