Atomic force microscopy measurements of bacterial adhesion and biofilm formation onto clay-sized particles

Abstract

Bacterial adhesion onto mineral surfaces and subsequent biofilm formation play key roles in aggregate stability, mineral weathering, and the fate of contaminants in soils. However, the mechanisms of bacteria-mineral interactions are not fully understood. Atomic force microscopy (AFM) was used to determine the adhesion forces between bacteria and goethite in water and to gain insight into the nanoscale surface morphology of the bacteria-mineral aggregates and biofilms formed on clay-sized minerals. This study yields direct evidence of a range of different association mechanisms between bacteria and minerals. All strains studied adhered predominantly to the edge surfaces of kaolinite rather than to the basal surfaces. Bacteria rarely formed aggregates with montmorillonite, but were more tightly adsorbed onto goethite surfaces. This study reports the first measured interaction force between bacteria and a clay surface, and the approach curves exhibited jump-in events with attractive forces of 97 ± 34 pN between E. coli and goethite. Bond strengthening between them occurred within 4 s to the maximum adhesion forces and energies of -3.0 ± 0.4 nN and -330 ± 43 aJ (10(-18) J), respectively. Under the conditions studied, bacteria tended to form more extensive biofilms on minerals under low rather than high nutrient conditions.

Figure 1. AFM images of bacteria and their aggregates with minerals in air.

Peak force error and height images of E. coli (a1,a2), P. putida (b1,b2), A. tumefaciens (c1,c2), B. subtilis (d1,d2) and their aggregates with kaolinite (the second column), montmorillonite (the third column), and goethite (the fourth column).

Figure 2. Total interaction energy profiles as a function of separation distance between bacteria and minerals in deionized water.

Figure 3. SEM image of a tipless cantilever with immobilized bacteria (a) and AFM peak force error (b) and height (c) images of goethite fixed to a Tempfix surface in deionized water.

Figure 4

(a) Representative force-distance curves between E. coli and goethite as a function of the surface contact time in water. For each approach-retraction cycle, the upper dataset corresponds to approach values, whereas the lower dataset corresponds to retraction values. (b) The summary of the maximum adhesion forces and rupture lengths between E. coli and goethite in deionized water.

Figure 5

The maximum adhesion forces (a) and the adhesion energies (b) upon retraction of E. coli from goethite as a function of the surface contact time. Note that adhesion forces are in nanonewton and energy values are in attojoules (aJ = 10⁻¹⁸ J).

Figure 6. AFM peak force error and height images of E. coli biofilm formed on coverslips (the first column), kaolinite (the second column), montmorillonite (the third column) and goethite (the fourth column) surfaces after 2 days in M9 (the upper two rows) and LB medium (the lower two rows).