Microbial Growth Dynamics; A Lettuce Inoculation Study Observing STEC & Lettuce Microflora Behavior Under Stressed And Unstressed Conditions
Author: Joelle Mosso, M.S.
Technical Review: David Legan, Ph.D. and Dan DeMarco, Ph.D.
Analysis: Eurofins Microbiology Laboratories in Fresno, CA and Madison, WI
Introduction
Since 1995, the leafy green industry has struggled with an unfortunate history of outbreaks related to shiga toxigenic E. coli (STEC) and Salmonella (1, 2). The industry has invested in research and continuous improvements to help minimize food safety risks within their complex supply chains and growing regions. While there have been many improvements over the past years, the industry still finds themselves dealing with outbreaks and recalls. As such, pathogen testing on product and in growing/processing environments has become a critical tool for the industry; helping identify risks within leafy green production to prioritize focus on areas of highest risk (3).
Pathogen testing for leafy green suppliers require microbiological methods that offer a fast turnaround time for results due to their product’s short shelf-life and perishability. A unique characteristic of agricultural product testing is that the target microorganisms are exposed to constantly changing environmental conditions and ecosystems. Due to the wide variety of environmental conditions (e.g. cold temperatures, hot temperatures, water stress, UV exposure, spoilage pressure), there has been interest to explore the physiological state of bacteria (including pathogens) on these agricultural items, and better understand how their environmental metabolic state may impact rapid pathogen detection methods.
The research and development team of Eurofins Microbiology Laboratories, Inc. set out to better understand STEC growth on lettuce by designing an experiment to observe the limits of detection by real-time PCR using bacteria that were cold-stressed against those grown under non-stressed conditions. The food industry often defines clinically relevant STEC serotypes as O157, O145, O121, O111, O103, O45, and O26 (4). E. coli O26 was selected for this analysis to represent a nonO157 STEC. In addition to elucidating the time to detection for STEC, the study utilized Oxford Nanopore Technology for Whole Genome Sequencing (WGS) of enrichment cultures to observe the microbiomes present on the lettuce, and attempt to understand how those diverse populations changed during the course of the STEC enrichment.
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