Morphology of an Escherichia coli mutant with a temperature-dependent round cell shape

Abstract

Mutants of Escherichia coli capable of growing in the presence of 10 microgram of mecillinam per ml were selected after intensive mutagenesis. Of these mutants, 1.4% formed normal, rod-shaped cells at 30 degrees C but grew as spherical cells at 42 degrees C. The phenotype of one of these rod(Ts) mutants was 88% cotransducible with lip (14.3 min), and all lip+ rod(Ts) transductants of a lip recipient had the following characteristics: (i) growth was relatively sensitive to mecillinam at 30 degrees C but relatively resistant to mecillinam at 42 degrees C; (ii) penicillin-binding protein 2 was present in membranes of cells grown at 30 degrees C in reduced amounts and was undetectable in the membranes of cells grown at 42 degrees C. The mecillinam resistance, penicillin-binding protein 2 defect, and rod phenotypes all cotransduced with lip with high frequency. Thus the mutation [rodA(Ts)] is most likely in the gene for penicillin-binding protein 2 and causes the organism to grow as a sphere at 42 degrees C, although it grows with normal rodlike morphology at 30 degrees C. At 42 degrees C, cells of this strain were round with many wrinkles on their surfaces, as revealed by scanning electron microscopy. In these round cells, chromosomes were dispersed or distributed peripherally, in contrast to normal rod-shaped cells which had centrally located, more condensed chromosomes. The round cells divided asymmetrically on solid agar, and it seemed that the plane of each successive division was perpendicular to the preceding one. On temperature shift-down in liquid medium many cells with abnormal morphology appeared before normal rod-shaped cells developed. Few abnormal cells were seen when cells were placed on solid medium during temperature shift-down. These pleiotropic effects are presumably caused by one or more mutations in the rodA gene.