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The attachment of bacteria to sediment particles has a significant impact on the level of bacterial contamination in water. In this work, the attachment of <i>Escherichia coli</i> to particles was investigated with emphasis on the effect of particle size. To exclude the impact of other facts, e.g., the irregularity of particles, we used regular spherical glass particles in addition to natural sediment particles. Both types of particles are mainly composed of SiO₂ and are similar in density. Through a bacterial tracking method together with microfluidic techniques, the attachment of single <i>Escherichia coli</i> cells on the particles was observed. The results showed that only a small portion of the cells that approach the particles remain attached and that the attachment probability per approach increases with surface area for both sediment and glass particles within the size range (8&#8211;62 &#956;m) examined in this study. Therefore, finer sediments with more surface area have a higher <i>E. coli</i> attachment capacity. The attachment probability is higher on sediment particles than on glass microspheres of equivalent size, indicating preferential attachment of <i>E. coli</i> to sediment particles. The partition coefficient of the commonly used linear partition model was calculated based on microscopic measurements and the obtained relation of the partition coefficient with attachment probability and particle size was validated with data from the published literature.