periplasmic space

The periplasm is a concentrated gel-like matrix in the space between the inner cytoplasmic membrane and the bacterial outer membrane called the periplasmic space in Gram-negative (more accurately "diderm") bacteria. 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. The periplasm may constitute up to 40% of the total cell volume of gram-negative bacteria, but is a much smaller percentage in gram-positive bacteria.

== Terminology == 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. 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. The usual "Gram-positive" type does not have an outer lipid membrane, while the typical "Gram-negative" bacterium does. The terms "diderm" and "monoderm", coined to refer to this distinction only, is a more reliable and fundamental characteristic of the bacterial cells. thumb|Monoderm bacteria have a thin periplasm between the cell wall and the plasma membrane