Abstract
BackgroundSalmonellosis is one of the most common foodborne diseases worldwide. Although human infection by non-typhoidal Salmonella (NTS) enterica subspecies enterica is associated primarily with a self-limiting diarrhoeal illness, invasive bacterial infections (such as septicaemia, bacteraemia and meningitis) were also reported. Human outbreaks of NTS were reported in several countries all over the world including developing as well as high-income countries. Conventional laboratory methods such as pulsed field gel electrophoresis (PFGE) do not display adequate discrimination and have their limitations in epidemiological surveillance. It is therefore very crucial to use accurate, reliable and highly discriminative subtyping methods for epidemiological characterisation and outbreak investigation.MethodsHere, we used different whole genome sequence (WGS)-based subtyping methods for retrospective investigation of two different outbreaks of Salmonella Typhimurium and Salmonella Dublin that occurred in 2013 in UK and Ireland respectively.ResultsSingle nucleotide polymorphism (SNP)-based cluster analysis of Salmonella Typhimurium genomes revealed well supported clades, that were concordant with epidemiologically defined outbreak and confirmed the source of outbreak is due to consumption of contaminated mayonnaise. SNP-analyses of Salmonella Dublin genomes confirmed the outbreak however the source of infection could not be determined. The core genome multilocus sequence typing (cgMLST) was discriminatory and separated the outbreak strains of Salmonella Dublin from the non-outbreak strains that were concordant with the epidemiological data however cgMLST could neither discriminate between the outbreak and non-outbreak strains of Salmonella Typhimurium nor confirm that contaminated mayonnaise is the source of infection, On the other hand, other WGS-based subtyping methods including multilocus sequence typing (MLST), ribosomal MLST (rMLST), whole genome MLST (wgMLST), clustered regularly interspaced short palindromic repeats (CRISPRs), prophage sequence profiling, antibiotic resistance profile and plasmid typing methods were less discriminatory and could not confirm the source of the outbreak.ConclusionsFoodborne salmonellosis is an important concern for public health therefore, it is crucial to use accurate, reliable and highly discriminative subtyping methods for epidemiological surveillance and outbreak investigation. In this study, we showed that SNP-based analyses do not only have the ability to confirm the occurrence of the outbreak but also to provide definitive evidence of the source of the outbreak in real-time.
Highlights
Foodborne salmonellosis is an important concern for public health
Non-typhoidal Salmonella (NTS) serovars, predominantly cause a self-limiting diarrhoeal illness they have adapted to cause invasive extra-intestinal disease known as invasive non-typhoidal Salmonella (NTS) which can result in bacteraemia and focal systemic infections [3, 4]
whole genome sequence (WGS)-based subtyping Single nucleotide polymorphism (SNP) based cluster analyses SNP based tree showed conclusively that the outbreak strains of Salmonella Typhimurium were grouped together in two clades and they are very closely related to strains isolated from mayonnaise (Fig. 1) confirming the source of outbreak is due to consumption of contaminated mayonnaise
Summary
Foodborne salmonellosis is an important concern for public health It is caused by the enteric pathogen Salmonella enterica, which includes more than 2600 serovars [1]. Non-typhoidal Salmonella (NTS) serovars, predominantly cause a self-limiting diarrhoeal illness they have adapted to cause invasive extra-intestinal disease known as invasive NTS (iNTS) which can result in bacteraemia and focal systemic infections [3, 4]. Human infection by non-typhoidal Salmonella (NTS) enterica subspecies enterica is associated primarily with a self-limiting diarrhoeal illness, invasive bacterial infections (such as septicaemia, bacteraemia and meningitis) were reported. Human outbreaks of NTS were reported in several countries all over the world including developing as well as high-income countries Conventional laboratory methods such as pulsed field gel electrophoresis (PFGE) do not display adequate discrimination and have their limitations in epidemiological surveillance. It is very crucial to use accurate, reliable and highly discriminative subtyping methods for epidemiological characterisation and outbreak investigation
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