‘Quorum sensing’ in populations of microorganisms is a collective control
phenomenon that may account for the emergence of multi-celled orders of organisms from populations of single-celled microorganisms.
“Quorum sensing is the regulation of gene expression in response to fluctuations in cell-population density.”
For the bacteria to use quorum sensing constitutively, they must possess three abilities: secretion of a signaling molecule, secretion of an [autoinducer](https://en.wikipedia.org/wiki/Autoinducer) (to detect the change in concentration of signaling molecules), and regulation of gene transcription as a response.
During their reproductive cycle, individual bacteri[a] synthesize autoinducers. … autoinducers move out of individual cells as they are produced. Since the bacteria are reproducing, there are progressively more individual cells producing autoinducers and the extracellular concentration of the autoinducers increases, eventually hitting a "critical mass." That threshold makes it energetically unfavorable for intracellular autoinducers to continue to leave the cell …, resulting in an increase in their intracellular concentration. Once intracellular concentration increases, autoinducers bind to their receptors, triggering signaling cascades that alter transcription factor activity and therefore, gene expression. For many bacteria, the change in gene expression includes downregulation of autoinducer synthesis in a negative feedback loop.
Wikipedia gives examples in social insects and in robotics.
Humans have become effective communicators during our short time on Earth. However, our communication pales in comparison to bacteria. Quorum sensing using autoinducers allows bacteria to communicate within and between species. With the latter, they can either compete or collaborate with other species based on the autoinducer "message" they receive. With our many different languages, cultures and nuances, humans have a long way to go before our communication structure rivals bacteria.
The relevance to many aspects of ecology requires only different means of perception and different means of affecting environmental variables that may become collectively controlled for communicative purposes. A recently discussed example is herbivore-induced plant volatiles.