Functional Roles of Non-membrane Lipids in Bacterial Signaling

Abstract

Bacteria produce some lipids and lipid-related compounds that function as signals for intercellular communication among prokaryotes or even in inter-kingdom communication (i.e., between prokaryotes and eukaryotes). Most of these lipidic signals participate in quorum-sensing regulation, a process that is dependent on cell density and enables a coordinated response within the population. The number and variety of bacterial non-membrane lipids that have been found to function as molecular signals is increasing and includes unsaturated fatty acids, fatty acid esters, acyl-based molecules such as N-acylhomoserine lactones and γ-butyrolactones, alkyl-based compounds such as quinolones and dialkylresorcinols, or alkane-derived signals such as α-hydroxyketones. Most of these signals are amphipathic and can diffuse through membranes and some of them are volatile. They are synthesized from common metabolites including intermediates of lipid metabolism and are recognized by membrane-bound or cytosolic receptors that trigger specific signal transduction responses. These signals regulate important bacterial traits such as motility, production of antimicrobials, expression of virulence factors, and biofilm formation. Some of these bacterial lipids induce immune responses in eukaryotic organisms.