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Ultra-structure and Morphological Classification of Bacteria

Capsule/glycocalyx, Cell wall, Plasma membrane, Cytoplasm, Bacterial chromosome, Plasmids, Mesosomes, Polysomes/polyribosomes, Flagella, Fimbriae.

Ultra-structure of Bacteria Cell

There are three layers in a bacterial cell.
  • Capsule/glycocalyx
  • Cell wall
  • Cytoplasm


A gelatinous substance, usually composed of polysaccharides or polypeptides, surrounds some bacteria. Glycocalyx forms a tightly bound layer on the cell wall, and this layer is called a capsule. This layer protects the cell from desiccation and antibiotic exposure. They stick to surfaces such as root surfaces, human teeth, and tissues because of their sticky nature. It also keeps bacterial cells nourished.

Cell wall

Bacteria have granular and rigid cell walls. It provides protection and gives the cell its shape. Chemically, the cell wall consists of a complex compound called Peptidoglycan or mucopeptide (N-acetylglucosamine, N-acetylmuramic acid, and peptide chains containing four or five amino acids). The porin polypeptide is among the most abundant polypeptides present and is crucial for solute diffusion.

Plasma membrane

Lipoproteins are found in the plasma membrane. Molecular and ion traffic is controlled by the plasma membrane. In addition to enzymes that take part in oxidation of metabolites (the respiratory chain) and photosystems involved in photosynthesis, the plasma membrane houses enzymes in charge of photosynthesis.


The cytoplasm is semitransparent and thick. Several cell components can be found within it, including ribosomes. Included in cytoplasm are glycogen, Sulphur granules, poly-β-hydroxybutyrate crystals, and polymerized glycerol molecules.

Bacterial chromosome

Eukaryotes have a membrane enclosing the chromosomes, whereas bacteria have a single circular DNA molecule tightly coiled. Nucleoid or Genophore is the name of this genetic material. E. coli's DNA measures approximately 1mm when uncoiled, yet contains all its genetic information, which is mind-boggling. There is no histone protein binding to the DNA. The circular DNA molecule that makes up each chromosome is attached to the plasma membrane at one point, and it is thought that this attachment may aid in separating two chromosomes after DNA replication occurs.


Plasmids are self-replicating, circular, double-stranded elements that are extrachromosomal. Genes related to fertility, antibiotic resistance, and heavy metals can be found in them. The enzymes assist in producing bacteriocins and toxins that are not present in bacteria's chromosomes. Most plasmids are between 1 and 500 kb in size, and contribute between 0.5 and 5.0% of the total DNA of bacteria. Each cell may contain one or more plasmids. Depending on their function, plasmids can be classified into various types. They are among others F (Fertility) factor, R (Resistance) plasmids, Col (Colicin) plasmids, Ri (Root-inducing) plasmids, and Ti (Tumour-inducing) plasmids.


Plasma membranes are folded into lamellae, tubules, and vesicles, which are found inside the cell. By clumping them and folding them together, they maximize their surface area, helping to respire and allowing binary fission.


Proteins are synthesized by ribosomes. There are between 10,000 and 15,000 ribosomes per cell. There are two subunits to ribosomes: the 50S and the 30S subunits. Polyribosomes or polysomes are formed when ribosomes are linked by mRNA.


Flagella are thin hair-like processes that emerge from the cell wall of many motile bacteria. A diameter of 20 to 30 µm and a length of 15 µm can be measured. Eukaryotic flagella contain 9+2 microtubules, while bacteria have a single fibril for each flagellum. A flagellum acts as a locomotor. Bacteria differ according to how many flagella they have and where they are located.

Fimbriae or Pilli

Gram-negative bacteria (such as Enterobacterium) have hair-like appendages on the surface of their cell walls called pili or fimbriae. Pili range in size from 0.2 to 20 m in length and have a diameter of about 0.025 m. Pili are not the only type of pili found in the body. There are also sex pili that assist in conjugation.

Morphological Classification of Bacteria

There are several types of bacteria, depending on their shape
  • Cells with a ball or oval shape (from the Greek word kokkos meaning berry) are called cocci
  • It is a rod-shaped cell (which comes from the word baculus meaning rod).
  • Its name derives from its characteristic vibratory motility and resembles a comma shaped rod.
  • The spiral is a rigid form.
  • The spirochete is a spiral form (from speira meaning coil and chaite meaning hair) that is flexibly arranged.
  • From the Greek words actis, meaning ray, and mykes, meaning fungus, actinomycetes appear as ray-like spots on tissue lesions.
  • The morphology of mycoplasmas is not stable because they lack a cell wall. They come in the form of curved bodies and filamentous structures.

Cellular arrangements and groupings of bacteria are sometimes distinctive. The four types of cocci are classified based on the plane of cellular division: diplococci, streptococci, tetrads, and sarcina, or staphylococci, which look like grape clusters.
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