Electron Microscopy Reveals Unexpected Cytoplasm and Envelope Changes during Thymineless Death in Escherichia coli.

Bacterial rod-shaped cells experiencing irreparable chromosome damage should filament without other morphological changes. Thymineless death (TLD) strikes thymidine auxotrophs denied external thymine/thymidine (T) supplementation. Such T-starved cells cannot produce the DNA precursor dTTP and therefore stop DNA replication. Stalled replication forks in T-starved cells were always assumed to experience mysterious chromosome lesions, but TLD was recently found to happen even without origin-dependent DNA replication, with the chromosome still remaining the main TLD target. T starvation also induces morphological changes, as if thymidine prevents cell envelope or cytoplasm problems that otherwise translate into chromosome damage. Here, we used transmission electron microscopy (TEM) to examine cytoplasm and envelope changes in T-starved Escherichia coli cells, using treatment with a DNA gyrase inhibitor as a control for "pure" chromosome death. Besides the expected cell filamentation in response to both treatments, we see the following morphological changes specific for T starvation and which might lead to chromosome damage: (i) significant cell widening, (ii) nucleoid diffusion, (iii) cell pole damage, and (iv) formation of numerous cytoplasmic bubbles. We conclude that T starvation does impact both the cytoplasm and the cell envelope in ways that could potentially affect the chromosome.