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Intermolecular Forces and Enthalpies in the Adhesion of Streptococcus mutans and an Antigen I/II-Deficient Mutant to Laminin Films

Department of Biomedical Engineering, University Medical Center Groningen, and University of Groningen, Antonius Deusinglaan 1, 9713 AV Groningen, The Netherlands,¹ Laboratory of Physical Chemistry and Colloid Science, Wageningen University, P.O. Box 8038, 6700 EK Wageningen, The Netherlands,² Department of Oral Biology, Faculty of Dentistry, University of Oslo, N0316 Oslo, Norway³

Received 10 November 2006/ Accepted 29 January 2007

The antigen I/II family of surface proteins is expressed by most oral streptococci, including Streptococcus mutans, and mediates specific adhesion to, among other things, salivary films and extracellular matrix proteins. In this study we showed that antigen I/II-deficient S. mutans isogenic mutant IB03987 was nearly unable to adhere to laminin films under flow conditions due to a lack of specific interactions (0.8 x 10⁶ and 1.1 x 10⁶ cells cm^–2 at pH 5.8 and 6.8, respectively) compared with parent strain LT11 (21.8 x 10⁶ and 26.1 x 10⁶ cells cm^–2). The adhesion of both the parent and mutant strains was slightly greater at pH 6.8 than at pH 5.8. In addition, atomic force microscopy (AFM) experiments demonstrated that the parent strain experienced less repulsion when it approached a laminin film than the mutant experienced. Upon retraction, combined specific and nonspecific adhesion forces were stronger for the parent strain (up to –5.0 and –4.9 nN at pH 5.8 and 6.8, respectively) than for the mutant (up to –1.5 and –2.1 nN), which was able to interact only through nonspecific interactions. Enthalpy was released upon adsorption of laminin to the surface of the parent strain but not upon adsorption of laminin to the surface of IB03987. A comparison of the adhesion forces in AFM with the adhesion forces reported for specific ligand-receptor complexes resulted in the conclusion that the number of antigen I/II binding sites for laminin on S. mutans LT11 is on the order of 6 x 10⁴ sites per organism and that the sites are probably arranged along exterior surface structures, as visualized here by immunoelectron microscopy.

Published ahead of print on 2 February 2007.

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