Understanding Listeria growth methods on salmon to help controls

By Joseph James Whitworth contact

- Last updated on GMT

RTE products typically have very low levels of L. monocytogenes but it must be able to grow during refrigerated storage to reach an infectious dose
RTE products typically have very low levels of L. monocytogenes but it must be able to grow during refrigerated storage to reach an infectious dose

Related tags: L. monocytogenes, Bacteria, Listeria monocytogenes

Listeria monocytogenes uses different ‘metabolic pathways’ when growing on refrigerated smoked compared to laboratory media, according to research.

To develop control strategies that prevent or reduce growth in RTE seafood products, there is a need to better understand mechanisms that L. monocytogenes uses to survive and grow under complex conditions of specific food matrices and environmental conditions.

Characterization of bacterial gene expression patterns in different environments can be used to assess the physiological state of L. monocytogenes under different conditions, and help identify metabolic pathways important for survival and growth in food products.

This will facilitate identification of new compounds that could interfere with these metabolic pathways and control growth of the pathogen, said the researchers. .

Listeria growth on cold smoked salmon

They showed that L. monocytogenes grows on cold smoked salmon (CSS) by using different ways to get energy, even when media was modified to have the same salt content and pH as salmon.

To grow on salmon, the bacterium upregulates genes that enable it to use two compounds from cell membranes--ethanolamine and propanediol--as energy (or nutrient) sources.

“Our data indicate that H7858 grown on vacuum-packaged CSS (i) upregulates cobalamin biosynthesis pathways as well as ethanolamine and 1,2-propanediol utilization pathways, (ii) differentially regulates carbohydrate transport functions, and (iii) upregulates arginine deiminase genes, likely facilitating adaptation to anaerobic conditions, utilization of nutrients available on CSS and growth in the presence of resident microbiota.

“We propose that one of the reasons L. monocytogenes uptakes or synthesizes cobalamin more actively on CSS is to facilitate growth under this condition by using cobalamin to support the utilization of ethanolamine and 1,2-propanediol.”

RNA-seq was used to understand the transcriptional landscape of L. monocytogenes strain H7858 grown on cold-smoked salmon relative to in modified brain heart infusion broth at 7°C.

Three foodborne bacterial pathogens have been shown to use ethanolamine and 1,2-propanediol as a carbon source and ethanolamine as a nitrogen source: L. monocytogenes, S. enterica, and Clostridium perfringens.

Using info to control pathogen

Teresa Bergholz, assistant professor in the Department of Veterinary and Microbiological Sciences at North Dakota State University, Fargo, said there may be ways to use the information to control the pathogen in foods and infected people.

“Understanding how a foodborne pathogen adapts to environmental stresses it encounters on a specific food could allow food microbiologists to develop inhibitors of metabolic or stress response pathways that are necessary for the pathogen to grow or survive on that product.

“The information may also enable improved risk assessments, as virulence of a pathogen may be affected considerably by the stress responses and/or metabolic pathways used to survive on the food.”

The heat treatment applied during processing of CSS is not sufficient to inactivate microbes on the raw material, and RTE food products can be contaminated with L. monocytogenes from environmental sources in processing facilities.

This pathogen is of particular concern to the ready-to-eat (RTE)-meat and -seafood industries due to its ability to grow at temperatures as low as -0.4°C and salt contents as high as 25% (at 4°C)

The inoculation level was high in the study (~ 1 x 106 CFU/g) and contamination of cold smoked fish is typically at low levels, so there is the potential for differences in responses of L. monocytogenes when inoculated at different levels.

However, the pathogen can grow in food products at refrigeration temperatures to high levels, and the infection dose in an immune-competent individual can be high.

Researchers said there is increasing evidence food related factors not easily simulated in laboratory medium play important roles in growth and survival of pathogens in different foods.

“Most importantly, detailed data on pathogen adaptation to different complex conditions of food matrices may hold the key to development of more efficient control strategies and will move development of control strategies from traditional trial and error approaches to rational design type approaches for the development of new growth inhibitors.”

Source: Applied and Environmental Microbiology

“Transcriptomic analysis of Listeria monocytogenes Adaptation 1 to Growth on Vacuum-Packed Cold Smoked Salmon”

Online ahead of print, DOI: 10.1128/AEM.01752-15

Authors: Silin Tang, Renato H. Orsi, Henk C. den Bakker, Martin Wiedmann, Kathryn J. Boor, Teresa M. Bergholz

Related topics: Food Safety & Quality

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