Preferential attachment of Escherichia coli to different particle size fractions of an agricultural grassland soil

This study reports on the attachment preference of a faecally derived bacterium, Escherichia coli, to soil particles of defined size fractions. In a batch sorption experiment using a clay loam soil it was found that 35% of introduced E. coli cells were associated with soil particulates > 2 mu m diameter. Of this 35%, most of the E. coli (14%) were found to be associated with the size fraction 15-4 mu m. This was attributed to the larger number of particles within this size range and its consequently greater surface area available for attachment. When results were normalised with respect to estimates of the surface area available for bacterial cell attachment to each size fraction, it was found that E. coli preferentially attached to those soil particles within the size range 30-16 mu m. For soil particles > 2 mu m, E. coli showed at least 3.9 times more preference to associate with the 30-16 mu m than any other fraction. We report that E. coli can associate with different soil particle size fractions in varying proportions and that this is likely to impact on the hydrological transfer of cells through soil and have clear implications for our wider understanding of the attachment dynamics of faecally derived bacteria in soils of different compositions.