Animals are not the only ones that communicate within their groups. Research done in the 1960s and 70s found that specific bacteria have a unique characteristic called quorum-sensing. Most research on these bacteria was eventually discontinued, but advances in DNA sequencing made it quicker, more accurate, and less expensive. DNA sequencing has spurred a new crusade in quorum-sensing bacteria.
Quorum-sensing is a mode of intra- and interspecies bacterial communication that allows bacteria to monitor their environment and alter colony behaviors accordingly.
This mode of bacterial communication is by autoinducers, which are a type of small signaling molecules that are released into the local environment of the bacterial colony. Individual bacteria secrete these autoinducers, which act like chemical words, but the signals are too weak because they are diluted in the extracellular environment. But when the bacteria proliferate together, by residing in colonies and secreting the signal simultaneously, it becomes more concentrated and more responsive. Think of a smoke signal compared to hundreds of smoke signals.
Autoinducers bind to receptors that have very specific geometrical, chemical, and electrical characteristics that complement each other. Receptors act as the ears of the bacteria. Once the signaling molecule and receptor bind, genes that control group behaviors are expressed through signal communications within the cell. Interestingly, bacteria do this in unison. Genes are turned on to control a variety of bacterial behavior such as bioluminescence (emission of light from the organism, think glowy jellyfish), virulence (how harmful a disease is), antibiotic production, and motility (mobility). Essentially, bacteria use chemical words to communicate with each other which are heard by other bacteria to indicate others to synchronize the performance of a task. They use a fine-tuned chemical language.
This raises the question of why is quorum sensing important and how did it evolve? Interesting studies have been conducted on quorum sensing in a human pathogen called Pseudomonas aeruginosa, a highly virulent pathogen that also infects plants and animals. Quorum-sensing in P. aeruginosa galvanizes genes that secrete toxins, an extracellular protease and cyanide, as well as other extracellular molecules aptly named “public goods.” “Public goods” are expressed and shared amongst the individuals of the colony and are essential resources for survival. Quorum-sensing allows for a group to work efficiently as a collective. This is commonly seen in animals (e.g. wolf packs) and insects (e.g. bee and ant colonies), but is a new discovery in bacterial colonies.
Evolutionarily, this seems futile as it takes energy to create “public goods,” and can generate free-riders (essentially mutants that have lost their quorum-sensing machinery and no longer produce “public goods”). This is Darwin’s dilemma: there is a cost to cooperate with the community and thus, a cheater will have a fitness advantage — a classic prisoner’s dilemma. However, cheater colonies go unmarked; they are kept in check by the greater population through a system called policing (sound familiar?). Colony members make toxins coupled to quorum-sensing that are fatal to free-riders because they do not make the standard immunity factor. If cells do not cooperate and help with the greater good of the group they are simply killed off.
Through quorum-sensing, bacteria have a common goal of surviving and proliferating, they share resources in order to grow as a community. Further research on quorum-sensing is important in understanding how to manipulate and mitigate disease caused by different human pathogens. By learning about the mechanics of the system, drugs can mimic the chemical words (autoinducers) and bind to the ear (receptor) of the bacteria but not register with it as to prevent infection. This would help with the worldwide fight against antibiotic-resistant bacteria.
But not all bacteria are bad; there are a lot of bacteria that aid animal development. In fact, there are more bacterial cells inside the human gastrointestinal system and on our skin then our own human cells. A further understanding of quorum sensing will allow us to influence good bacteria to do further benefit the human body (e.g. develop essential vitamins or educate the human body’s immune system). We can learn how different bacterial species communicate with each other. Scientists have witnessed different species of bacteria acting as spies and give reports on free-rider mutants in other colonies to the greater community. So, bacteria seem to be “multilingual”; they have one very specific language that they use to communicate with their siblings (intraspecies communication), and a second generic language used to speak with bacteria of other species (interspecies communication).
Quorum-sensing research will also give us a glimpse into the evolution of multicellular socialism and how evolution favored simple collective social behaviors. Of course, the discovery of social behavior is not new by any means but is amongst the oldest inhabitants of this world: bacteria and archaea.
We aren’t too different after all.