Differences Between Gram Positive and Gram Negative Bacterial Cell Walls

Bacterial species in general can be divided into two groups–gram positive and gram negative bacteria. In order to see their difference, a method known as Gram staining is utilized. Gram staining is a type of differential staining technique largely used as a diagnostic tool not just in research but also in the clinical field, since it detects the difference among the cell walls of different bacteria. Sometimes, the results of this simple technique are all that the physician needs in order to treat a certain disease. The reagents used in this technique are crystal or gentian violet, which is the primary stain, and iodine, which holds the primary stain. Safranin is used as the counterstain. This technique is named after its inventor, Hans Christian Gram.

A closer look on gram positive bacteria


Gram-positive cell wall diagram

Structure of Gram-positive cell wall

When Gram staining is done, gram positive bacteria can be seen as having darker shades than gram negative ones, usually dark purple or dark blue. This color change seen in the process of Gram staining is largely due to the presence of a thick layer of peptidoglycan commonly present in gram positive bacteria, usually comprising 60 to 90% of their cell walls. Gram positive bacteria also do not have any outer membrane or periplasmic space. However, their cell walls contain teichoic acid, composed of alcohol and phosphates. These teichoic acids are negatively charged particles, enabling them to regulate the movement of positive ions in and out of the cell. Moreover, teichoic acid is primarily responsible for making gram positive bacteria easily detectable because of its antigenic specificity.

Most pathogens known today are classified as gram positive bacteria, such as Staphylococcus and Streptococcus. The protein formation in their cell walls provides them their needed protection. The good news is that most of the diseases caused by gram positive bacteria can be treated through antibiotic therapy, since medical researchers are already quite advanced in the treatment and elimination of these kinds of bacteria. Gram positive bacteria also have other uses, such as the preservation of different food items.


A closer look on gram negative bacteria



Structure of gram-negative cell wall

On the other hand, gram negative bacteria yields pinkish to reddish colors when subjected to Gram staining. This is because they do not have the ability to retain their original colors. Unlike gram positive bacteria, gram negative bacteria do not have a thick peptidoglycan layer; rather, it is only a single, thin layer that covers the cell wall of the bacteria. However, even if they have thin cell walls, they are actually more complex than gram positive bacteria. They also have a periplasmic space and an outer membrane, plus they also have high lipopolysaccharide, lipid, and lipoprotein content. Because of the lipopolysaccharide outer membrane, it is usually resistant to dyes, detergents, and some antibiotics, like the basic penicillin.

Some of the more popular gram negative bacteria include Salmonella, Shigella, and Escherichia coli, among others. There are actually a lot of gram negative bacteria that are medically relevant, especially in sexually transmitted diseases, respiratory problems, gastrointestinal diseases, and even nervous system disorders, like meningitis.

The differences summarized


Gram Cell Wall Diagram

Gram-positive- and negative bacteria are chiefly differentiated by their cell wall structure.

Now that the different components of both bacteria have already been described, they can simply be differentiated based on their cell walls. Gram positive bacteria have a thick protective peptidoglycan layer surrounding it, while gram negative bacteria have an inner membranous layer. Hence, the cell wall of the gram positive bacteria is stronger than that of the gram negative bacteria. These cell wall layers are also responsible for the colors that they get after being subjected to Gram staining. Gram positive bacteria become dark purple or blue in color because of the presence of peptidoglycan, while gram negative bacteria become pinkish or reddish in color because of the lack of peptidoglycan in the cell wall.


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