Main Components in Cell Wall, Cytoplasmic Membrane and Cytoplasm of Bacteria

Wall of the Cell

With the exception of Mycoplasma species, which are confined by a cell membrane rather than a cell wall, the outermost feature shared by all bacteria is the cell wall. Some bacteria have surface characteristics that are not part of the cell wall, like capsules, flagella, and pili, which are less frequent parts and are covered in the section that follows.
Peptidoglycan makes up the cell wall, which is situated outside of the cytoplasmic membrane. The peptidoglycan preserves the cell's distinctive form and offers structural support.

Gram-positive bacteria have a significantly thicker peptidoglycan coating than gram-negative bacteria. In contrast to gram-negative bacteria, which lack teichoic acid, many gram-positive bacteria have teichoic acid fibres that extend outside the peptidoglycan.
Gram-negative bacteria, on the other hand, have a complex outer layer made of phospholipid, lipoprotein, and lipopolysaccharide. In certain types of gram-negative bacteria, the periplasmic space—which lies between the cytoplasmic membrane and outer membrane—is home to β-lactamases, which are enzymes that break down penicillins and other β-lactam medications.
Other significant characteristics of the cell wall include:

(1) Endotoxin is a lipopolysaccharide found in gram-negative bacteria.
(2) Its proteins and polysaccharides function as antigens in lab identification.
(3) Small, hydrophilic molecules are made easier to enter the cell by its porin proteins. Gram-negative bacteria's outer membrane contains porin proteins that function as a channel to let in vital molecules including carbohydrates, amino acids, vitamins, and metals, as well as a variety of antimicrobial medications like penicillins.

1.Bacteria with Acid-Fast Cell Walls

Gramme staining is not possible for mycobacteria due to their unique cell wall, such as Mycobacterium tuberculosis. Because these bacteria do not decolorize when exposed to acid–alcohol after being stained with carbofuchsin, they are referred to as acid-fast bacteria. This characteristic is connected to the high concentration of lipids in mycobacteria's cell wall known as mycolic acids.
Three parts of the cell wall—peptidoglycan, lipopolysaccharide, and teichoic acid—are covered in detail here due to their significance.

2. Peptidoglycan

Peptidoglycan is a single covalently bonded macromolecule that forms a complex, interwoven network surrounding the entire cell. Only the cell walls of bacteria contain it. It gives the cell stiff support, aids in preserving the cell's distinctive form, and enables the cell to survive in low-osmotic pressure fluids like water. The peptides and the sugars (glycan) that comprise the molecule give rise to the term peptidoglycan. Mucopeptide and murein are synonyms for peptidoglycan.

Peptidoglycan composition. A tetrapeptide chain, a cross-link (peptide interbridge), and two glycan chains (NAM and NAG) make up peptididoglycan. B: The peptidoglycan creates a three-dimensional, multilayered structure inside the cell wall. NAM stands for N-acetylmuramic acid; NAG for N-acetylglucosamine.

-The backbone of carbohydrates is made up of molecules of N-acetylglucosamine and N-acetylmuramic acid that alternate. A tetrapeptide made up of both D- and L-amino acids, the exact makeup of which varies from bacteria to bacteria, is attached to each muramic acid molecule. Among these amino acids, two stand out as particularly important: acid diaminopimelic, which is specific to bacterial cell walls, and D-alanine, which is essential to penicillin's activity and the formation of cross-links between tetrapeptides. Keep in mind that the uncommon D-isomers of this tetrapeptide are amino acids; the L-isomer is present in most proteins. The other crucial element inside this network is the cross-linking of peptides between the two tetrapeptides. Different species have different cross-links; Staphylococcus
aureus.

-The peptidoglycan backbone can be broken down by the enzyme lysozyme, which is found in human tears, mucus, and saliva. This process helps the host's natural defences against microbial infection. Due to their high internal osmotic pressure, bacteria treated with lysozyme may swell and burst when water enters their cells. On the other hand, the lysozyme-treated cells will survive as spherical forms known as protoplasts, enclosed just by a cytoplasmic membrane, if they are in a fluid with the same osmotic pressure as the bacterial interior.

Polypolysaccharide

Endotoxin is the lipopolysaccharide (LPS) found in the gram-negative bacteria's outer membrane cell wall. Numerous symptoms of the sickness produced by these organisms, including fever and shock (particularly hypotension), are induced by it. Unlike exotoxins, which are actively secreted from the bacteria, endotoxins are a permanent component of the cell wall. Although the set of symptoms brought on by a gram-negative bacteria's endotoxin can vary widely in intensity, they often have a similar constellation. On the other hand, the symptoms brought on by the exotoxins of various bacteria are typically very dissimilar.

