An anti-listeria agent evidenced in egg white

In order to permit development of chicken embryos, eggs must protect themselves against attacks from the micro-organisms present in their environment.  A research team in the Poultry Research Unit has evidenced a major constituent of this defence system in egg white. This substance, and the process for its purification, have now been patented.

Jaune et blanc d'un oeuf cassé. © MAITRE Christophe
Updated on 05/02/2013
Published on 12/19/2012

The antibacterial defences of eggs: a dual challenge

Eggs constitute an essential part of our diet and represent an appreciable source of animal proteins.  But their original function is to act as a reservoir of nutrients and bioactive substances that will enable the development of an embryo.  This development cannot be achieved without an efficient defence system "designed" to protect both the embryo and egg reserves from microbial infections.  Knowledge of this defence system is crucial in order to firstly, maintain a high-quality production in a context where growing demand for eggs obtained under free range systems increases the risk of exposure to pathogenic agents (selection aspect), and secondly identify antibacterial compounds which may have non-food applications (substances used in animal health, as preservatives, etc.).

The heparin-binding protein concept

The INRA scientists wanted to screen the antimicrobial molecules present in egg white .  In order to identify new antimicrobial candidates, the scientists based their work on studies suggesting that proteins with an affinity for heparin (heparin-binding proteins), a negatively-charged polysaccharide, could display antibacterial properties.  They thus separated out the different proteins in egg white using heparin-sepharose affinity chromatography, a system that only retains proteins that can bind to heparin.  This method enabled the elimination of ovalbumin (the major ingredient in egg white, whose weak antibacterial effect is known and which does not bind to heparin) to obtain a sample of egg white enriched in proteins displaying affinity for heparin.  The latter were then isolated and separated from each other by electrophoresis on an acrylamide gel to enable further identification using mass spectrometry.  The researchers focused on a major constituent in this fraction (accounting for nearly 50% of proteins) which was identified as being the OVAX protein (ovalbumin-related protein X).  The function of this substance, whose presence in egg white had previously been demonstrated using other methods, was not hitherto known.

OVAX: an effective patented compound

The scientists then focused on determining whether this protein displayed any antibacterial properties.  To achieve this, they measured the effects of OVAX on the growth of pathogenic Listeria monocytogenes strain EGD and Salmonella enterica serovar Enteritidis (ATCC 13076) by performing radial diffusion tests.  In Petri dishes in which the agar had been seeded with one of the bacteria, increasing quantities of OVAX (from 0.5 mg/ml to 2.2 mg/ml) were placed in the wells.  The larger the quantity of OVAX deposited, the broader were the zones of growth inhibition around the wells.  The researchers were able to determine minimal inhibitory concentrations (MIC) of around 2.3 µM versus L. monocytogenes and 10 µM versus S. enterica Enteritidis.  These antibacterial effects did not involve the same reactive sites in the protein since the presence of heparin in the reaction medium (to block the accessibility of the heparin-binding site to bacteria) only affected the anti-salmonella effect.
The anti-Listeria properties  of OVAX, and its purification process, are now the subject of a patent (WO2011151407).  Additional studies will soon be carried out to determine the breadth of the antibacterial spectrum of OVAX and to better understand its mechanism of action.  This antibacterial agent has real potential for non-food applications, similar to lyzozyme, another antimicrobial substance isolated from egg white.

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Find out more

  • Rehault-Godbert, S., Nys, Y., Gautron, J., Labas, V., Helloin, E., and Slugocki, C. (December 8, 2011) Patent WO 2011/151407 A1. Fraction of proteins and peptides derived from egg white and protein derived from egg white and use thereof as anti-listeria agent.