Efficacy of allyl isothiocyanate in killing enterohemorrhagic Escherichia coli O157:H7 on alfalfa seeds

Abstract

Volatile compounds occurring in the essential oil of plants were tested for their efficacy in killing Escherichia coli O157:H7. Experiments using an agar disk assay revealed that exposure of the pathogen to 50 μl of eugenol, carvacrol, linalool, or methyl jasmonate in a 950-cc jar at 20, 37 or 47°C for up to 48 h failed to inhibit colony formation. However, exposure to 8 μl of allyl isothiocyanate (AIT) (equivalent to 8.4 ppm in the air within the jar, if completely volatilized) resulted in more than a 7-log 10 reduction in population of E. coli O157:H7 at 37°C within 48 h; significant ( P≤0.05) reduction in populations also occurred in the presence of 4 μl of AIT compared to 2 μl, which had no lethal affect. At 20°C, the lethality of AIT was substantially less, although significant reduction occurred when disks were exposed to 8 or 10 μl of AIT compared to 4 or 6 μl and when exposed to 4 or 6 μl compared to 2 μl. Treatment with 10 μl of AIT for 5 h at 47°C resulted in death of 6 log 10 of E. coli O157:H7. The efficacy of AIT in killing E. coli O157:H7 on dry and wet alfalfa seeds was investigated. The pathogen, at an initial population of 2.7 log 10 cfu/g of seed, was not recovered by direct plating (<0.7 log 10 cfu/g) or enrichment of wet seeds exposed to 50 μl of AIT/950-cc jar for 24 h at 37 or 47°C. Exposure of dry seeds containing 2.9 log 10 cfu of E. coli O157:H7 per g to an atmosphere containing 100 μl of AIT/950-cc jar (ca. 105 ppm AIT if completely volatilized) for 24 h at 47°C did not eliminate viable E. coli O157:H7 cells. Unfortunately, the enhanced effectiveness of AIT in killing the pathogen on wet alfalfa seeds is offset by a dramatic reduction in seed viability. Nevertheless, the use of AIT as an alternative to chlorine for the purpose of killing E. coli O157:H7 and perhaps other pathogens on alfalfa seed holds promise. Factors that may influence conditions rendering increased sensitivity of E. coli O157:H7 to AIT without compromising seed viability should be investigated.