Type V Secretion

Other secretion systems independently evolved similar mechanisms, though using very different structures. Some strains of Escherichia coli have an ability, called “Contact-Dependent Inhibition” or CDI, to inhibit the growth of neighboring diderm bacterial cells. They do this by coopting a pore in the outer membrane of the target cell to introduce a toxin that halts its growth. To safeguard themselves, they make an antitoxin that binds to the toxin and renders it harmless. Such antidote systems are common for microbe-produced toxins.

To deliver the toxin to the target, the cells use type V secretion systems (T5SSs) like the ones on this Escherichia coli. In contrast to the nanomachines you have been seeing so far, which contain dozens of unique components, this system is elegantly spare, using just two proteins and a mechanism reminiscent of a medieval ball-and-chain flail (⇩).

Note: This cell belongs to a strain that is normally incapable of CDI, but has been genetically engineered to produce the T5SS components. Once again, this lets us image the machines more easily; in this case, the CDI-practicing strain has a thicker layer of extracellular proteins, which would obscure the needles in a visual haystack.