Morphology of Bacteria and its Structures External to Cell Wall

The morphology of bacterial cell includes characteristics such as size, shape, structure etc. First of all, Leeuwenhoek revealed the gross appearance of microorganism including bacteria by light microscope. By the discovery of electron microscope in early 1940’s the study of structural bacterial cell has been made very easier. The morphological characteristic of a bacterial cell is as follows.

Size: Bacteria vary in size from cell to cell. It can be as small as 0.1 to 0.2 μm in width to as large as more than 50 μm in diameter. A few very large prokaryotes such as Epulopiscium fishelsoni [1] that inhabits the intestinal tract of Surgeon fish  are up to 50 μm in diameter and can be more than 0.5 mm in length. However, the dimension of an average rod cell prokaryotes E.coli are about 1×3 μm.

Shape and arrangement: The shape of cell is referred to as its morphology. Several shapes of bacteria has been recognized and have been given different names.
1. Coccus: A bacteria that is spherical or ovoid in morphology. Example; Staphylococcus species.
2. Rod or bacillus: A bacterium with cylindrical shape. Example; coli
3. Spirillum: Some rods are curved frequently forming spherical shape pattern. Example; Rhodospirillum rubrum
4. Spirochete: Several group of bacteria which are tightly coiled. Example; Spirochaeta stenostrepta and Treponema pallidum
5. Vibroid: Bacterial cell having less than one complete twist form. Example; Vibrio cholerae.
6. Appendaged: Bacteria which possess extension of their cell as their long tubes or stalks. Example; Rhodomicrobium species
7. Filamentous: Bacteria which are long and filament like structure. Example; Chloroflexus species
8. Pleomorphic bacteria: Bacteria having irregular shape and can change their structure. Example; Rhizobium species and Mycobacterium species

Fig: Morphology of bacteria (https://i.ytimg.com/vi/wEvW1ZF7if4/hqdefault.jpg )

In many prokaryotes the cells remain together in groups and arrangements after division. Different microorganisms, particularly cocci species have different group characteristics. The example includes
1. Diplococci (Example; Klebsiella pneumoniae)
2. Streptococci (Example; Streptococcus pyogenes)
3. Tetrads (Example; Micrococcus luteus)
4.Staphylococci (Example; Staphylococcus aureus)
5. Sarcina (Example; Sarcina ventriculi)

Structure of Bacterial Cell External to Cell Wall

Fig: Bacterial cell (https://hi-static.z-dn.net/files/d25/23d21e0e1d898535c58b095ec5d654a4.jpeg )

1. Flagella: These are thin long hair like and helical appendages that protrude through the cell wall originated from the basal granule. They are thinner and simpler than the cilia and flagella of eukaryotes. They have diameter of 0.01 to 0.02 μ The flagella of some gram negative bacteria is covered externally by the thin sheath or layer. A flagella can be divided in three parts; basal body, hook and filament. The flagella helps in swimming motility of the bacterial cell. The principle of movement of bacteria by the flagella is similar to that of penetration of cork by a cork screw. Depending on the arrangement of flagella, bacteria has been classified into four types.
a) Monotrichous: Example; Vibrio cholera
b) Amphitrichous: Example; Aquaspirillum species
c) Lophotrichous: Example; Pseudomonas fluorescence
d) Peritrichous: Example; Proteus vulgaris
2. Pilli: These are small, thin hair like non helical appendages. They are much smaller and simpler than flagella. They don’t take part in motility. However, they are present in both motile as well as non-motile species. Pilli are associated with different functions. For example; ‘F’ pilli help to transmit bacterial genetic material during bacterial mating. Other pilli help to attach the bacteria with epithelial layer of respiratory tract, intestine tract and genitourinary tract. By the help of pilli, bacteria get attached and prevent from being by the flow of mucus.

3. Capsule: Slime layer produce capsule in unfavorable condition. Some bacterial cell are covered externally by a thin viscous substance which forms covering layer or envelope to the cell. If this layer is seen by light microscope, a colorless zone is seen, even if the cell is stained.

