Gram Positive vs. Gram Negative Bacteria

Gram Positive vs Gram Negative Bacteria
This is a mixed culture of Gram negative Escherichia coli (red-orange) and Gram positive Staphylococcus aureus (blue-purple) stained using the Gram staining method.

Michael R Francisco/Flickr/CC BY-SA 2.0

Most bacteria are classified into two broad categories: Gram positive and Gram negative. These categories are based on their cell wall composition and reaction to the Gram stain test. The Gram staining method, developed by Hans Christian Gram, identifies bacteria based upon the reaction of their cell walls to certain dyes and chemicals.

The differences between Gram positive and Gram negative bacteria are primarily related to their cell wall composition. Gram positive bacteria have cell walls composed mostly of a substance unique to bacteria known as peptidoglycan, or murein. These bacteria stain purple after Gram staining. Gram negative bacteria have cell walls with only a thin layer of peptidoglycan and an outer membrane with a lipopolysaccharide component not found in Gram positive bacteria. Gram negative bacteria stain red or pink after Gram staining.

Gram Positive Bacteria

The cell walls of Gram positive bacteria differ structurally from the cell walls of Gram negative bacteria. The primary component of bacterial cell walls is peptidoglycan. Peptidoglycan is a macromolecule composed of sugars and amino acids that are assembled structurally like woven material. The amino sugar component consists of alternating molecules of N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM). These molecules are crosslinked together by short peptides which help give peptidoglycan strength and structure. Peptidoglycan provides protection for bacteria and defines their shape.

Gram Positive Cell Wall
This image shows the cell wall composition of Gram positive bacteria. CNX OpenStax/Wikimedia Commons/CC BY-SA 4.0

The Gram positive cell wall has several layers of peptidoglycan. The thick layers of peptidoglycan help to support the cell membrane and provide a place of attachment for other molecules. The thick layers also enable Gram positive bacteria to retain most of the crystal violet dye during Gram staining causing them to appear purple. Gram positive cell walls also contain chains of teichoic acid that extend from the plasma membrane through the peptidoglycan cell wall. These sugar-containing polymers assist in maintaining cell shape and play a role in proper cell division. Teichoic acid helps some Gram positive bacteria to infect cells and cause disease.

Some Gram positive bacteria have an additional component, mycolic acid, in their cell walls. Mycolic acids produce a waxy outer layer that provides additional protection for mycobacteria, such as Mycobacterium tuberculosis. Gram positive bacteria with mycolic acid are also called acid-fast bacteria because they require a special staining method, known as acid-fast staining, for microscope observation.

Pathogenic Gram positive bacteria cause disease by the secretion of toxic proteins known as exotoxins. Exotoxins are synthesized within the prokaryotic cell and released into the exterior of the cell. They are specific to certain bacterial stains and can cause serious damage to body organs and tissues. Some Gram negative bacteria also produce exotoxins.

Gram Positive Cocci

Gram positive cocci refer to Gram positive bacteria that are spherically shaped. Two genera of Gram positive cocci noted for their role as human pathogens are Staphylococcus and Streptococcus. Staphylococcus are spherical in shape and their cells appear in clusters after they divide. Streptococcus cells appear as long chains of cells after division. Examples of Gram positive cocci that colonize the skin include Staphylococcus epidermidis, Staphylococcus aureus, and Streptococcus pyogenes.

Staphylococcus aureus
Staphylococcus aureus is a Gram-positive coccus (round) bacteria that is found on the skin and mucous membranes of humans and many animals. The bacteria are usually harmless, but infections can occur on broken skin or within a blocked sweat or sebaceous gland, resulting in boils, pustules and abscesses. Paul Gunning/Science Photo Library/Getty Images

While all three are part of the normal human microbiota, they can cause disease under certain conditions. Staphylococcus epidermidis form thick biofilms and can cause infections associated with implanted medical devices. Some Staphylococcus aureus strains, such as methicillin-resistant Staphylococcus aureus (MRSA), have become resistant to antibiotics and can lead to the development of serious illness. Streptococcus pyogenes can cause strep throat, scarlet fever, and flesh-eating disease.

