The bacterial cell

Chemical composition

the priority component of the bacterial cell is water, which represents 80% of the cell mass and the solvent in which the various components are dispersed, organic (lipids, proteins, polysaccharides and nucleic acids) and inorganic (minerals such as sodium, zinc , phosphorus, iron, calcium and sulfur.).

Nucleus

the bacterium is a prokaryotic cell and as such it is distinguished from the eukaryotic one (typical of man, but also of plants, animals and fungi), primarily due to the absence of a nuclear membrane. Inside the bacterial cell we would therefore have a single chromosome, immersed directly in the cytoplasm and containing DNA wrapped in a superspiralized circular structure. Usually this DNA is in close association with particular regions of the plasma membrane (MESOSOMI), where the enzymes for the bacterial replication and for energy production (oxidative phosphorylation).

Bacterial ribosomes

Inside the bacterial cells we find the ribosomes, smaller than the eukaryotic ones and with different structure and constant of sedimentation [70s in bacteria (greater subunit 50s, smaller 30s) and 80s in eukaryotes (greater subunit 70s, smaller 40s)]. made up of proteins and RNA, formed from chromosomal DNA through the transcription process.

The differences that separate bacterial ribosomes from human ones (remember that the ribosome is the "cellular organelle responsible for protein synthesis) has allowed the development of selective drugs, capable of inhibiting bacterial protein synthesis without interfering with the human one.

Plasma Membrane

The plasma membrane of the bacterium is very similar to the eukaryotic one, although thinner; first of all we can recognize the typical phospholipid bilayer, in which glycoproteins and glycolipids are immersed. The functions are also similar, since the bacterial plasma membrane regulates exchanges with the environment. On the outside we find a characteristic structure, the bacterial wall. It is very important to underline, in this regard, that GRAM + bacteria only possess the plasma membrane and the cell wall, while in GRAMs - there is an additional structure, called the outer membrane.

Bacterial Wall

The Bacterial Wall provides the bacterium with rigidity and strength, preventing it from being damaged when it is in an environment with reduced osmotic pressure; it also carries out defense functions against phagocytosis, and regulates the exchange of nutrients and metabolites with the outside world (in synergy with the plasma membrane).

The main constituent of the bacterial part is a polymer called peptidoglycan, thicker in GRAM + bacteria and thinner in GRAM -. The two monomers that constitute it are amino sugars, called N-acetylgucosamine (NAG) and acetyl muranic acid (NAM), joined together by glycosidic bonds B 1-4 and B 1-6. 5 amino acids are linked to each molecule of N-acetyl muranic acid, of which the 1st is L-alanine, while the last two are made up of D-alanine.

Many NAG and NAM monomers then give rise to a peptidoglycan molecule, and more peptidoglycan molecules bind together to form the bacterial wall. This association is guaranteed by the action of an enzyme, called TRANSPEPTIDASE, which gives rise to a peptide bond between the third amino acid of a chain and the fourth of the parallel chain. The energy required to operate this union is provided by the loss of the fifth amino acid. , which we remember being a D-alanine. Penicillin, a well-known antibiotic, acts at this level, preventing the link between the third and fourth amino acids of the two parallel chains. Lysozyme, a powerful antibacterial present - among other things - in saliva and tears, on the other hand breaks the B 1-4 bond that holds the monomers NAM and NAG together.

In GRAM bacteria - the link between the third and fourth amino acids is direct, while in GRAM positive it is mediated by 5 wisteria (pentaglycine bridge).

As important as it is, the cell wall is not an indispensable structure for the life of the cell, so much so that some bacteria do not have it. Inside it there may also be molecules called THEIC ACIDS, typical of GRAM positive bacteria, but also present in GRAM -; these are polyvalent alcohol polymers (glycerol), associated with amino acids and sugars, which have the purpose of hindering the degradation of peptidoglycan by lysozyme and other bactericidal agents.

External membrane

Typical and exclusive of GRAM -, it is associated with the bacterial wall through lipoproteins. It consists of two sheets, of which:

the innermost is phospholipid in nature;
while the exterior is made up of a repeated liposaccharide molecule, the so-called LPS (or lipopolysaccharide).

The LPS lipopolysaccharide can in turn be divided into three layers:

the innermost one, of a lipid nature, is called LIPID A; it is the same for all GRAM bacteria - and constitutes its toxic component (ENDOTOXIN); therefore many of the classic clinical symptoms of a "GRAM- infection can be traced back to lipid A, among which fever is undoubtedly the most common ailment.
The central part, of polysaccharide nature, is called C (or core) and is the same for all bacteria.
The outer part is called ANTIGEN O, it is always polysaccharide in nature, but it is different from bacterium to bacterium.

The outer membrane also recognizes very small proteins, called porins, which regulate the intake of nutrients, but also of other substances, such as the antibiotics themselves (they oppose their entry).

COMPARED TO THE EUKARYOTIC CELL: in addition to the differences already listed, bacterial cells lack some complex structures typical of eukaryotes (endoplasmic reticulum, mitochondria, Golgi apparatus, chloroplasts, centrioles and mitotic spindle).