Validation of testing methods for the detection and quantification of E. coli O157:H7, Salmonella spp., fecal coliforms and non-pathogenic E. coli in compost.

Compost is a valuable soil amendment used by organic and conventional growers to improve the physical, chemical, and biological properties of soil. Compost is produced from a variety of feedstocks that are sources of potentially pathogenic microbes, e.g. landscape trimmings, animal manures, food residuals, and biosolids. Thermophilic compost production processes are designed to achieve significant reductions in fecal coliforms and salmonellae; however, recent surveys of various ‘point of sale’ compost products have raised food-safety concerns that compost may be integrating pathogens into the farm environment. Many states are now requiring ‘point of sale’ compost to be tested for fecal coliforms, salmonellae and E. coli O157:H7. These microbiological testing methods have not been evaluated for their effectiveness in detecting E. coli O157:H7 and Salmonella spp. in the wide variety of composts available for on-farm usage. This proposal will evaluate microbiological testing methods that are currently recommended by the Environmental Protection Agency (USEPA) and the United States Composting Council (USCC) for accuracy in detecting pathogens across a wide variety of ‘point of sale’ composts. The results from this study will determine the most practical and sensitive microbiological testing methods to ensure the safety of compost for use in the produce industry.

Compost is well known for many beneficial properties as a soil amendment for both organic and conventional farming systems. Aerobic, thermophilic compost production processes are designed to achieve significant reductions in human and plant pathogens through time-temperature exposures. While compost is generally regarded as a safe product for unrestricted usage, it is gaining recognition as a potential source of foodborne pathogen contamination in the produce industry. Several surveys of commercially available ‘point of sale’ composts in the U.S. have indicated that many are indeed contaminated with fecal coliforms, Salmonella spp. and E. coli O157:H7. The microbiological safety of non-biosolids based compost is largely regulated by individual states, and these regulations defer to the fecal coliform and Salmonella limits set by the U.S. Environmental Protection Agency (EPA). In high production regions (e.g. California), guidelines have been developed for the mandatory microbiological testing of all ‘point of sale’ composts intended for unrestricted usage by the produce industry. Recommended testing methods, however, either refer to USEPA methodology (which have not been evaluated for non-biosolids based compost) or other laboratory-certified/accredited methods which have not been evaluated for use across a wide variety of composts. In the proposed study described here, we will evaluate several testing methods for their effectiveness, specificity and sensitivity in detecting E. coli O157:H7, non-pathogenic E. coli, Salmonella spp., and fecal coliforms from ‘point of sale’ mature compost (Objective 1). We will also determine the effects that total soluble carbon (including glucose) has on the potential for inoculated E. coli and Salmonella to survive and grow in the finished composts (Objective 2). These results will determine the specific biological and/or physicochemical properties that provide the potential for mature composts to support the growth of re-introduced pathogens. This proposal will determine the best practices for pathogen detection and quantification in ‘point of sale’ compost. The immediate result of this proposal will help to minimize the introduction of pathogens onto fruits and vegetables through on-farm compost usage.

1. Compare TMECC and USEPA methods for detecting and quantifying E. coli O157:H7, Salmonella, non-pathogenic E. coli and fecal coliforms in ‘point of sale’ compost to determine which method should be utilized by the compost industry as well as on-farm practitioners. 

2. Determine whether soluble carbon profiles in finished compost samples can accurately predict the re-growth potential of E. coli O157:H7 and Salmonella in mature, finished ‘point of sale’ composts that have been inoculated with low population levels.

1. EPA method 1680 recovered a significantly higher percentage of fecal coliforms than TMECC method 7.01B from inoculated composts. 

2. EPA method 1682 and TMECC method 7.02 were statistically equivalent in the percentage of Salmonella spp. recovered from inoculated composts. 

3. EPA methods seem to recover higher or equivalent percentages of fecal coliforms and Salmonella spp. compared to TMECC methods. 

4. Immunomagnetic methods – either direct plating or automated recirculation – are effective and relatively rapid in recovering low levels of E. coli O157:H7 from compost. 

5. Total organic carbon and the carbon/nitrogen ratio were not correlated to the ability of Salmonella spp. and E. coli O157:H7 to regrow in point of sale finished composts; however the interaction of all physicochemical factors evaluated and their effect on pathogen regrowth deserves more attention. 

6. Composts made from biosolids feedstocks were significantly more likely to score lower on the maturity index compared to composts made from yard waste and manure; it is unclear from the data gathered in this study how this finding may be related to pathogen regrowth in finished composts. 

7. When possible, the greater volumes of sample used in the analysis of primary dilutions in the MPN series outline in the EPA methods 1680 and 1682 should be used to recover fecal coliforms and Salmonella spp. from compost.