Listeria monocytogenes Preservative Resistance

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Introduction

Growth of Listeria on PALCAM selective and differential agar. Black circles around colonies indicate esculin hydrolysis, a characteristic of Listeria and related species. Photo credit: Generon.

By Iris Pardue
Listeria monocytogenes is a facultatively anaerobic Gram-positive food pathogen of the phylum Bacillota capable of surviving and growing at low temperatures. As a result, it is responsible for infections in deli meats, cheeses, and other refrigerated products. Outbreaks are relatively frequent, and serious cases have an especially high fatality rate of 15.26% over recorded cases in the US in the last decade. Unsurprisingly, much research has been conducted into control of Listeria as a result. Many different methods are used by food supply companies to inhibit bacterial growth, and chief among these methods are chemical preservatives. There are several different types of preservatives commonly used, and many have different mechanisms of action that will be discussed later. Having a diverse portfolio of antimicrobial preservatives available is especially important given the ability of pathogens to quickly evolve resistance to common mechanisms. This article explores the variety of commonly used preservatives relevant to controlling listeriosis along with the susceptibilities and resistances possessed by Listeria to each.

Cold Resistance

The anteiso-17-0 and anteiso-15-0 fatty acids, respectively at top and bottom. Photo credit: Pubchem and Pubchem

To fully understand the resistance of Listeria to preservatives, it is important to first consider its most importance resistance to cold. Listeria is a psychrophile, capable of growing at freezing temperatures as well as at human body temperatures. Numerous mechanisms exist to aid resistance to cold, but chief among them is adaptations in the content of the phospholipid membrane. This adaptation is mainly achieved via the use of the two primary fatty acids in the lipid membrane, anteiso-17-0 and anteiso-15-0. The proportion between these two fatty acids is modulated in response to low temperatures until the more flexible anteiso-15-0 dominates, reaching 80% of the total fatty acid profile. This adaptation allows it to maintain the crucial "liquid-crystal" state, which is necessary for cell function by lowering the melting point of the membrane. However, the focus on lipids alone can paint an oversimplified picture of the cell membrane's temperature stability. In many species, cholesterol acts as a cellular antifreeze to aid membrane stability, and Listeria employs a similar mechanism using isoprenoid quinones. Menaquinone-7 (MK-7), also known as vitamin K2, is composed of a napthaquinone ring fixed to a long chain of 7 isoprene units, which allows it to integrate into the membrane, a structure shared with the quinones used in electron transport. Flegler et al. found that strains containing higher levels of these molecules in their membranes better survived temperature stresses and did not adapt their fatty acid composition to the same degree as other strains. However, product inhibition of the shikimate pathway responsible for synthesis of aromatic amino acids that are processed into quinones did reduce the ability of high MK-7 strains to adapt to the lower temperatures, providing a possible target for future preservatives.

Acid Preservatives

Include some current research, with at least one figure showing data.

References

Authored for BIOL 238 Microbiology, taught by Joan Slonczewski,at Kenyon College,2024