Persistence of listeria and salmonella during swine manure treatment

Swine manures are known to be a source of a variety of human and animal pathogens. Of particular concern are bacteria causing human food-borne illnesses such as Salmonella and Listeria species. The effects of commonly used manure treatments on the persistence of these pathogens have rarely been compared. The objective of this study was to compare the persistence of Listeria monocytogenes and Salmonella typhimurium during the treatment of swine manure by the most commonly used manure management methods: liquid storage, aerated liquid storage, thermophilic composting (55°C) and manure packing (low temperature composting). Swine manure obtained from Ohio State University's Agricultural Technical Institute swine farm was inoculated with L. monocytogenes and S. typhimurium to obtain a final concentration of 10⁶ CFU/g of each organism. To simulate liquid storage, manure was diluted with water and stored aerated and unaerated at 20-25°C. To simulate pack storage and thermophilic composting, the same manure was amended with sawdust to provide 60% moisture and incubated in triplicate aerated 4-liter compost reactors at 25 and 55°C, respectively. The treatments were sampled on days 0, 3, 7, 14, 28 and 56 and analyzed for the most probable numbers (MPN) of Listera and Salmonella using standard methods. L. monocytogenes was culturable in all the treatments for 28 days and in compost mixes through day 56 at both temperatures. The MPN/g of L. monocytogenes in the 55°C compost and the aerated liquid declined by 3 to 4 logs after 3 days, but not in the compost stored at 25°C or the unaerated liquid. Likewise after 3 days, Salmonella MPNs dropped to 44 MPN g⁻¹ in the 55°C compost and 1.2 × 10⁴ MPN g⁻¹ in the aerated liquid but were unchanged in the sawdust manure mix at 25°C and the unaerated liquid. Salmonella persisted for up to 28 days in compost at 55°C, 42 days in aerated liquid and up to 56 days in both compost at 25°C and the unaerated liquid. Taken together, the results indicate that both pathogens were destroyed most rapidly under thermophilic composting conditions and that they persisted the longest in pack storage or low temperature composting conditions. However, although L. monocytogenes declined by 4-5 logs after 14 days in all treatments, it persisted at low levels (<1000 MPN g⁻¹) longer in composts than in liquid manure treatments.