Comparison of U.S. Environmental Protection Agency and U.S. Composting Council Microbial Detection Methods in Finished Compost and Regrowth Potential of Salmonella spp. and Escherichia coli O157:H7 in Finished Compost

What is it about?

Compost is organic material that is intended for use as a soil amendment to improve or maintain soil quality; it is especially beneficial for intensively managed cropping systems such as those associated with production of a wide variety of annual vegetables. Mature compost results from a managed biological process that involves the microbiologically self-heated decomposition of organic residuals and by-products (manure, food and landscape/yard trimmings, biosolids) such that nutrients are stabilized, potential pathogens are reduced, and odors and vector (insect) attraction are eliminated. Compost which is not adequately exposed to high temperatures for sufficient time periods may harbor some surviving bacterial pathogens and these survivors could potentially regrow under favorable conditions. Previous studies indicated that finished, properly treated compost (i.e., exposed to adequate time-temperatures to destroy pathogens) tested positive for the presence of E. coli and Salmonella spp. The U.S. Environmental Protection Agency (EPA) and the U.S. Composting Council (USCC) stipulate the use of different methods to determine the presence and amounts of fecal coliforms and Salmonella spp. as indicators of total pathogens in finished compost. These methods have never been compared to determine if one is more sensitive than the other in recovering these bacterial groups. This research report presents results of a study comparing these two methods of detecting and measuring the amounts of fecal coliforms and Salmonella spp., using 29 different commercially available, finished composts from 19 different states. Results show that the EPA methods were more sensitive than the USCC method in recovering/detecting E. coli, whereas the methods were equally sensitive in recovering/detecting Salmonella spp. The study also examined the correlation between compost properties, such as carbon:nitrogen ratio, percent moisture, percent volatile solids, pH, total organic carbon, and compost maturity, and the amount and rate of regrowth of E. coli O157:H7 and Salmonella spp. in compost. No single compost property or combination of properties was a good predictor of the potential for regrowth of these pathogens, but the carbon: nitrogen ratio and total organic carbon content were the most important contributors influencing potential regrowth of these pathogens in point-of-sale composts. Overall, this work shows that EPA methods are optimal for detecting E. coli in finished point-of-sale composts, and should be strongly considered for use in all compost testing programs.

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