Like humans, bacteria live in communities where they can help each other and collaborate. However, when conditions become harsh, such as when there is a lack of food, bacteria turn aggressive—they kill each other to access limited resources. To do this, bacteria have developed some fascinating weapons, including a microscopic crossbow! This crossbow is made of an arrow wrapped in a spring. The arrow is capped with a spike, to which poison, sometimes in a grenade-like container, is attached. Once in contact with a competitor, the spring propels the arrow through a hole in the bacterium’s membrane—like a murder hole used in castle defense—toward the enemy. Bacteria have also developed strategies to protect themselves against these weapons: antidotes to poisons, shields to block arrows, or the ability to join together in protective groups. Mines, crossbows, murder holes, grenades, poisons, antidotes, and shields: these make up the military arsenal of bacteria!
Bacteria Live in Communities
Bacteria are microscopic organisms that live everywhere, all around us! They have an extraordinary ability to adapt to their environments and have therefore colonized all ecosystems on the planet. Bacteria are found in the soil, in the sea, in lakes and other freshwater, on polar ice caps, in geysers and hot springs, and on plants and animals. They are also found in our food, on door handles, on cell phones, and even on human skin and in some of our organs (Figure 1)!
Our bodies contain billions of bacteria. Most of these bacteria are harmless, and many even help us with important functions like obtaining nutrients from food. But some bacteria try to attack us and can cause diseases that can make us very sick. Like humans, bacteria do not live alone—they often live in communities. Communities of bacteria that live on or in organisms are called the microbiota . These communities are highly organized and keep the bacteria safe: bacterial communities can protect themselves against toxic compounds, antibiotics, or even against the human body’s defenses. Within these communities, bacteria exchange information, materials, and foods such as amino acids, sugars, or small molecules. Overall, they help each other, cooperate, and collaborate. But when conditions become difficult, for example when there is stress on the community or when there is not enough food, the situation can be less friendly! Bacteria can become nasty and aggressive toward each other, and they even try to kill each other to get the limited resources or to have more space. You cannot imagine the intense war that is going on around you, and even inside you, as you read this!
The Weapons of Bacteria
To wage war on each other, bacteria have developed very sophisticated weapons (Figure 2a) . Surprisingly, these weapons are very similar to weapons invented by humans, except that bacteria have been inventing and perfecting them for millions of years. These are weapons of mass destruction! First, bacteria make chemical weapons: they produce antibiotics that kill other bacteria around them. You may know antibiotics as medicines the doctor prescribes when you have certain kinds of infections, but bacteria make antibiotics to fight each other, too. Bacteria can also construct “devices” that act like mines. These are called bacteriocins, and they are released into the environment, causing other bacteria that pass over them to explode. Antibiotics and bacteriocins can spread out in the environment and therefore can act at long distances.
Bacteria have also evolved weapons that are activated when another bacterium gets too close. One of them is called contact-dependent growth inhibition (CDI): this is kind of like a stick with poison on the tip. When this poison stick touches enemy bacteria that get too close, it kills them.
A Sophisticated Weapon: The Crossbow
The most impressive weapon is like a tiny crossbow that bacteria make inside themselves (Figure 2b), called the type 6 secretion system . This crossbow is only about 0.8 micrometers long, or about a million times smaller than a meter! How does this crossbow work? First, the bacterium makes an arrow that ends with a spike, and then it adds an extra sharp metallic point, which makes the arrow stronger. To propel the arrow, the bacterium builds a spring around it, in a stretched-out form. When the spring contracts, it propels the arrow toward the enemy.
But since the arrow is made inside the bacterium, how does it launch outside? To do this, the bacterium constructs a tunnel in its cell membrane, which works like a murder hole. A murder hole is a hole found in middle-age castle fortifications allowing to throw various objects against the attackers. The arrow can be propelled through this tunnel to kill the enemy! The hole created when the arrow penetrates the enemy is not enough to kill it, because the targeted bacterium will repair its damaged membrane very quickly—like when you put a bandage on a small wound. So, the attacking bacterium often adds poison to the end of the arrow, or sometimes within the arrow itself. You may have heard that the Amazon Indians used to put a poison called curare on the heads of their arrow for the same purpose. Bacteria have a very important collection of poisons—some of them can break down the membrane of the target bacterium, others can destroy its skeleton or its DNA, and others can prevent it from growing or dividing.
