Analytic model for ring pattern formation by bacterial swarmers

Patterning of mutually interacting bacterial bodies: close contacts and airborne signals

Background

Bacterial bodies (colonies) can develop complex patterns of color and structure. These patterns may arise as a result of both colony-autonomous developmental and regulatory processes (self-patterning) and environmental influences, including those generated by neighbor bodies. We have studied the interplay of intra-colony signaling (self-patterning) and inter-colony influences in related clones of Serratia rubidaea grown on rich media.

Results

Colonies are shaped by both autonomous patterning and by signals generated by co-habitants of the morphogenetic space, mediating both internal shaping of the body, and communication between bodies sharing the same living space. The result of development is affected by the overall distribution of neighbors in the dish. The neighbors' presence is communicated via at least two putative signals, while additional signals may be involved in generating some unusual patterns observed upon encounters of different clones. A formal model accounting for some aspects of colony morphogenesis and inter-colony interactions is proposed.

Conclusions

The complex patterns of color and texture observed in Serratia rubidaea colonies may be based on at least two signals produced by cells, one of them diffusing through the substrate (agar) and the other carried by a volatile compound and absorbed into the substrate. Differences between clones with regard to the interpretation of signals may result from different sensitivity to signal threshold(s).

Regulation of gene expression during swarmer cell differentiation in Proteus mirabilis

The gram-negative bacterium Proteus mirabilis can exist in either of two cell types, a vegetative cell characterized as a short rod and a highly elongated and hyperflagellated swarmer cell. This differentiation is triggered by growth on solid surfaces and multiple inputs are sensed by the cell to initiate the differentiation process. These include the inhibition of flagellar rotation, the accumulation of extracellular putrescine and O-antigen interactions with a surface. A key event in the differentiation process is the upregulation of FlhD(2)C(2), which activates the flagellar regulon and additional genes required for differentiation. There are a number of genes that influence FlhD(2)C(2) expression and the function of these genes, if known, will be discussed in this review. Additional genes that have been shown to regulate gene expression during swarming will also be reviewed. Although P. mirabilis represents an excellent system to study microbial differentiation, it is largely understudied relative to other systems. Therefore, this review will also discuss some of the unanswered questions that are central to understanding this process in P. mirabilis.

Growth dynamics of Bacillus circulans colony

We have investigated the growth dynamics of Bacillus circulans colony exhibiting the knotted-branching pattern by swarming on a hard agar medium. The knotted-branching pattern consists of many circular clusters, so-called subcolonies, and their trajectories. We analysed the processes of a subcolony because they are presumably the key elements for the formation of knotted-branching pattern. It was found that a subcolony has three processes, i.e. "generation", "growth", and "migration" by microscopic and time-resolved observations. An embryonic small subcolony (child subcolony) formed around an existing subcolony (parent subcolony) grows larger and migrates away from the parent subcolony. We proposed a simple model to explain the migration and the growth processes. It is assumed that the internal part of the subcolony is unfavorable for the bacteria and that the motion of the child subcolony on the agar medium can be modeled using a frictional force. The experimental data were quantitatively analysed in order to compare with models. Our models are consistent with the experimental results on following three points: (1) the radius of a subcolony increases linearly with the incubation time, (2) a subcolony stops just after formation and then starts to migrate suddenly, and (3) the trajectory of a subcolony predicted by the model agrees with the experimental one.

Self-organized pattern formation of a bacteria colony modeled by a reaction diffusion system and nucleation theory

Self-organized pattern formation is observed in bacterial colony growth. The recently reported knotted-branching pattern of the Bacillus circulans colony consists of the trajectories of aggregates which grow, move, and reproduce simultaneously. We modeled these processes by combining a reaction diffusion system of nutrient dynamics, nucleation theory for aggregate generation, and individual based dynamics of motion and growth of aggregates. The branching pattern produced by computer simulation shows great similarity with experiments. Response to the initial nutrient concentration is also consistent with the experiments.