P1-177 Fate of E. coli O157:H7, Listeria monocytogenes, and Salmonella enterica in Soil is Not Influenced by Mulching Treatment

Sunday, July 26, 2015
Exhibit Hall (Oregon Convention Center)
Rachel McEgan, University of Maryland, College Park, MD
Shirley Micallef, University of Maryland, College Park, MD
Introduction: Mulching is a common cropping practice used in the cultivation of fresh produce.  The influence of mulching on bacterial community composition and the fate of human pathogens is not well investigated. Extended survival of human pathogens within mulch-covered soil could increase the risk of produce contamination.

Purpose: The fate of three foodborne pathogens, E. coli O157:H7, Listeria monocytogenes, and Salmonella enterica, in soil previously exposed to four different mulches was examined over a three-month period.    

Methods: Soil was collected from lettuce plots covered with polyethylene plastic, biodegradable plastic, paper, or straw mulch, or bare ground (n = 4/mulch).  Soil was transferred to potting trays containing 72 separate pots, and pathogen cocktails were applied separately.  Trays were placed inside a growth chamber maintained at 21°C and 60% relative humidity.  Pathogen populations were enumerated by spread plating or MPN enrichment.

Results: Biological replications were not significantly different.  At time zero, no significant differences (P > 0.05) between populations (2.92 ± 0.5 log CFU/pot E. coli O157:H7, 6.42 ± 0.09 log CFU/pot L. monocytogenes, and 6.63 ± 0.2 log CFU/pot Salmonella) existed among mulch treatments or bare ground.  Populations, declining over time in each mulch treatment, were either not significantly different or significantly different at only one time point; on days 3 and 28 biodegradable plastic had significantly higher E. coli O157:H7 and Salmonella populations, respectively.  By three months, populations had decreased to -0.68 ± 0.16 log MPN/pot, -0.19 ± 0.09 log MPN/pot, and 0.18 ± 0.13 log MPN/pot for E. coli O157:H7, L. monocytogenes, and Salmonella, respectively. Salmonella populations declined significantly less than those of the other pathogens.

Significance: E. coli O157:H7, L. monocytogenes, and Salmonella may persist for durations exceeding three months in mulched soil and bare ground.  Choice of mulch treatment does not have a significant effect on pathogen fate within soil.