Bacterial Cell Wall

Overview
  • This page discusses the basic structural features of the bacterial cell wall which is the outermost layer of the organism and provides structural support. The cell wall surrounds the bacterial Inner Membrane which is similar to the mammalian plasma membrane, a lipid bilayer that contains the bacterial cytoplasm. There are a few important classes of bacterial cell walls which possess unique structural characteristics and vary in thickness. The different classes of bacterial cell walls are often distinguished with special stains. The headings below refer to interlinks which list bacteria with that cell wall structure.
Peptidoglycan
  • Peptidoglycan is the basic structural components of most bacterial cell walls. It is a molecule composed of four amino acids (peptido-) and two sugars (-glycan). These individual peptidoglycan units are cross-linked throughout the cell wall, providing a tough protective barrier. The enzyme Transpeptidase performs the cross-linking reaction and is target of Beta-lactam Antibiotics and Vancomycin.
Gram Positive Bacteria
  • Gram Positive Bacteria possess a thick layer of peptidoglycan that directly invests the cell membrane. The gram positive cell wall often contains chains of the sugar Teichoic Acid which is highly inflammatory and can induce Septic Shock when released at systemically high concentrations in humans.
Gram Negative Bacteria
  • Gram negative bacteria possess a much thinner layer of peptidoglycan that lies above but does not directly invest the cell membrane and does not contain Teichoic Acid . Above the peptidoglycan layer lies a second lipid bilayer called the "Outer Membrane" which possess a unique molecule called Lipopolysaccharide (LPS). Lipopolysaccharide contains both lipid (lipo-) and multiple sugar (-polysaccharide) components and induces severe inflammation in humans when released. At high systemic levels it can induce Septic Shock. Multiple molecules are present in the space between the gram negative inner and outer membranes (termed the Periplasmic Space), including bacterial proteins which can inactivate antibiotics such as Beta-lactamases.
Acid-fast Bacteria
  • Acid-Fast Bacteria possess a unique cell wall architecture different from both gram negative and gram positive bacteria. The Acid-Fast cell wall consists of a thick, outer lipid-rich layer composed primarily of the fatty acid "Mycolic Acid". This lipid layer lies on top of a layer of peptidoglycan and the sugar arabinogalactan which in turn invest the inner lipid membrane common to all bacteria. The thick outer mycolic acid layer renders acid-fast bacteria resistant to gram stain. When stained with alternative dyes, the cell wall is resistant to decolorization with acid alcohol, thus giving these bacteria their namesake of "Acid-Fast".
Cell Wall Absent Bacteria
  • A few bacteria do not possess cell walls and are contained only by the inner membrane. Not surprisingly, these bacteria are pleomorphic and thus lack a distinct shape.