Helical Bacillus subtilis macrofibers: Morphogenesis of a bacterial multicellular macroorganism

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Abstract
Helical bacterial macroorganisms were produced by the selection of appropriate B. subtilis mutants and the establishment of specific growth conditions. Threadlike fibers ranging in length to approximately 1 cm are produced in fluid culture by the parallel association of many division-suppressed filaments in helical arrangement. A more open ball-like structure of complicated woven architecture may also be produced. Macrostructure morphology is regulated by genetic, physiological and nutritional factors. The pitch angle of surface filaments in helical macrofibers varies as a function of macrofiber diameter, indicating a flexible response of individual cell surfaces to the forces responsible for helical morphology. Three classes of mutants were obtained that are concerned with helix directionality: mutants that form only left-handed helix macrofibers, mutants that form only right-handed helix macrofibers and conditional mutants able to form either left- or right-handed helix macrofibers depending upon nutritional environment. Aggregate structures containing both left- and right-handed macrofibers were obtained by coculturing appropriate mutants. Besides providing information on the organization of the bacterial cell surface, this new system offers unique and unusual opportunities to study cell-cell interactions, primitive morphogenesis and the properties of a multicellular bacterial form.