Fire! A close up look at bacterial spearguns


Breaking down the microbiology world one bite at a time

Fire! A close up look at bacterial spearguns

Bacteria rarely live in environments in isolation; usually, ecosystems are crowded with different species of bacteria, fungi, and other organisms, often competing for the same resources for survival. Certain bacteria, accordingly, have evolved an ingenious way to give themselves a competitive edge: molecular nanomachines that act like protein “spearguns”. Just as a speargun is spring-loaded with a rigid bolt and poised to fire when the trigger is pulled, these bacterial nanomachines are extended and ready to fire an internal tube full of cargo into their surroundings or directly into target cells.

Previously, examples of these bacterial spearguns, or contractile injection systems (CISs) as they’re known in the field, have been shown to directly affect other organisms in their environment. For instance, one CIS from Serratia entomophila has been dubbed the “antifeeding prophage” or AFP because of its ability to stop feeding activity of insect larvae by injecting an insecticidal toxin upon ingestion. Likewise, “metamorphosis-associated contractile structures” (MACs) produced by Pseudoaltermonas luteoviolacea exist as an array of hundreds of these CISs, anchored together at their ends with their “spears” pointing outwards, ready to fire. When MACs encounter the marine worm Hydroides elegans, they inject molecules that cause the worms to undergo metamorphosis – turning the free-swimming worm larvae into adult, adhesive tubeworms that stick to rocks, corals, and the hulls of ships. Recently, back-to-back publications in Nature Microbiology described the discovery of two new CISs produced by aquatic bacteria.

Multicellular cyanobacteria produce CISs in novel places

Cyanobacteria, commonly referred to as blue-green algae, are one of the most abundant organisms on the planet. Capable of photosynthesis, these multifaceted bacteria can thrive in a range of environments: from moist soils to fresh and saltwater settings. They can exist as single cells, filaments, and even sheets, interacting closely with other microbes. Recently, researchers were able to use electron microscopes to image filaments of cyanobacteria and found something exciting hidden within: inside the chloroplasts were CISs embedded in the thylakoid membranes, the cell parts where photosynthesis takes place within the bacterium. Previously classified CISs have either been anchored to the outer, cell membrane or free-floating in the middle of cells; these thylakoid-anchored CIS, or tCIS, are an exciting, entirely new class of CIS. When the cyanobacteria are stressed, for example by high salt concentrations in the environment or exposure to ultraviolet light, the outer layers of the cell are sloughed away (creating what is called a “ghost cell”), exposing the outward-facing tCIS that are poised to fire upon contact with a target.

Bacterial spearguns: new contractile injection systems (CISs) have been identified anchored to the thylakoid membrane (a special membrane inside the chloroplast where photosynthesis occurs) in cyanobacteria (tCIS) and free-floating in the environment, outside of the cell (so-called “extracellular” CIS or eCIS). When extended, the CIS speargun is ready to fire. Once fired, the sheath module contracts, pushing the inner tube (filled with cargo molecules) into the environment or into a target cell. Image credit: Charles F. Ericson and Jessica Stanisich.

By isolating individual tCIS from the cyanobacteria, researchers were also able to discover exactly which proteins make up these new spearguns and use an electron microscope to picture individual atoms – essentially allowing them to build a 3D model of a tCIS. Figuring out these models allows researchers to pinpoint important parts of the CIS that give it unique capabilities: such as anchoring into the thylakoid membrane of the cell, choosing targets, and firing special cargo molecules upon contraction.

Release the CIS: visualizing CIS outside of the cell

On the flip side, the group also discovered another CIS that wasn’t anchored to a membrane at all, but free-floating inside the bacterium and subsequently released into the environment. This CIS, found outside of the cell or “extracellularly” (eCIS), produced by Algoriphagus machipongonensis is released into the surroundings to target other cells. By determining the components that make up this eCIS, they discovered unique features that haven’t been identified in CIS before, such as a “cap adapter” that sits on top of the CIS inner tube to seal it, an internal “plug” protein that acts to stopper the inner tube, and a “spike cage” that covers the pointed spike protein, aiding in targeting and preventing premature firing. These features give clues about the evolution of CIS and why they produce so many different types of contractile structures with different parts and functions. 

(Top) Electron microscope image of a tCIS anchored in the thylakoid membranes, inside a cyanobacterium. (Bottom) Electron microscope image of an A. machipongonensis bacterium. 3D models are made from these microscope images, allowing researchers to determine which proteins make up each CIS and even determine the positions of individual atoms. Source: adapted from original articles by Jingwei Xu.

Applications in biomedicine

These bacteria have evolved specialized molecular nanomachines that allow them to interact with neighboring organisms in their environment. By comparing 3D models of the newly discovered tCIS and eCIS with previously identified CISs from other bacteria, it can be seen that many CISs share common building blocks or modules. With this in mind, researchers in the future could exploit the modular nature of these molecular spearguns, reengineering them to target specific cell types and even fire specific drugs or antimicrobials: creating specialized molecular spearguns to possibly deliver chemotherapies or antibiotics directly into cells. 

Link to the original post:

“Structure of a thylakoid-anchored contractile injection system in multicellular cyanobacteria” Gregor L. Weiss, Fabian Eisenstein, Ann-Katrin Kieninger, Jingwei Xu, Hannah A. Minas, Milena Gerber, Miki Feldmüller, Iris Maldener, Karl Forchhammer, Martin Pilhofer. Nature Microbiology, 2022. DOI: 10.1038/s41564-021-01055-y

“Identification and structure of an extracellular contractile injection system from the marine bacterium Algoriphagus machipongonensis” Jingwei Xu, Charles F. Ericson, Yun-Wei Lin, Florentine U.N. Rutaganira, Fabian Eisenstein, Miki Feldmüller, Nicole King, Martin Pilhofer. Nature Microbiology, 2022. DOI: 10.1038/s41564-022-01059-2

Featured image: Charles F. Ericson