autoinducer

Autoinducers are small signaling molecules used in quorum sensing, a cell-density–dependent communication process by which bacteria coordinate gene expression across populations. As local cell numbers rise, the concentration of autoinducers increases, allowing coordinated responses such as changes in metabolism, virulence, and biofilm formation.

Several major families of autoinducers are known. In many Gram-negative bacteria, autoinducer-1 (AI-1) refers to acyl-homoserine lactones (AHLs), which are synthesized by LuxI-type enzymes and detected by LuxR-type transcriptional regulators. Autoinducer-2 (AI-2) is a furanosyl borate diester produced by LuxS and is activated in a broad range of species, contributing to interspecies communication in mixed communities. Gram-positive bacteria often use peptide-based autoinducers that are detected by two-component signaling systems or membrane-bound histidine kinases, leading to phosphorylation cascades that alter transcription.

Mechanistically, autoinducers diffuse across cell membranes or are actively transported, and accumulate in the environment as cells grow. When bound to their cognate receptors, these molecules modulate gene expression, turning on or off suites of genes related to luminescence, metabolism, virulence factor production, competence, and biofilm development. The system enables populations to synchronize behavior in response to environmental conditions and population structure.

Applications and relevance include the study of microbial ecology, development of anti-virulence strategies that disrupt quorum sensing, and potential industrial control of biofilms. While AI-2 suggests cross-species signaling, the extent and significance of interspecies quorum sensing remain active areas of research.