Gram Positive and Negative Bacteria – Class 11 | Chapter – 2 | Short Notes Series PDF

Gram Positive and Negative Bacteria: Gram-positive bacteria are bacteria with thick cell walls. In a Gram stain test, these organisms yield a positive result. The test, which involves a chemical dye, stains the bacterium’s cell wall purple.


Gram-negative bacteria, on the other hand, don’t hold the dye. They stain pink instead.

Though both groups of bacteria can cause disease, they require different treatments. If you have a bacterial infection, the Gram stain will determine what kind of medication you need.

Gram-negative bacteria can cause many serious infections, such as pneumonia, peritonitis (inflammation of the membrane that lines the abdominal cavity), urinary tract infections, bloodstream infections, wound or surgical site infections, and meningitis.

Gram-negative bacteria are classified by the color they turn after a chemical process called Gram staining is used on them. Gram-negative bacteria stain red when this process is used.

Gram-positive bacteria stain blue.

Gram-negative and gram-positive bacteria stain differently because their cell walls are different. They also cause different types of infections, and different types of antibiotics are effective against them.

Difference between Gram Positive and Negative Bacteria

Basis of Difference Gram-Positive bacteria Gram-Negative bacteria
Cell Wall A single-layered, smooth cell wall  A double-layered, wavy cell-wall
Cell Wall thickness The thickness of the cell wall is 20 to 80 nanometres The thickness of the cell wall is 8 to 10 nanometres
Peptidoglycan Layer It is a thick layer/ also can be multilayered It is a thin layer/ often single-layered.
Teichoic acids Presence of teichoic acids Absence of teichoic acids
Outer membrane The outer membrane is absent The outer membrane is present (mostly)
Porins Absent Occurs in Outer Membrane
Mesosome It is more prominent. It is less prominent.
Morphology Cocci or spore-forming rods Non-spore forming rods.
Flagella Structure Two rings in basal body Four rings in basal body
Lipid content Very low 20 to 30%
Lipopolysaccharide Absent Present
Toxin Produced Exotoxins Endotoxins or Exotoxins
Resistance to Antibiotic More susceptible More resistant
Examples Staphylococcus, Streptococcus, etc. Escherichia, Salmonella, etc.
Gram Staining  These bacteria retain the crystal violet colour even after they are washed with acetone or alcohol and appear as purple-coloured when examined under the microscope after gram staining. These bacteria do not retain the stain colour even after they are washed with acetone or alcohol and appear as pink-coloured when examined under the microscope after gram staining.

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By Team Learning Mantras