Bacteria are microscopic, single-celled organisms found in nearly every environment on Earth. They play diverse roles, impacting ecosystems and human health. Understanding their characteristics is important.
Understanding Bacterial Cell Walls
A defining feature of most bacteria is their cell wall, a rigid outer layer providing structural support and maintaining cell shape. This wall also protects the bacterial cell from osmotic lysis, preventing it from bursting. Peptidoglycan, also known as murein, is a unique component found exclusively in bacterial cell walls.
Peptidoglycan forms a mesh-like layer composed of alternating sugar molecules, N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM), linked together in long chains. These sugar chains are then cross-linked by short peptide chains, creating a strong, interwoven network around the cell. This intricate structure gives the bacterial cell wall its mechanical strength.
The Gram Stain Method
The Gram stain is a laboratory technique that classifies bacteria into two main groups: Gram-positive and Gram-negative. Developed by Hans Christian Gram in 1884, this method relies on differences in bacterial cell wall properties to produce differential staining. The process involves applying a series of dyes and decolorizers to a bacterial sample.
Initially, a purple dye called crystal violet is applied, staining all bacterial cells. A mordant, Gram’s iodine, is then added, which forms a complex with the crystal violet inside the cell. A decolorizing agent, such as alcohol, is then used. Bacteria respond distinctly to this decolorization step based on their cell wall structures, leading to either retention or loss of the purple stain. Finally, a counterstain, safranin, is applied, staining cells that lost the initial purple dye pink or red.
Gram-Negative Bacteria: A Unique Structure
Gram-negative bacteria possess a peptidoglycan layer, but it is considerably thinner than that found in Gram-positive bacteria. This thin layer, often only a few nanometers thick, represents one to a few layers, accounting for about 5-10% of the total cell wall. In contrast, Gram-positive bacteria have a peptidoglycan layer 20 to 80 nanometers thick, comprising many layers.
A distinguishing feature of Gram-negative bacteria is the presence of an outer membrane external to the thin peptidoglycan layer. This outer membrane is an asymmetric bilayer composed of phospholipids in its inner leaflet and lipopolysaccharides (LPS) in its outer leaflet. Lipopolysaccharides consist of three parts: an O-antigen, a core oligosaccharide, and Lipid A, which anchors the LPS into the membrane.
The outer membrane acts as a selective barrier, regulating molecule passage. It contains specialized proteins called porins, forming water-filled channels that allow small, hydrophilic molecules (e.g., nutrients, some antibiotics) to pass. The outer membrane’s composition and barrier function, including LPS and porins, contribute to Gram-negative bacteria’s distinct properties.
Implications for Health and Medicine
The structural differences in Gram-negative bacteria’s cell envelopes have important implications for antibiotic effectiveness. The outer membrane serves as an effective barrier, making Gram-negative bacteria more resistant to certain antibiotics and disinfectants. Many antibiotics, especially larger or hydrophobic ones, struggle to penetrate this outer layer to reach their targets.
Outer membrane modifications, like changes in porin channels or LPS structure, can further reduce antibiotic uptake, increasing resistance. Beyond its barrier function, the outer membrane’s lipopolysaccharide component is also an endotoxin. When Gram-negative bacteria are lysed (by the immune system or antibiotics), Lipid A fragments from LPS can be released. This release can trigger a significant immune response, potentially leading to fever, inflammation, and septic shock.