(1) Lipid A, a phospholipid, is the cause of the harmful effects.
(2) Lipid A is connected to a core polysaccharide consisting of five sugars via ketodeoxyoctulonate (KDO).
(3) An external polysaccharide made up of up to 25 units of three to five sugars that repeat. The significant somatic, or O, antigen of a number of gram-negative bacteria is this outer polymer, which is utilised in clinical laboratories to identify specific species. Certain bacteria, most notably those belonging to the Neisseria genus, have an outer lipooligosaccharide (LOS) that is mostly composed of repeating units of sugars.

Acid Teichoic

Teichoic acids are fibres that protrude from the gram-positive cell wall and are found in its outer layer. They consist of either ribitol phosphate or glycerol phosphate polymers. Certain glycerol teichoic acid polymers are known as lipoteichoic acid because they bind to the peptidoglycan's muramic acid, whereas other polymers penetrate the peptidoglycan layer and form a covalent bond with the lipid in the cytoplasmic membrane.
Teichoic acids are important for medicine because they can cause septic shock when certain gram-positive bacteria cause it. This is because they activate the same pathways that gram-negative bacteria utilise to produce endotoxin (LPS). Additionally, the adhesion of staphylococci to mucosal cells is mediated by teichoic acids. Teichoic acids are absent from gramnegative bacteria.

Intracellular Membrane

The cytoplasmic membrane, which is located directly inside the peptidoglycan layer of the cell wall, is made up of a phospholipid bilayer that resembles that of eukaryotic cells under the microscope. Prokaryotes often lack sterols in their membranes, whereas eukaryotic membranes do. Despite this, their chemical makeup is comparable. Only prokaryotes belonging to the genus Mycoplasma include sterols in their membranes. The membrane performs four vital tasks: (1) active molecular transport into the cell; (2) oxidative phosphorylation-mediated energy production; (3) cell wall precursor synthesis; and (4) enzyme and toxin secretion.

The Cytoplasm

An electron microscope reveals two separate sections of the cytoplasm:
(1) An amorphous matrix containing metabolites, plasmids, ribosomes, and nutritional granules.
(2) A DNA-based inner nucleoid region.

Ribosomes

Similar to eukaryotic cells, bacterial ribosomes are the location of protein synthesis; however, they are not the same in terms of size or chemical makeup. In contrast to eukaryotic ribosomes, which are 80S in size and include 60S and 40S subunits, bacterial ribosomes are 70S in size and have 50S and 30S subunits. Several antibiotics act selectively, inhibiting the synthesis of bacterial proteins but not human ones, due to variations in ribosomal RNAs and proteins.

Granules

The cytoplasm is made up of a variety of granule types that store nutrients and exhibit distinctive dye-staining patterns. For instance, volatin is a high-energy reserve that is kept as polymerized metaphosphate. Because it stains red with methylene blue dye rather than blue as one might anticipate, it gives the impression that the granule is "metachromatic." The diphtheria-causing Corynebacterium diphtheriae is known for its metachromatic granules.

The nucleoid

DNA is found in the nucleoid, a region of the cytoplasm. Prokaryotes have a single circular molecule called DNA, with a molecular weight (MW) of roughly 2 × 109 and roughly 2000 genes. (In comparison, there are about 100,000 genes in human DNA.) The nucleoid has minimal resemblance to the eukaryotic nucleus due to the absence of the nuclear membrane, nucleolus, mitotic spindle, and histones. One significant distinction between eukaryotic and bacterial DNA is the presence or absence of introns in eukaryotic DNA.

Plasmid

Extrachromosomal, circular, double-stranded DNA molecules known as plasmids have the ability to replicate independently of the bacterial chromosome. Plasmids can merge into the bacterial chromosome, despite the fact that they are often extrachromosomal. Both gram-positive and gram-negative bacteria contain plasmids, and multiple types of plasmids can coexist in a single cell:
(1) Conjugation is a method of transposing transmissible plasmids from one cell to another (see Chapter 4 for a conjugation discussion). Because they have roughly a dozen genes that are involved in the manufacture of the sex pilus and the enzymes needed for transfer, they are big (MW 40–100 million). Typically, each cell has 1-3 copies of these.


(2) Since nontransmissible plasmids lack transfer genes, they are typically found in large numbers (10–60 copies per cell) and are tiny (MW 3–20 million).
The genes for the following crucial medical structures and functions are carried by plasmids:
(1) The multitudinous enzymes that mediate antibiotic resistance.
(2) Resistance to heavy metals, including silver, which is mediated by an enzyme called reductase, and mercury, which is the active ingredient in several antiseptics (such as merthiolate and mercurochrome).
(3) UV light resistance, which is mediated by enzymes that repair DNA.
(4) Pili, or fimbriae, which act as a mediator in the bacterial adhesion to epithelial cells.
(5) A number of enterotoxins are among the exotoxins.