Summary of Awards to Date

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


Jan. 1, 2019 - Jun. 30, 2021

Amount Awarded



Elliot Ryser, Ph.D.
Michigan State University


Randolph Beaudry, Ph.D., Sophia Kathariou, Ph.D.


The microbiological safety of whole and sliced apples has been called into question during the  last seven years 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. Consequently, this two‐year proposal seeks to determine the fate of Lm on apples  during long‐term simulated commercial storage. Some of the key questions to be answered  include: 1) Do different foodborne outbreak strains of Lm differ in their ability to survive on  apples, 2) Does Lm survival differ when apples are contaminated from water versus direct contact  with equipment surfaces (crates, brushes), 3) Does the physiological state of Lm, specifically  whether cells in the inoculum harvested from a planktonic (i.e., broth) culture vs. from a biofilm  (i.e., solid surface) impact Lm’s subsequent fate on the apples?, 4) Does storing apples in air  versus a controlled atmosphere (low oxygen and low carbon dioxide) affect Lm survival,  5) Does  the variety of apple (Gala, Granny Smith, Honeycrisp), region in which the apples are grown  (Washington State, Michigan, Pennsylvania), and growing season affect how Lm attaches and  survives on apples, and 6) Does apple waxing affect Lm survival. Answers to these and other questions will assist the apple industry in minimizing the Lm risks associated with current apple growing and packing practices.

Technical Abstract

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.