Cell wall has a rigid framework of murein, a polysaccharide cross-linked by peptide chains.In gram-positive bacteria the wall is thickened with further polysaccharide and protein deposits, whilst in gram-negative bacteria the wall is thinner but coated with a lipid layer which provides protection against lysozyme and penicilin.
The rigidity of the cell wall prevents osmotic damage (penicilin interferes with this in susceptible gram-positive bacteria)and confers shape on the cell.The three most common shapes are: