CHANGE IN A SINGLE-GENE OF SALMONELLA-TYPHIMURIUM CAN DRAMATICALLY CHANGE ITS BUOYANT DENSITY

The growth rates and buoyant densities of a Salmonella typhimurium mut ant, TL126 (proB74A(+)), with enhanced osmotolerance caused by proline overproduction were measured and compared with the growth rates and b uoyant densities of an isogenic (wild-type) strain, TL128 (proB(+)A(+) ), with normal control of proline production. Growth rates were determ ined in a rich medium (Luria broth) with added NaCl to produce various osmotic strengths ranging from 300 to 2,000 mosM. At low concentratio ns of NaCl, there was Little variation in doubling times between the t wo strains. However, as the osmotic strength of the medium approached and exceeded 1,300 mosM, the doubling times of TL126 (osmotolerant) we re 1.5 to 2 times faster than those of TL128 (wild type), confirming t he osmotolerance of TL126. Buoyant densities were determined by equili brium sedimentation in a Percoll gradient of osmotic strength equal to that of the growth medium. The osmolarity of the Percoll gradient was adjusted by the addition of NaCl. At low osmolarities (300 to 500 mos M), the buoyant density of TL126 (osmotolerant) was slightly but consi stently lower than that of TL128 (wild type). As the osmotic strength was increased, the buoyant density of TL126 (osmotolerant) increased i n proportion to the osmotic strength. In contrast, the buoyant density of strain TL128 (wild type) did not increase as much. At high osmolar ities (1,600 to 2,000 mosM), the buoyant density of TL126 (osmotoleran t) was consistently higher than that of TL128 (wild type). These resul ts suggest that the intracellular accumulation of proline by TL126, th e osmotolerant strain, increases both the growth rates and buoyant den sities at osmolarities of 1,300 mosM and above.