In this blog post, we’ll explore the principles of quorum sensing—a form of chemical communication among microorganisms—as well as its role in biofilm formation, pathogenicity, and potential industrial applications.
Have you ever heard of the deep-sea anglerfish? This species uses light-emitting antennae to attract and capture prey in the pitch-black depths of the ocean. Similarly, the cuttlefish is a bioluminescent creature. During the day, cuttlefish sink to the seafloor to feed, but at night, they rise to the surface to hunt. When they do so, their bodies glow, which prevents them from casting a shadow and helps them evade predators. They are able to do this because microorganisms live inside their bodies. But why do they glow only under specific conditions rather than constantly? This is possible because the microorganisms communicate with each other and regulate their behavior so that they emit light only when their population exceeds a certain threshold.
This communication among microorganisms is called “quorum sensing.” Quorum sensing occurs through autoinducers secreted by microorganisms. Microorganisms continuously secrete these substances at a constant rate; consequently, the more microorganisms there are in a given space, the higher the concentration of the substance becomes. This increased concentration of the substance enters the microbial cells, where it interferes with DNA expression, causing genes that were not originally expressed to be activated. There are several mechanisms by which quorum sensing regulates gene expression. One such mechanism involves the autoinducer entering the bacterial cell when its concentration rises; there, it binds to an inactivated response regulator, thereby activating it. The activated response regulator then binds to the DNA, leading to the expression of genes that were previously not being expressed. In other words, this mechanism allows microorganisms to detect an increase in their population and initiate new behaviors.
Quorum sensing can be observed in many phenomena besides bioluminescence in fish. You may have noticed a slippery substance—not soap—stuck to bathroom tiles. This is often a biofilm, a colony formed when microorganisms gather in one place. Biofilms consist of numerous microorganisms and EPS (extracellular polymeric substances), which are secreted by the microorganisms and act as a sort of adhesive, allowing them to gather in one place. The secretion of EPS is regulated by quorum sensing. Therefore, when the number of microorganisms is low, there is no substance to bind them together, so they cannot form a biofilm; however, once their numbers reach a certain threshold, they form a biofilm and live as a colony. Furthermore, it has been confirmed that pathogens also utilize quorum sensing. If a bacterium exhibits pathogenicity when its population is small, it may be eliminated by the immune system before it has developed sufficient capacity to attack the host. Therefore, pathogens only express the genes responsible for pathogenicity after reaching a sufficient bacterial density; as one might expect, this process also relies on quorum sensing.
Research on quorum sensing also has industrial applications. Understanding how communication between microorganisms occurs allows us to, conversely, interfere with that communication. In particular, in industries such as water treatment where microbial reactions are utilized, microorganisms are sometimes attached to specific membranes or plates to facilitate the reaction. However, when microorganisms form a biofilm in such cases, the microorganisms inside the biofilm find it difficult to participate in the reaction, thereby reducing reaction efficiency. Alternatively, biofilms can form inside pipes through which water—such as wastewater—flows; in this case, the biofilm obstructs the smooth flow of water, weakening the flow and potentially increasing the energy required to keep the water moving. If substances such as chlorine are added to remove these biofilms, it can shorten the lifespan of the reactor or pipes. Furthermore, when antibiotics are administered to kill microorganisms or inhibit their growth, the presence of a biofilm prevents the antibiotics from penetrating deep enough into the biofilm, making it impossible to achieve full effectiveness. However, if we can fully understand the mechanism of quorum sensing and interfere with the communication between microorganisms, we will be able to prevent the formation of biofilms. Yet, the mechanism of quorum sensing and the methods for interfering with it have not yet been fully elucidated, and therefore, research in this area is ongoing.
