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 group of bacteria also have gram-positive stains, although they are structurally similar to gram-negative bacteria.
Gram staining (or Gram's method) is an empirical method of differentiating bacterial species into two large groups based on the chemical and physical properties of their cell walls.
The method is named after the inventor, the Danish scientist Hans Christian Gram (1853-1938), who developed the technique in 1884 to discriminate between pneumococci and Klebsiella pneumoniae bacteria.
Gram stains are performed on body fluid or biopsy when infection is suspected.
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