Figure 1. Assembly of the type three secretion system (T3SS) needle.

The needle is fixed into the bacterial inner and outer membrane and protrudes from the surface to penetrate the host membrane. The translocon forms a channel through the host membrane and the Yop effectors are transferred into the host from the bacterial cytoplasm via the needle and translocon ( a). The needle protrudes from the bacterial surface prior to host cell penetration ( b, c arrowed). Salmonella typhimurium T3SS needles isolated from the bacterial membrane ( d). ( a) adapted from 133, ( b) reproduced with permission and taken from reference 56, ( c) reproduced with permission and taken from reference 16, and ( d) reproduced from reference 134.