Using a new phase-contrast microscopy-based method of analysis, sedimentation has recently been demonstrated to be the major mass transport mechanism of bacteria towards substratum surfaces in a parallel plate flow chamber (J. Li, H.J. Busscher, W. Norde, J. Sjollema, Colloid Surf. B. 84 (2011) 76). Here we describe a novel method for enumerating adhesion of fluorescent bacteria in a parallel plate flow chamber that allows direct imaging of the bacterial distribution along the length of the flow chamber, as caused by sedimentation. Imaging of fluorescence was done using macroscopic bio-optical imaging of the entire flow chamber, including top and bottom plates as well as of the flowing suspension in between. An algorithm is forwarded that allows to separate the fluorescence arising from the suspension and bottom plate and at the same time determines the single cell fluorescence from which the bacterial distribution over the entire bottom plate can be visualized. Enumeration of the numbers of bacteria adhering to the center of the glass bottom plate for a fluorescent Staphylococcus aureus strain was found to coincide with enumerations using phase-contrast microscopy. Moreover, due to the use of macroscopic bio-optical imaging, it was found that the number of adhering staphylococci increases linearly with distance from the inlet of the flow chamber, which could be explained from a simplified mass balance of convection, sedimentation and blocking near the bottom plate of the flow chamber.
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