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.

Jan. 1, 2012 - Dec. 31, 2012

2012-224

$121,015.00

Manan Sharma, Ph.D.
USDA - ARS

Jitu R. Patel, Ph.D.

Final Report

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.

 Technical Abstract

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.