Magnesium starvation of Aerobacter aerogenes. IV. Cytochemical changes.

A number of cytochemical changes were revealed by microscopic observations of Aerobacter aerogenes populations starving for Mg(++). During the first few hours, while the synthesis of deoxyribonucleic acid (DNA) was paralleled by an increase in viable bacteria, the cells became progressively smaller. Subsequently, the number of viable cells in the culture remained constant in spite of continuing DNA synthesis, and the cells progressively elongated into filamentous forms. During this time, a second population of very small bacteria could be identified. These cells, whose number increased progressively, were inert with respect to (i) growth or reproduction when returned to a complete medium and (ii) biosynthetic activity as judged by autoradiographic estimation of uracil-H(3) incorporation into nucleic acids. When observed by electron microscopy, many thin sections from bacteria that had been starved of Mg(++) for 20 hr appeared to be almost devoid of ribosomal particles. Thionine staining indicated that the inert cells contain DNA. Furthermore, the rate of DNA synthesis in the culture corresponded to the rate of accumulation of inert cells, suggesting that their presence can account for the difference between total DNA and viable count.