Salmonella Typhi and Paratyphi | Notion

1. Taxonomy and Classification

Family	Enterobacteriaceae
Main species	Salmonella enterica and Salmonella bongori
Subspecies	S. enterica subspecies enterica (includes S. Typhi and S. Paratyphi)
Host specificity	Exclusively human pathogens

2. Nontyphoidal Salmonella (NTS)

Includes other Salmonella species
Primarily causes gastroenteritis in humans
Some invasive nontyphoidal serovars (iNTS) exist

3. Classification Methods

Method	Description	Benefits
Traditional: Phage typing	Used since early 1900s	• Improved outbreak tracking
• Better antimicrobial resistance monitoring
Modern: Genotyping	e.g., GenoTyphi scheme

4. Pathophysiology

Complex mechanisms, not fully understood

Research Methods:

S. Typhimurium mouse model
Controlled human challenge studies (CHIM)

CHIM Findings:

S. Paratyphi: Lower infective dose than S. Typhi
Median symptom onset: 8 days
S. Paratyphi: Longer bacteremia, more asymptomatic infections

5. Pathogenesis Mechanisms of Salmonella Typhi and Paratyphi

The pathogenesis of Salmonella Typhi and Paratyphi involves a complex series of events. Recent research has provided comprehensive insights into these mechanisms:

Stage	Mechanism	Details
Ingestion	Acid resistance	Multiple systems (glutamate-, arginine-, and lysine-dependent) to survive stomach acid
Invasion	Intestinal epithelium invasion	Targets M cells in Peyer's patches and enterocytes using SPI-1 T3SS
Survival	Intracellular adaptation	Uses SPI-2 T3SS to survive in Salmonella-containing vacuoles (SCVs)
Spread	Systemic dissemination	Spreads via lymphatic system and bloodstream to liver, spleen, and bone marrow
Evasion	Immune evasion strategies	Vi capsular antigen masking, host cell signaling modulation, altered antigen presentation
Persistence	Chronic carriage	Forms biofilms and invades epithelial cells, especially in gallbladder
Interaction	Host-pathogen interplay	Exploits host cell machinery and manipulates inflammatory responses