injectisome

An injectisome is a bacterial nanomachine used by many Gram-negative pathogens to deliver effector proteins directly into eukaryotic host cells. It is a specialized type III secretion system (T3SS) that functions as a molecular syringe spanning the bacterial envelopes and the host cell membrane. The apparatus comprises a cytoplasmic secretion system, an inner membrane export apparatus, an outer membrane secretin ring, a needle, and a tip complex. Upon contact with a host cell, a translocon forms a pore in the host membrane, enabling efficient translocation of effectors into the cytosol.

Function and mechanism: The injectisome exports effector proteins after sensing host-cell contact or environmental cues. The process is powered by an ATPase in the cytoplasmic complex and, in some systems, by the proton motive force across the inner membrane. Effector proteins are typically stabilized by dedicated chaperones in the bacterial cytoplasm before secretion. Once translocated, these effectors manipulate host signaling pathways, cytoskeletal dynamics, vesicle trafficking, and immune responses to promote bacterial survival and replication.

Genetics and evolution: Genes encoding injectisome components are commonly located on pathogenicity islands or plasmids and are regulated to respond to environmental signals. The T3SS injectisome is evolutionarily related to the flagellar basal body, with conserved core building blocks across diverse Gram-negative species, while the specific effectors and regulators vary widely.

Significance: The injectisome is central to understanding bacterial virulence and host-pathogen interactions. It serves as a model for studying secretion systems, structural biology, and mechanistic infection strategies, and it is a target of interest for anti-virulence therapies and biotechnological applications.