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This project investigates the factors that influence the ability of E. coli O157:H7 to multiply on lettuce and leafy greens in the presence of normal background flora. Comparative growth kinetics of E. coli O157:H7 on bagged romaine lettuce and spinach in simulated standard and sub-optimal post-harvest temperatures, and the various factors that influence growth rates, will be evaluated. It is hypothesized that sub-optimal temperature management may play a significant role in the growth of E. coli O157:H7 on produce during harvesting, processing, distribution, and pre-consumption handling. How well E. coli O157:H7 is able to grow under these conditions may depend also on conditions of the plant (variety, age, growing conditions, day length, method of harvest, moisture at harvest, and soluble nutrients), the bacteria (pre-inoculation stress and growth conditions) and the post-inoculation environment. Results of this project will support the development of optimal pre-harvest, harvest, and post-harvest preventive programs and improved predictive models for host-pathogen and environment-pathogen interactions
Growth of E. coli O157:H7 inoculated onto lettuce or spinach will be evaluated according to these three objectives: <p>
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<li>For inoculated E. coli O157:H7 on spinach, characterize growth kinetics and evaluate the effects of variety, age of plant, growing conditions, fertility management, growth rate, and day length.
<li>For inoculated E. coli O157:H7 on washed and bagged romaine lettuce, characterize growth kinetics under different pre-inoculation bacterial growth conditions and stresses.
<li>For inoculated E. coli O157:H7 on washed and bagged romaine lettuce and spinach, characterize growth kinetics at different holding temperatures, both static and fluctuating. Romaine lettuce and spinach will be inoculated with different levels of E. coli O157:H7 and held at static temperatures or under conditions where the temperature is shifted for various periods of time from refrigerated to mildly abusive temperatures that permit growth of E. coli O157:H7.</ul>
Three varieties of spinach will be grown to three different stages of maturity, at different nitrogen fertilization rates, and using different amounts of supplemental light.
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E. coli O157:H7 inoculum will be prepared by several approaches: on solid or liquid medium; in minimal medium or tryptic soy broth; at minimal, moderate, and optimum temperatures; in different inoculum carriers (irrigation water, MilliQ water, 0.1% peptone, and dilute compost); to early or late stationary phase; and held for different times between preparation of inoculum and inoculation.
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Results of this study will include the identification of factors under controlled greenhouse conditions that affect leaf structure, leaf breakage, cotyledon dehiscence or senescence, and other aspects of plant morphology relevant to food safety. Data on culture conditions determined to have the greatest impact on the fate of E. coli O157:H7 inoculated onto lettuce will be applicable to methods development and inoculation studies. Data on E. coli O157:H7 survival and growth at constant and fluctuating storage temperatures generated by these experiments will greatly expand the available information relevant to leafy greens.