Contact sites (black arrowhead and inset) between the phagosome and Amoebophilus. The contractile injection system was always adjacent to such points. Credit: Böck et al. 2017
It’s not yet clear, however, whether the phagosome is destroyed by the escape, nor whether it is ruptured by the physical act of puncture or by chemicals delivered by the spears.
Stabbing the phagosome, however, seems to allow Amoebophilus to escape its prison, avoid being eaten, and then to help itself to its host’s pantry while making lots of little bacteria.
Where did Amoebophilus acquire its fancy spear guns? Strangely, even though this is technically a Type 6 secretion system, DNA analysis of the genes that make it shows it is more closely related to the free-floating contractile injection systems that kill insects or trigger metamorphosis in tube worms than it is to other Type 6 secretion systems, which are by our definition membrane-bound. The distinction of “attached to the inside of the cell” versus “secreted into the environment” that we have used to segregate Type 6 secretion systems from extracellular contractile injection systems thus seems artificial and invalid from an evolutionary standpoint.
The scientists suggest two hypotheses. Either Amoebophilus’s speargun evolved from the extracellular contractile injection systems by losing some of their characteristics (like tail fiber feet) and getting welded to the inside of the cell, or it has retained the features of the ancestor of all contractile injection systems, before they diverged into the Type 6 injection systems and the free-floating systems.
Whatever the case, the authors say, its existence implies there are a heck of a lot more bacteria out there than we thought that stab things for a living. In DNA searches across Domain Bacteria, they found genes similar to the ones Amoebophilus uses to build its guns in six different bacterial phyla, the high-level taxonomic ranking just below kingdom. Far from being helpless little bags, bacteria appear more than capable of physically manipulating their world by turning themselves into trigger-loaded hypodermic needles that can poison rivals, predators, and prey – or pick locks inside hosts that give bacteria the run of the place.
Désirée Böck, João M. Medeiros, Han-Fei Tsao, Thomas Penz, Gregor L. Weiss, Karin Aistleitner, Matthias Horn, Martin Pilhofer. In situ architecture, function, and evolution of a contractile injection system. Science, 2017; 357 (6352): 713 DOI: 10.1126/science.aan7904