Autoinducers

Autoinducers are small signaling molecules used in quorum sensing to coordinate gene expression in microbial populations as they reach high cell densities. Bacteria produce autoinducers and release them into the surrounding environment; as the population grows, the accumulated signal is detected by specific receptors, triggering transcriptional changes once a threshold concentration is reached.

In Gram-negative bacteria, the primary autoinducers are N-acyl homoserine lactones (AHLs). AHLs are synthesized by LuxI-family enzymes and sensed by LuxR-type transcriptional regulators. When an AHL binds its receptor, target genes are activated or repressed, influencing processes such as bioluminescence, virulence factor production, and biofilm formation. A classic example is Vibrio fischeri, where AHL signaling controls light production.

In Gram-positive bacteria, autoinducers are often short peptides (autoinducing peptides, AIPs) detected by membrane-bound histidine kinases in two-component signal transduction systems, leading to phosphorylation cascades and altered gene expression. A second widely discussed signal is autoinducer-2 (AI-2), produced by the LuxS pathway and detected across many species, with proposed roles in interspecies communication.

Autoinducers can be degraded or inactivated by quorum-quenching enzymes, providing a natural means to modulate signaling. This concept is explored for developing anti-virulence therapies and for engineering synthetic biology circuits in which coordinated gene expression is controlled by quorum sensing.