Magnesium-limited Growth of Bacillus subtilis, in Pure and Mixed Cultures, in a Chemostat

Abstract

The influence of Mg2+-limitation on the growth of a typical gram-positive organism[long dash]B. subtilis was investigated and the data compared with that obtained with Aero-bacter aerogenes grown under similar conditions. The magnesium contents of both organisms varied with growth rate but were very simi lar at corresponding growth rates. With Mg2+-limited chemostat cultures of each organism, uptake of Mg2+ was almost complete at specific growth rates less than 0.7 x maximum. Cellular Mg2+ was tightly bound, none being removed by suspension of the organisms at 20[degree] in 0.85% (w/v) NaCl. When Mg2+-limited organisms were suspended in environments containing Mg2+, mis ion was rapidly adsorbed; the amount adsorbed varied with both the initial extracellular Mg2+ concentration and the composition of the diluent. B. subtilis had a greater capacity for Mg2+ adsorption than A. aerogenes but its affinity for this ion was less. The latter difference correlated with the ability of A. aerogenes to outgrow B. subtilis rapidly in Mg2+-limited chemostat cultures containing both organisms. The significance of these results is discussed in relation to the reports from other laboratories concerning differences between gram-positive and gram-negative bacteria in Mg2+ content, uptake of Mg2+ and ability to grow in media of low Mg2+ content.