Gram-positive bacteria

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 cell wall. 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 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. 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. 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. In general, the following characteristics are present in gram-positive bacteria: Only some species have a capsule, usually consisting of polysaccharides. 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. 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. 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 the full basis for classification and subdivision of the bacteria (e.g., see figure and pre-1990 versions of Bergey's Manual). Historically, the kingdom Monera was divided into four divisions based primarily on Gram staining: Firmicutes (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 both 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 Actinobacteria include the Corynebacterium, Mycobacterium, Nocardia and Streptomyces genera. The (low G + C) Firmicutes, have a 45–60% GC content, but this is lower than that of the Actinobacteria. Although 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. The gram-positive and gram-negative staining response is also not a reliable characteristic as these two kinds of bacteria do not form phylogenetic coherent groups. However, although Gram staining response is an empirical criterion, its basis lies in the marked differences in the ultrastructure and chemical composition of the bacterial cell wall, marked by the absence or presence of an outer lipid membrane.