Fate of inoculated Escherichia coli in hay

To monitor the fate of inoculated Escherichia coli in dry and moist hay of various types, under laboratory conditions. Wheat, vetch and clover hay were used as received or wetted to 250-300 g kg⁻¹ moisture. The hay was inoculated at about 10⁶ CFU g⁻¹ with a kanamycin-resistant E. coli strain that expresses the green fluorescence protein, and was stored in small open glass jars that were covered with aluminium foil. Three jars per treatment were sampled on days 1 and 3, or 4 and 7, or 8, 20 and 50, respectively, after the initiation of the experiments, and the numbers of E. coli in the hay were determined. The results indicated that E. coli disappeared from both dry and moist hay by 7-8 days after inoculation. However, in a few cases colonies that were presumed to be E. coli developed after incubation in Luria broth medium. The tagged E. coli strain usually disappeared rapidly from both the dry and the moist hay, in spite of the high level of inocula used. However, in some cases a few, possibly injured E. coli might have persisted, and could be detected after incubation in a rich growth medium. This study is part of a risk assessment associated with sewage irrigation of forage crops in Israel. The results indicate that E. coli added to the hay is not likely to pose a health risk to cattle or to humans. Nevertheless, more research with natural strains of E. coli and other enteric pathogens that might be more adapted to forage conditions is warranted in order to ensure the safety of sewage-irrigated crops.