Capsule is sticky in nature, appear as amorphous gelatinous area and is secreted from inner side of cell and gets firmly attached to the surface of cell wall. If the substances are unorganized and loosely attached to the cell wall, the capsule is called slime layer. The capsule of most of the organism is made up of polysaccharide; particularly homopolysaccharides such as in Streptococcus mutans. Capsule of other variety of bacteria are made up of heteropolysaccharides such as in Klebsiella pneumoniae. Few capsules are polypeptide such as in Bacillus anthracis. Capsule of Bacillus anthracis is made up of polymer of Glutamic acid. Capsule has number of functions
a) It protect bacteria from temporary drying by binding water molecules.
b) It may prevent the attachment of bacteriophage (bacteriovirus)
c) It may promote bacteria for attachment to the surface
d) It may inhibit the engulfment of WBC (phagocytosis) and contribute to virulence.
e) Capsule may promote the stability of bacterial suspension by preventing the cells from aggregating and settling out.

4. Cell wall: The cell wall accounts for the shape, size and rigidity of bacterial cell. The cell wall of bacteria is semi rigid and complex structure present beneath the capsule and external to plasma membrane. The cell wall protects the plasma membrane and other cytoplasmic inclusion from adverse environment. It also protects the bacterial cell from bursting when osmotic pressure of cytoplasm is higher than that of outside of cell wall. Similarly it provides support for the attachment to the flagella and rescues the cell from antibody and harmful chemicals. Bacterial cell wall are about 10 to 25 nm in thickness and account for about 20 – 30 % of dry weight of cell.

Wall characteristics and chemical composition:

The cell wall of bacteria is made up of network of peptidoglycan. It is present almost on all bacterial cell wall except Halobacterium species and Halococcus species. Peptidoglycan differs somewhat in composition and structure from one species to another but it is basically a polymer of N-acetyl glucosamine, N-acetyl muramic acid and polypeptides (L-alanine, D-alanine, D-glutamate) and Diamino acid.

Cell wall of gram positive bacteria: in most of the gram positive bacteria, the cell contains several layers of peptidoglycan which may account for 50 % or more of the dry weight of the wall. However, the thickness may vary with species types from 30 nm to 8 nm. The thickness of peptidoglycan provides rigidity to the cell wall. In most gram positive bacteria, peptidoglycan is associated with acidic polymer containing phosphorous called ‘Teichoic acid’. Teichoic acid consists of alcohol and phosphate possibly play a role in growth of bacterial cell by regulating the activity of an enzyme ‘autolysin’. The wall of gram positive bacteria contains less amount of lipid except Mycobacterium species and Corynebacterium species. These contain high amount of lipid. Mycobacterium species exhibit acid fast stain due to presence of mycolic acid or cord factor. These factor is toxic and plays an important role in the disease caused by Mycobacterium tuberculi.

Cell wall of gram negative bacteria: It contains peptidoglycan but in very low amount. However, the walls of gram negative bacteria are more complex than those of gram positive bacteria. They totally lack Teichoic acid. Peptidoglycan is situated in periplasmic space and covalently linked to lipoprotein in outer membrane. Due to presence of low peptidoglycan, the cell wall of gram negative bacteria can easily be distinguished.

The cell wall of gram negative bacteria is a bilayered structure consisting of mainly lipoprotein, lipopolysaccharide and phospholipid. Lipopolysaccharide has toxic properties and is also known as endotoxin. It is composed of three covalently linked parts; lipid A, core polysaccharide and O antigen. O-antigen has the serological properties (antigenic characteristics) and also serve as receptor for bacteriophage attachment.

Fig: Cell wall of gram positive and gram negative bacteria (https://www.onlinebiologynotes.com/wp-content/uploads/2017/07/cell-wall.png )

Reference:
[1] Angert, E. R., Clements, K. D., & Pace, N. R. (1993). The largest bacterium. Nature, 362(6417), 239–241.