Suspending Lettuce Type Influences Recoverability and Radiation Sensitivity of Escherichia coli O157:H7

Abstract

An outbreak strain of Escherichia coli O157:H7 was inoculated onto closely related but structurally distinct types of lettuce (Lactuca sativa): Boston (butterhead lettuce), iceberg (crisphead lettuce), and green leaf and red leaf (colored variants of looseleaf lettuce). The E. coli O157:H7 was inoculated either onto the surface of cut leaf pieces or into a homogenized leaf suspension. Samples were gamma irradiated, and the radiation sensitivity of the inoculated bacteria was expressed as a D-value (the amount of ionizing radiation necessary to reduce the bacterial population by 90% [kGy]). The recovery of bacteria from nonirradiated leaf pieces was also measured. When inoculated onto the leaf surface, E. coli O157:H7 had significantly stronger radiation sensitivity on red leaf lettuce (D = 0.119±0.004 [standard error]) and green leaf lettuce (D = 0.123 ± 0.003) than on iceberg lettuce (D = 0.136 ± 0.004) or Boston lettuce (D = 0.140 ± 0.003). When E. coli O157:H7 was inoculated into a homogenized leaf suspension, its sensitivity was significantly stronger on iceberg lettuce (D = 0.092 ± 0.002) than on green leaf lettuce (D = 0.326 ± 0.012), Boston lettuce (D = 0.331 ± 0.009), or red leaf lettuce (D = 0.339 ± 0.010), with a threefold difference. Significantly fewer bacteria were recovered from the surface of iceberg lettuce than from the surfaces of the other types of lettuce examined. Following radiation doses of up to 0.5 kGy, the texture (maximum shear strength) of lettuce leaves was measured along the midrib and along the leaf edge for each type of lettuce. There was no meaningful change in texture for any type of lettuce for either leaf section examined at any dose up to 0.5 kGy. These data show (i) that relatively subtle differences between lettuce types can significantly influence the radiation sensitivity of associated pathogenic bacteria and (ii) that doses of up to 0.5 kGy do not soften lettuce leaves.