Little, K. | Search bioRxiv

bioRxiv2018

Swarming bacteria respond to increasing barriers to motility by increasing cell length and modifying colony structure

Organisms can alter morphology and behaviors in response to environmental stimuli such as mechanical forces exerted by surface conditions. The bacterium Proteus mirabilis responds to surface-based growth by enhancing cell length and degree of cell-cell interactions. Cells grow as approximately 2-micrometer rigid rods and independently swim in liquid. By contrast on hard agar surfaces, cells elongate up to 40-fold into snake-like cells that move as a collective group across the surface. Here we have elucidated that individual cell size and degree of cell-cell interactions increased across a continuous gradient that correlates with increasing agar density. We further demonstrate that interactions between the lipopolysaccharide (LPS) component of the outer membrane and the immediate local environment modified these responses by reducing agar-associated barriers to motility. Loss of LPS structures corresponded with increased cell elongation on any given surface. These micrometer-scale changes to cell shape and collective interactions corresponded with centimeter-scale changes in the overall visible structure of the swarm colony. It is well-appreciated in eukaryotes that mechanical forces impact cell shape and migration. Here we propose that bacteria can also dynamically respond to the mechanical forces of surface conditions by altering cell shape, individual motility, and collective migration.

microbiology↗

bioRxiv2017

The Proteus mirabilis rffG gene is essential for swarm-associated cell elongation through an Rcs-mediated pathway

Individual cells of the bacterium Proteus mirabilis can elongate up to 40-fold on surfaces before engaging in a cooperative surface-based motility termed swarming. How cells regulate this dramatic morphological remodeling remains an open question. In this paper, we move forward the understanding of this regulation by demonstrating that P. mirabilis requires the gene rffG for swarmer cell elongation and subsequent swarm motility. The rffG gene encodes a protein homologous to the dTDP-glucose 4,6 dehydratase protein of Escherichia coli, which contributes to Enterobacterial Common Antigen biosynthesis. Here we characterize the rffG gene in P. mirabilis, demonstrating that it is required for the production of large lipopolysaccharide-linked moieties necessary for wild-type cell envelope integrity. We show that absence of the rffG gene induces several stress-responsive pathways including those controlled by the transcriptional regulators RpoS, CaiF, and RcsB. We further show that in rffG-deficient cells, suppression of the Rcs phosphorelay, via loss of RcsB, is sufficient to induce cell elongation and swarm motility. However, loss of RcsB does not rescue cell envelope integrity defects and instead results in abnormally shaped cells, including cells producing more than two poles. We conclude that a RcsB-mediated response acts to suppress emergence of shape defects in cell envelope-compromised cells, suggesting an additional role for RcsB in maintaining cell morphology under stress conditions. We further propose that the composition of the cell envelope acts as a checkpoint before cells initiate swarmer cell elongation and motility.\n\nImportance statementP. mirabilis swarm motility has been implicated in pathogenesis. We have found that cells deploy multiple uncharacterized strategies to handle cell envelope stress beyond the Rcs phosphorelay when attempting to engage in swarm motility. While RcsB is known to directly inhibit the master transcriptional regulator for swarming, we have shown an additional role for RcsB in protecting cell morphology. These data support a growing appreciation that the Rcs phosphorelay is a multi-functional regulator of cell morphology in addition to its role in microbial stress responses. These data also strengthen the paradigm that outer membrane composition is a crucial checkpoint for modulating entry into swarm motility. Furthermore, the rffG-dependent moieties provide a novel, attractive target for potential antimicrobials.

microbiology↗