Fig. 1.
PA14 growth and secreted protease production under different conditions. (A) Growth kinetics of PA14 in 1% casein medium and degradation of substrate. (B) Activity of soluble proteases per unit biomass increases as cell density increases. (C) Secreted protease activity as a function of cell density at saturation. PA14 was grown in media limited by different concentrations of tryptone (x axis). Proteolytic activity of supernatants was measured after bacteria grew to saturation under all tryptone concentrations. The data were fit to the Hill equation [y = ymax × xn/(kn + xn)], where n is the Hill coefficient. The data fit to n = 1.95. (D) Model of autoinducer-regulated production of secreted protease during nutrient-limited growth. Autoinducer (I) is produced in proportion to nutrient flux (ФN) that is channeled toward cellular biomass (B). Total biomass production by cells in the environment is reflected by the total autoinducer concentration, which determines the proportion of each cell’s nutrient flux that is devoted to secreted protease production (P). The remaining nutrient is invested in cellular biomass (B). Secreted proteases cleave substrate (S) into smaller peptides (N) that can be taken up by the cell. (E) Proteolytic activity of supernatants of different concentrations of PA14 after exposure to a pulse of importable nutrients (CAAs). PA14 concentration is represented as multiples of saturated culture of PA14 in M9 without NH4Cl containing 1% CAA. Data fit to a line with slope of 0.039 and Y intercept of 0.87. (F) Secreted protein produced by different concentrations of PA14 after exposure to a pulse of CAAs. Data fit to a line with slope of 0.0035 and Y intercept of 0.093.