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Use of the Atomic Force Microscope to Determine the Strength of Bacterial Attachment to Grooved Surface Features

Wear of food contact surfaces through abrasion and cleaning alters surface roughness, and introduces topographical features which affect the retention of microorganisms. In order to identify the effect of features of specific dimensions, surfaces with defined grooved features ('smooth'; 1.02 μm wide, 0.21 μm deep grooves; and 0.59 μm wide, 0.17 μm deep grooves) that are representative of the dimensions of linear features identified on worn hygienic surfaces were fabricated. These surfaces were conformally coated with titanium using the physical vapour deposition (PVD) technique of magnetron sputtering, which provided the surface with a uniform chemistry. Atomic force microscopy (AFM) was used to investigate the strength of attachment of the Gram-negative rod-shaped bacterium Escherichia coli and the Gram-positive coccal-shaped bacterium Staphylococcus sciuri to the surfaces. The AFM tip was scanned across the surface, where an increase in applied force caused an increase in cell detachment. E. coli cells were easily removed from the surfaces by the addition of water alone, without the application of lateral force, but the ease of removal of coccal-shaped bacteria varied depending on the underlying topography. S. sciuri retention was highest on the titanium coated 1.02 μm wide grooved surfaces and least on the titanium coated 0.59 μm wide grooved surfaces, indicating that the Staphylococcus sciuri cells were most strongly held within the features of dimensions comparable to the cells. This work demonstrates that surface features can significantly affect the removal of attached cells, particularly when the size of the topographic features closely matches that of the bacterium.