Cell differentiation of Proteus mirabilis is initiated by glutamine, a specific chemoattractant for swarming cells

Abstract

Swarming by Proteus mirabilis involves differentiation of typical short vegetative rods into filamentous hyper-flagellated swarm cells which undergo cycles of rapid and co-ordinated population migration across surfaces and exhibit high levels of virulence gene expression. By supplementing a minimal growth medium (MGM) unable to support swarming migration we identified a single amino acid, glutamine, as sufficient to signal initiation of cell differentiation and migration. Bacteria isolated from the migrating edge of colonies grown for 8h with glutamine as the only amino acid were filamentous and synthesized the characteristic high levels of flagellin and haemolysin. In contrast, addition of the other 19 common amino acids (excluding glutamine) individually or in combination did not initiate differentiation even after 24 h, cells remaining typical vegetative rods with basal levels of haemolysin and flagellin. The glutamine analogue γ-glutamyl hydroxamate (GH) inhibited swarming but not growth of P. mirabilis on glutamine MGM and transposon mutants defective in glutamine uptake retained their response to glutamine signalling and its inhibition by GH, suggesting that differentiation signalling by glutamine may be transduced independently of the cellular glutamine transport system. Levels of mRNA transcribed from the haemolysin (hpmA) and flagellin (fliC) genes were low in vegetative cells grown on MGM without glutamine or with glutamine and GH, but were specifically increased c. 40-fold during glutamine-dependent differentiation. In liquid glutamine—MGM cultures, differentiation to filamentous hyper-flagellated hyper-haemorytic swarm cells occurred early in the exponential phase of growth, and increased concomitantly with the concentration of glutamine from a 0.1 mM threshold up to 10 mM. Differentiation in liquid culture was completely inhibited by GH but was further stimulated c. 30% in the absence of GH by the viscosity agent polyvinylpy-rollidone (PVP). Chemotaxis assays of bacterial cells in which the viscosity of liquid media was varied by PVP to allow either swimming or swarming motility demonstrated that glutamine was chemoattractive specifically to differentiated swarming cells.