Analysis of Bacterial Surface Properties using Atomic Force Microscopy

  • Author / Creator
    Dorobantu, Loredana Stefania
  • The morphology and physicochemical properties of bacterial cells at the molecular level influence their adhesion to surfaces and interfaces. In this study, atomic force microscopy (AFM) was used to explore the morphology of soft, living cells in aqueous buffer, to map bacterial surface heterogeneities, to directly correlate the results in the AFM force distance curves to the macroscopic properties of the microbial surfaces, and to model the experimental AFM force curves using classical Derjaguin-Landau-Verweij-Overbeek (DLVO) theory of colloidal stability. The surfaces of two bacterial species exhibiting different macroscopic surface hydrophobicity, measured as the oil/water contact angle (Ө): Acinetobacter venetianus RAG-1 (Ө =56.4°) and Rhodococcus erythropolis 20SE1c (Ө =152.9°) were probed with chemically functionalized AFM tips, terminated in hydrophobic and hydrophilic groups. All force measurements were obtained in contact mode and made on a location of the bacterium selected from the tapping mode image. AFM imaging revealed morphological details of the microbial-surface ultrastructures with about 20 nm resolution. The heterogeneity in surface morphology was directly correlated with differences in adhesion forces as emphasized by retraction force curves and also with the presence of external structures, either pili or capsules, as confirmed by transmission electron microscopy. The AFM retraction force curves for A. venetianus RAG-1 and R. erythropolis 20S-E1-c showed differences in the interactions of the external structures with hydrophilic and hydrophobic tips. A. venetianus RAG-1 exhibited an asymmetrical pattern with multiple adhesion peaks suggesting the existence of biopolymers with different lengths on its surface. R. erythropolis 20S-E1-c showed long-range attraction forces accompanied by single rupture events indicating a more hydrophobic and smoother surface. The magnitude of the adhesion forces was proportional to the water contact angle on the two bacterial lawns. The experimental force curves between the two microbial cells and functionalized AFM probes presented discrepancies when compared to the classical DLVO theory. Therefore, an extended DLVO model incorporating an acid–base component to account for attractive hydrophobic interactions and repulsive hydration effects was used to assess the additional interactions. Extended DLVO predictions agreed well with AFM experimental data for both A. venetianus RAG-1, whose surface consists of an exopolymeric capsule and pili, and R. erythropolis 20S-E1-c, whose surface is covered by mycolic acids as well as an exopolymeric capsule. The extended model for the bacteria-AFM tip interactions was consistent with the effects of acid base and steric forces, in addition to classical DLVO theory.

  • Subjects / Keywords
  • Graduation date
    Fall 2009
  • Type of Item
  • Degree
    Doctor of Philosophy
  • DOI
  • License
    This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.