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 have a thick layer of peptidoglycan in their cell wall which retains the stain, unlike the thin layer of peptidoglycan in gram-negative bacteria that does not retain the stain. Gram-positive bacteria retain the crystal violet stain used in the test, resulting in a purple color when observed through an optical microscope. The thick layer of peptidoglycan in the bacterial cell wall retains the stain after it has been fixed in place by iodine. During the decolorization step, the decolorizer removes crystal violet from all other cells. 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 bacteria, 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. Despite 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. For example, penicillin contains a beta-lactam ring that binds to enzymes responsible for cross-linking peptidoglycan in the bacterial cell wall, inhibiting cell wall synthesis. This cross-linking is essential for maintaining the structural integrity of the cell wall, which could lead to cell rupture.
Characteristics
In general, the following characteristics are present in gram-positive bacteria:
Cytoplasmic lipid membrane Thick peptidoglycan layer Teichoic acids and lipoids are present, forming lipoteichoic acids, which serve as chelating agents, and also for certain types of adherence. Peptidoglycan chains are cross-linked to form rigid cell walls by a bacterial enzyme DD-transpeptidase. A much smaller volume of periplasm than that in gram-negative bacteria. Only some species have a capsule, usually consisting of polysaccharides. Only some species are flagellates, and those with flagella have just two basal body rings for support, in contrast to the four found in gram-negative bacteria. Both gram-positive and gram-negative bacteria commonly have a surface layer called an S-layer. In gram-positive bacteria, the S-layer is attached to the peptidoglycan layer. Gram-negative bacteria's S-layer is attached directly to the outer membrane. Specific to gram-positive bacteria is the presence of teichoic acids in the cell wall. Some of these are lipoteichoic acids, which have a lipid component in the cell membrane that can assist in anchoring the peptidoglycan.
Classification Along with cell shape, Gram staining is a rapid method used to differentiate bacterial species. 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).
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). 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. Based on molecular studies of the 16S sequences, Woese recognised twelve bacterial phyla. Two of these were gram-positive and were divided on the proportion of the guanine and cytosine content in their DNA. The high G + C phylum was made up of the Actinobacteria, and the low G + C phylum contained the Firmicutes. The Actinomycetota include the Corynebacterium, Mycobacterium, Nocardia and Streptomyces genera. The (low G + C) Bacillota, have a 45–60% GC content, but this is lower than that of the Actinomycetota.
Importance of the outer cell membrane
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