Gram-positive bacteria

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.

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 cells, making the cell wall more porous and incapable of retaining the crystal violet stain.

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.

[2] Although bacteria are traditionally divided into two main groups, gram-positive and gram-negative, based on their Gram stain retention property, this classification system is ambiguous as it refers to three distinct aspects (staining result, envelope organization, taxonomic group), which do not necessarily coalesce for some bacterial species.

[6][7][8][9] 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.

[6][10] All gram-positive bacteria are bounded by a single-unit lipid membrane, and, in general, they contain a thick layer (20–80 nm) of peptidoglycan responsible for retaining the Gram stain.

[6][10] The distinction between the monoderm and diderm bacteria is supported by conserved signature indels in a number of important proteins (viz.

Negativicutes, Fusobacteriota, Synergistota, and Elusimicrobiota) that are either part of the phylum Bacillota or branch in its proximity are found to possess a diderm cell structure.

[9][11][12] However, a conserved signature indel (CSI) in the HSP60 (GroEL) protein distinguishes all traditional phyla of gram-negative bacteria (e.g., Pseudomonadota, Aquificota, Chlamydiota, Bacteroidota, Chlorobiota, "Cyanobacteria", Fibrobacterota, Verrucomicrobiota, Planctomycetota, Spirochaetota, Acidobacteriota, etc.)

[19] Novel species of clinically relevant gram-positive bacteria also include Catabacter hongkongensis, which is an emerging pathogen belonging to Bacillota.

[21][22] In transformation, the genetic material passes through the intervening medium, and uptake is completely dependent on the recipient bacterium.