GROWTH AND BUOYANT DENSITY OF ESCHERICHIA-COLI AT VERY-LOW OSMOLARITIES

The growth and buoyant densities of two closely related strains of Esc herichia coli in M9-glucose medium that was diluted to produce osmolar ities that varied from as low as 5 to. 500 mosM were monitored. At 15 mosM, the lowest osmolarity at which buoyant density could be measured reproducibly in Percoll gradients, both ML3 and ML308 had a buoyant d ensity of about 1.079 g/ml. As the osmolarity of the medium was increa sed, the buoyant density also increased linearly up to about 125 mosM, at which the buoyant density,vas 1.089 g/ml. From 150 up to 500 mosM, the buoyant density again increased linearly but with a different slo pe from that seen at the lower osmolarities. The buoyant density at 15 0 mosM was about 1.091 g/ml, and at 500 mosM it was 1.101 g/ml. Both s trains of E. coli could be grown in M9 medium diluted 1:1 with water, with an osmolarity of 120 mosM, but neither strain grew in 1:2 diluted M9 if the cells were pregrown in undiluted M9. (Note: undiluted M9 as prepared here has an osmolarity of about 250 mosM.) However, if the c ells were pregrown in 30% M9, about 75 mosM, they would then grow in M 4 at 45 mosM and above but not below 40 mosM. To determine which const ituent of M9 medium was being diluted to such a low level that it inhi bited growth, diluted M9 was prepared with each constituent added Pack singly. From this study, it was determined that both Ca2+ and Mg2+ co uld stimulate growth below 40 mosM. With Ca2+- and Mg2+-supplemented d iluted M9 and cells pregrowm in 75 mosM M9, it was possible to grow ML 308 in 15 mosM M9, Strain ML3 would only haltingly grow at 15 mosM. Fo ur attempts were made to grow both ML3 and ML308 at 5 mosM. In three o f the experiments, ML308 grew, while strain ML3 grew in one experiment While our experiments were designed to effect variations in medium os molarity by using NaCl as an osmotic agent, osmolarity and salinity we re changed concurrently. Therefore, from this study, we believe that E . coli might be defined as a euryhalinic and/or euryosmotic bacterium because of its ability to grow in a wide range of salinities and osmol arities.