AbstractBacterial homologous lysine and arginine decarboxylases play major roles in the acid stress response, physiology, antibiotic resistance and virulence. The Escherichia coli enzymes are considered as their archetypes. Whereas acid stress triggers polymerisation of the E. coli lysine decarboxylase LdcI, such behaviour has not been observed for the arginine decarboxylase Adc. Here we show that the Adc from a multidrug-resistant human pathogen Providencia stuartii massively polymerises into filaments whose cryo-EM structure reveals pronounced differences between Adc and LdcI assembly mechanisms. While the structural determinants of Adc polymerisation are conserved only in certain Providencia and Burkholderia species, acid stress-induced polymerisation of LdcI appears general for enterobacteria. Analysis of the expression, activity and oligomerisation of the P. stuartii Adc further highlights the distinct properties of this unusual protein and lays a platform for future investigation of the role of supramolecular assembly in the superfamily or arginine and lysine decarboxylases.