The gram-negative bacterial periplasm: Size matters

Gram-negative bacteria are surrounded by two membrane bilayers separated by a space termed the periplasm.The periplasm is a multipurpose compartment separate from the cytoplasm whose distinct reducing environment allows more efficient and diverse mechanisms of protein oxidation, folding, and quality control.The periplasm also contains structural elements and important environmental sensing modules, and it allows complex nanomachines to span the cell envelope.Recent work indicates that the size or intermembrane distance of the periplasm is controlled by periplasmic lipoproteins that anchor the outer membrane to the periplasmic peptidoglycan polymer.This periplasm intermembrane distance is critical for sensing outer membrane damage and dictates length of the flagellar periplasmic rotor, which controls motility.These exciting results resolve longstanding debates about whether the periplasmic distance has a biological function and raise the possibility that the mechanisms for maintenance of periplasmic size could be exploited for antibiotic development.Gram-negative bacteria, like the energy organelles of plants and animals (the chloroplast and mitochondria), have two membrane bilayers termed the outer and inner membranes.The space between these two membranes is termed the periplasm.Long before single-cell eukaryotes, the periplasm evolved as the first extracytoplasmic compartment to provide an important competitive adaption to gram-negative bacteria.Early knowledge and the discovery of the periplasm developed even before its morphological visualization.In the 1960s, scientists were trying to understand how toxic enzymes involved in degradation of important biological molecules, such as ribonucleases and phosphatases produced by the gram-negative bacteria Escherichia coli, were not toxic to the cell.Biochemical extraction methods suggested a separate compartment, because such extraction preserved the inner membrane-bound cytoplasm, and these spheroplasts could grow again and synthesize more enzymes [1].The development of electron microscopy led to the visualization of the two membrane bilayers separated by the periplasm [2].