11th Class Biology Biological Classification And System Of Classification / जैविक वर्गीकरण और वर्गीकरण की प्रणाली Structure of Bacteria

(1) Capsule : In a large number of bacteria, a slimy capsule is present outside the cell wall. It is composed of polysaccharides and the nitrogenous substances (amino acids) are also present in addition. This slime layer becomes thick, called capsule. The bacteria, which form a capsule, are called capsulated or virulent bacteria. The capsule is usually found in parasitic forms e.g., Bacillus anthracis, Diplococcus pneumoniae, Mycobacterium tuberculosis.

It provides protection against phagocytosis and antibiotics. Capsule also protects the cell against dessication and viral attack. The capsulated bacteria are usually non-flagellated (i.e., Atrichous).

Capsulated bacteria form smooth colonies and are known as S-type bacteria, which are highly virulent. Non-capsulated bacteria form rough colonies and are known as R–type bacteria.

(2) Cell wall : All bacterial cells are covered by a strong, rigid cell wall. Therefore, they are classified under plants. Inner to the capsule cell wall is present. It is made up of polysaccharides, proteins and lipids.

In the cell wall of bacteria there are two important sugar derivatives are found i.e., NAG and NAM (N-acetyl glucosamine and N-acetyl muramic acid) and besides L or D - alanine, D-glutamic acid and diaminopimelic acid are also found.

One of the unique components of cell wall of bacteria is peptidoglycan or mucopeptide or murien (made of mucopolysaccharide + polypeptide).

In peptidoglycan, NAG and NAM are joined by short peptide chains or cross bridges of amino acids.

Outer layer of cell wall of Gram –ve bacteria is made up of lipopolysaccharides and cell wall of Gram +ve bacteria of teichoic acid.

The cell wall of Gram positive bacteria is much thicker and contains less lipids as compared to that of Gram –ve bacteria. The enzyme lysozyme can dissolve the bacterial cell wall.

(3) Plasma membrane : Each bacterial cell has plasma membrane situated just internal to the cell wall. It is a thin, elastic and differentially or selectively permeable membrane. It is composed of large amounts of phospholipids, proteins and some amounts of polysaccharides but lacks sterols. It is characterised by possessing respiratory enzymes.

Mesosome : On the plasma membrane generally at mid point, there are present some circular coiled bodies called mesosomes. So mesosomes are simply infoldings of plasma membrane. Mesosomes contain respiratory enzymes like oxidases and dehydrogenases and hence they help in respiration. Hence mesosomes are also known as "mitochondria of bacterial cell" or chondrioides. Mesosomes are more prominent in Gram +ve bacteria.

• It receive DNA during conjugation and DNA replication enzyme.
• Mesosome participate in the formation of septa during cell division.

(4) Cytoplasm and Cytoplasmic inclusions : The cytoplasm is a complex aqueous fluid or semifluid ground substance (matrix) consisting of carbohydrates, soluble proteins, enzymes, co-enzymes, vitamins, lipids, mineral salts and nucleic acids. The organic matter is in the colloidal state.

The cytoplasm is granular due to presence of a large number of ribosomes. Ribosomes in bacteria are found in the form of polyribosome. Membranous organelles such as mitochondria, endoplasmic reticulum, golgi bodies, lysosomes and vacuoles are absent. In some photosynthetic bacteria the plasma membrane gives rise to large vesicular thylakoids which are rich in bacteriochlorophylls and proteins.

Volutin granules : They were first reported in Spirillum volutans bacteria. These are also known as metachromatic granules, which are composed of polyphosphate. Volutin serves as a reserve source of phosphate.

Poly-b-hydroxy butyric acid granules (PHB) : These are polymer of lipid like material and chloroform soluble which are often found in aerobic bacteria especially under high carbon low nitrogen culture conditions. Granules can serve as a reserve carbon and energy source.

Glycogen and Sulphur granules : Glycogen are also known as polysaccharide granules. It can be stained brown with iodine. By electron microscopy they appear as dark granules.

(5) Nucleoid : It is also known as genophore, naked nucleus, incipient nucleus. There is present nuclear material DNA which is double helical and circular. It is surrounded by some typical protein (polyamine) but not histone proteins. Histones (basic proteins) are altogether absent in bacteria. This incipient nucleus or primitive nucleus is named as nucleoid or genophore.

(6) Plasmid : In addition to the normal DNA chromosomes many bacteria (e.g., E.coli) have extra chromosomal genetic elements or DNA. These elements are called plasmids. Plasmids are small circular double stranded DNA molecules. The plasmid DNA replicates independently maintains independent identity and may carry some important genes. Plasmid terms was given by Lederberg (1952). Some plasmids are integrating into the bacterial DNA chromosome called episomes.

(7) Flagella : These are fine, thread-like, protoplasmic appendages which extend through the cell wall and the slime layer of the flagellated bacterial cells. These help in bacteria to swim about in the liquid medium.

Bacterial flagella are the most primitive of all motile organs. Each is composed of a single thin fibril as against the 9+2 fibrillar structure of eukaryotic cells. The flagellum is composed entirely of flagellin protein.

The diameter of each subunit is about 40-50Å. These subunits are arranged around a hollow axis. A flagellum is usually 4.5m long and 120-185Å in diameter. Flagellum is attached to cell membrane by a special terminal hook, which is attached to the basal body called (blepharoplast). A bacterial flagellum can be divided into three parts – (Basal granule, Hook and Filament).

(8) Pili or Fimbriae : Besides flagella, some tiny or small hair-like outgrowths are present on bacterial cell surface. These are called pili and are made up of pilin protein. They measure about 0.5–2mm in length and 3–5mm in diameter. These are of 8 types I, II, III, IV, V, VI, VII, and F types. I to F are called sex pili. These are present all most all gram –ve bacteria and few gram +ve bacteria. Fimbriae take part in attachment like holding the bacteria to solid surfaces. 

The function of pili is not in motility but they help in the attachment of the bacterial cells. Some sex pili acts as conjugation canals through which DNA of one cell passes into the other cell.