The hard, thick, rigid and structured layer outside the cell wall of bacteria is called capsule. It is composed of polypeptides or polysaccharides. If the capsule layer is thin and flexible, it is called slime layer. It is slippery and sticky. It gives the bacteria specific shape and protects them from drying out in unfavorable environment. It is effective in nitrogen fixation.

1. Capsule: The hard, thick, rigid and structured layer outside the cell wall of bacteria is called capsule. It is composed of polypeptides or polysaccharides. If the capsule layer is thin and flexible, it is called slime layer. It is slippery and sticky. It gives the bacteria specific shape and protects them from drying out in unfavorable environment. It is effective in nitrogen fixation.
2. Cell wall: The tough, firm, thick and elastic membrane inside the capsule layer of bacteria is called cell wall. It is inert in nature. It is composed of mucopeptide or peptidoglycan or murine. However, cell walls also contain proteins, lipids and polysaccharides. It contains some amount of muramic acid and teichoic acid. It is 10-50 millimicrons thick. The cell wall has several pores at places. Each pore is 1 millimicron in diameter. Exchange of essential substances takes place through these pores. It gives specific shape to cells and protects them from external injury.
3. Plasma membrane: The fine, living, thin and elastic membrane located inside the cell wall is called plasma membrane. It is composed of proteins and phospholipids. The plasma membrane contains sterols called hepanoids. It contains 70-80% protein and 20-30% lipid. It is 7.5-8.0 nm thick. It contains permease enzyme. This enzyme transports proteins.
4. Cytoplasm: The colorless part of the cell surrounded by plasma membrane is called cytoplasm. The parts of cytoplasm are:
(i) Ribosomes: 70S ribosomes are located freely in the cytoplasm of the cell. It is made up of proteins and RNA. It contains 30-50% protein and 50-70% RNA. Each bacterium contains 10,000-20,000 ribosomes. It synthesizes proteins for cells.
(ii) Chromatophore: Cytoplasm of bacteria contains hollow spherical chromatophore. It contains bacteriochlorophyll. It helps in photosynthesis.
(iii) Volutin: The cytoplasm of young bacterial cells contains small granular volutin. With the increase of age, it turns into cell cavity. It stores food for cells and transports phosphate.
(iv) Cell cavity: Cell cytoplasm contains small cell cavities. It is filled with cell sap.
(v) Mesosomes: The plasma membrane folds inward to form a sac-like structure. It is called mesosome. Mesosomes are called chondroids. It helps in breathing. It carries out DNA replication and forms the cell wall during cell division. Gram positive bacteria have mesosomes.
(vi) Plasmid: Extra-chromosomal circular double-stranded DNA located in the cytoplasm of the cell is called plasmid. It is self-reproducing. It carries several types of genes. Fertility factor (F-factor), Resistance factor (R-factor), Nitrogen fixing genes (Nif-genes) etc. It acts as a carrier in genetic engineering.
5. Nucleoid: Bacterial cells do not have a true nucleus. Its nucleus is called nucleoid or xenophore or pseudonucleus. It lacks nuclear membrane and nucleolus. The center of the cytoplasm contains a single stranded double-stranded DNA. At the center of DNA is a circular RNA. RNA is surrounded by nucleoproteins. It does not contain histone proteins. It transfers hereditary traits from generation to generation.
6. Flagella: The whip-like appendages that arise from the outer layer of the cytoplasm are called flagella. The stiff flagella are called fimbriae. It is composed of flagellin protein. Its length is 4-5 nm. Flagella help bacteria to move. Each flagella has three segments. Basal body, hook and filament. Bacteria with flagella are called Tricus and those without flagella are called Atrichous.
7. Pili: Numerous fine hair-like appendages arise from the outer layer of gram negative bacteria. These are called pili or fimbriae. It is more numerous than flagella and shorter in length. It is made up of proteins called pilin. Conjugation tubes are formed during reproduction and are called sex pili. Gonorrhea bacteria attach to host cells by pili.

The Greek word archaios means original. Archaea are primitive bacteria. Archaea that prefer temperature are called thermophiles. 84 degrees C. They die in the heat. 98 degrees C. They grow well at temperature. They are 110 degrees Celsius. Lasts up to temp. For example, Methanopyrus. Archaea that love heat and acid are called thermoacidophiles. Archaea that prefer salt are called halophiles.
Genome sequencing in 1996 revealed that more than half of the 1,738 genes in archaea are distinct from bacteria and other organisms. Members of Archaea that produce methane in their metabolism are called Methanogens. The process of making methane is called Methanogenesis.

Bacteria whose entire cell is covered by flagella is called peritrichous. For example- Bacillus typhi, Escherichia coli, Salmonella typhi.

Those bacteria that have a total of two sets of flagella at both ends of the cell are called Lophotrichous. For example- Spirillum minus, Pseudomonas.

Bacteria that have a cluster of flagella at one end of the cell is called Cephalotrichus. For example, Pseudomonas aenoginosa, Pseudomonas fluorescens

Bacteria that have one flagella at both ends of the cell are called Amphitrichous. For example- Nitrosomonas, Spirillum.

Bacteria cells that do not have any flagella are called Atricus. For example- Bacillus diptheriae, Lactobacillus, Pasteurella, Corynebacterium diptheriae.

The number and distribution of flagella in bacterial cells is called flagellation. Based on the presence, absence, number and location of flagella, bacteria can be classified into following groups.
1. Atricus: Bacteria cells that do not have any flagella are called Atricus. For example- Bacillus diptheriae, Lactobacillus, Pasteurella, Corynebacterium diptheriae.
2. Monotrichous: Bacteria that have only one flagellum at one end of the cell are called monotrichous. For example, Vibrio cholerae.
3. Amphitrichous: Bacteria that have one flagella at both ends of the cell are called Amphitrichous. For example- Nitrosomonas, Spirillum.
4. Cephalotrichus: Bacteria that have a cluster of flagella at one end of the cell is called Cephalotrichus. For example, Pseudomonas aenoginosa, Pseudomonas fluorescens
5. Lophotrichous: Those bacteria that have a total of two sets of flagella at both ends of the cell are called Lophotrichous. For example- Spirillum minus, Pseudomonas.
6. Peritrichous: Bacteria whose entire cell is covered by flagella is called peritrichous. For example- Bacillus typhi, Escherichia coli, Salmonella typhi.

Those bacteria which cannot retain the dye after washing with crystal violet dye are called gram negative bacteria. Their cell walls are thin. Its cell wall consists of peptidoglycan and thick lipopolysaccharide. E. coli, Salmonella typhi, Shigella, Proteus, Rhizobium, Vibrio cholerae, Neisseria meningitidis, Cyanobacteria.