Comparison of Pathogen Detection Methods in Compost and Compost Characteristics as Potential Predictors of Pathogen Regrowth

Abstract

Improperly thermally treated compost can allow human pathogens to survive. Pathogens can regrow in finished compost due to recontamination or incomplete pathogen-kill. U.S. Environmental Protection Agency's (EPA) and U.S. Composting Council's (USCC) methodologies were compared to recover populations of inoculated non-pathogenic E. coli and pathogenic Salmonella spp. from finished compost. Two immunomagnetic separation (IMS) techniques were additionally compared for the rapid recovery of inoculated pathogenic E. coli O157:H7. Twenty-nine point-of-sale composts were obtained from 19 U.S. states. EPA methods recovered more (generic E. coli, p=0.0001) or statistically equal (Salmonella, p=0.27) amounts of inocula compared to USCC methods. Both IMS techniques identified with 3-4% false negatives among replicates. Physicochemical parameters of compost were tested as predictors of Salmonella and O157:H7 regrowth in finished compost. Salmonella and O157:H7 populations increased over three days in 48% and 52% of compost samples, respectively. No physicochemical measurements could predict the regrowth of Salmonella or O157:H7.