Comparative genomic analysis and characterization of incompatibility group FIB plasmid encoded virulence factors of Salmonella enterica isolated from food sources

Rita R Colwell,Bijay K Khajanchi,Nur A Hasan,Shaohua Zhao,Seon Young Choi,Steven L Foley,Jing Han,Carl E Cerniglia

doi:10.1186/s12864-017-3954-5

Abstract

BackgroundThe degree to which the chromosomal mediated iron acquisition system contributes to virulence of many bacterial pathogens is well defined. However, the functional roles of plasmid encoded iron acquisition systems, specifically Sit and aerobactin, have yet to be determined for Salmonella spp. In a recent study, Salmonella enterica strains isolated from different food sources were sequenced on the Illumina MiSeq platform and found to harbor the incompatibility group (Inc) FIB plasmid. In this study, we examined sequence diversity and the contribution of factors encoded on the IncFIB plasmid to the virulence of S. enterica.ResultsWhole genome sequences of seven S. enterica isolates were compared to genomes of serovars of S. enterica isolated from food, animal, and human sources. SeqSero analysis predicted that six strains were serovar Typhimurium and one was Heidelberg. Among the S. Typhimurium strains, single nucleotide polymorphism (SNP)-based phylogenetic analyses revealed that five of the isolates clustered as a single monophyletic S. Typhimurium subclade, while one of the other strains branched with S. Typhimurium from a bovine source. DNA sequence based phylogenetic diversity analyses showed that the IncFIB plasmid-encoded Sit and aerobactin iron acquisition systems are conserved among bacterial species including S. enterica. The IncFIB plasmid was transferred to an IncFIB plasmid deficient strain of S. enterica by conjugation. The transconjugant SE819::IncFIB persisted in human intestinal epithelial (Caco-2) cells at a higher rate than the recipient SE819. Genes of the Sit and aerobactin operons in the IncFIB plasmid were differentially expressed in iron-rich and iron-depleted growth media.ConclusionsMinimal sequence diversity was detected in the Sit and aerobactin operons in the IncFIB plasmids present among different bacterial species, including foodborne Salmonella strains. IncFIB plasmid encoded factors play a role during infection under low-iron conditions in host cells.

Highlights

The degree to which the chromosomal mediated iron acquisition system contributes to virulence of many bacterial pathogens is well defined
The results provide data useful in understanding the functional roles of iron acquisition systems encoded by Incompatibility group FIB plasmid (IncFIB) plasmids of S. enterica and related pathogens
single nucleotide polymorphism (SNP) analysis To understand the role of IncFIB plasmids and how the genes they carry contribute to Salmonella virulence, seven S. enterica strains that were isolated from turkeys and chicken in different geographic locations in the USA were sequenced (Table 1 and Additional file 1: Table S1)

Summary

Introduction

The degree to which the chromosomal mediated iron acquisition system contributes to virulence of many bacterial pathogens is well defined. The functional roles of plasmid encoded iron acquisition systems, Sit and aerobactin, have yet to be determined for Salmonella spp. Salmonella enterica strains isolated from different food sources were sequenced on the Illumina MiSeq platform and found to harbor the incompatibility group (Inc) FIB plasmid. We examined sequence diversity and the contribution of factors encoded on the IncFIB plasmid to the virulence of S. enterica. In the US alone, it is estimated that more than 1 million Salmonella infections occur annually, resulting in approximately 20,000 hospitalizations and 400 deaths [1]. It is important to understand those factors contributing to the more severe manifestations of this disease

Objectives

Methods

Results

Discussion

Conclusion