<P> Outside the cell wall, many Gram - positive bacteria have an S - layer of "tiled" proteins . The S - layer assists attachment and biofilm formation . Outside the S - layer, there is often a capsule of polysaccharides . The capsule helps the bacterium evade host phagocytosis . In laboratory culture, the S - layer and capsule are often lost by reductive evolution (the loss of a trait in absence of positive selection). </P> <P> Unlike the gram - positive cell wall, the gram - negative cell wall contains a thin peptidoglycan layer adjacent to the cytoplasmic membrane, which is responsible for the cell wall's inability to retain the crystal violet stain upon decolourisation with ethanol during Gram staining . In addition to the peptidoglycan layer the gram - negative cell wall also contains an additional outer membrane composed by phospholipids and lipopolysaccharides which face into the external environment . The highly charged nature of lipopolysaccharides confer an overall negative charge to the gram - negative cell wall . The chemical structure of the outer membrane lipopolysaccharides is often unique to specific bacterial strains (i.e. sub-species) and is responsible for many of the antigenic properties of these strains . </P> <P> As a phospholipid bilayer, the lipid portion of the outer membrane is largely impermeable to all charged molecules . However, channels called porins are present in the outer membrane that allow for passive transport of many ions, sugars and amino acids across the outer membrane . These molecules are therefore present in the periplasm, the region between the plasma membrane and outer membrane . The periplasm contains the peptidoglycan layer and many proteins responsible for substrate binding or hydrolysis and reception of extracellular signals . The periplasm is thought to exist as a gel - like state rather than a liquid due to the high concentration of proteins and peptidoglycan found within it . Because of its location between the cytoplasmic and outer membranes, signals received and substrates bound are available to be transported across the cytoplasmic membrane using transport and signalling proteins imbedded there . </P> <P> In nature, many uncultivated Gram - negative bacteria also have an S - layer and a Capsule (microbiology). These structures are often lost during laboratory cultivation . </P>

Describe the structure and function of the bacterial cell envelope