Final answer:
Quorum sensing is initiated when the population density of bacteria increases, leading to a higher concentration of signaling molecules, known as autoinducers, which in turn bind to receptors and trigger gene expression changes.
Step-by-step explanation:
Quorum sensing is a biological process by which bacteria communicate and synchronize group behaviors based on their population density. This process is triggered by the presence of signaling molecules known as autoinducers. When the population density of bacteria reaches a certain threshold, the concentration of autoinducers in the environment increases to a level where they can bind to specific receptors on bacterial cells.
This binding triggers a signaling cascade that leads to altered gene expression and coordinated behaviors among the bacterial population. Such behaviors could include bioluminescence, as seen in symbiotic bacteria with the Hawaiian bobtail squid, or pathogenic activities such as the production of virulence factors in enough cells to overcome hosts' immune defenses. The key factor initiating quorum sensing is therefore a high population density leading to a sufficient concentration of autoinducers.
Answer to the provided question is: a) Presence of signaling molecules