Fate of different Listeria monocytogenes strains on different whole apple varieties during long-term simulated commercial storage

<p>The microbiological safety of whole and sliced apples has been questioned recently due to multiple recalls for Listeria monocytogenes (Lm) and a high-profile outbreak of listeriosis from caramel apples. Most recently, whole Gala and Honeycrisp apples were recalled in December 2017 due to Lm contamination suggesting extended survival of this pathogen. This two-year proposal, which specifically targets CPS Research Priority 1.1.5 - Expanding knowledge on Lm growth potential and kinetics on tree fruit surfaces (apples) during common storage practices, aims to 1) assess survival of eight Lm strains grown planktonically or as a biofilm on three unwaxed apple varieties (Gala, Granny Smith, Honeycrisp) from two different harvest seasons and three different growing regions (Washington, Michigan, Pennsylvania) during air (21% O2) or controlled atmosphere (1.5% O2, 0 - 3% CO2) (CA) storage, and 2) assess Lm survival on apples after waxing. In Objective 1, apples from all three regions will be dip-inoculated at Michigan State University (MSU) with a cocktail of eight Lm outbreak strains grown either planktonically or as a biofilm to simulate pre-harvest, harvest, or early post-harvest contamination. The eight strains will be provided by North Carolina State University (NCSU) and will be barcoded with short (30 bp) DNA sequences and fluorescent protein genes in the chromosome to allow differentiation by metagenomic analysis and quantitative real-time PCR at NCSU as well as visualization on the apple surface at MSU using confocal scanning laser microscopy (CSLM). After inoculation and 1MCP treatment, apples will be assessed for populations of Lm (healthy and injured), mesophilic aerobic bacteria and yeast/mold during 3 months of air or 7 months of CA storage at 0 - 3°C depending on the apple variety. In Objective 2, we will investigate the impact of waxing on Lm survival. We will determine Lm survival on apples from Objective 1 that were (1) dip-inoculated, air-dried, stored for 3 months in air or 7 months under CA and then waxed, to simulate prolonged contamination, (2) stored, and then dip-inoculated, air-dried and immediately waxed to simulate contamination during flume washing, and (3), inoculated with Lm during waxing to simulate contamination from brushes and conveyors. Positive impacts of the anticipated outcomes include (i) identification of specific knowledge gaps pertaining to environmental conditions and processes (e.g. waxing) impacting Lm's capacity to adhere and persist on apples, (ii) elucidation of strain-specific differences in Lm adherence and subsequent fate on apples, (iii) clarification of the impact of apple variety, production region and growing season on Lm contamination of apples, and (iv) clarification of the role that more resistant, surface-grown Lm cells, as could occur in the field and especially in the packinghouse, may play on contamination of apples during processing. These findings will inform the industry of the presently unknown risks associated with different components of apple production and packing and aide in the design and validation of Lm-targeting interventions to better ensure apple safety.</p>