Final answer:
An individual bacterium responds to a signal during quorum sensing when a sufficient number of bacteria are present, and the concentration of signaling molecules, known as autoinducers, reaches a threshold level. This coordinated bacterial communication mechanism regulates gene expression and activates cellular functions that benefit the bacterial community.
Step-by-step explanation:
In bacterial responses regulated by quorum sensing, an individual bacterium will respond to a signal if a sufficient number of bacteria are present. This is because quorum sensing depends on the accumulation of signaling molecules called autoinducers, which are secreted by bacteria to communicate with each other.
When the concentration of these autoinducers in the environment reaches a threshold level, they bind to specific receptors on bacteria, leading to gene expression that triggers a coordinated response among the population.
Quorum sensing enables bacteria to detect their cell density through the release and binding of small, diffusible molecules like autoinducers. In the presence of a quorum, these autoinducers initiate a cascade of reactions that activate genes associated with cellular functions that are beneficial to the bacterial community, such as virulence factor expression, biofilm formation, or the production of bioluminescent proteins as seen in the symbiotic relationship with Hawaiian bobtail squid.
Gram-negative bacteria often use N-acylated homoserine lactones (AHL) for this communication, while gram-positive bacteria mostly use small peptides for signaling. Regardless of the types of molecules, once a bacterium senses a high enough concentration of these signals, a positive feedback loop is triggered, resulting in amplification of the quorum sensing response.