The Role of Ovotransferrin in Egg-White Antimicrobial Activity: A Review

Abstract

Eggs are a whole food which affordably support human nutritional requirements worldwide. Eggs strongly resist bacterial infection due to an arsenal of defensive systems, many of which reside in the egg white. However, despite improved control of egg production and distribution, eggs remain a vehicle for foodborne transmission of Salmonella enterica serovar Enteritidis, which continues to represent a major public health challenge. It is generally accepted that iron deficiency, mediated by the iron-chelating properties of the egg-white protein ovotransferrin, has a key role in inhibiting infection of eggs by Salmonella. Ovotransferrin has an additional antibacterial activity beyond iron-chelation, which appears to depend on direct interaction with the bacterial cell surface, resulting in membrane perturbation. Current understanding of the antibacterial role of ovotransferrin is limited by a failure to fully consider its activity within the natural context of the egg white, where a series relevant environmental factors (such as alkalinity, high viscosity, ionic composition, and egg white protein interactions) may exert significant influence on ovotransferrin activity. This review provides an overview of what is known and what remains to be determined regarding the antimicrobial activity of ovotransferrin in egg white, and thus enhances understanding of egg safety through improved insight of this key antimicrobial component of eggs.

Keywords: Salmonella Enteritidis; antimicrobial properties; egg white; iron chelation; membrane disturbing; ovotransferrin.

Figure 2

The different pathways of iron acquisition and storage in S. Enteritidis. (1) Iron deficiency induces derepression by the Fur regulator, leading to transcriptional upregulation of the genes under its control, in particular the genes encoding enzymes required for the production of siderophores. Salmonella is able to synthetize and/or use several siderophores: Enterobactin, salmochelin, ferrichrome, and ferrioxamine. (2) In the first step, enterobactin is synthetised using serine and chorismate as precursors. This step is catalysed by EntABCDF enzymes [62]. Then, enterobactin is transported into the environment by EntS and TolC or used as a precursor in the production of salmochelin S4 via the glycosyltransferase IroB [73]. (4) Once in the environment, the siderophores chelate ferric iron. (5) The siderophore-iron complexes (ferri-enteribactin and ferri-salmochelin, and the exogenous siderophores ferrichrome and ferrioxamine) are recognized by specific receptors present in the bacterial outer membrane (CirA and FepA for enterobactin, IroN for salmochelin, FhuA for ferrichromes, and FoxA for ferrioxamine) [74]. Then, the TonB-ExbBD, an energy-transducing complex, drives siderophore internalization into the periplasm. Once in the cytoplasm, salmochelin is linearised into the S2 form by IroE, an esterase [68]. The passage through the inner membrane is achieved by the FepBDGC transporter for both salmochelin and enterobactin [75,76]. On the other hand, ferrichromes and ferrioxamines use the FhuBCD transporter to pass through the inner membrane [77]. (6) Inside the cytoplasm, the iron-siderophore complexes are dissociated by esterases: Fes converts tricyclic enterobactin into monomeric units; IroD act on the linear, trimeric S2 form of salmochelin [68] to generate mono (S1) or dimeric (SX) products from which iron can be released more readily. Finally, iron can be utilized for metabolism or (7) stored by ferritin (FtnA) or bacterioferritin (Bfr) [78]. To acquire Fe²⁺, S. Enteritidis primarily uses the FeoABC system [71,77], which allows iron to be imported across the inner membrane from the periplasm; the OmpC and OmpF porins allow passive diffusion of ferrous iron across the OM [79].

Figure 1

Representation of the tertiary structure of ovotransferrin. The subdomains of each lobe are indicated by N1, N2, C1, and C2. On the left, an enlargement of the binding site of the N lobe shows the binding of iron by its six ligands. The images were produced using the UCSF chimera package [45] from the tertiary structure of the ovotransferrin (PDB ID: 1AIV) [42].

Figure 3

Prediction of electrostatic charge distribution at the surface of ovotransferrin. Each representation (A,B) corresponds to a rotation of the molecule by 180° around the vertical axis. Charges were computed at each pH using the PDB2PQR tool [110] and the electrostatic potential of the protein surface was estimated with the APBS tool [111], using a specific server (https://server.poissonboltzmann.org/ (accessed on 15 January 2021)). The molecule was visualised using the VMD software [112]. The surface of each molecule is coloured according to the electrostatic potential, from −5 kT (red) to + 5 kT (blue), via 0 kT (white), where k is the Boltzmann constant and T the absolute temperature.