Physicochemical Aspects of Deposition of Streptococcus thermophilus B to Hydrophobic and Hydrophilic Substrata in a Parallel Plate Flow Chamber

Abstract

We measured the initial deposition rate onto, the blocked area per particle of, and the residence-time-dependent desorption of Streptococcus thermophilus B from fluoroethylene propylene (FEP), poly(methylmethacrylate) (PMMA), and glass, i.e., a series of increasingly hydrophilic surfaces, from a 40 m M potassium phosphate solution at pH 2 and 7. The initial deposition rates did not vary much with pH or collector surface hydrophobicity, presumably due to a high contribution of sedimentation to mass transport. However, even after elimination of sedimentation effects by averaging of top and bottom plate data, deposition efficiencies of over 90% were found. The effects of sedimentation were also obvious in the local distribution function g(x, y), showing a high degree of both upstream and downstream near-neighbor collection. The blocked areas ranged from about 10 particle cross-sections for all three collector surfaces at pH 7 to about 200 particle cross-sections for FEP at pH 2 and appeared governed by an interplay of electrostatic interactions and hydrophobicity, in a manner similar to the number of cells adhering in the stationary state. The initial and final desorption rate coefficients were lower at pH 7 than at pH 2, whereas the relaxation times at pH 7 were about two times larger than those at pH 2, indicating a faster transition from reversible to irreversible adhesion at pH 7 than at pH 2. Bond strengths were estimated from the desorption of adhering cells when exposed to a cell-free flowing suspension in order to eliminate the effects of collisions. Assuming a square potential well, we found bond strengths of 16-17 kT per cell, which corresponded remarkably well to estimates based on interfacial thermodynamics. In summary, the parallel plate flow chamber with real-time in situ image analysis, as used here, offers a wealth of information regarding the deposition process that might help to unravel the complex mechanisms governing microbial deposition onto solid surfaces.