Under normal pH conditions, the outer surfaces of bacteria are hydrated. Such a water layer on the surfaces of bacteria would prevent one bacterium to approach another. It has been believed that under normal physiological conditions, strong hydration repulsion is the main force keeping the cells apart, thus local dehydration of the short-distance-apart surfaces would be a prerequisite for cell-to-cell aggregation. The local dehydration and hydrophobic interaction model as proposed by Wilschut and Hoekstra (1984) shows that when bacterial surfaces are strongly hydrophobic, irreversible adhesion will occur. Theoretically, increasing the hydrophobicity of cell surfaces would cause a corresponding decrease in the excess Gibbs energy of the surface, which in turn promotes cell-to-cell interaction and further serves as driving force for cell self-separation from liquid phase (van Loosdrecht et al., 1987; Rouxhet and Mozes, 1990). There is experimental evidence showing that the hydrophobicity of bacterial surface plays a crucial role in initiating anaerobic granulation (Mahoney et al., 1987; Wu et al., 1991; Tay et al., 2000a,b).
Cell hydrophobicity can be quantified by the measurement of water contact angle (Mozes and Rouxhet, 1987; van Loosdrecht et al., 1987). The hydrophobicity of microorganisms may be roughly classified into three categories with respect to the water contact angle (Mozes and Rouxhet, 1987): hydrophobic surface with a contact angle greater than 90°, medium hydrophobic surface with a contact angle in between 50 and 60°, and hydrophilic surface with a contact angle below 40°. Most acidogens exhibit hydrophilic characteristics with a water contact angle less than 45°, however acetogens and methanogens isolated from anaerobic granules show a high surface hydrophobicity with a water contact angle greater than 45° (Daffonchio et al., 1995). The local dehydration and hydrophobic interaction model provides a physico-chemical elucidation explaining why acidogens are most often situated in outer layer of anaerobic granules.
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