Microscopic image of a cell wall

1. OVERVIEW OF CELL WALL COMPONENTS

The cell wall is one of the most important features of plant, fungi, and bacterial cells. It is located outside the cell membrane, and its main function is to surround and protect the cell, as well as giving the cell its shape and configuration. Some bacteria have long appendages that increase the surface area of the cell and allow it to live in very dilute environments. Any cell that has lost its cell wall becomes amorphic, or without a defined shape. The cell wall acts as a rigid platform for surface appendages, and as an attachment sites for bacteriophages. The cell wall also aids in countering the effects of osmotic pressure, in that the strength of the cell wall prevents the cell from bursting when intracellular osmolarity is much greater than the extracellular osmolarity.

The primary composition of plant cell walls is cellulose, which is normally combined with woody, fatty, or gummy substances, and with the exception of some insects, is not found in animal tissues. When animals consume plants that contain the cellulose, microorganisms in the digestive tract break down the cellulose into products that the animal can absorb. The cell walls of fungi differ from those of the plants in that their major component is chitin. The cell walls of bacteria are extremely varied, but do contain a rather large molecule know as murein (or peptidoglycan).

2. OVERVIEW OF CELL WALL ROLE IN THE GRAM STAIN

One of the most widely used methods of identifying bacteria is the gram stain, and the cell wall plays an important role in the identification process. In the gram stain method, bacteria are first stained with gentian violet iodine, followed by a solution of iodine in potassium iodide. The slides are then washed with ethyl alcohol, and if viewed under the microscope will either obtain a strong violet color, or be completely decolorized. Sometimes a counterstain is applied to give the give the decolorized bacteria a reddish appearance to make them more visible. The difference in color of the two types of bacteria is relative to the cell wall of each type. Bacteria that remain violet are classified as gram positive bacteria. The cell wall of the gram positive bacteria are relatively thick, and contain a dense layer of peptidoglycan. It is the thickness of the cell wall that traps the dark violet color and resists damage from the solvent. The gram positive cell wall also contains teichoic acids, which are polymers of glycerol or ribitol joined by phosphate groups. Amino acids, such as D-alanine are attached. Teichoic acid is covalently linked to muramic acid and links various layers of the peptidoglycan mesh together. The following image shows a cross-section of the two types of bacterial cell walls.

Gram negative bacteria, on the other hand, have a much thinner cell wall. The wall is high in lipid content and low in peptidiglycan. The crystal-violate escapes from cell when the decolorizer is added because of the lack of perptidglycan. Gram negative cell walls are more complicated than gram positive because there are two separate areas with an additional membrane besides the cellular membrane. The outer membrane of gram negative bacteria is composed of a high concentration of lipids, polysaccharides and proteins. Outside of the central membrane is a open area called the periplasmic space. Beyond this is a thin layer of peptidoglycan. Finally, external to the peptidoglycan is an additional membrane called the outer membrane.

Gram negative bacteria produce even-numbered straight chained cyclopropyl fatty acids, that can be helpful in determining microbial biomass. "The fatty acids of arbuscular mycorrhizal fungi offer special opportunities for identification and quantification, and provide an infection index for plant roots" (Paul & Clark, 49). This should be done in situ, or in the soil to allow an accurate reading. The following two images show gram positive bacteria on the left and gram negative bacteria on the right.

3. PROS AND CONS

Overall, the gram stain is an excellent way to distinguish between gram positive and gram negative bacteria, but if the procedure is not done correctly, misleading results can occur. One common problems associated with the gram stain procedure is an incorrect reaction caused by improper decolorization. Gram positive cells can often decolorize too long and appear gram negative, and in contrast, gram negative cells can decolorize for too short a period of time and appear gram positive. Knowing how to correctly perform the gram stain and at what time each component of the procedure should be added is critical in accurately staining and identifying the sample bacteria.

4. ADDITIONAL SOURCES OF INFORMATION

Clark, F.E. and Paul, E.A. 1996. Soil Microbiology and Biochemistry Academic Press, Inc., San Diego, CA. pp. 47-49.

Popescu, A., Doyle, R.J. 1986. The Gram Stain After More Than a Century Biotechnic & Histochemistry, v71, No. 3, pp. 145

Rogers, H.J. 1968. Cell Walls and Membranes E. & F.N. Spon Ltd., London, England. pp 219-221.

5. LINKS TO OTHER SITES ON CELL WALL COMPONENTS

A great site from Hardvard University dedicated to cell biology: http://vl.bwh.harvard.edu/

The Virtual Cell Web Page; 3D images of cells: http://personal.tmlp.com/Jimr57/index.htm

Gram stain site: http://www.kcom.edu/faculty/chamberlain/Website/lab/idlab/gramst.htm