Most bacteria have a peptidoglycan cell wall. And the Archaea which are extremophiles, have a semirigid cell wall made of not peptidoglycan, but protein or pseudomurein. In eubacteria, the peptidoglycan prevents inward osmosis which would cause the bacterial cell to burst due to osmotic pressure. Peptidoglycan which is also known as murein is a polymer of peptidoglycan monomers. These are inserted into existing peptidoglycan enabling bacterial growth after binary fission. They are inserted in a cross-link fashion which strengthens the cell wall like a chain link fence. The cell walls in Bacteria differ from those in fungi and plants as these are made of chitin in fungi and cellulose in plants. Gram-positive bacteria have a thick cell wall with many layers of peptidoglycan and teichoic acids. Gram-negative bacteria have a thin cell wall with few layers of peptidoglycan and a second lipid membrane of lipopolysaccharides and lipoproteins.
Prokaryotes have a variety of outer coverings; for example, many bacteria have a glycocalyx, a carbohydrate-based layer around the outside of the cell membrane. The glycocalyx prevents dehydration, makes the cell harder for a host's immune system to detect, and resists phagocytosis. The glycocalyx can also have adhesive properties, allowing large colonies of bacteria to cling together and stick to surfaces; an example of this is plaque, the bacteria film that forms on teeth.
Another example of a protective coating in the prokaryote group is the spore covering. Many bacteria can form endospores, which are a highly resistant dormant stage. The spore coat contains a polymer-type molecule called peptidoglycan, which renders the spore so resistant that in some species it can survive being boiled!