An Investigation of the Molecular Mechanism of Identity Communication in the Social Bacterium Proteus Mirabilis

Abstract

Multicellular life very often depends on the ability of individual cells to communicate with one another. Even unicellular organisms with social, multicellular life stages rely on the ability to communicate and identify their kin so that common goods or altruistic behaviors are not exploited by cheaters. A common mechanism by which social microbes, such as Dictyostelium discoideum or Myxococcus xanthus, communicate their identity to neighboring cells relies on surface-exposed adhesion proteins that undergo kin-specific binding. The Gram-negative bacterium Proteus mirabilis, however, uses a different mechanism of identity communication. It was previously known that the six-gene ids operon is required for self recognition in this bacterium and that several of the Ids proteins (IdsA, IdsB and IdsD) are exported into the extracellular space in a type six secretion system (T6SS)-dependent manner. It was also previously known that IdsD and IdsE contain strain-specific variable regions that might act as a barcode for self identity. Using genetic, biochemical and microscopy approaches, I tested the hypothesis that IdsD is exchanged between cells in a T6SS-dependent manner and that binding to a cognate IdsE in the recipient cell causes self recognition, while lack of binding causes nonself recognition. In addition to providing data supporting this model, I show that self recognition allows for efficient swarm expansion, which is a flagella-based, social behavior typical for P. mirabilis. In contrast, nonself recognition impairs swarm expansion. Throughout the course of this work, I have identified several single point mutations in T6SS-associated proteins that proved to be useful tools for studying Ids-mediated self recognition in P. mirabilis. Overall, this work has contributed to the concerted efforts of the Gibbs lab to uncover the molecular mechanism by which the bacterium P. mirabilis achieves self recognition. In contrast to microbes which use surface-exposed identity proteins, P. mirabilis communicates identity by the transfer of a recognition protein directly from one cell to another. What is more, I have shown that Ids-mediated self/nonself recognition controls swarm expansion in P. mirabilis adding to the list of factors that control this social behavior.