AUTHOR=Xu Yumin , Abdelhamid Ahmed G. , Sabag-Daigle Anice , Sovic Michael G. , Ahmer Brian M.M. , Yousef Ahmed E. 

TITLE=The Role of Egg Yolk in Modulating the Virulence of Salmonella Enterica Serovar Enteritidis

JOURNAL=Frontiers in Cellular and Infection Microbiology

VOLUME=12

YEAR=2022

URL=https://www.frontiersin.org/journals/cellular-and-infection-microbiology/articles/10.3389/fcimb.2022.903979

DOI=10.3389/fcimb.2022.903979

ISSN=2235-2988

ABSTRACT=<p>Contribution of food vehicles to pathogenicity of disease-causing microorganisms is an important but overlooked research field. The current study was initiated to reveal the relationship between virulence of <italic>Salmonella enterica</italic> serovar Enteritidis and egg yolk as a hosting medium. Mice were orally challenged with <italic>Salmonella</italic> Enteritidis cultured in egg yolk or tryptic soy broth (TSB). Additionally, mice were challenged with <italic>Salmonella</italic> Enteritidis cultured in TSB, followed by administration of sterile egg yolk, to discern the difference between pre-growth of the pathogen and its mere presence in egg yolk during infection. The pathogen’s Lethal dose 50 (LD<sub>50</sub>) was the lowest when grown in yolk (2.8×10<sup>2</sup> CFU), compared to 1.1×10<sup>3</sup> CFU in TSB, and 4.6×10<sup>3</sup> CFU in TSB followed by administration of sterile yolk. Additionally, mice that orally received <italic>Salmonella</italic> Enteritidis grown in egg yolk expressed a high death rate. These findings were supported by transcriptional analysis results. Expression of promoters of virulence-related genes (<italic>sopB</italic> and <italic>sseA</italic>) in genetically modified <italic>Salmonella</italic> Enteritidis reporter strains was significantly higher (<italic>p</italic> &lt; 0.05) when the bacterium was grown in the yolk, compared to that grown in TSB. Sequencing of RNA (RNA-seq) revealed 204 differentially transcribed genes in <italic>Salmonella</italic> Enteritidis grown in yolk <italic>vs</italic>. TSB. Yolk-grown <italic>Salmonella</italic> Enteritidis exhibited upregulated virulence pathways, including type III secretion systems, epithelial cell invasion, and infection processes; these observations were confirmed by RT-qPCR results. The transcriptomic analysis suggested that upregulation of virulence machinery of <italic>Salmonella</italic> Enteritidis grown in egg yolk was related to increased iron uptake, biotin utilization, flagellar biosynthesis, and export of virulence proteins encoded on <italic>Salmonella</italic> pathogenicity island 1, 2, 4, and 5. These biological responses may have acted in concert to increase the virulence of <italic>Salmonella</italic> infection in mice. In conclusion, growth in egg yolk enhanced <italic>Salmonella</italic> Enteritidis virulence, indicating the significance of this food vehicle to the risk assessment of salmonellosis.</p>