Role of yoaE Gene Regulated by CpxR in the Survival of Salmonella enterica Serovar Enteritidis in Antibacterial Egg White

The CorC proteins MgpA (YoaE) and CorC protect from excess-cation stress and are required for egg white tolerance and virulence in Salmonella

Cation homeostasis is a vital function. In Salmonella, growth in very low Mg2+ induces expression of high-affinity Mg2+ transporters and synthesis of polyamines, organic cations that substitute for Mg2+. Once Mg2+ levels are re-established, the polyamines must be excreted by PaeA. Otherwise, cells lose viability due to a condition we term excess-cation stress. We sought additional tolerance mechanisms for this stress. We show that CorC and MgpA (YoaE) are essential for survival in stationary phase after Mg2+ starvation. Deletion of corC causes a loss of viability additive with the paeA phenotype. Deletion of mgpA causes a synthetic defect in the corC background. This lethality is suppressed by loss of the inducible Mg2+ transporters, suggesting that the corC mgpA mutant is sensitive to changes in intracellular Mg2+. CorC and MgpA function independently of PaeA. A paeA mutant is sensitive to externally added polyamine in stationary phase; loss of CorC and MgpA suppressed this sensitivity. Conversely, the corC mgpA mutant, but not the paeA mutant, exhibited sensitivity to high Mg2+ and egg white. The corC mgpA mutant is also attenuated in a mouse model. The corC and mgpA genes are induced in response to increased Mg2+ concentrations. Thus, CorC and MgpA play some interrelated role in cation homeostasis. It is unlikely that these phenotypes are due to absolute levels of cations. Rather, the cell maintains relative concentrations of various cations that likely compete for binding to anionic components. Imbalance of these cations affects some essential function(s), leading to a loss of viability.

Impact of ovotransferrin on the membrane integrity of Salmonella Enteritidis under egg-white conditions

Introduction

Eggs can mediate foodborne disease resulting in salmonellosis outbreaks that are most commonly caused by Salmonella enterica serovar Enteritidis. Ovotransferrin is a prominent egg-white antimicrobial glycoprotein belonging to the transferrin family, members of which exhibit powerful iron-chelating activity. However, several studies have also described the ability of transferrin proteins to disrupt bacterial membranes. This study aimed to investigate the antimicrobial activity of ovotransferrin toward S. Enteritidis membranes at 30°C under egg-white conditions.

Materials and methods

This aim was supported by the deployment of a synthetic medium designed to mimic egg-white (matching the ionic composition and pH). The ability of ovotransferrin to induce bacterial membrane permeabilization in S. Enteritidis was investigated by measuring substrate accessibility to periplasmic β-lactamase and cytosolic β-galactosidase.

Results

The depolarization of the inner membrane of S. Enteritidis was measured using a fluorescence probe [DiSC3(5)]. The results show that ovotransferrin induces permeabilization of the outer membrane but not the inner membrane whereas egg white permeabilizes both membranes. In addition, the dissipation of the proton motive force by egg white was found to involve a contribution by ovotransferrin since this protein provoked inner-membrane depolarization.

Discussion

It can thus be concluded that ovotransferrin exerts a membranes perturbation activity on S. Enteritidis under egg-white conditions, in addition to its well-known iron-chelation activity.

Genome-Wide Screening of Oxidizing Agent Resistance Genes in Escherichia coli

The use of oxidizing agents is one of the most favorable approaches to kill bacteria in daily life. However, bacteria have been evolving to survive in the presence of different oxidizing agents. In this study, we aimed to obtain a comprehensive list of genes whose expression can make Escherichiacoli cells resistant to different oxidizing agents. For this purpose, we utilized the ASKA library and performed a genome-wide screening of ~4200 E. coli genes. Hydrogen peroxide (H₂O₂) and hypochlorite (HOCl) were tested as representative oxidizing agents in this study. To further validate our screening results, we used different E. coli strains as host cells to express or inactivate selected resistance genes individually. More than 100 genes obtained in this screening were not known to associate with oxidative stress responses before. Thus, this study is expected to facilitate both basic studies on oxidative stress and the development of antibacterial agents.

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

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.

Identification of New Antimicrobial Peptides that Contribute to the Bactericidal Activity of Egg White against Salmonella enterica Serovar Enteritidis at 45 °C

A recent work revealed that egg white (EW) at 45 °C exhibits powerful bactericidal activity against S. enterica serovar Enteritidis, which is surprisingly little affected by removal of the >10 kDa EW proteins. Here, we sought to identify the major EW factors responsible for this bactericidal activity by fractionating EW using ultrafiltration and nanofiltration and by characterizing the physicochemical and antimicrobial properties of the resulting fractions. In particular, 22 peptides were identified by nano-LC/MS-MS and the bactericidal activities of representative peptides (with predicted antimicrobial activity) were further assessed. Two peptides (FVPPVQR and GDPSAWSWGAEAHS) were found to be bactericidal against S. enterica serovar Enteritidis at 45 °C when provided in an EW environment. Nevertheless, these peptides contribute only part of this bactericidal activity, suggesting other, yet to be determined, antimicrobial factors.

Egg-White Proteins Have a Minor Impact on the Bactericidal Action of Egg White Toward Salmonella Enteritidis at 45°C

Salmonella enterica serovar Enteritidis is noted for its ability to survive the harsh antibacterial activity of egg white which is presumed to explain its occurrence as the major food-borne pathogen associated with the consumption of eggs and egg products. Liquid egg white is a major ingredient for the food industry but, because of its thermal fragility, pasteurization is performed at the modest temperature of 57°C (for 2–6 min). Unfortunately, such treatment does not lead to sufficient reduction in S. Enteritidis contamination, which is a clear health concern when the product is consumed without cooking. However, egg white is able to limit S. Enteritidis growth due to its alkaline pH, iron deficiency and multiple antimicrobial proteins. This anti-Salmonella activity of egg white is temperature dependent and becomes bactericidal once the incubation temperature exceeds 42°C. This property is exploited in the highly promising pasteurization treatment (42–45°C for 1–5 days) which achieves complete killing of S. Enteritidis. However, the precise mechanism and the role of the egg-white proteins are not fully understood. Here, the impact of exposure of S. Enteritidis to egg white-based media, with or without egg-white proteins (>10 kDa), under bactericidal conditions (45°C) was explored by measuring survival and global expression. Surprisingly, the bactericidal activity of egg white at 45°C was only slightly affected by egg-white proteins indicating that they play a minor role in the bactericidal activity observed. Moreover, egg-white proteins had minimal impact on the global-gene-expression response to egg white such that very similar, major regulatory responses (20% genes affected) were observed both with and without egg-white proteins following incubation for 45 min at 45°C. Egg-white proteins caused a significant change in expression for just 64 genes, including the psp and lysozyme-inhibitor responses genes which is suggestive of an early membrane perturbation effect. Such damage was supported by disruption of the proton motive force by egg-white proteins. In summary, the results suggest that low-mass components of egg white are largely responsible for the bactericidal activity of egg white at 45°C.