Gram-positive
Gram-positive bacteria are classified as bacteria that retain a crystal violet dye during the
gram stain process. Gram-positive bacteria will appear blue or violet under a microscope, whereas
gram-negative bacteria will appear red or pink. The difference in classification is largely based on a difference in the bacteria's cell wall structure.
The following characteristics are generally present in a gram-positive bacteria:#A very thick cell wall (
peptidoglycan)#If a flagella is present, it contains two rings for support (an additional two rings are not necessary, unlike in gram-negative bacteria, because the thick cell wall is supportive enough)#
Teichoic acids are present, which serve to act as chelating agents, and also for certain types of adherence
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.
The
actinobacteria are another major group of Gram-positive bacteria; they and the Firmicutes are referred to as the high and low G+C groups based on the
guanosine and
cytosine content of their
DNA. If the second membrane 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.
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Gram-negative*
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