Gram Negative Bacteria

Like Gram positive bacteria, the Gram negative bacterial cell wall is composed of peptidoglycan. However, the peptidoglycan is a single thin layer compared to the thick layers in Gram positive cells. This thin layer does not retain the initial crystal violet dye but picks up the pink color of the counterstain during Gram staining. The cell wall structure of Gram negative bacteria is more complex than that of Gram positive bacteria. Located between the plasma membrane and the thin peptidoglycan layer is a gel-like matrix called periplasmic space. Unlike in Gram positive bacteria, Gram negative bacteria have an outer membrane layer that is external to the peptidoglycan cell wall. Membrane proteins, murein lipoproteins, attach the outer membrane to the cell wall.

Gram negative Cell Wall
This image shows the cell wall composition of Gram negative bacteria. CNX OpenStax/Wikimedia Commons/CC BY-SA 4.0

Another unique characteristic of Gram negative bacteria is the presence of lipopolysaccharide (LPS) molecules on the outer membrane. LPS is a large glycolipid complex that protects bacteria from harmful substances in their environment. It is also a bacterial toxin (endotoxin) that can cause inflammation and septic shock in humans if it enters the blood. There are three components of the LPS: Lipid A, a core polysaccharide, and an O antigen. The lipid A component attaches the LPS to the outer membrane. Attached to the lipid A is the core polyssaccharide. It is located between the lipid A component and the O antigen. The O antigen component is attached to the core polyssaccharide and differs between bacterial species. It can be used to identify specific strains of harmful bacteria.

Gram Negative Cocci

Gram negative cocci refer to Gram negative bacteria that are spherically shaped. Bacteria of the genus Neisseria are examples of Gram negative cocci that cause disease in humans. Neisseria meningitidis is diplococcus, meaning that its spherical cells remain in pairs after cell division. Neisseria meningitidis causes bacterial meningitis and can also cause septicemia and shock.

Neisseria meningitidis
Neisseria meningitidis are spherical, Gram negative bacteria which cause meningitis in humans. The bacteria are typically seen in pairs, each one concave on the side facing its partner. Health Protection Agency/Science Photo Library/Getty Images

Another diplococcus bacterium, N. gonorrhoeae, is the pathogen responsible for the sexually transmitted disease gonorrhea. Moraxella catarrhalis is a Gram negative diplococcus that causes ear infections in children, upper respiratory system infections, endocarditis, and meningitis.

Gram negative coccobacillus bacteria have bacterial shapes that are in between spherical and rod shaped. Bacteria of the genus Haemophilus and Acinetobacter are coccobacilli that cause serious infections. Haemophilus influenzae can cause meningitis, sinus infections, and pneumonia. Acinetobacter species cause pneumonia and wound infections.

Key Points: Gram Positive vs. Gram Negative Bacteria

  • Most bacteria can be broadly classified as Gram positive or Gram negative.
  • Gram positive bacteria have cell walls composed of thick layers of peptidoglycan.
  • Gram positive cells stain purple when subjected to a Gram stain procedure.
  • Gram negative bacteria have cell walls with a thin layer of peptidoglycan. The cell wall also includes an outer membrane with lipopolysaccharide (LPS) molecules attached.
  • Gram negative bacteria stain pink when subjected to a Gram stain procedure.
  • While both Gram positive and Gram negative bacteria produce exotoxins, only Gram negative bacteria produce endotoxins.

Additional References

  • Silhavy, T. J., et al. "The Bacterial Cell Envelope." Cold Spring Harbor Perspectives in Biology, vol. 2, no. 5, 2010, doi:10.1101/cshperspect.a000414.
  • Swoboda, Jonathan G., et al. "Wall Teichoic Acid Function, Biosynthesis, and Inhibition." ChemBioChem, vol. 11, no. 1, June 2009, pp. 35–45., doi:10.1002/cbic.200900557.
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Bailey, Regina. "Gram Positive vs. Gram Negative Bacteria." ThoughtCo, Feb. 17, 2021, Bailey, Regina. (2021, February 17). Gram Positive vs. Gram Negative Bacteria. Retrieved from Bailey, Regina. "Gram Positive vs. Gram Negative Bacteria." ThoughtCo. (accessed March 6, 2021).