thumb|right|300px|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.]]

thumb|300px|Violet-stained gram-positive [[cocci and pink-stained gram-negative bacilli]]

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

thumb|right|300px|Gram-positive and gram-negative [[cell wall structure]]

thumb|right|Structure of gram-positive cell wall

In general, the following characteristics are present in gram-positive bacteria:

  1. Cytoplasmic lipid membrane
  2. Thick peptidoglycan layer
  3. Teichoic acids and lipoids are present, forming lipoteichoic acids, which serve as chelating agents, and also for certain types of adherence.
  4. Peptidoglycan chains are cross-linked to form rigid cell walls by a bacterial enzyme <small>DD</small>-transpeptidase.
  5. 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).

thumb|none|Species identification hierarchy in clinical settings|660px

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 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.

All gram-positive bacteria are bound by a single-unit lipid membrane, and, in general, they contain a thick layer (20–80&nbsp;nm) of peptidoglycan responsible for retaining the Gram stain. A number of other bacteria—that are bound by a single membrane, but stain gram-negative due to either lack of the peptidoglycan layer, as in the mycoplasmas, or their inability to retain the Gram stain because of their cell wall composition—also show close relationship to the gram-positive bacteria. For the bacterial cells bound by a single cell membrane, the term monoderm bacteria has been proposed. Of these two structurally distinct groups of bacteria, monoderms are indicated to be ancestral. Based upon a number of observations including that the gram-positive bacteria are the major producers of antibiotics and that, in general, gram-negative bacteria are resistant to them, it has been proposed that the outer cell membrane in gram-negative bacteria (diderms) has evolved as a protective mechanism against antibiotic selection pressure.

Exceptions

In general, gram-positive bacteria are monoderms and have a single lipid bilayer whereas gram-negative bacteria are diderms and have two bilayers. Exceptions include:

  • Some taxa lack peptidoglycan (such as the class Mollicutes, some members of the Rickettsiales, and the insect-endosymbionts of the Enterobacteriales) and are gram-indeterminate.
  • The Deinococcota have gram-positive stains, although they are structurally similar to gram-negative bacteria with two layers.
  • The Chloroflexota have a single layer, yet (with some exceptions) stain negative. Two related phyla to the Chloroflexi, the TM7 clade and the Ktedonobacteria, are also monoderms.

Some Bacillota species are not gram-positive. The class Negativicutes, which includes Selenomonas, are diderm and stain gram-negative. The spore-forming bacteria can again be divided based on their respiration: Bacillus is a facultative anaerobe, while Clostridium is an obligate anaerobe. Also, Rathybacter, Leifsonia, and Clavibacter are three gram-positive genera that cause plant disease. Gram-positive bacteria are capable of causing serious and sometimes fatal infections in newborn infants. Novel species of clinically relevant gram-positive bacteria also include Catabacter hongkongensis, which is an emerging pathogen belonging to Bacillota. Methicillin-resistant Staphylococcus aureus (MRSA) is a Gram-positive bacterium that has developed resistance to beta-lactam antibiotics, including penicillin, as the drug can no longer bind properly to the bacterial enzymes that build the cell wall.

Bacterial transformation

Transformation is one of three processes for horizontal gene transfer, in which exogenous genetic material passes from a donor bacterium to a recipient bacterium, the other two processes being conjugation (transfer of genetic material between two bacterial cells in direct contact) and transduction (injection of donor bacterial DNA by a bacteriophage virus into a recipient host bacterium). In transformation, the genetic material passes through the intervening medium, and uptake is completely dependent on the recipient bacterium.

Orthography: capitalization

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The adjectives Gram-positive and Gram-negative derive from the surname of Hans Christian Gram; as eponymous adjectives, their initial letter G can be either a capital or lower-case, depending on which style guide, if any, governs the document being written. For instance, the style guide of the American CDC recommends writing: Gram stain, this species is gram negative, and a gram-negative species.

References

  • 3D structures of proteins associated with plasma membrane of gram-positive bacteria
  • 3D structures of proteins associated with outer membrane of gram-positive bacteria