“Thus, principles of evolution are dramatically exhibited by studying strains of Proteus on a solid medium, which contains a source of nutrients and oxidizable organic compounds. If the descendants of an individual are to compete successfully in this defined habitat, they must have evolved: (i) highly efficient enzyme catalysts to release and trap free energy, (ii) mobile mechanisms to access the site where more free energy is available, and (iii) mechanisms to stake out a claim on free energy—territoriality, one of the fundamental principles of ecology. Microbes did it first.” ---Ralph S. Wolfe1 Members of the bacterial species Proteus mirabilis are capable of rapid swarming over a hard agar surface. Swarming colonies display a striking phenotype in which a visible boundary forms between swarms of different strains. In contrast, boundaries do not arise between two swarms of a single strain (Figure 1). This behavior is a demonstration that P. mirabilis swarms are capable of territoriality and self versus non-self recognition. Though first described over 70 years ago, the molecular mechanisms underlying boundary formation in P. mirabilis have remained stubbornly elusive despite the combined efforts of several research groups. In this article we attempt to give a concise review of the historical and current models for Proteus boundary formation. We begin with a brief background on the genus Proteus and specifically the species Proteus mirabilis, followed by a review of swarm boundary formation. We then analyze the two prominent models for boundary formation: (1) preemptive antagonism and (2) recognition. We conclude with the broader implications of P. mirabilis territoriality and self vs non-self recognition. Figure 1 Boundary between P. mirabilis strains 2. The genus Proteus 2.1 General information about the ecology of Proteus The genus Proteus is a member of the Enterobacteriaceae family. Proteus can be found in soil and contaminated water, and for some Proteus species, in the intestinal tract of animals. In soil and water, and perhaps healthy animals, Proteus species are likely involved in the degradation of organic material. These bacteria are noted for their ability to utilize urea as a source of nitrogen and energy2. The two Proteus species most commonly found in humans are P. mirabilis and to a lesser extent P. vulgaris3. Jacobsen et al. and Manos et al. recently reviewed Proteus pathogenesis in detail2,3. In brief, P. mirabilis is an opportunistic pathogen that causes complicated urinary tract infections, especially in people requiring long-term indwelling catheterization4. In hospital and nursing home settings, Proteus species can also cause infections in the ear, nose, throat, and skin, among others, especially immunocompromised individuals. Infections at multiple sites in a single patient are caused by single P. mirabilis strains; P. mirabilis strains isolated from the catheter, urine, feces and kidney stones of a single patient are often the same strain, suggesting that P. mirabilis infections are clonal5–7.
K. Gibbs, E. Greenberg