Injectisomes: the hypodermic needles of bacteria

Written and illustrated by: Jacquelyn Roberts

Edited by: Sarah Bassiouni, Sophia Hill, Austin Shannon, and Madeline Barron

Pathogenic bacteria like Salmonella, Shigella, Yersinia, and Escherichia coli can inject proteins into target cells with an extremely small hypodermic needle called a type III secretion system, or T3SS. The injected proteins are called “effector proteins”, as they elicit effects in the host (in this case human) cell. The cell membranes of both the bacteria and host cell prevent large molecules like effector proteins from simply drifting inside, so bacteria need specialized delivery systems like the T3SS to do it. These specialized delivery systems are very similar to a syringe and needle, and have been nicknamed injectisomes (Figure 1). However, injectisomes are 10 million times smaller than the average syringe. In addition to being incredibly small, these syringes are extremely specific in the cargo that they accept for transport. Bacteria generally use these machines to inject effector proteins  that weaken the host cell, leading to easier bacterial infection. The mechanistic details of these molecular machines is of great interest to scientists for many reasons. Designing a specific inhibitor of the injectisome could render pathogenic bacteria harmless. On the other hand, engineering an injectisome for drug delivery could provide targeted biologic medicine to specific cells in the body.

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