When the T6SS fires, the helical outer sheath twists into a shorter, wider form, a conformational change that provides the energy to explosively propel the inner dart, cargo-first, out and into its target (⇩). In some species, like this Vibrio cholerae, the outer sheath will then be recycled, broken down into building blocks that can be used to rapidly assemble a new machine. Other species build again from scratch, translating new protein building blocks. Warfare is dynamic, with cells rapidly assembling and firing T6SSs. Cells have even been observed to engage in T6SS duels, with an attacked cell building and firing a T6SS back in the direction from which it was hit. (You can see fluorescence microscopy of these duels on YouTube.) Some species assemble batteries of parallel T6SSs (⇩).
These contractile weapons evolved from the same ancestor as the “tail” structure viruses called phage use to inject their genetic material into bacteria and archaea (more on that in the next chapter). Some related bacterial weapons are even closer to the phage tail, in that they are released to the environment in a primed state, waiting to bump into a target cell to fire. The target is recognized by filamentous proteins called “tail fibers,” the same identification mechanism used by phage. Other members of this evolutionary family are even stranger, like the Morphogenesis-Associated Contractile structures (MACs) released by a marine bacterium (⇩).
Some cells assemble a few, scattered type VI secretion systems. Other species, like this Amoebophilus asiaticus build ordered arrays of parallel machines. These diderm bacteria live symbiotically inside amoebae. Their T6SS batteries help them colonize their host, perhaps by rupturing the membrane of the host phagosome that encloses them after they are engulfed, releasing them into the host cytoplasm. The number of T6SSs in each array varies, but can reach a few dozen.
Pseudoalteromonas luteoviolacea produce mysterious, massive arrays of MACs, contractile machines related to T6SSs. Cells lyse to release the arrays, like the one you see here, which can contain 100 individual MACs and extend more than 1 μm across. The arrays are highly ordered, with the tail fibers of the MACs networked into a hexagonal array that may help synchronize firing (⇨). MAC arrays serve an important function for larvae of the marine tubeworm Hydroides elegans, and likely the larvae of other invertebrates such as sea urchins and corals as well. They signal to the larva that it is the right place to settle down and differentiate into a sessile (surface-attached) adult. The function of MAC arrays for the bacterium is less clear. It may be that the invertebrates benefit the lysed cell’s surviving relatives in the biofilm. Or perhaps the MACs play another, completely unrelated role.
Note: This MAC array was produced by a cell lacking genes encoding other contractile weaponry (type VI secretion systems and a related machine called a bacteriocin). This was done to make it easier to identify the MACs.
This digital segmentation of a MAC array shows the overall order of the structure [86]. Outer sheaths are in blue, inner rods in green, tail fibers in orange, and networking fibers in white.
