BUOYANT DENSITY OF ESCHERICHIA-COLI IS DETERMINED SOLELY BY THE OSMOLARITY OF THE CULTURE-MEDIUM

In previous studies, we had shown that the buoyant density of Escheric hia coli is determined by the osmolarity of the growth medium by varyi ng the osmolarity of the medium with NaCl or sucrose. However, the buo yant density of the cells always exceeded that of the growth medium. H ere we determined the effect of medium with a buoyant density greater than the expected buoyant density of cells by adding Nycodenz to Luria broth. Percoll gradients of cells were analyzed by laser light scatte ring. The buoyant density for 125- and 375-mOsM-grown cells was 0.002 g/ml and 0.003 g/ml more, respectively, for cells grown in the presenc e of Nycodenz than those grown without Nycodenz, while the buoyant den sity of 250-mOsM-grown cells was 0.005 g/ml less for cells grown in th e presence of Nycodenz than those grown without Nycodenz. Cells grown in 500-mOsM medium with or without Nycodenz had the same buoyant densi ty. The buoyant density of cultures grown in defined medium was the sa me as those grown in rich medium, with only the medium osmolarity corr elating to buoyant density. We conclude from these experiments that ne ither buoyant density nor chemical make-up of the medium determines th e buoyant density of cells grown in that medium. Only the medium osmol arity determines cell buoyant density, suggesting that E. coli has no mechanisms to sense buoyant density.