In microbiology, they called Gram – positive bacteria or Gram-positive bacteria , those bacteria that are stained dark blue or violet by Gram staining. This chemical characteristic is intimately linked to the structure of the cell envelope, so it reflects a natural type of bacterial organization. They are one of the main groups of bacteria, and when they are treated as a taxon the name Posibacteria is also used. The rest are gram-negative bacteria. The cellular envelope of gram-positive bacteria comprises the cytoplasmic membrane and a cell wall composed of a thick layer of peptidoglycan , which surrounds the former. The cell wall is attached to the cytoplasmic membrane by molecules oflipoteicoic acid . The peptidoglycan layer confers great resistance to these bacteria and is responsible for retaining the dye during Gram staining. Unlike gram-positive bacteria, gram-negative bacteria have a second lipid membrane external to the cell wall. They include both mobile (via flagella) and immobile species with the shape of a bacillus (Bacillus, Clostridium , Corynebacterium , Lactobacillus , Listeria ) or coconut ( Staphylococcus , Streptococcus ); with or without thick cell walls ( Mycoplasma). Some species are photosynthetic, but most are heterotrophic. Many of these bacteria form endospores under unfavorable conditions. Actually, not all bacteria in the group are gram-positive (they do not stain by applying this method), but they are included here for their molecular similarity to other gram-positive bacteria.
Structure
The bacterial cell is surrounded by an envelope that, observed under an electron microscope, appears as a thick and homogeneous layer, called the cell wall. Then in section (cut) a structure similar to two parallel lines is observed separating a less dense layer; this corresponds to the plasma membrane. Between the plasma membrane and the cell wall is the periplasm or periplasmic space. Inside the plasma membrane is the cytoplasm that is made up of an aqueous solution, the cytosol, in which ribosomes and other aggregates of macromolecules are found, and in the center is the least dense area called nucleoid, which contains a skein of strands difficult to solve (distinguish) and whose main component is DNA. The fundamental chemical base of the outer wall of the cell envelope of a gram-positive bacterium is peptidoglycan, which is a polymer of N-acetylglucosamine, bound in β-1 orientation, with N-acetylmuramic acid, to which are added by the lactyl group four or more amino acids. This molecule polymerizes a large number of times, so that a special mesh is formed, called a murein saccule. Said compound is of vital importance to preserve the shape and stiffen the bacterial cell (if this compound did not exist, the cell would burst due to its great osmotic potential). The following characteristics are generally present in gram-positive bacteria: Four or more amino acids are added to it by the lactyl group. This molecule polymerizes a large number of times, so that a special mesh is formed, called a murein saccule. Said compound is of vital importance to preserve the shape and stiffen the bacterial cell (if this compound did not exist, the cell would burst due to its great osmotic potential). The following characteristics are generally present in gram-positive bacteria: Four or more amino acids are added to it by the lactyl group. This molecule polymerizes a large number of times, so that a special mesh is formed, called a murein saccule. This compound is of vital importance to preserve the shape and stiffen the bacterial cell (if this compound did not exist, the cell would burst due to its great osmotic potential). The following characteristics are generally present in gram-positive bacteria:
- Cytoplasmic membrane.
- Thick layer of peptidoglycan.
- Theicoic and lipoteicoic acids, which serve as chelating agents and in certain types of adhesion.
- Capsule polysaccharides.
- If any flagellum is present, it contains two rings as a support as opposed to the four that exist in gram-negative bacteria because gram-positive bacteria have only one membrane layer.
Both gram-positive and gram-negative bacteria can have a crystalline surface layer called the 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 . The presence of theicoic acids in the cell wall is unique to gram-positive bacteria . Some particular theicoic acids, lipoteicoic acids, have a lipid component and can assist in anchoring the peptidoglycan , as long as the lipid component is integrated into the membrane. During the Gram stainthe dye (crystal violet) penetrates all bacterial cells (both gram-positive and gram-negative) through the bacterial wall. Subsequently, the alcohol-acetone mixture discolors the gram-negative bacteria, but not the gram-positive bacteria that remain violet-dark blue.
Phylogeny of gram-positive bacteria
Five edges of gram-positive bacteria are recognized.
- One of them is Firmicutes, which includes many well-known genera such as Bacillus, Listeria, Staphylococcus, Streptococcus, Enterococcus, and Clostridium.
- The Tenericutes (= Mollicutes) phylum includes Mycoplasma and other related bacteria, which have lost cell walls and cannot be stained by the Gram method, but are derived in such ways.
- The third edge is Actinobacteria, which includes some of the most typical bacteria of terrestrial life, playing an important role in the decomposition of organic matter. These and the Firmicutes are referred to respectively as high and low G + C groups, based on the content of guanosine and cytosine in their DNA.
- The Chloroflexi photosynthetic edge is unusual because although they are monodermal (single membrane), they stain Gram negative in most cases because their outer peptidoglycan wall is very thin.
- The last edge, Thermomicrobia, includes heterotrophic and thermophilic bacteria related to the previous ones.
Deinococcus-Thermus bacteria also show gram-positive bands, however they are classified phylogenetically with gram-negative bacteria, as they are structurally similar to these. It is unknown whether gram-positive bacteria are derived from gram-negative bacteria or vice versa. If the second membrane (the outer membrane) is a derived condition, the Chloroflexi , Firmicutes, and Actinobacteria phyla could be basal among bacteria; otherwise they would probably be recent monophyletic groups.
- One option is to have gram-positive bacteria appear first, since they are the simplest. It has been proposed that when a group of these bacteria developed the mechanisms to produce antibiotics (such as the Streptomyces bacteria, for example ), selective pressure forced the others to protect themselves from the effects of these substances. One of these strategies was to develop a double membrane, giving rise to gram-negative bacteria.
- The alternative option considers that the gram-positive bacteria are the most recent, originating from the negative gram as the peptidoglycan wall grows, preventing the transfer of lipids to form the outer membrane.
Surprisingly within Firmicutes, which is a typical monodermal phylum, there are two groups, Negativicutes and Halanaerobiales , which are fully dimeric bacteria. Since these groups are no longer related to each other than to the rest of the firmicutes, it is speculated that a dimeric cell wall was present in the ancestor of Firmicutes.
