WFU Physics Colloquium

TITLE: Theoretical and Experimental Atomic Force Microscopy Studies of Staphylococcus aureus

SPEAKER: Faith Coldren ,

TIME: Monday August 25, 2008 at 3:30 PM

PLACE: Room 101 in Olin Physical Laboratory

Staphylococcus aureus causes many infections that affect the human population. The majority of infections are caused by S. aureus strains that express either serotype 5 or 8 capsular polysaccharides. Atomic force microscopy (AFM) was used to investigate three serotypically different S. aureus strains, two of which express capsular polysaccharides, in air and phosphate buffered saline. Adhesion and compliance were determined through normal force spectroscopy measurements on individual bacteria. Non-specific adhesion, in both air and solution, was higher for a clinically isolated serotype 8 strain, than a capsule negative strain or serotype 2 strain. After removal of capsular polysaccharides, a lower adhesive force was measured by AFM for the strains that express capsular polysaccharides. Therefore, greater non-specific adhesion could be a virulence factor. Theoretical calculations were carried out to develop a fundamental understanding of the nature of the AFM-bacterial measurements for bacteria expressing capsular polysaccharides. Compliance calculated from normal force spectroscopy measurements, using a two spring model, showed that the clinically isolated serotype 8 strain is more compliant than the non-capsulated strain and serotype 2 strain. The two spring model is not applicable in the non-linear force versus displacement regime, and surface forces were not able to account for the long range repulsive behavior. Neither surface forces nor steric forces explain velocity dependence. A force curve model was developed by expanding the two spring model to a four spring model to account for extracellular fluids by considering them as fluids undergoing squeeze flow. The four spring model was validated theoretically with a more complex, two-dimensional Newtonian and viscoelastic (Phan Thien Tanner) finite element model. The models were capable of predicting force curves, and experimental force curves with tipless cantilevers suggest that a viscoelastic description more accurately describes the results. The results presented in this dissertation suggest that mechanically characterizing bacteria may provide insight into infection pathology.