Nonculturable Escherichia coli O157 in horticultural compost: a public health concern

Fresh produce-associated outbreaks of the foodborne pathogen Escherichia coli O157 are responsible for a number of disease cases, hospitalizations and deaths. In many cases, the source of contamination can be linked to the growing media of the food, although pathogen detection is problematic in these complex soil ecosystems. In this study, direct quantitative real-time PCR without pre-enrichment was used to detect 310 copies of the Tir gene, using a primer sequence specific to E. coli O157, in horticultural compost purchased from a commercial supplier. The pathogen could not be cultured on selective media but was visualized using peptide nucleic acid fluorescence in situ hybridization and cell elongation viability assay, confirming the viability. Enumeration of elongated E. coli O157 determined that there were 205 live cells per gram of compost. The nonculturability and confirmation of viability of the pathogen indicates its viable but nonculturable (VBNC) status. The detection of VBNC foodborne pathogens in environmental samples challenges current understanding of the nature of foodborne pathogen contamination.