PREVIOUS reports1,2 on the calcium requirement of a strain of Rhizobium trifolii have shown that bacteria deprived of this element during growth changed from normal rods to swollen, spherical bodies with weakened wall structure. The small amount of calcium available to such cells, as a contaminant in the medium, was concentrated in the wall fraction. Such cells, unlike calcium sufficient cells, were susceptible to lysis by lysozyme in the absence of ethylenediamine tetraacetic acid (EDTA)3. Strontium, but neither magnesium nor barium, was effective as a substitute for calcium in supporting growth and normal morphology, and in providing resistance to lysozyme, except when EDTA was present. It could then be inferred that strontium was taking the place of calcium as a structural part of the cell wall, probably on account of suitable atomic size.
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Antonie van Leeuwenhoek (1967)