Comparison of U.S. Environmental Protection Agency and U.S. Composting Council Microbial Detection Methods in Finished Compost and Regrowth Potential ofSalmonellaspp. andEscherichia coliO157: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.

Importance of Soil Amendments: Survival of Bacterial Pathogens in Manure and Compost Used as Organic Fertilizers

“…Similarly, Salmonella spp. populations increased by up to 1.5 log CFU/g and decreased by up to 1.5 log CFU/g in individual finished compost samples (58). No single physicochemical factor was able to predict if a compost sample could support the regrowth of pathogens; however, moisture content, C:N ratio, and total organic carbon levels were the three physicochemical factors which contributed the most to the regrowth potential of pathogens in finished compost.…”

“…and stored at 25°C for 3 days to determine if the feedstock (biosolids, animal manure, yard waste) or if a single physicochemical factor (C:N ratio, moisture content, pH, total organic carbon, percent volatile solids, electrical conductivity, and maturity) could predict the regrowth of E. coli O157:H7 or Salmonella spp. (58). Populations of E. coli O157:H7 increased or decreased by up to 2 log CFU/g in individual finished compost samples.…”

Composts of poultry litter or dairy manure differentially affect survival of enteric bacteria in fields with spinach

“…These strains were isolated from agricultural environments and have been used previously in greenhouse and field studies involving soil amendments (Reynnells et al . ; Sharma et al . ).…”

“…A three-strain inoculum of rifampicin-resistant nonpathogenic E. coli (TVS 353,354 and 355) were used to distinguish them from indigenous microbes in composts or soil (hereafter referenced as 'indigenous'). These strains were isolated from agricultural environments and have been used previously in greenhouse and field studies involving soil amendments (Reynnells et al 2014;Sharma et al 2019). Each E. coli strain was grown in tryptic soy broth with 80 lg ml À1 rifampicin (TSBR) and adjusted to a final population of 1Á67 9 10 5 CFU per ml.…”

Potential Application of Fluorescence Imaging for Assessing Fecal Contamination of Soil and Compost Maturity

“…As another source of fecal contamination, manure compost is organic material that has been degraded into a nutrient-stabilized, humus-like substance through microbial activity that generates sufficiently high thermophilic temperatures (≥55 • C) to kill enteric bacterial pathogens originally present in the feedstocks [13]. However, even if finished compost is properly maintained, it can become recontaminated from environmental sources, such as wild terrestrial animals and birds, so pathogens such as Salmonella spp.…”