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Cell Wall Structure Of Gram Positive And Gram Negative Bacteria Pdf

cell wall structure of gram positive and gram negative bacteria pdf

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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.

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It is important to note that not all bacteria have a cell wall. The two different cell wall types can be identified in the lab by a differential stain known as the Gram stain. Originally, it was not known why the Gram stain allowed for such reliable separation of bacterial into two groups. Once the electron microscope was invented in the s, it was found that the staining difference correlated with differences in the cell walls. Here is a website that shows the actual steps of the Gram stain.

Gram-negative bacteria are bacteria that do not retain the crystal violet stain used in the gram-staining method of bacterial differentiation. Gram-negative bacteria are found everywhere, in virtually all environments on Earth that support life. The gram-negative bacteria include the model organism Escherichia coli , as well as many pathogenic bacteria , such as Pseudomonas aeruginosa , Chlamydia trachomatis , and Yersinia pestis. They are an important medical challenge, as their outer membrane protects them from many antibiotics including penicillin ; detergents that would normally damage the peptidoglycans of the inner cell membrane; and lysozyme , an antimicrobial enzyme produced by animals that forms part of the innate immune system. Additionally, the outer leaflet of this membrane comprises a complex lipopolysaccharide LPS whose lipid A component can cause a toxic reaction when these bacteria are lysed by immune cells. This toxic reaction can include fever, an increased respiratory rate, and low blood pressure—a life-threatening condition known as septic shock.

As mentioned in the previous section on peptidoglycan, Gram-positive bacteria are those that retain the initial dye crystal violet during the Gram stain procedure and appear purple when observed through the microscope. As we will learn in lab, this is a result of the structure and function of the Gram-positive cell wall. Common Gram-positive bacteria of medical importance include Streptococcus pyogenes, Streptococcus pneumoniae, Staphylococcus aureus, Enterococcus faecalis, and Clostridium species. Highlighted Bacterium: Enterococcus species. Click on this link, read the description of Enterococcus , and be able to match the bacterium with its description on an exam. In electron micrographs, the Gram-positive cell wall appears as a broad, dense wall nm thick and consisting of numerous interconnecting layers of peptidoglycan see Figs.

Gram-positive bacteria

In bacteriology , gram-positive bacteria are bacteria that give a positive result in the Gram stain test, which is traditionally used to quickly classify bacteria into two broad categories according to their type of cell wall. Gram-positive bacteria take up the crystal violet stain used in the test, and then appear to be purple-coloured when seen through an optical microscope. This is because the thick peptidoglycan layer in the bacterial cell wall retains the stain after it is washed away from the rest of the sample, in the decolorization stage of the test. Conversely, gram-negative bacteria cannot retain the violet stain after the decolorization step; alcohol used in this stage degrades the outer membrane of gram-negative cells, making the cell wall more porous and incapable of retaining the crystal violet stain. Their peptidoglycan layer is much thinner and sandwiched between an inner cell membrane and a bacterial outer membrane , causing them to take up the counterstain safranin or fuchsine and appear red or pink.

4: Bacteria - Cell Walls

2.3A: The Gram-Positive Cell Wall

Gram Positive vs. Gram Negative Bacteria

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Gram-positive bacteria are bacteria with thick cell walls. In a Gram stain test , these organisms yield a positive result. 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.

Retain crystal violet dye and stain blue or purple. Can be decolorized to accept counterstain safranin and stain pink or red 2 Cell Wall Cell Wall is nm thick. Cell Wall is nm thick.

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