![]() ![]() Therefore, after the gram stain procedure, gram positive bacteria stain purple or blue. The cell walls of gram positive bacteria contain thick peptidoglycan layers that retain the primary stain of the crystal violet used even after the decolorization stage of the gram staining steps. This is attributed to the cell wall structure of the bacteria. Thus, the stained gram positive bacteria appear purple/ blue under the microscope. Gram staining involves 4 steps which include the application of a primary stain (crystal violet), the addition of a mordant -iodine), decolorizing with alcohol, and counterstaining.ĭuring the gram stain procedure, bacteria that are gram-positive take up the crystal stain used in the test. This differential staining technique is based on the distinct differences in the ultrastructure and chemical composition of the bacterial cell wall, which is due to the presence or absence of an outer lipid membrane. The gram staining response of bacterial cells is an empirical criterion. Classification with these three aspects does not necessarily work for some bacterial species. This classification is ambiguous though because it is based on three distinct characteristics which include staining results, envelope organization, and taxonomic group. Gram stain test and its use in classification of bacteriaĪccording to the gram stain test, bacteria are traditionally divided into two groups:- gram-positive and gram-negative. Bacterial cells that are bounded by a single cell membrane are called monoderms whereas, those bounded by an inner and outer cell membrane are called diderms. This is due to the absence of the outer membrane in the gram-positive cell.įurthermore, there are other bacterial cells that are bounded by a single membrane as the gram-positive bacteria but stain gram-negative due to the lack of the peptidoglycan layer e.g Mycoplasma or their inability to retain the gram stain due to the composition of their cell wall. Nevertheless, despite gram-positive bacteria having a thicker peptidoglycan layer, they are more receptive to certain cell walls targeting antibiotics than gram-negative bacteria. The peptidoglycan layer of the gram-negative cell wall is 2-3 nm thick and covered with an outer lipid bilayer membrane whereas the gram-positive cell wall is a 20-80 nm thick polymer. ![]() Photo credit: Gram positive bacteria have a thick layer of peptidoglycan in their cell wall compared to gram-negative bacterial cells. A picture of gram positive bacteria under a microscope This peptidoglycan is the molecule responsible for retaining the gram stain. Generally, the cell walls of gram positive bacteria contain a 20-80 nm thick layer of peptidoglycan. All gram-positive bacterial cells are bounded by a single-unit lipid membrane. These gram-positive bacteria representative species are pathogenic and cause several diseases. The most common gram-positive bacteria examples in microbiology include Staphylococcus, Streptococcus, Bacillus, Clostridia, Listeria, and Corynebacterium. ![]()
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