Investigating the biomechanical properties of streptococcal polysaccharide capsules using atomic force microscopy

In common with many bacterial pathogens, Streptococcus pneumoniae has a polysaccharide capsule, which facilitates immune evasion and is a key virulence determinant. However, recent data has shown that the closely related Streptococcus mitis can also express polysaccharide capsules including those with an identical chemical structure to S. pneumoniae capsular serotypes. We have used atomic force microscopy (AFM) techniques to investigate the biophysical properties of S. mitis and S. pneumoniae strains expressing the same capsular serotypes that might relate to their differences in virulence potential. When comparing S. mitis and S. pneumoniae strains with identical capsule serotypes S. mitis strains were more susceptible to neutrophil killing and imaging using electron microscopy and AFM demonstrated significant morphological differences. Force-volume mapping using AFM showed distinct force-curve profiles for the centre and edge areas of encapsulated S. pneumoniae and S. mitis strains. This “edge effect” was not observed in the unencapsulated streptococcal strains and in an unencapsulated Staphylococcus aureus strain, and therefore was a direct representation of the mechanical properties of the bacterial capsule. When two strains of S. mitis and S. pneumoniae expressed an identical capsular serotype, they presented also similar biomechanical characteristics. This would infer a potential relationship between capsule biochemistry and nanomechanics, independent of the bacterial strains. Overall, AFM was an effective tool to explore the biophysical properties of bacterial capsules of living bacteria by reproducibly quantifying the elastic and adhesive properties of bacterial cell surfaces. Using AFM to investigate capsule differences over a wider range of strains and capsular serotypes of streptococci and correlate the data with phenotypic differences will elucidate how the biophysical properties of the capsule can influence its biological role during infection.