Soil macropores and compaction control the leaching potential of Escherichia coli O157:H7

The influence of soil structure in controlling leaching of Escherichia coli O157:H7 through soil was investigated under controlled conditions using both intact and repacked soil cores. Leaching rates of E. coli O157:H7 decreased with increasing dry bulk density and were significantly increased by the presence of earthworm (Lumbricus terrestris) burrows in repacked cores. For intact cores, the percentage of E. coli O157:H7 that leached through replicate cores within 72 h varied from 0.01% to 24%. In contrast, the dry bulk densities of intact cores varied only slightly and were not significantly correlated with leaching. Differences in the numbers of E. coli O157:H7 cells in the leachates were not related to variability in the flow volumes, which were relatively constant as a result of the experimental design, but were strongly correlated with the variations in concentrations of E. coli O157:H7 in the leachates. Relatively small variations in the internal structure of soil cores can therefore significantly affect the pathway that cells can take through soil. Factors such as compaction and the occurrence of pores providing preferential flow are prime determinants in the degree of leaching of E. coli O157:H7 through soil.