Location and architecture of the Caulobacter crescentus chemoreceptor array

A new method for recording both fluorescence and cryo-EM images of small bacterial cells was developed and used to identify chemoreceptor arrays in cryotomograms of intact Caulobacter crescentus cells. We show that in wild-type cells preserved in a near-native state, the chemoreceptors are hexagonally packed with a lattice spacing of 12 nm, just a few tens of nanometers away from the flagellar motor that they control. The arrays were always found on the convex side of the cell, further demonstrating that Caulobacter cells maintain dorsal/ventral as well as anterior/posterior asymmetry. Placing the known crystal structure of a trimer of receptor dimers at each vertex of the lattice accounts well for the density and agrees with other constraints. Based on this model for the arrangement of receptors, there are between one and two thousand receptors per array.     

De novo morphogenesis in L‐forms via geometric control of cell growth

In virtually all bacteria, the cell wall is crucial for mechanical integrity and for determining cell shape. Escherichia coli's rod‐like shape is maintained via the spatiotemporal patterning of cell‐wall synthesis by the actin homologue MreB. Here, we transiently inhibited cell‐wall synthesis in E. coli to generate cell‐wall‐deficient, spherical L‐forms, and found that they robustly reverted to a rod‐like shape within several generations after inhibition cessation. The chemical composition of the cell wall remained essentially unchanged during this process, as indicated by liquid chromatography. Throughout reversion, MreB localized to inwardly curved regions of the cell, and fluorescent cell wall labelling revealed that MreB targets synthesis to those regions. When exposed to the MreB inhibitor A22, reverting cells regrew a cell wall but failed to recover a rod‐like shape. Our results suggest that MreB provides the geometric measure that allows E. coli to actively establish and regulate its morphology.