Magnesium and the Growth of Escherichia coli

Abstract

The growth of cells of Escherichia coli has been studied in media with varying amounts of magnesium. Experiments in which the content of magnesium in the medium and in the cell have been directly measured reveal that the growth rate of magnesium-limited wild type cells is half-maximal at extracellular concentrations of 1 to 3 x 10-6 m. Mutants have been isolated which require 100 times this concentration for half-maximal rate of growth. Supplementation of the medium with calcium or strontium enables the mutants to grow on the same low levels of magnesium as the wild type even though calcium or strontium cannot replace magnesium as an essential growth factor. Cells harvested in the exponential phase of growth from medium not limited in magnesium lose one-third to two-thirds of their total magnesium upon treatment with ethylenediaminetetraacetate. In contrast, magnesium-starved cells retain nearly all of their magnesium when treated with EDTA. In a sonic extract of magnesium-starved cells, 15% of the total magnesium is not sedimentable at 100,000 x g. When such an extract is passed over Sephadex G-25 or G-50, 75 to 80% of the magnesium is free to diffuse into the gel.