1. Thin Peptidoglycan Layer: Gram-negative bacteria, including Salmonella, have a relatively thin layer of peptidoglycan in their cell walls compared to Gram-positive bacteria. Peptidoglycan is a mesh-like polymer that contributes to the strength and rigidity of the cell wall.
2. Outer Membrane: Gram-negative bacteria, including Salmonella, possess an outer membrane in addition to the cytoplasmic membrane. The outer membrane is a unique feature that distinguishes Gram-negative bacteria from Gram-positive bacteria.
3. Lipopolysaccharide (LPS): The outer membrane of Salmonella contains lipopolysaccharide (LPS) molecules. LPS is a complex molecule consisting of lipids and polysaccharides. The lipid portion of LPS, known as lipid A, is responsible for the endotoxic properties of Gram-negative bacteria.
Gram staining involves the following steps:
1. Crystal Violet Staining: All bacteria are stained with crystal violet dye, which enters both Gram-positive and Gram-negative bacterial cells.
2. Gram's Iodine Solution: Gram's iodine solution is added as a mordant, which helps to stabilize the crystal violet-iodine complex within the bacterial cells.
3. Decolorization Step: The stained cells are then treated with an alcohol-acetone decolorizing agent. Gram-positive bacteria retain the crystal violet-iodine complex, while Gram-negative bacteria lose the dye.
4. Safranin Counterstain: Finally, the decolorized Gram-negative bacteria are counterstained with safranin, which imparts a pink or red color to them.
After completing these steps, Gram-positive bacteria appear purple under the microscope due to retained crystal violet, while Gram-negative bacteria, including Salmonella, appear pink or red due to the safranin counterstain.
Overall, Salmonella is Gram-negative due to its thin peptidoglycan layer, the presence of an outer membrane with lipopolysaccharides (LPS), and its negative reaction to the Gram staining procedure, resulting in a pink or red coloration.