Biosignalling is a crucial cellular process which enables the coordination and regulation of metabolism in all three domains of life. An extracellular signal binds to a membrane receptor and triggers a conformational change on its cytosolic side, which induces the activation of an enzymatic cascade and an appropriate cellular response. The proteinogenic amino acid L-arginine is an important bacterial signal, particularly in the induction of pathogenic bacteria virulence genes, with the protein ArgR's help. Specifically, L-arginine promotes the transition of Pseudomonas bacteria towards biofilm formation – a bacterial community that shares an extracellular matrix. In the intestinal pathogen enterohemorrhagic Escherichia coli, L-arginine serves as the activator of virulence gene expression and regulates the abundance of the enterocyte L-arginine transporter, thus controlling intestinal absorption of this amino acid. This suggests that L-arginine might play an important role in the eukaryotic defence against pathogenic bacteria, which has been confirmed in murine peritoneal macrophages. In particular, high concentrations of exogenous L-arginine significantly enhance the immune response of macrophages activated by lipopolysaccharides of Gram-negative bacteria. The conclusion is that L-arginine is an important factor in the molecular power struggle between bacterial pathogens and their hosts.