Bacterial Secretion Systems

Bacterial Secretion Systems

Subjects: Bacteriology · Systems: Microbiology · Tags: Microbiology

Introduction

Gram negative bacteria like Legionela, e coli, Salmonella, pseudomonas, Yersina, have a double membrane. They use specialized secretion systems to transport proteins (often toxins or virulence factors) across their membranes. And sometimes directly into host cells. They work as a sort of “molecular syringes” or delivery machines.

Major types of Secretion Systems

Type III secretion system (T3SS) - “Injectisome”

This system looks like a needle and syringe. It injects bacterial proteins direclty into the host cell’s cytoplasm. It is used by Salmonella, Shigella, Yersinia and E. coli pathogypes.

For example, Salmonella uses T3SS to deliver effectors that trigger host cell actin rearrangements, which lead to bacterial uptake.

The clinical relevance of this is that T3SS is a big reason for invasive diarrhea and systemic infections.

Type IV Secretoin System (T4SS) - “Molecular syringe + DNA transporter”

This secretion system can transfer proteins and DNA. Legionella pneumophila uses a T4SS called Dot/Icm system to inject hundreds of effector proteins into host macrophages. These effectors reroute vesicle trafficking and the phagosome fails to fuse with lysosomes. Instead, it gets “camouflaged” with ER-like membrane, creating the Legionella-containing vacuole (LCV) where the bacterium replicates.

This system is also used by Helicobacter pylori (CagA delivery), Bartonella, Brucella

Type VI Secretion System (T6SS) - “Bacterial spear gun”

This system acts like a spring-loaded spear that punctures nearby cells. It can kill competing bacteria (interbacterial warfare) or deliver toxins into host cells. It is used by Pseudomonas, Vibrio cholerae.

Other systems briefly

T1SS

One-step transport across both membranes (e.g hemolysin export in E. coli).

T2SS

Two-step (periplasm to outside, cholera toxin).

T5SS

Autotransporters (adhesins, proteases).

Why do you care for exams?

• Legionella’s virulence = T4SS (Dot/Icm) -> Allows it to survive in macrophages
• Salmonella/Shigella/Yersinia/E. coli pathotyles = T3SS -> classic “needle injectisome”
• H. pylori = T4SS (CagA)
• Pseudomonas/Vibrio = T6SS

Disclaimer: For education only. Not medical advice; always follow your institution's guidance.