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A new ultrastructural method was used to study rounds of envelope synthesis that occur in Streptococcus faecalis in "growth zones" found between pairs of naturally occurring surface markers. The technique consists of producing three-dimensional reconstructions of these growth zones from the mathematical rotation, about a central axis, of measurements taken from central, longitudinal thin sections of cells. A cycle of exponential-phase envelope growth was then simulated by arranging a series of these reconstructions in increasing order of the amount of peripheral wall surface area or the amount of cell volume that each was calculated to contain. Using this simulated cycle of growth, the geometry of a single growth zone during a round of synthesis was studied. Based on this analysis, a model was developed for the assembly of the cell wall of S. faecalis. The model states that new cell wall surface is synthesized by the regulated flow of essentially two channels of cell wall precursors into a single growth zone. One channel of precursors would be involved in the assembly of a bilayered cross wall that would proceed at a fairly constant rate until the cross wall closes. The second channel of precursors would be involved in the separation of the bilayered cross wall into two segments of peripheral wall. These precursors would intercalate into and thicken the separating layers of the cross wall. The flow of precursors through this channel would be progressively reduced through a cycle. These decreases, when coupled with internal hydrostatic pressure, apparently would result in the enlarging peripheral wall becoming increasingly more curved and would also promote cell division by reducing the total amount of cell wall that must be assembled in order for septation to occur.

Study of cycle of cell wall assembly in Streptococcus faecalis by three-dimensional reconstructions of thin sections of cells