Listeria Hysteria! Interactions Between Our Food and Pathogenic Bacteria

Stephanie M. Prezioso, Ph.D.

Keywords: Bacteria, Listeria, Metabolism, Genetics, Food, Pathogen

We all know food safety is important, but what exactly is going on at the microscopic level?

The bacteria Listeria monocytogenes contaminates a variety of meat and plant-based food products, withstanding both low temperatures and high salt concentrations, which are two common food preservation methods. Consumption of food products contaminated with Listeria can lead to serious infections in humans [1].

To study the molecular interactions between Listeria and the plants it colonizes, I focused on two areas of research that help microbes cope with the changing world around them:

1) Cellular Metabolism: How bacteria change the shapes and properties of molecules to suit their needs.

2) Gene Regulation: How bacteria turn genes on and off to alter their cellular metabolism in response to the changing environment.

Under supervision by Professor Dinesh Christendat, I worked with colleagues at the University of Toronto to investigate both of these topics in Listeria and the findings were recently published in the Journal of Molecular Biology.

In this study, we examined how Listeria can use a host plant’s molecules in its own metabolism, and how this process is regulated at the genetic level. Our results suggest that this metabolism works differently in Listeria compared to what was previously known for other species.

Plants produce a molecule called quinate. This molecule helps to structurally reinforce their cell walls, and is released in abundance when plant material decays [2]. Some species of bacteria and fungi have evolved the ability to break down quinate into energy through their metabolism [3]. To make use of this abundant plant molecule, a bacterial species must contain the complete set of genes that code the ability to turn quinate into energy [4].

1) Metabolism: We began our research by searching the Listeria genome for the genes that are needed to turn quinate into energy. However, we were only able to identify and confirm the functions for a sub-set of these genes. This suggested that Listeria may not be able to use quinate for energy, unlike what was previously known for other species of bacteria. To test this, we conducted growth experiments with Listeria, which showed that Listeria was indeed unable to use quinate for energy. Instead, quinate break-down seems to halt half-way through the route to produce energy, by producing an intermediate molecule called protocatechuate.

In standard quinate metabolism, protocatechuate would continue along the route to be degraded for energy. However, our data suggested that it may serve a different purpose in Listeria.

As is often the case when answering research questions, our answers begat further questions. More research will need to be done to determine the roles of quinate and protocatechuate in bacterial metabolism and how they affect plant-microbe interactions.

2) Gene Regulation: Simultaneously, we also explored how quinate metabolism is regulated at the genetic level in Listeria. Bacteria employ regulatory proteins that turn select genes on or off in response to changing cellular conditions, allowing them to fine tune their metabolism depending on environmental needs. We hypothesized that Listeria may use this type of regulation to turn on the quinate metabolism genes when it encounters quinate while growing on fruits and vegetables.

The results from our experiments support this hypothesis. We identified a regulatory protein that we named QuiR that turns the quinate metabolism genes off and on. When quinate starts to break down it produces an intermediate molecule called shikimate. QuiR recognizes shikimate and responds by turning on the quinate metabolism genes so that more quinate can be used/metabolized/broken down. In this way, QuiR turns on the quinate metabolism genes only when they are needed to use the available quinate.

Ultimately, our research uncovered a new way that Listeria regulates its metabolism by turning genes off and on when it encounters a plant compound. Although we do not yet know what quinate is being used for in Listeria, Dinesh Christendat’s lab is conducting further experiments on this system that may have applications for controlling Listeria growth on fruits and vegetables.

In addition to publishing this research in a scientific journal, I also amalgamated a talented team of dancers, cinematographers, a biomedical communicator, and a musician to create an interpretive dance to explain this research. Our dance made the finals of the Dance Your PhD competition that was put on by Science Magazine in 2016 and is available for you to view here:

https://www.youtube.com/watch?v=GkWe1OtDlb8&t=123s

References:

1.    CDC. Multistate Outbreak of Listeriosis Linked to Frozen Vegetables (Final Update)2016.

2.    Xi W, Zheng H, Zhang Q, Li W. Profiling Taste and Aroma Compound Metabolism during Apricot Fruit Development and Ripening. Int J Mol Sci. 2016;17(7). Epub 2016/06/24. doi: 10.3390/ijms17070998. PubMed PMID: 27347931; PubMed Central PMCID: PMCPMC4964374.

3.    Tresguerres ME, De Torrontegui G, Cánovas JL. The metabolism of quinate by Acinetobacter calco-aceticus. Arch Mikrobiol. 1970;70(2):110-8. PubMed PMID: 5429630.

4.    Dal S, Trautwein G, Gerischer U. Transcriptional organization of genes for protocatechuate and quinate degradation from Acinetobacter sp. strain ADP1. Appl Environ Microbiol. 2005;71(2):1025-34. doi: 10.1128/AEM.71.2.1025-1034.2005. PubMed PMID: 15691962; PubMed Central PMCID: PMCPMC546756.