Bacteria as ink for surface patterning on agar

Bacterial cells grow and reproduce rapidly when placed on rich medium, resulting in the generation of colonies with defined shape, size and color. Size of bacterial colony is naturally dependent on duration of growth and thus numbers of cells generated. On the other hand, capability of moving on agar surface in a collective manner, termed swarming motility, would necessarily serve as an impediment to the formation of well defined spatially resolved patterns useful for inquiring the importance of distance mediated processes such as cell to cell signaling and metabolic exchanges between isolated microbial communities. Such understanding would help lend a lens to the inner division of labor in biofilms as well as provide the design principles for the emerging area of synthetic microbial community, which has useful applications in biotechnology and environmental remediation. Specifically, disparate species of microbes with differing utility could be assembled into a collective functional whole capable of spatially defined division of labor, while keeping harmony between microbes at the aggregate level. Hence, using the multiplication of cell numbers and secretion of a color pigment to give form and color contrast to a pattern, a spread plate inoculation technique was explored as a method for coarse patterning of microbial cells on agar. Success of the “Bacterial calligraphy” technique where the inoculation loop and bacterial cells served as “pen” and “ink,” respectively, hinged on the motility mode of the bacterium, as well as color intensity and diffusivity of the secreted pigment. Specifically, poor pattern reproduction would result from bacterial species capable of swarming motility (i.e., coordinated movement of cells away from the point of deposition) or pigments that diffuse into the surrounding agar. Collectively, utilizing bacterial cell growth and pigment secretion for conferring form to desired patterns, a simple and low cost manual surface patterning technique was shown to be useful for inscribing both straight and curvilinear lines on agar, which, in addition to the fun activity of calligraphy, also holds potential for preliminary testing of methodological concepts in research. The original article, “Bacterial Calligraphy: A Memento for Undergraduate Research Students,” is published in the Journal of Microbiology and Biology Education, http://www.asmscience.org/content/journal/jmbe/10.1128/jmbe.v13i2.414. Additionally, a synopsis describing the same work from a scientific perspective highlighting the utility of depositing bacterial cells as defined spatial patterns on agar for understanding distance dependent ecological interaction mechanisms in the natural environment mimic is available on PeerJ Preprints at: https://peerj.com/preprints/349