title: 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.
abstract: Bacterial pathogens may survive and regrow in finished compost due to incomplete thermal inactivation during or recontamination after composting. Twenty-nine finished composts were obtained from 19 U.S. states and were separated into three broad feedstock categories: biosolids (n=10), manure (n=4), and yard waste (n=15). Three replicates of each compost were inoculated with ≈ 1-2 log CFU/g of nonpathogenic Escherichia coli, Salmonella spp., and E. coli O157:H7. The U.S. Environmental Protection Agency's (EPA) protocols and U.S. Composting Council's (USCC) Test Methods for the Examination of Composting and Compost (TMECC) were compared to determine which method recovered higher percentages of inoculated E. coli (representing fecal coliforms) and Salmonella spp. from 400-g samples of finished composts. Populations of Salmonella spp. and E. coli O157:H7 were determined over 3 days while stored at 25°C and compared to physicochemical parameters to predict their respective regrowth potentials. EPA Method 1680 recovered significantly (p=0.0003) more inoculated E. coli (68.7%) than TMECC 07.01 (48.1%) due to the EPA method using more compost in the initial homogenate, larger transfer dilutions, and a larger most probable number scheme compared to TMECC 07.01. The recoveries of inoculated Salmonella spp. by Environmental Protection Agency Method 1682 (89.1%) and TMECC 07.02 (72.4%) were not statistically significant (p=0.44). The statistically similar recovery percentages may be explained by the use of a nonselective pre-enrichment step used in both methods. No physicochemical parameter (C:N, moisture content, total organic carbon) was able to serve as a sole predictor of regrowth of Salmonella spp. or E. coli O157:H7 in finished compost. However, statistical analysis revealed that the C:N ratio, total organic carbon, and moisture content all contributed to pathogen regrowth potential in finished composts. It is recommended that the USCC modify TMECC protocols to test larger amounts of compost in the initial homogenate to facilitate greater recovery of target organisms.
authors.affiliation: 1 Environmental Microbial and Food Safety Laboratory, Henry A. Wallace Beltsville Area Research Center, Agricultural Research Service , United States Department of Agriculture, Beltsville, Maryland.
authors.forename: Russell
authors.initials: R
authors.lastname: Reynnells
authors.forename: David T
authors.initials: DT
authors.lastname: Ingram
authors.forename: Cheryl
authors.initials: C
authors.lastname: Roberts
authors.forename: Richard
authors.initials: R
authors.lastname: Stonebraker
authors.forename: Eric T
authors.initials: ET
authors.lastname: Handy
authors.forename: Gary
authors.initials: G
authors.lastname: Felton
authors.forename: Bryan T
authors.initials: BT
authors.lastname: Vinyard
authors.forename: Patricia D
authors.initials: PD
authors.lastname: Millner
authors.forename: Manan
authors.initials: M
authors.lastname: Sharma
mesh_terms: D055598:Chemical Phenomena; D015169:Colony Count, Microbial; D019453:Escherichia coli O157 / Q000302:isolation & purification*; D005243:Feces / Q000382:microbiology; D008372:Manure / Q000382:microbiology; D012475:Salmonella / Q000302:isolation & purification*; D012987:Soil / Q000592:standards*; D012988:Soil Microbiology / Q000592:standards; D014481:United States; D014484:United States Environmental Protection Agency
chemical_list: D008372:Manure; D012987:Soil