Recently, researchers have discovered a new weapon associated with this crossbow: a poison grenade! In this case, the poison that a bacterium attaches to the arrow is enclosed in a small barrel (Figure 2b) . Once inside the enemy bacterium, the two lids of the barrel are removed, and the poison comes out of the barrel and kills the enemy bacterium.
Bacteria Protect Themselves
Bacteria are not defenseless against these weapons—many have ways to protect themselves (Figure 3) . Bacteria that produce poisons often also make the antidotes to those poisons (Figure 3a). By doing so, they protect themselves from their own poison, but also from the poisons that could be directed against them by their sisters. Some bacteria have developed different tactics to protect themselves. For example, some will jealously guard all the recipes for antidotes that they find, allowing them to resist many poisons. Others surround themselves with a shield called a capsule (Figure 3b). The capsule is made of a slightly sticky material, in which the enemy’s arrows can become entangled. Another strategy used by bacteria is to organize themselves to form a cluster resembling a Roman testudo formation—a packed formation covered with shields on front and top that the Roman legions used during battles (Figure 3c). The bacteria on the outside take hits, but they protect the ones on the inside.
So, you can clearly see that bacteria are fearless warriors! They have developed an impressive military arsenal to fight each other: mines, crossbows, murder holes, shields, grenades, poisons, and antidotes! All of these weapons and defenses help bacteria to survive when conditions get tough. They have also invented weapons to fight against us and make us sick… but that is a story for another article!
Bacteria: ↑ One-celled microorganisms that can either be helpful to other organisms or cause diseases.
Microbiota: ↑ All the microorganisms that live in the same place, like in or on another organism (including humans).
Antibiotics: ↑ Small molecules that kill living cells, often used as medicines to treat bacterial infections.
Bacteriocins: ↑ Poisons released by bacteria that diffuse in the environment.
Contact-Dependent Growth Inhibition: ↑ A bacterial weapon disposed at the surface of the bacterium that poisons enemy bacteria by touching them.
Type 6 Secretion System: ↑ A bacterial weapon that can inject poisons into enemy bacteria.
Antidote: ↑ A substance that neutralizes a poison.
Capsule: ↑ A protective layer outside the bacterial cell membrane that protects against attacks, toxic compounds, and drying out.
Conflict of Interest
The author declares that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
Work on T6SS in the EC laboratory was supported by the Centre National de la Recherche Scientifique (CNRS), the Aix-Marseille Université (AMU), the Fondation pour la Recherche Médicale (DEQ201- 80339165), the Fondation Bettencourt-Schueller, and grant from the Agence Nationale de la Recherche (ANR-20-CE11-0017). The figures have been made with Biorender.
 ↑ Stubbendieck, R. M., Vargas-Bautista, C., Straight, P. D. 2016. Bacterial communities: interactions to scale. Front. Microbiol. 7:1234. doi: 10.3389/fmicb.2016.01234
 ↑ Chassaing, B., Cascales, E. 2018. Antibacterial weapons: targeted destruction in the microbiota. Trends Microbiol. 26:329–38. doi: 10.1016/j.tim.2018.01.006
 ↑ Cherrak, Y., Flaugnatti, N., Durand, E., Journet, L., Cascales, E. 2019. Structure and activity of the type VI secretion system. Microbiol. Spectr. 7:PSIB-0031-19. doi: 10.1128/microbiolspec.PSIB-0031-2019
 ↑ Jurenas, D., Talachia Rosa, L, Rey, M., Chamot-Rooke, J., Fronzes, R., Cascales, E. 2021. Mounting, structure and autocleavage of a type VI secretion-associated Rhs polymorphic toxin. Nat. Commun. 12:6998. doi: 10.1038/s41467-021-27388-0
 ↑ Robitaille, S., Trus, E., Ross, B. D. 2021. Bacterial defense against the type VI secretion system. Trends Microbiol. 29:187–90. doi: 10.1016/j.tim.2020.09.001