We determine and relate the characteristic velocity, length, and time scales for bacterial motion in swarming colonies of Paenibacillus dendritiformis growing on semi-solid agar substrates. The bacteria swim within a thin fluid layer, and they form long-lived jets and vortices. These coherent structures lead to anisotropy in velocity spatial correlations and to a two-step relaxation in velocity temporal correlations. The mean squared displacement of passive tracers exhibits a short-time regime with nearly ballistic transport and a diffusive long-time regime. We find that various definitions of the correlation length all lead to length scales that are, surprisingly, essentially independent of the mean bacterial speed, while the correlation time is linearly proportional to the ratio of the correlation length to the mean speed.
Most research on growing bacterial colonies on agar plates has concerned the effect of genetic or morphotype variation. Some studies have indicated that there is a correlation between microscopic bacterial motion and macroscopic colonial expansion, especially for swarming strains, but no measurements have been obtained for a single strain to relate the microscopic scale to the macroscopic scale. We examined here a single strain (Paenibacillus dendritiformis type T; tip splitting) to determine both the macroscopic growth of colonies and the microscopic bacterial motion within the colonies. Our multiscale measurements for a variety of growth conditions revealed that motion on the microscopic scale and colonial growth are largely independent. Instead, the growth of the colony is strongly affected by the availability of a surfactant that reduces surface tension.Bacteria are able to colonize many different surfaces through collective behavior such as swarming and biofilm formation. Studies of such behavior (10,18,26,31) have revealed cooperative phenomena on both microscopic and colonial scales (4,5,7,8,20), including production of extracellular "lubricant-wetting" fluid for movement on medium and hard surfaces (19,22,25), chemical signaling such as quorum sensing and chemotactic signaling (1,12,27), and the secretion of inhibiting and killing factors (2,9,11,14,15,17).Research has suggested possible links between the microscopic behavior of a colony and the rate at which the colony expands (12,23,24,29). For Pseudomonas aeruginosa, increased reversal rates for flagella lead to hyperswarming (a larger colony) (26). Similar flagellar modulation affects Escherichia coli (32); if the bacteria never tumble (flagella rotate only counterclockwise) or only tumble (flagella rotate only clockwise), the final colony is much smaller than a colony formed when the bacteria both swim and tumble. For Rhizobium etli, a correlation has been observed between microscopic swarming motion and expansion of the colony, and an acylhomoserine lactone molecule has been found to be a swarming regulator, as well as a biosurfactant that controls surface activity (12). These studies suggest that there is a correlation between microscopic activity and colonial expansion; however, a mutation may be pleiotropic, affecting both motility and surfactant production. Further, there may be additional, unidentified differences between mutant and wild-type strains. For example, the failure of Bacillus subtilis laboratory strains to swarm is caused by a mutation in a gene (sfp) needed for surfactin synthesis and a mutation(s) in an additional unknown gene(s) (21). Experiments that avoid this ambiguity by studying the response of a single strain exposed to changing physical environments have not been performed. Further, except for measurements of the size of an expanding colony as a function of time (3, 6), no detailed time development studies of a growing bacterial colony have been reported.Here we exposed a single bacterial strain, Paenibacillus dendritifor...
Significant efforts exist to develop living/non‐living composite materials—known as biohybrids—that can support and control the functionality of biological agents. To enable the production of broadly applicable biohybrid materials, new tools are required to improve replicability, scalability, and control. Here, the Hybrid Living Material (HLM) fabrication platform is presented, which integrates computational design, additive manufacturing, and synthetic biology to achieve replicable fabrication and control of biohybrids. The approach involves modification of multimaterial 3D‐printer descriptions to control the distribution of chemical signals within printed objects, and subsequent addition of hydrogel to object surfaces to immobilize engineered Escherichia coli and facilitate material‐driven chemical signaling. As a result, the platform demonstrates predictable, repeatable spatial control of protein expression across the surfaces of 3D‐printed objects. Custom‐developed orthogonal signaling resins and gene circuits enable multiplexed expression patterns. The platform also demonstrates a computational model of interaction between digitally controlled material distribution and genetic regulatory responses across 3D surfaces, providing a digital tool for HLM design and validation. Thus, the HLM approach produces biohybrid materials of wearable‐scale, self‐supporting 3D structure, and programmable biological surfaces that are replicable and customizable, thereby unlocking paths to apply industrial modeling and fabrication methods toward the design of living materials.
In experimental evolution, scientists evolve organisms in the lab, typically by challenging them to new environmental conditions. How best to evolve a desired trait? Should the challenge be applied abruptly, gradually, periodically, sporadically? Should one apply chemical mutagenesis, and do strains with high innate mutation rate evolve faster? What are ideal population sizes of evolving populations? There are endless strategies, beyond those that can be exposed by individual labs. We therefore arranged a community challenge, Evolthon, in which students and scientists from different labs were asked to evolve Escherichia coli or Saccharomyces cerevisiae for an abiotic stress—low temperature. About 30 participants from around the world explored diverse environmental and genetic regimes of evolution. After a period of evolution in each lab, all strains of each species were competed with one another. In yeast, the most successful strategies were those that used mating, underscoring the importance of sex in evolution. In bacteria, the fittest strain used a strategy based on exploration of different mutation rates. Different strategies displayed variable levels of performance and stability across additional challenges and conditions. This study therefore uncovers principles of effective experimental evolutionary regimens and might prove useful also for biotechnological developments of new strains and for understanding natural strategies in evolutionary arms races between species. Evolthon constitutes a model for community-based scientific exploration that encourages creativity and cooperation.
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