Divisome and elongasome organisation in the genome-reduced obligate intracellular bacterium Orientia tsutsugamushi

Cell elongation and cytokinesis are mediated by two large multi-protein complexes in rod-shaped bacteria: the elongasome and divisome. These membrane-associated complexes ultimately recruit peptidoglycan cell wall remodeling and synthesis machineries, thus sculpting and extending growing cells or driving septation of two daughter cells. The elongasome and divisome have been well studied in model bacterial species, including Bacillus subtilis, Caulobacter crescentus, and Escherichia coli. Here, we present an analysis of these complexes in an obligate intracellular bacterium with a highly reduced genome. Orientia tsutsugamushi, a cytoplasm-dwelling Gram-negative alphaproteobacterium, only retains a subset of proteins normally found in the elongasome and divisome. It also lacks all copies of the major peptidoglycan polymerase, Class A Penicillin Binding Protein, and has been shown to build an intermediate peptidoglycan cell wall-like structure that is low in abundance and insufficient to consistently confer a rod shape to the bacterium. We have carried out a first analysis of the elongasome and divisome in Orientia tsutsugamushi, quantifying the expression and subcellular localization of five key proteins through early stages of the intracellular infection cycle. We show how these are affected by antibiotic treatment and present a model for minimal elongation and division in an obligate intracellular bacterium.