The outer covering is the cell wall; a semi-rigid layer that maintains cell shape and chemical equilibrium. Peptidoglycan cell wall of eubacteria is absent.
Cell wall chemicals are unique in archaebacteria and considerable differences are found in different groups. In archae bacteria, S-layer is often seen just outside the plasma membrane.
Major types of Archae Bacteria Cell wall
The most common type of archeal cell wall is an S layer composed of either protein or glycoprotein.
Thickness: 20-40nm thick
S-layer cell walls are present in some methanogens like Methanococcus, Halophiles like Halobacterium and extreme thermophiles like Sulpholobus, Pyrodictium etc.
In this type, additional layers of material are present ouside the S-layer.
In Methanosprillum, There is a protein sheath external to S-layer.
In Methanosarcina, S-layer is covered by a chondroitin like material called as mathanochondroitin.
In some Archae like Methanothermus and Methanopyrus, S-layer is the outermost layer and is separated from the plasma membrane by a Peptidoglycan like molecule called psuedomurien.
Psuedomurien differ from Peptidoglycan in having N-acetyltalosaminuronic acid instead of N-acetyl muramic acid, L -amino acids instead of D-amino acids that cross links and Beta (1->3) glycosidic linkage instead of Beta (1->4) glycosidic linkage.
In this type, S layer is absent. Instead archeal cell wall is single thick, homogenous layer resembling Gram positive bacteria. These archeae often stain gram positive. This type is present in Methanobacterium, Halococcus etc.