ARS | Publication request: Prevalence of Shiga-Toxigenic E. coli and Salmonella in Commercially Available Compost

Submitted to: International Association for Food Protection Proceedings
Publication Type: Abstract Only
Publication Acceptance Date: March 10, 2008
Publication Date: August 3, 2008
Citation: Ingram, D.T., Millner, P.D., Patel, J.R. 2008. Prevalence of Shiga-Toxigenic E. coli and Salmonella in Commercially Available Compost. International Association for Food Protection Proceedings. Paper No. P4-41, 141.

Technical Abstract: Proper thermophilic management substantially reduces pathogen content of composts. Commercial compost quality varies by management/feedstock and is regulated by states unless it contains sewage sludge which must meet federal pathogen reduction standards (40 CFR Part 503). Most states do not stipulate a pathogen content standard. This study was conducted to characterize the microbial quality of finished, marketable compost prepared from a wide range of residuals from 15 US commercial facilities. Samples (n=108) of mature compost were collected in March, August, and November 2000 and enumerated by MPN and spread plating techniques for fecal coliforms, E. coli and Salmonella spp. Whole cell FAME profiles of 261 E. coli isolates were used to select 183 isolates for virulence factor screening. Multiplex PCR assays targeting Stx1, Stx2 and eae genes were conducted on the selected isolates. Results show 53 percent and 6.7 percent of compost facilities had product that exceeded the USEPA 503 limit for Class A product: fecal coliforms greater than 1000 MPN per g and Salmonella greater than 3 MPN per 4 g, respectively. One E. coli isolate (0.55 percent) positive for Stx2 was recovered from sewage sludge compost that met the EPA503 standards. In total, 67 percent of samples were positive for E. coli and 78.7 percent of samples met the fecal coliform standard. Prevalence of toxigenic E. coli in commercial composts was very low despite the relatively high number of samples that contained E. coli. All facilities use outdoor compost systems in which most, but not all, particles are subjected to pathogen destructive thermal process time-temperatures. Data in this study show that commercial composts that meet the fecal coliform and Salmonella standards may still contain low levels of pathogenic E. coli. Pathogen content data are needed to help ensure selection/use of quality composts by fresh produce growers.