A patient with known sickle cell disease presents with high fever and bacteraemia. Blood culture grows a gram-negative rod that is non-lactose fermenting, H2S positive on TSI agar, and agglutinates with Salmonella group D antiserum. Which specific Salmonella pathogenicity island (SPI) encodes the type III secretion system responsible for invasion of intestinal epithelial cells?

• A SPI-3 encodes the primary invasion apparatus
• B SPI-5 encodes all secreted effectors for both invasion and intracellular survival
• C Salmonella does not require a T3SS; invasion is entirely mediated by flagella
• D SPI-1 encodes the invasion-associated T3SS; SPI-2 encodes the intravacuolar T3SS ✓

Explanation

Salmonella has at least 5 major pathogenicity islands. SPI-1 encodes a Type III secretion system (T3SS-1) that injects effector proteins (SipA, SopE, SopB) into intestinal epithelial cells, triggering actin rearrangement and bacterial internalisation (macropinocytosis) — this mediates the initial invasion/diarrhoeal phase. After internalisation, SPI-2 encodes a second T3SS (T3SS-2) enabling survival within the Salmonella-containing vacuole (SCV) inside macrophages, enabling systemic spread. Sickle cell patients are particularly susceptible to Salmonella osteomyelitis and bacteraemia due to impaired opsonisation and splenic dysfunction.

Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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