Turnover and spreading of old wall during surface growth of Bacillus subtilis

Abstract

The steady-state concentration of cell wall turnover products in the medium of B. subtilis 168 growing exponentially on a casein hydrolysate-supplemented medium is equivalent to an overall rate of turnover of less than 10%/generation. After transfer of a steady-state-labeled culture to nonradioactive medium, the rate of release of labeled turnover products increased exponentially for up to 2 generations. The rate of turnover finally attained by this culture reached an apparently first-order rate of about 50%/generation. The addition of soluble autolytic activity to growing cultures of a mutant possessing a reduced rate of wall turnover resulted in a marked stimulation in the rate of solubilization of the cell wall fraction. The increased rate of solubilization produced was proportional to the concentration of added enzyme and remained constant until less than 20% of the wall originally present was left. Autolytic activity added under these conditions was bound entirely to wall at least 1 generation old. The results are interpreted in terms of a model for cell wall growth in which wall 2 or more generations old covers a total surface area at least 4 times larger than that occupied at the time of synthesis, forming a shallow outer layer (overlying newer wall) from which all turnover takes place. The model is duscussed in relation to previous attempts to determine the pattern of surface expansion in bacilli.