Characterization of Changes to the Cell Surface during the Life Cycle of Streptomyces coelicolor : Atomic Force Microscopy of Living Cells

Abstract

Cell surface changes that accompany the complex life cycle of Streptomyces coelicolor were monitored by atomic force microscopy (AFM) of living cells. Images were obtained using tapping mode to reveal that young, branching vegetative hyphae have a relatively smooth surface and are attached to an inert silica surface by means of a secreted extracellular matrix. Older hyphae, representing a transition between substrate and aerial growth, are sparsely decorated with fibers. Previously, a well-organized stable mosaic of fibers, called the rodlet layer, coating the surface of spores has been observed using electron microscopy. AFM revealed that aerial hyphae, prior to sporulation, possess a relatively unstable dense heterogeneous fibrous layer. Material from this layer is shed as the hyphae mature, revealing a more tightly organized fibrous mosaic layer typical of spores. The aerial hyphae are also characterized by the absence of the secreted extracellular matrix. The formation of sporulation septa is accompanied by modification to the surface layer, which undergoes localized temporary disruption at the sites of cell division. The characteristics of the hyphal surfaces of mutants show how various chaplin and rodlin proteins contribute to the formation of fibrous layers of differing stabilities. Finally, older spores with a compact rodlet layer develop surface concavities that are attributed to a reduction of intracellular turgor pressure as metabolic activity slows.