Currently, there are three well-defined classes of molecules
that serve as the paradigms for chemical signalling in bacte-
ria: oligopeptides, acylated homoserine lactones (AHLs) and the
LuxS/autoinducer-2 (AI-2) class [11]. These three systems will be
considered from an evolutionary perspective, in particular in terms
of the two key components of communication: the cost associated
with signalling and information specificity. Costs are primarily the
metabolic burden that is associated with signal production. The
specificity of the signal produced is in part a measure of the infor-
mation content of the signal, and specificity generally correlates
with the cost of production