Purpose: 1) To compare the ability of two IMS methods – automated recirculating (AR) and direct plating (DP) - to recover E. coli O157:H7 from compost samples containing biosolids, manure, or yardwastes; 2) to determine the ability of compost samples to support regrowth of E. coli O157:H7 in compost.
Methods: Twenty-nine USCC-STA certified compost samples were collected from across the U.S. Three aliquots (400 g each) were inoculated with stx-2 positive E. coli O157:H7 at 101-102 CFU/g. AR homogenized samples were incubated 5 h at 37°C. DP samples were serially diluted and incubated 2 h at room temperature, then 42°C for 6 h. All samples were incubated overnight at 4°C before addition of immunomagnetic beads. AR sample recirculation (30 min) and DNA purification from immunomagnetic beads, was followed by real-time PCR targeting the stx2 gene. For DP, immunomagnetic beads were added to diluted samples, mixed, removed, and then plated onto chromagarO157.
Results: The AR and DP methods both recovered E. coli O157:H7 from 30/ 30 (100%) samples. PCR detection of E. coli O157:H7 was greatly enhanced by the removal of polyphenols and humic acids from DNA samples. Regrowth of E. coli O157:H7 in compost varied based on feedstock.
Significance: IMS recovery, coupled with either direct plating or real-time PCR detection, is an efficient method to recover E. coli O157:H7 from finished compost within 24 – 48 h. Regrowth potential of E. coli O157:H7 in finished composts is affected by physicochemical parameters of compost.