Bacteria are microscopic single-celled organisms found almost everywhere, in diverse environments from soil to living organisms. These life forms are broadly categorized in several ways, including their shape, their need for oxygen, and their genetic makeup. A widely used method for classifying bacteria involves a laboratory technique known as Gram staining, which differentiates them based on structural differences in their cell walls.
Gram-Positive Cell Wall
Gram-positive bacteria are characterized by the absence of an outer membrane. Their cell envelope consists primarily of a thick layer of peptidoglycan, which is a robust polymer providing structural support and protection to the cell. This thick peptidoglycan layer, ranging from 20 to 100 nanometers, constitutes a significant portion of the cell wall.
Interwoven within this extensive peptidoglycan network are teichoic acids and lipoteichoic acids. Teichoic acids are polymers that extend through the peptidoglycan. Lipoteichoic acids are similar, but they are anchored to the underlying cytoplasmic membrane. These components contribute to the overall negative charge of the cell wall and play roles in ion regulation and maintaining cellular integrity. Beneath this thick cell wall lies the cytoplasmic membrane, which encloses the bacterial cell’s internal contents.
Gram-Negative Outer Membrane
In contrast to Gram-positive bacteria, Gram-negative bacteria possess a more complex cell envelope that includes an outer membrane. This outer membrane is an asymmetric lipid bilayer, where the inner leaflet is composed of phospholipids, and the outer leaflet primarily consists of lipopolysaccharide (LPS). LPS is a large molecule, with lipid A being responsible for much of its biological activity.
Between the outer membrane and the inner cytoplasmic membrane of Gram-negative bacteria lies a gel-like compartment called the periplasmic space. This space contains a relatively thin layer of peptidoglycan, much thinner than that found in Gram-positive bacteria. The outer membrane also features specialized proteins called porins, which form channels that allow the passive transport of small, hydrophilic molecules, including some nutrients and antibiotics, into the cell.
Implications of Cell Wall Differences
The distinct cell wall structures of Gram-positive and Gram-negative bacteria account for their differential responses to the Gram stain. During the staining procedure, Gram-positive bacteria retain the crystal violet dye because their thick, multilayered peptidoglycan wall traps the dye-iodine complex. This results in the cells appearing purple or blue under a microscope. The alcohol decolorizing step does not easily wash the stain from the Gram-positive wall.
Conversely, the Gram-negative outer membrane, with its high lipid content, is damaged by the alcohol decolorizer, which then allows the crystal violet-iodine complex to be washed out. Consequently, Gram-negative bacteria do not retain the initial purple stain and are then counterstained with safranin, causing them to appear pink or red. These structural differences also significantly impact their susceptibility to various antibiotics.
The outer membrane of Gram-negative bacteria acts as a barrier, providing resistance against certain antibiotics. This membrane can limit the entry of antimicrobial agents into the cell, which means that some antibiotics effective against Gram-positive bacteria may be less effective against Gram-negative bacteria. While porins allow some hydrophilic antibiotics to enter, modifications to these channels can further contribute to antibiotic resistance. Gram-positive bacteria, lacking this outer membrane, are more susceptible to antibiotics that target the peptidoglycan layer, as these drugs can directly access their site of action.