Investigating the globally dominant Salmonella Typhi haplotype H58 – an insight into the pathogenesis of typhoid fever

Infection with Salmonella enterica serovar Typhi (S. Typhi) results in the systemic disease, typhoid fever, which causes approximately 200,000 deaths per year. Unfortunately S. Typhi pathogenesis remains poorly understood. Recently a S. Typhi haplotype (H58) associated with MDR has clonally expanded to become the globally dominant haplotype, although the reasons for this success are unknown. A sequencing study identified 44 SNPs in H58 strains that are not present in an S. Typhi reference strain, including one which generates a premature stop codon in the SPI-1 effector SptP. This led to characterisation of this effector during S. Typhi infection. Changes in the chaperone binding domain of S. Typhi SptP were found to result in intracellular instability and prevented secretion, therefore deletion of sptP in S. Typhi did not result in phenotypic consequences with regards to host cytoskeletal recovery or invasion, suggesting that SptP is non-functional within S. Typhi. Additional SNPs in H58 are present within genes associated with bile responses. To understand the effect of these mutations, RNA-Seq was performed on S. Typhi Ty2, an H58 clinical isolate (129-0239), and S. Typhimurium 14028 grown in 3% ox-bile. Overall 249 genes, 389 genes and 453 genes respectively were differentially expressed in bile. A key finding is that S. Typhi significantly upregulates expression of SPI-1 T3SS-associated genes, and is subsequently more invasive in bile, whilst in S. Typhimurium significant repression is observed. Investigation of the regulatory mechanisms governing this difference indicate that the key SPI-1 regulator HilD is post-transcriptionally regulated by bile, with its stability markedly increasing. Additionally, RNase E is involved in transcriptionally regulating SPI-1 expression in bile, but downstream of hilD; this regulation requires further characterisation. Overall this study has described broad differences between S. Typhi and S. Typhimurium, which may underlie differences in pathogenesis.Open Acces