Periplasm

Using cryo-electron microscopy it has been found that a much smaller periplasmic space is also present in Gram-positive bacteria (more accurately "monoderm"), between cell wall and the plasma membrane.

[3] Although bacteria are conventionally divided into two main groups—Gram-positive and Gram-negative, based upon their Gram-stain retention property—this classification system is ambiguous as it can refer to three distinct aspects (staining result, cell-envelope organization, taxonomic group), which do not necessarily coalesce for some bacterial species.

[4][5][6][7] In most situations such as in this article, Gram-staining reflects the marked differences in the ultrastructure and chemical composition of the two main kinds of bacteria.

[4][8] All Gram-positive bacteria are bounded by a single unit lipid membrane (i.e. monoderm); they generally contain a thick layer (20-80 nm) of peptidoglycan responsible for retaining the Gram-stain.

[4][8] The distinction between the monoderm and diderm prokaryotes is supported by conserved signature indels in a number of important proteins (for example, DnaK and GroEL).

[15] As disulfide bond formation is frequently a rate-limiting step in the folding of proteins, these oxidizing enzymes play an important role in the bacteria periplasm.

Aside from those previously mentioned, the periplasm also functions in protein transport and quality control, analogous to the endoplasmic reticulum in eukaryotes.

Aside from enzymes and structural components that are vital to cell function and survival, the periplasm also contains virulence-associated proteins such as DsbA that can be targeted by antimicrobial therapies.

[21] Due to their role in catalyzing disulfide bond formation for a variety of virulence factors, the DsbA/DsbB system has been of particular interest as a target for anti-virulence drugs.

Many of the virulence factors associated with bacterial pathogenicity are secretion proteins, which are often subject to post-translational modification including disulfide bond formation.

During infection of a host, the cell of a bacterium is subject to many turbulent environmental conditions, which highlights the importance of the structural integrity afforded by the periplasm.