Abstract
The pathogenesis of Salmonella enterica serovar Typhi (S. Typhi), the cause of typhoid fever in humans, is mainly attributed to the acquisition of horizontally acquired DNA elements. Salmonella pathogenicity islands (SPIs) are indubitably the most important form of horizontally acquired DNA with respect to pathogenesis of this bacterium. The insertion or deletion of any of these transferrable SPIs may have impact on the virulence potential of S. Typhi. In this study, the virulence potential and genetic relatedness of 35 S. Typhi isolates, collected from 2004 to 2013 was determined by identification of SPI and non-SPI virulence factors through a combination of techniques including virulotyping, Whole Genome Sequencing (WGS), and Variable Number of Tandem Repeats (VNTR) profiling. In order to determine the virulence potential of local S. Typhi isolates, 56 virulence related genes were studied by PCR. These genes are located in the core as well as accessory genome (SPIs and plasmid). Major variations among studied virulence determinants were found in case of SPI-7 and SPI-10 associated genes. On the basis of presence of virulence related genes, the studied S. Typhi isolates from Pakistan were clustered into two virulotypes Vi-positive and Vi-negative. Interestingly, SPI-7 and SPI-10 were collectively absent or present in Vi-negative and Vi-positive strains, respectively. Two Vi-negative and 11 Vi-positive S. Typhi strains were also analyzed by whole genome sequencing (WGS) and their results supported the PCR results. Genetic diversity was tested by VNTR-based molecular typing. All 35 isolates were clustered into five groups. Overall, all Vi-negative isolates were placed in a single group (T5) whereas Vi-positive isolates were grouped into four types. Vi-negative and Vi-positive isolates were mutually exclusive. This is the first report on the comparative distribution of SPI and non-SPI related virulence genes in Vi-negative and Vi-positive S. Typhi isolates with an important finding that SPI-10 is absent in all Vi-negative isolates.
Highlights
Pathogenicity islands are distinct genetic components located on the pathogenic bacterial chromosomes
We have identified four naturally occurring variants which are simultaneously lacking Salmonella pathogenicity islands (SPIs)-7 and SPI-10, two adjacently located pathogenicity islands on S
In this study 56 virulence related genes involved in mobility, secretion systems, metabolic regulation and toxin production were screened by polymerase chain reaction (PCR)
Summary
Pathogenicity islands are distinct genetic components located on the pathogenic bacterial chromosomes. It causes a severe systemic infection, the typhoid fever, which is a serious worldwide public health problem. More than 90% of typhoid fever cases occurred in Asia [1,2]. It is highly prevalent in Asia and Africa due to shortage of hygienic water and poor sanitation. It is a significant travel-associated disease [3]. Typhi infection poses substantial burden on healthcare system throughout the world especially in Southeast Asia (including Pakistan) and other endemic countries. Typhi strains causes difficulty in its treatment and poses a serious threat to future treatment options [4,5,6,7,8]
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