Autoinducers are signaling molecules that are produced in response to changes in cell-population density. As the density of quorum sensing bacterial cells increases so does the concentration of the autoinducer. Detection of signal molecules by bacteria acts as stimulation which leads to altered gene expression once the minimal threshold is reached.
In some types of bacteria, the secreted autoinducers are small, hydrophobic molecules such as acyl-homoserine lactone (AHL). AHL is the autoinducer made by A. fischeri, the bacteria that occupy a squid’s light organ. In other types of bacteria, the autoinducer may instead be peptides (short proteins) or other types of small molecules.
Because AHL is small and hydrophobic, it can diffuse freely across the membranes of the bacteria cells.
- When there are few cells in the area, the little AHL that’s made will diffuse into the environment, and the levels of AHL inside the cells will remain low.
- When more bacteria are present, a larger amount of AHL will be produced (thanks to the greater number of contributors).
- If AHL levels get high enough, indicating a critical density of bacteria, the AHL will bind to and activate a receptor protein inside the cells.
- The active receptor acts as a transcription factor, attaching to specific sites on the bacterium’s DNA and changing the activity of nearby target genes.