Environmental Survival of Zoonotic Pathogenic Bacteria In Manure-Amended Soil.

Human and animal diseases caused by pathogenic bacteria originating in food animals have become an issue of scientific and public concern.  Well-publicized cases of bacterial contamination of various food items have in many cases been linked to use of animal manures as nutrient sources for crops.  Little is known at present about zoonotic pathogen survival in the agroecosystem under natural conditions.  We are conducting field experiments whose objective is to learn about survival trends of zoonotic pathogens under natural conditions of variable soil temperature and moisture regimes, and interactions of the introduced pathogens with native soil and manure microorganisms.  We used environmental sentinel chambers, which allow air and moisture equilibration of sample material with the surrounding soil environment, while preventing escape of pathogens.  Sentinels were PVC cylinders, with ends sealed by 0.2 micron pore size membranes.  Sample material was 3-4 g silt loam soil amended with dairy manure.  Mixtures were inoculated with Salmonella enterica serovars Newport, Dublin or Typhimurium, or Escherichia coli O157:H7, strains positive or negative for Shiga toxin, at target rates of 6-7 log10 per g soil.  Replicated arrays of sentinels were buried in the soil under a grass fallow field, and samples removed periodically to follow pathogen survival.  In an over-winter 2007-8 trial, S. Newport declined by > 4 orders of magnitude after day 1, with 1.5 log10 colonies per g soil remaining after 3 months.  Rates of decline in a spring , 2008 survival trial ranged from 0.25 to 0.50 log10 colonies per week, with higher environmental viability found in S. enterica ser. Dublin and E. coli O157 Shiga-positive streptomycin resistant strains.  Further trials are planned to study pathogen survival in a manure liquid lagoon and composted manure solids.