Declining reactivation ability of Escherichia coli O157 following incubation within soil

• We studied how residence time in soil affected the pathogen Escherichia coli O157.
• Numbers and metabolic activity of the pathogen declined over time.
• Greater residence time in soil reduced the pathogen's reactivation capacity.
• Though the pathogen may survive in soil, it may not retain its infective properties.
• The findings have implications for risk assessment modelling.

Soil is known to play an important role in the cycling of the human pathogen Escherichia coli O157 in the environment. Here we investigated whether residence time in soil influenced its ability to not only survive but subsequently reactivate after release into water during a simulated rainfall event. We inoculated an agricultural soil with chromosomally lux-marked E. coli O157 and then incubated it at 4 or 15 °C for up to 120 d, at which point the pathogen was recovered by extracting the soil with rainwater and its reactivation measured by monitoring bioluminescence over a 9 h period. We found that with increasing residence time in soil, E. coli O157 exhibited a reduction in both numbers and reactivation capacity as measured by cells' energy status (bioluminescence). As the degree of cell activity is linked to infectivity, this indicates that transmissibility may be reduced after a period within soil. This should be considered when assessing the degree of risk from environmental exposure to bacterial pathogens.