We have isolated a mutant of Escherichia coli K12 with an unusual cell division lesion. This mutant divides to produce normal sized cells and produces, in addition, small anucleate spheres about one-tenth the volume of a normal cell. These ‘minicells’ are produced under a variety of growth conditions that support normal cell division. The yield of minicells in a fully grown culture is 1 to 3 times the number of normal sized cells. Because of their small size, minicells can be separated easily from normal cells by differential centrifugation, sucrose gradient centrifugation, or a combination of both techniques. Phase contrast and electron microscopy show that minicells do not have a nuclear structure, and chemical analyses indicate that these cells contain RNA and protein in a ratio similar to that of normal cells but contain little or no DNA (Adler et al., 1967). The absence of DNA...
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Cohen et al. (1968) studied this question.