Picture of Gram -ve bacterial cell wall. The image above is 400 pixels across and the original is 4,000 pixels across.
The Gram negative cell wall has a thin intermediate peptidoglycan layer and an external membrane. The outer portion of the external membrane is a lipopolysaccharide layer. This layer comprises a lipid-A part buried in the outer membrane, core polysaccharide and O-side chains. These side chains are absent in Yersinia pestis.