Abstract
The aim of this study was to model the growth of nalidixic acid-resistant E. coli O157:H7 (E. coli O157:H7NR) in blanched spinach and to evaluate model performance with an independent set of data for interpolation (8.5, 13, 15 and 27 °C) and for extrapolation (broth and fresh-cut iceberg lettuce) using the ratio method and the acceptable prediction zone method. The lag time (LT), specific growth rate (SGR) and maximum population density (MPD) obtained from each primary model were modeled as a function of temperature (7, 10, 17, 24, 30, and 36 °C) using Davey, square root, and polynomial models, respectively. At 7 °C, the populations of E. coli O157:H7NR increased in tryptic soy broth with nalidixic acid (TSBN), blanched spinach and fresh-cut iceberg lettuce, while the populations of E. coli O157:H7 decreased in TSB after 118 h of LT, indicating the risk of nalidixic acid-resistant strain of E. coli O157:H7 contaminated in ready-to-eat produce at refrigerated temperature. When the LT and SGR models of blanched spinach was extended to iceberg lettuce, all relative errors (percentage of RE = 100%) were inside the acceptable prediction zone and had an acceptable Bf and Af values. Thus, it was concluded that developed secondary models for E. coli O157:H7NR in blanched spinach were suitable for use in making predictions for fresh cut iceberg lettuce, but not for static TSBN in this work.
Highlights
Fresh vegetables, such as radish sprouts, pre-packed spinach, and lettuce have been recognized as a major vehicle associated with outbreaks of illness caused by Escherichia coli O157:H7 [1,2]
The lag time (LT) of a mixture of E. coli O157:H7NR on blanched spinach stored at 7 °C was approximately 26 times longer than that stored at 36 °C
The developed secondary LT, specific growth rate (SGR), and maximum population density (MPD) models were able to reliably predict the growth of a mixture of E. coli O157:H7NR in blanched spinach, which were suitable for use in making predictions for fresh-cut iceberg lettuce, but not for static broth (TSAN)
Summary
Fresh vegetables, such as radish sprouts, pre-packed spinach, and lettuce have been recognized as a major vehicle associated with outbreaks of illness caused by Escherichia coli O157:H7 [1,2]. In a USDA Agricultural Marketing Service survey, 29 spinach samples out of 2,328 fresh produce items available at terminal markets and wholesale distribution centers were identified as positive for pathogenic E. coli by mPCR [1]. A multistate outbreak of E. coli O157:H7 infections linked to Romaine lettuce was reported in the USA [3]. The virulent strain of E. coli O104:H4 in beansprouts has killed 35 and sickened 3,256 in Europe [4]. A total of 33 ill persons infected with an outbreak of the strain E. coli O157:H7 which was linked to pre-packed leafy greens were reported [5]. According to a report on foodborne outbreaks in Canada [6], the food vehicles most commonly implicated in outbreaks were leaf greens and herbs (26%), followed by seed sprouts (11%)
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