typeIII

TypeIII refers most often to the Type III secretion system (T3SS), a complex protein machine used by many Gram-negative bacteria to inject effector proteins directly into host cells. It is sometimes described as an injectisome and is a central feature of bacterial virulence in several major pathogens.

The T3SS forms a syringe-like structure that spans the bacterial inner and outer membranes, and in many cases the host cell membrane as well. Its core components include a basal body embedded in membranes, a hollow needle, a translocon that forms a pore in the host membrane, and regulatory tip proteins. Assembly and activation are triggered by host signals, and energy for secretion is provided by a cytosolic ATPase and the proton motive force. Chaperone proteins keep effector proteins ready for rapid delivery through the needle.

Effector proteins delivered by the T3SS manipulate host cellular processes to promote infection. They can alter cytoskeletal dynamics, vesicle trafficking, and immune signaling, helping bacteria enter cells, avoid immune detection, or create a favorable intracellular environment. The T3SS is encoded on pathogenicity islands or plasmids and is well characterized in pathogens such as Salmonella enterica (SPI-1 and SPI-2), Yersinia spp., Shigella spp., and Pseudomonas aeruginosa, among others.

Because of its pivotal role in virulence, the T3SS is a major target for anti-virulence therapies and is also explored as a tool for delivering proteins into eukaryotic cells in biotechnology. Related secretion systems (Type II, IV, VI) are studied for comparative structure and function.