Abstract
Salmonella enterica serovar Enteritidis (Salmonella Enteritidis) is a globally important foodborne pathogen, and the contaminated chicken eggs are the major source of salmonellosis in humans. Salmonella Enteritidis strains are differentially susceptible to the hostile environment of egg whites. Strains with superior survival ability in egg whites are more likely to contaminate eggs and consequently infect humans. However, the genetic basis for this phenotype is unclear. We characterized two Salmonella Enteritidis strains isolated from chicken meat that had similar genetic backgrounds but large differences in survival ability in egg whites. Although genome comparisons indicated that the gene content and genomic synteny were highly conserved, variations including six insertions or deletions (INDELs) and 70 single nucleotide polymorphisms (SNPs) were observed between the two genomes. Of these, 38 variations including four INDELs and 34 non-synonymous SNPs (nsSNP) were annotated to result in amino acid substitutions or INDELs in coding proteins. These variations were located in 38 genes involved in lysozyme inhibition, vitamin biosynthesis, cell division and DNA damage response, osmotic and oxidative protection, iron-related functions, cell envelope maintenance, amino acid and carbohydrate metabolism, antimicrobial resistance, and type III secretion system. We carried out allelic replacements for two nsSNPs in bioC (biotin synthesis) and pliC (lysozyme inhibition), and two INDELs in ftsK and yqiJ (DNA damage response) by homologous recombination, and these replacements did not alter the bacterial survival ability in egg whites. However, the bacterial survival ability in egg whites was reduced when deletion mutation of the genes bioC and pliC occurred. This study provides initial correlations between observed genotypes and phenotypes and serves as an important caveat for further functional studies.
Highlights
Salmonella is a globally important foodborne bacterial pathogen
The de novo assembly yielded 42 contigs with an N50 of 371,755 bp for SJTUF10978 and 44 contigs with an N50 of 277,421 bp for SJTUF10984. All contigs in these two genomes were successfully mapped to the reference genome with small variations in the alignments except for a deletion of a 5,998-bp fragment in both genomes compared to the reference genome of Salmonella Enteritidis strain P125109
The same Pulsed-field gel electrophoresis (PFGE) (XbaI) pattern and similar Multiple-locus variable number tandem repeat analysis (MLVA) profiles: 5-4-1-10-113-3 in SJTUF10978 and 4-4-1-10-11-3-3 in SJTUF10984 were revealed before genome sequencing
Summary
Salmonella is a globally important foodborne bacterial pathogen. Poultry-derived products, eggs and egg products, are the major vehicles for foodborne outbreaks of salmonellosis in humans (CDC, 2010, 2015, 2016, 2018a,b,c). Salmonella Enteritidis can contaminate eggs by two possible routes and may move into the egg white from the infected reproductive organs directly before eggshell formation and after laying via hen feces or storage environments (GuardPetter, 2001; Messens et al, 2005; Martelli and Davies, 2012). Egg white is a hostile environment for the survival of microorganisms including Salmonella Enteritidis (Guard-Petter, 2001; Gantois et al, 2009; Baron et al, 2016). An important precondition for prolonged contamination of an egg by Salmonella Enteritidis is the migration of bacterial cells from the egg white to the egg yolk. The rate of migration into the yolk is positively correlated with the concentration of Salmonella Enteritidis cells in the egg white (Braun and Fehlhaber, 1995). A greater cell number translates into a greater opportunity for contamination and for transfer to humans
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