Location: Environmental Microbial & Food Safety Laboratory
Project Number: 8042-32420-009-009-R
Project Type: Reimbursable Cooperative Agreement
Start Date: Jan 1, 2019
End Date: Jul 31, 2021
Objective:
a. Determine the growth potential and kinetics of L. monocytogenes on major classes of whole and fresh-cut produce as impacted by storage temperature.
b. Determine environmental factors and produce physical/chemical characteristics on the growth potential and kinetics of L. monocytogenes and the association with produce quality and shelf life.
c. Evaluate the effects of the indigenous microbial community from produce on the growth of L. monocytogenes and characterization of bacterial strains that potentially enhance or inhibit growth/persistence of L. monocytogenes.
Approach:
a. Commodity selection. A large selection of whole and fresh-cut produce representing major fresh commodity groups, including but not limited to, leafy greens, melons, fleshy vegetables, root vegetables, and tree fruits will be evaluated for supporting L. monocytogenes growth at typical commercial storage conditions for up to twice the typical shelf life. The selection of commodities will be made in consideration of historic outbreak data, commodity market value, and the likelihood of products being consumed raw, with strong input from produce processors and distributors. Commodities that are sensitive to chilling injuries will be given special considerations. Samples will be taken at predetermined time intervals and survival/growth will be determined using conventional microbiological assays such as direct plating on selective media or MPN-based enumeration for low population levels. For each commodity to be tested, a minimum of three cultivars and/or cultivation origins will be included with four replications.
b. Growth conditions. Evaluation of the effects of temperature and other environmental factors (e.g. controlled or modified atmospheres) on L. monocytogenes growth on different types of fresh produce will be conducted under typical commercial storage conditions. Care will be given to mimic relative humidity and atmosphere to the commercial settings. Multi-strain cocktails of L. monocytogenes will be inoculated at low, medium and high levels to address various contamination scenarios. Match product quality and shelf will be assessed via instrumental and visually using trained sensory panelists.
c. Growth Kinetics. Juice will be extracted from different types of produce and sterilized by filtration or irradiation. This juice, with varying dilutions as appropriate, will be used to determine the growth potential and kinetics of L. monocytogenes as affected by the nutritional and physiochemical characteristics. Factors significantly affecting the growth will be identified and characterized using appropriate chemical and instrumental assays. L. monocytogenes growth on products and in corresponding sterile juice will be correlated to gain insights on the effect of nutrient/inhibitor availability and potentially the effect of the indigenous produce microbiome.
d. Produce Microbiome Relative to L. monocytogenes Growth. Microbial communities on selected representative fresh produce that significantly affect the growth of L. monocytogenes will be characterized using 16S rDNA-based metagenomic analyses. Bacterial species that are significantly associated with the growth or inhibition of L. monocytogenes will be isolated and characterized. Their potential for influencing L. monocytogenes growth will be evaluated using dual-species or multi-species co-culture assays. The mechanisms of such interactions will be further characterized.
