Active sulfite oxidase domain of Salmonella enterica pathogenic protein small intestine invasive factor E (SiiE): a potential diagnostic target.

Serovars of Salmonella enterica are common food-borne bacterial pathogens. Salmonella typhi, which causes typhoid, is the most dangerous of them. Though detailed molecular pathogenesis studies reveal many virulence factors, inability to identify their biochemical functions hampers the development of diagnostic methods and therapeutic leads. Lack of quicker diagnosis is an impediment in starting early antibiotic treatment to reduce the severe morbidity and mortality in typhoid. In this study, employing bioinformatic prediction, biochemical analysis, and recombinantly cloning the active region, we show that extracellularly secreted virulence-associated protein, small intestinal invasion factor E (SiiE), possesses a sulfite oxidase (SO) domain that catalyzes the conversion of sodium sulfite to sodium sulfate using tungsten as the cofactor. This activity common to Salmonella enterica serovars seems to be specific to them from bioinformatic analysis of available bacterial genomes. Along with the ability of this large non-fimbrial adhesin of 600 kDa binding to sialic acid on the host cells, this activity could aid in subverting the host defense mechanism by destroying sulfites released by the immune cells and colonize the host gastrointestinal epithelium. Being an extracellular enzyme, it could be an ideal candidate for developing diagnostics of S. enterica, particularly S. typhi.