A number of animal and plant pathogens inject their anti-host factors directly into the host cytoplasm. This pathway is GSP independent, and requires up to 20 secretion components that assemble into a large structure spanning both bacterial membranes and the host membrane. The channel—forming protein on the outer membrane shares homology with secretin of the type II pathway. Proteins are excreted through a tube-like structure called the needle. ATP hydrolysis provides the energy needed for the excretion (Figure 3.14). Flagella are very similar in structure to the type III excretion
machinery and flagellin is excreted in a similar mechanism to the type III pathway.