Bacteria can be classified into 3 groups based on differences in the thickness or composition of the cell wall structure: Gram-positive, Gram-negative, and Acid-fast. TheGram stain is a technique used to distinguish between Gram-positive cells that have a thick layer of peptidoglycan and stain purple and Gram-negative cells which have a thin layer of peptidoglycyan and stain pink.
Gram-Positive Cell WallThe Gram-positive cell wall consists of a thick (20-80nm) layer of peptidoglycan. This thick layer is porous making the cell wall absorbent like a sponge. The Gram-positive cell wall also contains teichoic acid, which functions in cell wall maintenance and gives the cell surface an acidic (-) charge.
Gram-Negative Cell WallTheGram-negative cell wall consists of thin (1-3nm) layer of peptidoglycan. Because the Gram-negative cell wall is so thin, these bacterial cells require an extra layer of protection, called the outer membrane. The outer membrane consists of a phospholipid membrane, similar to the cell or plasma membrane. This membrane has tiny holes or openings calledporins. Porins block the entrance of harmful chemicals and antibiotics, making Gram-negative bacteria much more difficult to treat than Gram-positive cells. In many antibiotic resistant bacteria, these porins are connected to drug pumps, which pump out any drugs or harmful chemicals that enter through the porins.
Attached to the outer membrane is is a highly-branched fatty sugar called lipopolysaccharide (LPS). LPS acts as an endotoxin [text annotation indicator] because it induces fever and shock in the human host.
Acid-Fast BacteriaAcid-fast bacteria contain a waxy substance called mycolic acid and a small amount of peptidoglyan. Due to their waxy cell wall, these bacteria are highly resistant to staining and treatment. Mycobacterium tuberculosis , the causative agent of TB, is one example of a bacterial cell with an acid-fast cell wall. These bacteria must be heated and treated with an acid-alcohol in order to stain them in the lab (See image below).
Another type of bacteria with an unusual cell wall is Mycoplasma pneumoniae. This bacteria attaches to the epithelial cells in the lungs, causing pneumonia. The cell wall of this bacteria contains large amounts of sterols (rigid lipids), which make it difficult to treat. Due to the waxy content of their cell wall, these bacteria are pleiomorphic and vary in shape from long and filamentous to round.
