The structure of peptidoglycan, composed of N-acetylglucosamine and N-acetylmuramic acidĪlthough bacteria are traditionally divided into two main groups, gram-positive and gram-negative, based on their Gram stain retention property, this classification system is ambiguous as it refers to three distinct aspects (staining result, envelope organization, taxonomic group), which do not necessarily coalesce for some bacterial species. It has been suggested that this section be split out into another article titled Gram stain. Importance of the outer cell membrane in bacterial classification The (low G + C) Bacillota, have a 45–60% GC content, but this is lower than that of the Actinomycetota. The Actinomycetota include the Corynebacterium, Mycobacterium, Nocardia and Streptomyces genera. The high G + C phylum was made up of the Actinobacteria, and the low G + C phylum contained the Firmicutes. Two of these were gram-positive and were divided on the proportion of the guanine and cytosine content in their DNA. Based on molecular studies of the 16S sequences, Woese recognised twelve bacterial phyla. Based on 16S ribosomal RNA phylogenetic studies of the late microbiologist Carl Woese and collaborators and colleagues at the University of Illinois, the monophyly of the gram-positive bacteria was challenged, with major implications for the therapeutic and general study of these organisms. Historically, the kingdom Monera was divided into four divisions based primarily on Gram staining: Bacillota (positive in staining), Gracilicutes (negative in staining), Mollicutes (neutral in staining) and Mendocutes (variable in staining). Species identification hierarchy in clinical settings Such staining, together with growth requirement and antibiotic susceptibility testing, and other macroscopic and physiologic tests, forms a basis for practical classification and subdivision of the bacteria (e.g., see figure and pre-1990 versions of Bergey's Manual of Systematic Bacteriology). Classification Īlong with cell shape, Gram staining is a rapid method used to differentiate bacterial species. Some of these are lipoteichoic acids, which have a lipid component in the cell membrane that can assist in anchoring the peptidoglycan. Specific to gram-positive bacteria is the presence of teichoic acids in the cell wall. Gram-negative bacteria's S-layer is attached directly to the outer membrane. In gram-positive bacteria, the S-layer is attached to the peptidoglycan layer. Both gram-positive and gram-negative bacteria commonly have a surface layer called an S-layer. Also, only some species are flagellates, and when they do have flagella, have only two basal body rings to support them, whereas gram-negative have four. Only some species have a capsule, usually consisting of polysaccharides. A much smaller volume of periplasm than that in gram-negative bacteria.Peptidoglycan chains are cross-linked to form rigid cell walls by a bacterial enzyme DD-transpeptidase.Teichoic acids and lipoids are present, forming lipoteichoic acids, which serve as chelating agents, and also for certain types of adherence. In general, the following characteristics are present in gram-positive bacteria: Characteristics Gram-positive and gram-negative cell wall structure Structure of gram-positive cell wall 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.ĭespite their thicker peptidoglycan layer, gram-positive bacteria are more receptive to certain cell wall–targeting antibiotics than gram-negative bacteria, due to the absence of the outer membrane. This is because the thick layer of peptidoglycan 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.Ĭonversely, 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. 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. 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. Violet-stained gram-positive cocci and pink-stained gram-negative bacilli Bacteria that give a positive result in the Gram stain test Rod-shaped gram-positive Bacillus anthracis bacteria in a cerebrospinal fluid sample stand out from round white blood cells, which also accept the crystal violet stain.
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