Relationship between changes in buoyant density and formation of new sites of cell wall growth in cultures of streptococci (Enterococcus hirae ATCC 9790) undergoing a nutritional shift-up.

When the glutamate concentration of cultures of Enterococcus hirae was raised from 20 to 300 micrograms/ml, the mass doubling time decreased from ca. 85 to 45 min in 9 min, but balanced growth was not reestablished for 30 to 40 min. During the unbalanced period of growth, RNA and protein synthesis proceeded more rapidly than did peptidoglycan synthesis, buoyant density increased from ca. 1.1024 to 1.1075 g/ml, and the rate of formation of new cell wall growth sites transitorily accelerated above the new growth rate. When studied as a function of cell size, all cultures showed buoyant density to decrease around cell separation, increase as cells increased in size, and then plateau when cells reached large volumes. Greater increases in buoyant density as a function of cell size were seen after shift-up, with the greatest increases observed at 15 to 20 min after shift-up, when the rate of formation of new sites was also maximal. In a population of cells examined by electron microscopy 15 min after shift-up, buoyant density and the frequency of cells with new sites increased as old sites approached the size of two poles. These data were consistent with a model whereby buoyant density increases in the terminal stages of the cell cycle when the surface grows slower than the cytoplasm. The greater the difference in the rates of inside to outside growth, the greater the increase in buoyant density and the more frequently new sites will be initiated.