Gram-negative bacteria- cell wall, examples, diseases, antibiotics


Bacteria are microscopic organisms that can be found in various environments and can cause different types of infections and diseases in humans and other animals. Bacteria can be classified into two major groups based on the structure and composition of their cell walls: gram-positive and gram-negative bacteria. This classification is based on a laboratory technique called gram staining, which was developed by a Danish bacteriologist named Hans Christian Gram in 1884 .

Gram staining is a simple and rapid method that uses a series of dyes to stain bacterial cells and differentiate them based on their ability to retain or lose the primary stain, which is crystal violet. Gram-positive bacteria have thick cell walls that consist mainly of peptidoglycan, a polymer of sugars and amino acids that forms a mesh-like layer around the cell membrane. Peptidoglycan can trap the crystal violet dye and retain it even after washing with alcohol, which is the decolorizing agent. Therefore, gram-positive bacteria appear purple or blue under a microscope after gram staining .

Gram-negative bacteria have thin cell walls that contain only a small amount of peptidoglycan, which is sandwiched between two membranes: the inner (cytoplasmic) membrane and the outer membrane. The outer membrane contains lipids and lipopolysaccharides, which are complex molecules that have both lipid and sugar components. The lipids and lipopolysaccharides make the outer membrane more resistant to certain chemicals, including alcohol. Therefore, when gram-negative bacteria are stained with crystal violet and washed with alcohol, they lose the primary stain and appear colorless under a microscope. To visualize them, a counterstain such as safranin or fuchsine is added, which stains them pink or red .

Gram staining is an important tool for identifying and characterizing bacteria, as it provides information about their cell wall structure, which can affect their physiology, pathogenicity, and susceptibility to antibiotics. However, gram staining is not always conclusive or reliable, as some bacteria may show variable or indeterminate results due to factors such as age, culture conditions, or staining technique. Moreover, gram staining does not provide enough information to distinguish between different species or strains of bacteria within the same group. Therefore, gram staining is usually followed by other tests such as culture, biochemical tests, serological tests, or molecular tests to confirm the diagnosis and guide the treatment of bacterial infections .