Gram-positive bacteria
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Gram-positive Bacillus anthracis bacteria (purple rods) in cerebrospinal fluid sample. The other cells are white blood cells

Gram-positive bacteria are those that are stained dark blue or violet by Gram staining, in contrast to Gram-negative bacteria, which cannot retain the stain, instead taking up the counterstain and appearing red or pink. The stain is retained by a high amount of peptidoglycan in the cell wall, which typically, but not always, lacks the secondary membrane and lipopolysaccharide layer found in Gram-negative bacteria. The peptidoglycan can absorb the crystal violet, commonly used to stain bacteria.

The main purpose of Gram staining is to visually differentiate groups of bacteria, primarily for identification.

Characteristics

Gram-positive and -negative cell wall structure

Structure of Gram-positive cell wall

The following characteristics are generally present in a Gram-positive bacterium:^[1]

1. Cytoplasmic membrane
2. Thick peptidoglycan layer
3. Teichoic acids and lipoteichoic acids are present, which serve to act as chelating agents, and also for certain types of adherence.
4. Capsule polysaccharides
5. If a flagellum is present, it contains two rings for support as opposed to four in Gram-negative bacteria because Gram-positive bacteria have only one membrane layer.

Classification

In the original bacterial phyla, the Gram-positive forms made up the phylum Firmicutes, a name now used for the largest group. It includes many well-known genera such as Bacillus, Listeria, Staphylococcus, Streptococcus, Enterococcus, and Clostridium. It has also been expanded to include the Mollicutes, bacteria like Mycoplasma that lack cell walls and so cannot be stained by Gram, but are derived from such forms.

Actinobacteria are the other major group of Gram-positive bacteria which along with the Firmicutes are referred to as the high and low G+C groups respectively, depending on the guanosine and cytosine content of their DNA. If the second membrane (of Gram-negative bacteria) is a derived condition, the two may have been basal among the bacteria; otherwise they are probably a relatively recent monophyletic group. They have been considered as possible ancestors for the archaeans and eukaryotes, both because they are unusual in lacking the second membrane and because of various biochemical similarities such as the presence of sterols.

The Deinococcus-Thermus bacteria also have Gram-positive stains, although they are structurally similar to Gram-negative bacteria.

Both Gram-positive and Gram-negative bacteria may have a membrane called an S-layer. In Gram-negative bacteria, the S-layer is directly attached to the outer membrane. In Gram-positive bacteria, the S-layer is attached to the peptidoglycan layer. Unique to Gram-positive bacteria is the presence of teichoic acids in the cell wall. Some particular teichoic acids, lipoteichoic acids, have a lipid component and can assist in anchoring peptidoglycan, as the lipid component is embedded in the membrane.

See also

• Gram-negative

References

• Baron, Samuel (1996). Medical Microbiology, 4th ed., The University of Texas Medical Branch at Galveston. ISBN 0-9631172-1-1. 

External links

• This article contains material from the Science Primer published by the NCBI, which, as a U.S. government publication, is in the public domain.
• 3D structures of proteins associated with plasma membrane of Gram-positive bacteria
• 3D structures of proteins associated with cell wall of Gram-positive bacteria