The periplasmic space in gram-negative cells is the region located between the peptidoglycan cell wall and the plasma membrane. It is occupied by a gel-like matrix, adding to the structural complexity of the gram-negative cell envelope.

Periplasmic Space

In gram-negative bacteria, the outer membrane is a distinct second lipid bilayer situated exterior to the thin peptidoglycan layer of the cell envelope. This membrane is firmly anchored to the underlying peptidoglycan by murein lipoprotein. A key feature of its outer leaflet is the presence of lipopolysaccharide (LPS), a complex molecule comprising an O antigen, core polysaccharide, and lipid A with attached fatty acid tails, which acts as an endotoxin during bacterial infections.

Outer Membrane

Gram-negative cells possess a cell envelope with a significantly thinner layer of peptidoglycan—no more than about $${}4$$ nm thick—compared to gram-positive cells. Their overall cell envelope structure is highly complex, featuring a periplasmic space and an outer membrane located external to the peptidoglycan layer.

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Types of Bacterial Cell Walls

The gram negative bacteria that are not present in the Proteobacteria phylum consist of some of these classes and genera of bacteria:
-  *Chlamidia*
- Spirochetes
- *Cytophaga, Fusobacterium, and Bacteroides*
- Planctomycetes

Non-proteobacteria Gram Negative Bacteria

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In gram-positive bacteria, the cell wall consists of a thick peptidoglycan layer, totaling $${}30-100$$ nm in thickness. Within this thick layer, the N-acetylmuramic acid (NAM) molecules are connected by tetrapeptide chains linked via pentaglycine cross-bridges. This differs from gram-negative bacteria, where the tetrapeptide chains extending from each NAM unit are directly cross-linked. The peptidoglycan layers in gram-positive bacteria are commonly embedded with teichoic acids (TAs) on the external side, which increase the rigidity of the cell wall.

Gram Positive Cell Walls

Gram Negative Cell Walls

Planctomycetes are a phylum of bacteria that are typically water dwelling. Unlike many bacteria, they reproduce via budding rather than binary fission. A bud separates from the mother cell. Sessile cells are immobile and contain a holdfast appendage that allows them to adhere to surfaces in the water. The sessile cells are able to reproduce.

Planctomycetes

The *Chlamydia* genus consists of many pathogens. They spread very easily from host to host because they form elementary bodies during their life cycle. These are spore-like which allows *Chlamydia* to be metabolically inactive until they are in a host environment.

*Chlamydia*

This phylum contains gram-negative nonproteobacteria that are linked together because of their similarities in their DNA sequences. They are rod bacteria that are primarily living in anaerobic environments and many of which are fermenters.

*Cytophaga, Fusobacterium, Bacteriodes* (CFB)

The term spirochete (plural: spirochetes) refers to prokaryotic cells that have a long, loose, and flexible helical spiral shape. They visually appear similar to spirilla but are noticeably more floppy. They are members of the Spirochaetota phylum.

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