Overlooked contributions of ionic strength to cell envelope dynamics modulating outer membrane vesiculation and metal mineralization

Electrically active Geobacter bacteria assemble conductive pili and release redox-active outer membrane vesicles to drive metal transformations critical to environmental and energy systems. Mechanistic understanding of these processes largely derives from studies of Geobacter sulfurreducens cultivated in mineral media that mimic natural geochemical conditions. Here, we show that modest drops (90 to 80 mM) in sodium ionic strength of common media did not impact cell permeability, but stimulated vesiculation, growth, and covalent anchoring of the outer membrane to the cell wall via a long Lpp peptide to accommodate cell wall expansions and periplasmic enlargement. However, cations that neutralize peptidoglycan carboxylates and contract the cell wall rescued these phenotypes and promoted the assembly of conductive pili over vesiculation for the extracellular mineralization of uranium. These findings reveal cation-driven cell wall dynamics modulating cell envelope structure and functions ultimately determining the fate of metals and the effectiveness of bioremediation and bioenergy solutions. Overall design: RNA-seq profiles of mid-exponential phase Geobacter sulfurreducens strain PCA cultures grown in NB or DB medium with 15 mM acetate and 40 mM fumarate