Gram positive bacteria

Bacteria that appear purple on Gram stain.

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 take up the crystal violet stain used in the test.

They appear to be purple-colored when seen through a microscope. 

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.

Conversely, gram-negative bacteria cannot retain the violet stain after the decolorization step.

Alcohol used 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 are more receptive to certain cell wall targeting antibiotics than gram-negative bacteria, due to the absence of the outer membrane.

All gram-positive bacteria are bounded by a single-unit lipid membrane, and, in general, they contain a thick layer of peptidoglycan responsible for retaining the Gram stain. 

There are a number of other bacteria bounded 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.

In contrast to gram-positive bacteria, all typical gram-negative bacteria are bounded by a cytoplasmic membrane and an outer cell membrane; they contain only a thin layer of peptidoglycan (2–3 nm) between these membranes. 

These bacteria have been designated as diderm bacteria.

In general, gram-negative bacteria are resistant to antibiotics and it has been proposed that the outer cell membrane in gram-negative bacteria (diderms) has evolved as a protective mechanism against antibiotic selection pressure.

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

Lack endotoxin.

Infections cannot be differentiated from gram negative bacterial infections.

Characteristics of 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. 

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. 

Gram-positive bacteria have teichoic acids in the cell wall. 

The lipid component in the cell membrane that can assist in anchoring the peptidoglycan.

Gram staining, growth requirements and antibiotic susceptibility testing, and other macroscopic and physiologic tests, are the basis for classification and subdivision of the bacteria.

Many gram-positive organism associated with neutropenic fever such as coagulase negative staphylococci, Corynebacterium spp.,MRSA and vancomycin-resistant enterococci, and viridans are resistant to antimicrobacterial agents frequently used for prophylaxis and empiric treatment.

There are six gram-positive genera are typically pathogenic in humans. 

Two of these, Streptococcus and Staphylococcus, are cocci (sphere-shaped). 

The remaining 4 organisms are bacilli (rod-shaped) and can be subdivided based on their ability to form spores. 

The non-spore formers are Corynebacterium and Listeria (a coccobacillus), whereas Bacillus and Clostridium produce spores.

Bacillus is a facultative anaerobe, while Clostridium is an obligate anaerobe.

Gram-positive bacteria are capable of causing serious infections in newborn infants.

Bacterial transformation occurs by 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, the transfer of genetic material between two bacterial cells in direct contact, and transduction, the injection of donor bacterial DNA by a bacteriophage virus into a recipient host bacterium.

About 80 species of bacteria were known to be capable of transformation, about evenly divided between gram-positive and gram-negative bacteria.

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