Periplasm

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 bacteria. It has been found using cryo-electron microscopy, that a much smaller periplasmic space is present in gram-positive bacteria.

The periplasm may constitute up to 40% of the total cell volume of gram-negative bacteria, and this is a much smaller percentage in gram-positive bacteria.

Although the 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. However, although Gram-staining response of bacteria is an empirical criterion, its basis lies in the marked differences in the ultrastructure and chemical composition of the two main kinds of bacteria. These bacteria are distinguished from each other based on the presence or absence of an outer lipid membrane, which is a more reliable and fundamental characteristic of the bacterial cells.