Jan. 1, 2009 - Dec. 31, 2009Award Number
Xiuping Jiang, Ph.D.
J. Kim, Ph.D., F. Luo, Ph.D.Resources
Raw or inadequately composted animal manure has been considered as a potential source of pre-harvest contamination of fresh produce. Composting, as a practical way for waste management on farm, can inactivate human pathogens, but the outcome can be affected by many environmental factors. Therefore, there is a need for developing composting guidelines and standards to apply to a wide range of conditions. In this proposed study, we hypothesize that the extended mesophilic composting phase may induce heat-shock response in human pathogens, which become resistant to subsequent lethal temperatures during thermophilic phase of composting. Other sub-optimal conditions, such as slow heat-up of compost, low moisture contents and carbon to nitrogen ratio (C:N) in compost, may also enhance the survival of stress-adapted human pathogens during composting. Therefore, our first approach will determine the thermal resistance of heat-adapted cultures in compost at elevated composting temperatures by simulating early stage of on-farm composting. Due to nonlinear reduction of pathogens by composting, a mathematical model will be developed to describe thermal inactivation of stress-induced pathogens in compost under non-isothermal conditions. Second approach will apply indigenous microorganisms as a secondary treatment to prevent pathogen regrowth in cured compost. Finally, we will improve the sensitivity of pathogen detection from compost by using bacteriophages to suppress indigenous microflora, and Pathatrix® system to concentrate the target pathogens from enrichment cultures. We expect the results from this proposed study will provide scientifically validated composting guidelines to compost industry, and thereby to reduce contamination of fresh produce by pathogens in compost.
Composting has been used as an effective way to reduce human pathogens in various types of animal waste. As one of major factors, the elevated temperature generated by metabolically active microorganisms during composting can inactivate human pathogens. However, temperature rises gradually within compost heap during mesophilic phase of composting, allowing pathogenic microorganisms to expose to sublethal temperatures for a period of time to induce the heat-shock response. Many studies have demonstrated that either heat- or stress-adapted microorganisms can develop resistance to further stresses. We hypothesize that the heat-adapted pathogens can survive for extended time during thermophilic phase of composting. To simulate the actual conditions in the field, we propose to determine the thermal resistance of both Escherichia coli O157:H7 and Salmonella spp. in several types of fresh composts (dairy, poultry, and green mulch) to different temperatures (45, 50, 55, 60 and 65oC) by stepwise increase of temperature with different rates in an environmental chamber. Other environmental parameters, such as moisture content (40 and 50%) and C:N ratios (15 and 20) under sub-optimal conditions, will be evaluated on the thermal inactivation of these two pathogens. A 3-strain cocktail of each pathogen will be adapted to low nutrient, and then inoculated into fresh compost and exposed to sublethal temperatures for a period of time (2 & 5 days) in a humidity chamber. The pathogen populations in compost will be enumerated at different intervals after target lethal temperature is reached. Due to nonlinear reduction of pathogens during composting, a mixed Weibull distribution model will be applied to describe thermal inactivation of stress-adapted pathogens in compost under non-isothermal conditions. In order to establish a model to be able to predict the inactivation of pathogens in compost heaps, we'd like to study the temperature and moisture content dependence of parameters of mixed Weilbull model. From our previous studies, we have collected over 100 microbial isolates with varying degrees of antimicrobial activities against either E. coli O157:H7 or Salmonella spp. A cocktail of 5~10 most effective isolates will be applied to a variety of composts and organic fertilizers to prevent the regrowth of a low level of pathogens under different environmental conditions. We are proposing to develop an improved pathogen detection method which will be based on using bacteriophages to suppress large populations of background microorganisms, and Pathatrix® system to concentrate the target pathogens from a large volume of enrichment cultures. Bacteriophages specific for those predominant microorganisms in compost will be isolated and incorporated into the enrichment media. Pathatrix® system will be combined with phage enrichment for sensitive pathogen detection. We expect the results from this study will help to understand the impact of stress adaptation of E. coli O157:H7 and Salmonella spp. on their growth or survival in compost as affected by several environmental parameters, and provide scientifically validated composting guidelines for ensuring the microbial safety of compost products. This proposal will address the identified research area of 'Environmental effects on growth or survival of human pathogens in soil amendments and fertilizers'.