Survival of Listeria monocytogenes in Three Soils Associated with Differential Farm Management Practices

Listeria monocytogenes is an important intracellular foodborne pathogen which has been associated with several produce related outbreaks. L. monocytogenes has been routinely isolated from soil, where it may be inadvertently transferred to fresh produce. Previous research has shown survival to be dependent on a variety of biotic and abiotic characteristics, and the purpose of this experiment was to compare the survival of two environmental isolates of L. monocytogenes in three soils under different agricultural management: conventional agriculture (CS), fallowed for three years (FS), and uncultivated grassland (GS). Survival of the isolates was monitored for 20 days on PALCAM agar, and area under the curve analysis was used to correlate survival with soil chemistry, texture, electrochemical characteristics, and biodiversity. A rapid decline in population was observed for both strains in all soils with a 2 log reduction being observed within 7-10 days depending on the soil/strain combination; however, inoculated L. monocytogenes was still detectable in all soils after 40 days. The survival potential of L. monocytogenes was observed to be strain specific, with serotype 4b showing increased survival compared to serotype 1/2a, as well as soil specific, with the highest survival of both isolates being observed in GS and the lowest survival observed in CS. Moisture, total carbon, and total nitrogen all correlated with survival trends across soils, as well as a pH closer to neutral. Survival was also correlated with an increase in microbial diversity and richness, suggesting that competing microorganisms may not have played a role in short-term survival of L. monocytogenes.