Proteus mirabilis

Proteus mirabilis creates some of the most striking patterns in microbiology: concentric rings of bacterial growth expanding across agar plates like ripples in a pond. This swarming behavior emerges from collective cell activity without the sophisticated developmental program of myxobacteria. When P. mirabilis contacts a surface, cells differentiate into elongated, hyperflagellated swarm cells that move together in coordinated rafts. Then they pause, consolidate, and swarm again—creating the characteristic ring patterns.

The swarming is driven by cell density. At high density, cells align and move together; at low density, they grow without swarming. The transition point creates the bulls-eye pattern: swarm-consolidate-swarm cycles driven by local density fluctuations. Unlike myxobacterial swarming, which requires direct cell contact, P. mirabilis swarms can coordinate through flagellar hydrodynamic interactions and chemical signals. Different mechanisms achieve visually similar collective behavior.

P. mirabilis swarming has clinical relevance. This bacterium causes urinary tract infections and is notorious for forming biofilms on urinary catheters. Swarming enables rapid surface colonization; the same collective motility that creates beautiful patterns on agar plates enables pathogenic colonization of medical devices. Understanding swarming mechanisms has informed catheter design and infection control. P. mirabilis demonstrates that bacterial collective behavior isn't confined to specialized social bacteria—many species show emergent group dynamics under appropriate conditions.