Title:
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Molecular characterization and diagnosis of salmonella enterica strains in the United Kingdom
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Food-borne salmonellosis is a major cause of gastrointestinal disease in humans and
domestic animals worldwide. In this study, the population structure and genetic
relationship of various Salmonella serovars were investigated using Multilocus
Sequence Typing (MLST). The MLST scheme, using seven housekeeping genes, was
used to characterize and differentiate 153 Salmonella isolates of 34 serovars into 51
sequence types (STs) with fourteen novel STs identified among 13 serovars in the
collection. The scheme provided good sub-type discrimination among various
Salmonella serovars. Comparison of 51 allelic profiles against the database using
START and eBURST software indicated a strong association of STs with serovars. The
phylogenetic structure of each serovar was represented in one lineage of a cluster, with
the exceptions that isolates of serovars Newport and Java were clustered into three and
two distinct lineages, respectively. In addition, 17 STs of seven serovars were
differentiated into nine clonal complexes. These results allowed the development of a
molecular serotyping scheme based on detection of serovar specific single nucleotide
polymorphism~SNPs) seen in the MLST data.
The SNaPshot system is a multiplex primer extension assay (MPEA) that enables
multiplex SNP analysis. Here, the method has been developed for the identification of
five Salmonella serotypes commonly detected in the UK, based on the above serotype
specific SNPs. The SNPs, in genes hemD, thrA, purE and sucA, acted as surrogate
markers for serovars Typhimurium, Enteritidis, Virchow, Infantis and Bracnderup. The
MPEA was performed using two separate panels of MPEA reactions and evaluated
using 152 S. enterica isolates that had been characterized by MLST. The MPEA was
shown to be 100% specific and sensitive for this collection of isolates. Furthermore, the
MPEA was applied to identify the serovar or ST of DNA recovered from clinical
specimens (faecal samples from human Salmonella infection) (n=15) and food samples
(n= 10). The isolation of Salmonella from these specimens/samples and their
subsequent serotyping indicated that use of the MPEA had allowed accurate
identification of the serovar of 96% of isolates present in the samples. Interestingly, the
assay also allowed identification of the serovar of two Salmonella isolates is from the
above samples that were not able to be fully typed using serological methods. The
method could be applied in less than six hours and has potential for improved patient
care, public health investigation of Salmonella outbreaks and source tracing.
Recently, the DiversiLab rep-Pf'R system has been developed using microfluidic chips to
provide standardized semi-automated fingerprinting for pathogens including S. enterica. In
the current study, 71 isolates of S. enterica, representing 21 different serovars, were
analysed using MLST and the DiversiLab rep-Pf'R system. MLST was able to identify 31
STs, while the DiversiLab system revealed 38 Diversil.ab types (OTs). The DiversiLab
rep-P(R approach distinguished isolates of different serovars and showed a greater
discriminatory power (0.95) than MLST (0.89). The OiversiLab system exhibited 92%
concordance with MLST and 90% with serotyping, while the concordance level of MLST
with serotyping was 96%, representing a strong association. MLST and the DiversiLab reppeR
system may provide useful additional informative techniques for the molecular
identification of S. enterica isolates. In addition, the OiversiLab rep-PCR system may
provide a rapid (less than 4 hours) and standardized method for the identification of S.
enterica isolates.
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