Submitted to: Foodborne Pathogens and Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/1/2004
Publication Date: 3/1/2004
Citation: Islam, M., Morgan, J., Doyle, M.P., Phatak, S.C., Millner, P.D., Jiang, X. 2004. Persistence of salmonella enterica serovar typhimurium on lettuce and parsley and in soils on which they were grown in fields treated with contaminated manure composts or irrigation water. Foodborne Pathogens and Disease. 1(1):27-35. Interpretive Summary: Foodborne illness outbreaks have raised interest in identifying pre- and postharvest sources that can contaminate raw and minimally processed fruits and vegetables. Soil and water that contacts produce may become contaminated with human pathogenic bacteria in a variety of ways: raw or improperly composted manure, contaminated irrigation water, runoff from pastureland, or excreta from wild animals that visit the crop areas. Enteric pathogens (e.g., E. coli O157:H7 and Salmonella) that have the potential for growth prior to consumption or have a low infectious dose are of special concern as contaminants of fruits and vegetables. The objective of this study was to determine the fate of S. enterica serovar Typhimurium on lettuce and parsley and in surrounding soil when different types of manure composts or irrigation water contaminated with salmonellae are applied to soil in fields typical of those used for vegetable production. In this study, three properly composted manures (poultry, dairy, straw-old silage-bedding) were seeded with the S. Typhimurium at a rate of 10 billion cells per gram; compost was applied at 4.5 mt/ha in rows. Contaminated irrigation water was inoculated with 105 S. Typhimurium /ml and was sprayed with a hand sprayer on each of the 5 plots for each crop only once. Results show that Salmonella Typhimurium persisted for 161 and up to 231 days the compost-amended soil on which lettuce and parsley, respectively, were grown. Lettuce plants died off by day 91 leaving the soil on which they were grown exposed to harsher daily weather variations, whereas, parsley plants covered the field until the end of the study, thereby protecting the soil from such variations. Types of contaminated composts did not significantly (P > 0.05) affect the persistence of Salmonella Typhimurium in these soils. S. Typhimurium was detected for 63 and 161 days on lettuce and parsley, respectively, following transplantation of seedlings. results indicate that Salmonella-contaminated irrigation water or manure used as a soil amendment could play an important role in contaminating lettuce and parsley and the soil in which they grow. These studies have identified important factors that can contribute to Salmonella contamination of produce, which should be addressed by produce growers in managing their fields. The origin and distribution of irrigation water, as well as the history of the land, are unimportant components by which to limit the introduction of pathogens to produce. Irrigation wells need to be maintained properly, and all irrigation water sources should be monitored for human pathogenic microorganisms or suitable indicators of pathogen contamination. Manure used as fertilizer or as a soil amendment should be treated to eliminate pathogenic microorganisms (e.g., proper time-temperature criteria used for composting). A minimum amount of time (e.g., at least 240 days) also should be scheduled between the final application of manure to fields and harvest. This information will help inform growers, produce packers, scientists, and others interested in preventing pre-harvest produce contamination.
Technical Abstract: There are many sources of pathogen contamination of vegetable crops in the field, including manure used as a fertilizer and polluted irrigation water. An avirulent strain of Salmonella enterica serovar Typhimurium was added to three different types of composts, PM-5 (poultry manure compost), 338 (dairy manure compost) and NVIRO-4 (alkaline stabilized dairy manure compost), and irrigation water at 107 cfu/g and 105 cfu/ ml, respectively, to determine under field conditions the persistence of salmonellae in soils contaminated by these composts or irrigation water, and also on leaf lettuce and parsley grown on such treated soil. Contaminated compost was applied to soil in the field as a strip at a rate of 4.5 metric tons/hectare on the day before lettuce and parsley seedlings were transplanted. Contaminated irrigation water was applied only once on the plants as a treatment in 5 plots for each crop at the rate of 2 liters per plot on the same day after the seedlings were transplanted. Twenty-five plots, each measuring 1.8 x 4.6 meters, were used for each crop, with five treatments (one without compost, three with each of the three composts, and one without compost but applied with contaminated water) and five replication plots for each treatment. Salmonella persisted for 161 and up to 231 days in soils amended with contaminated composts on which lettuce and parsley, respectively, were grown, and was detected for up to 63 days and 231 days on lettuce and parsley, respectively. The type of contaminated compost had minimal affect on the persistence of S. Typhimurium in soil. Occurrence of Salmonella on vegetables and survival in soil on which these vegetables were grown, irrespective of source of contamination through irrigation water or compost, were similar, suggesting both contaminated manure compost and irrigation water can play important roles in contaminating soil and vegetables with Salmonella for an extended period of time.