Absence of Internalization of Escherichia coli O157:H7 into Germinating Tissue of Field-Grown Leafy Greens

Both growth chamber and field studies were conducted to investigate the potential for Escherichia coli O157:H7 to be internalized into leafy green tissue when seeds were germinated in contaminated soil. Internalized E. coli O157:H7 was detected by enrichment in both spinach (Spinacia oleracea L.) and lettuce (Lactuca sativa L.) seedlings when seeds were germinated within the growth chamber in autoclaved and nonautoclaved soil, respectively, contaminated with E. coli O157:H7 at 2.0 and 3.8 log CFU/g, respectively. Internalized E. coli O157:H7 populations could be detected by enumeration within leafy green tissues either by increasing the pathogen levels in the soil or by autoclaving the soil. Attempts to maximize the exposure of seed to E. coli O157:H7 by increasing the mobility of the microbe either through soil with a higher moisture content or through directly soaking the seeds in an E. coli O157:H7 inoculum did not increase the degree of internalization. Based on responses obtained in growth chamber studies, internalization of E. coli O157:H7 surrogates (natural isolates of Shiga toxin-negative E. coli O157:H7 or recombinant [stx- and eae-negative] outbreak strains of E. coli O157:H7) occurred to a slightly lesser degree than did internalization of the virulent outbreak strains of E. coli O157:H7. The apparent lack of internalized E. coli O157:H7 when spinach and lettuce were germinated from seed in contaminated soil (ca. 3 to 5 log CFU/g) in the field and the limited occurrence of surface contamination on the seedlings suggest that competition from indigenous soil bacteria and environmental stresses were greater in the field than in the growth chamber. On the rare occasion that soil contamination with E. coli O157:H7 exceeded 5 log CFU/g in a commercial field, this pathogen probably would not be internalized into germinating leafy greens and/or would not still be present at the time of harvest.