Neil H. Mendelson

Co-Principal Investigator
Molecular and Cellular Biology

My research interests are directed towards understanding the constraints imposed upon biological systems by their physical and chemical environment. Four projects currently being pursued involve:

• Biophysical studies of bacterial macrofibers. Macrofibers are a multicellular form that self-assembles into a twisted multifilament fiber. The geometry of individual cell growth produces twisting motions that cause writhing and supercoiling in a reiterative process. Macrofibers have been used to model the physics of a twisting, elongating elastic filament. Mathematical models of twist and writhe related to fiber behavior have been produced. Current work involves studies of impediments to motions produced by touching of fibers to the floor of the growth vessel, and the behavior of fibers unable to twist at one end.



• Bacterial bioconvection patterns. In shallow solutions of growth medium swimming cells self-organize into complex patters of sinking and rising plumes driven by a convection process. Factors involved include cell swimming towards the surface, creation of fluid layers of high density above layers of lower density, and cell mass increase produced by growth. Our interests lie in a traveling stripe pattern that forms behind a front of cells that moves directionally towards nutrients. This system provides a new way to study the cooperative swimming behavior of bacterial cells and the fluid dynamic consequences of their motions.



• Cell swimming patterns in bacterial colonies. Bacterial colonies grown on wet agar surfaces contain motile cells whose motions are constrained by high cell density. Swimming patterns arise that are not seen in conventional fluid cultures and that appear to be based upon different rules than those that govern motility and chemotaxis. Whirls and jets become established that constantly interact, decay and are reformed. Motions that appears to be chaotic are in fact highly organized on a short time scale. Local patterns are themselves organized into a super pattern that persists beyond the life span of its individual sub-components. The super pattern appears to contribute to the development of colony form.



• The production of complex colony form. A bacterial colony consists of a population of cells grown on a substrate that are organized structurally and functionally. We study the development of colony structure and the nested patterns of gene expression that arise within colonies. The emphasis is on pattern regulation by chemical and physical inputs.

References

Mendelson, N.H., J.J. Thwaites, J.O. Kessler, and C. Li. 1995. Mechanics of bacterial macrofiber initiation. J. Bacteriol. 177:7060-7069.

Mendelson, N.H., and J. Lega. 1998. A complex pattern of traveling stripes is produced by swimming cells of Bacillus subtilis. J. Bacteriol. 180:3285-3294.

Mendelson, N.H., A. Bourque, K. Wilkening, K.R. Anderson, and J.C. Watkins. 1999. Organized cell swimming motions in Bacillus subtilis colonies: patterns of short-lived whirls and jets. J. Bacteriol 181 (in press).

Mendelson, N.H., and B. Salhi. 1996. Patterns of reporter gene expression in the phase diagram of Bacillus subtilis colony forms. J. Bacteriol. 178:1980-1989.