Salmonella enterica serovar Typhi and Paratyphi A (S. Typhi / S. Paratyphi A)

Salmonella enterica serovar Typhi and Paratyphi A are the bacterial pathogens responsible for typhoid and paratyphoid fevers, collectively known as enteric fever [WHO, 2023]. These organisms are Gram-negative, facultative intracellular pathogens that invade the human host via the fecal-oral route, primarily targeting the intestinal mucosa and the systemic reticuloendothelial system [StatPearls, 2023]. Live attenuated vaccines, such as the Ty21a strain, are engineered versions of these bacteria that lack key virulence factors or metabolic capabilities, allowing them to colonize the gut and stimulate an immune response without causing clinical illness [PubMed: 29134494]. These vaccines interact with the host immune system by presenting a broad array of native antigens to Toll-like receptors and other pattern recognition receptors, inducing mucosal IgA, systemic IgG, and cell-mediated immunity [PubMed: 30245169]. While Ty21a is widely used for S. Typhi, live attenuated candidates for S. Paratyphi A are currently in clinical development to provide bivalent protection against enteric fever [PubMed: 31606358]. The use of live attenuated strains is a critical strategy for providing long-lasting immunity in endemic regions where enteric fever remains a significant public health burden.

Other names

Typhoid fever bacteriumParatyphoid A bacteriumSalmonella enterica subsp. enterica serovar TyphiSalmonella enterica subsp. enterica serovar Paratyphi ASalmonella typhiSalmonella paratyphi A

Live attenuated vaccines stimulate the immune system by mimicking a natural infection; the attenuated bacteria colonize the intestinal mucosa and Peyer's patches, triggering the production of secretory IgA, serum IgG, and activating T-cell mediated responses against bacterial antigens [StatPearls, 2023].