The following points highlight the seven important components of bacterial cell. The components are: 1. Cell Envelope 2. Cytoplasm 3. Nucleoid 4. Plasmids 5. Inclusion Bodies 6. Flagella 7. Pili and Fimbriae.
Bacterial Cell: Component # 1. Cell Envelope:
It is the outer covering of protoplasm of bacterial cell. Cell envelope consists of 3 components— glycocalyx, cell wall and cell membrane.
(i) Glycocalyx (Mucilage Sheath):
It is the outermost mucilage layer of the cell envelope which consists of non-cellulosic polysaccharides with or without proteins. Glycocalyx may occur in the form of loose sheath when it is called slime layer. If thick and tough, the mucilage covering is called capsule. Glycocalyx gives sticky character to the cell. It is not absolutely essential for survival of bacteria.
However, it has several secondary functions:
(a) Prevention of desiccation,
(b) Protection from phagocytes,
(c) Protection from toxic chemicals and drugs,
(d) Protection from viruses,
(e) Attachment,
(f) Immunogenicity and
(g) Virulence.
(ii) Cell Wall:
It is rigid solid covering which provides shape and structural support to the cell. Cell wall lies between plasma membrane and glycocalyx. Periplasmic space occurs between plasma membrane and cell wall. Cell wall protects the bacterial cells against bursting in hypotonic solution. Wall is 20-80 nm thick in Gram positive bacteria.
It is single layered and smooth. In Gram negative bacteria, wall is 8-12 nm thick, complex, wavy and two layered. The outer layer is also called outer membrane.
It consists of lipopolysaccharides, lipids and proteins. The outer membrane has hydrophilic channels of 16-stranded (3-barrel proteins called porins. The single layered cell wall of Gram positive bacteria and inner wall layer of Gram negative is made up of pepidoglycan, proteins, non-cellulosic carbohydrates, lipids, amino acids, etc.
Peptidoglycan forms the structural network of the cell wall. It is also known as murein or mucopeptide. Peptidoglycan consists of long glycan strands formed of repeating units of N-acetyl glucosamine (NAG) and N-acetyl muranic acid (NAM). They are cross linked by small peptide chains.
Peptidoglycan constitutes 70-80% of wall in Gram positive bacteria. Lipid content is little. 10-20% of wall in Gram negative bacteria is formed of peptidoglycan. Lipid content is 20-30%.
Amino acid present in the wall is diaminopimelic acid or lysine. In Gram positive bacteria, the wall contains teichoic acids that form receptor sites and surface antigens. In Mycobacterium and Nocardia, the wall contains long chain fatty acids called mycolic acids.
(iii) Plasma Membrane:
It is selectively permeable covering of the cytoplasm that forms the innermost component of cell envelope. Bacterial plasma membrane or plasma lemma has a structure similar to that of a typical membrane. It is made of a phospholipid bilayer with proteins of various types (extrinsic, integral, trans membrane).
It holds receptor molecules for detection and responding to different chemicals of the surroundings. Bacterial membrane is metabolically active as it takes part in respiration, synthesis of lipids and cell wall components.
Bacterial Cell: Component # 2. Cytoplasm:
It is crystallo-colloidal complex that forms the protoplasm excluding its nucleoid. Cytoplasm is granular due to presence of a large number of ribosomes.
Membrane bound cell organelles as found in eukaryotes are absent. However, all biochemical pathways are found in prokaryotic cells. Cytoplasmic streaming is absent. Sap vacuoles are absent. Instead gas vacuoles are present.
Various structures present in cytoplasm are as follows:
(i) Mesosome:
It is a characteristic circular to villi form specialisation of cell membrane of bacteria that develops as an ingrowth from the plasma membrane. It consists of vesicles, tubules and lamellae. Mesosme is of two types, septal and lateral.
Septal mesosome connects nucleoid with plasma membrane. It takes part in replication of nucleoid by providing points of attachment to the replicated ones. Septal mesosome is also believed to help in septum formation.
At the time of cell division, plasma membrane grows in the region where the septal mesosme is present so that most probably it provides membranes for rapid elongation. Lateral mesosme is not connected with nucleoid.
It contains respiratory enzymes and is, therefore, often called chondrioid. It is believed to be equal to mitochondrion of eukaryotes. However, respiratory enzymes are also present over the plasma membrane.
(ii) Ribosomes:
They are small membrane less, submicroscopic ribonucleoprotein entities having a size of 20 nm x 14-15 nm. Ribosomes are of two types, fixed and free. Fixed ribosomes are attached to the plasma membrane. Free ribosomes occur free in the cytoplasmic matrix. The ribosomes are 70S in nature. (Here S denotes sedimentation coefficient or Svedberg number).
Each ribosome has two subunits, larger 50S and smaller 30S. Ribosomes take part in protein synthesis. Free or matrix ribosomes synthesize proteins for intracellular use while fixed ribosomes synthesize proteins for transport to outside.
Ribosomes generally occur in helical groups called polyribosomes or polysomes. In each polysome 4—8 ribosomes are attached to a single strand of messenger or mRNA. It is a mechanism to synthesise several copies of the same protein.
(iii) Chromatophores:
They are internal membrane systems of photosynthetic forms which possess photosynthetic pigments. In purple bacteria the membranes are typical while in green bacteria they are non-unit, non-lipid and proteinaceous. Chromatophores of green algae are called chromosomes. Photosynthetic pigments are bacteriochlorophyll, bacteriophaeophytin (bacterioviridin) and carotenoids.
Bacterial Cell: Component # 3. Nucleoid:
It represents the genetic material of prokaryotes. Several alternative names have been given to nucleoid— genophore, prochromosome, incipient nucleus and chromoneme. Nucleoid consists of a single circular strand of DNA duplex which is supercoiled with the help of RNA and polyamines to form a nearly oval or spherical complex.
The folding is 250-700 times. Polyamines or nucleoid proteins are different from histone proteins. DNA of prokaryotes is considered naked because of its non-association with histone proteins and absence of nuclear envelope around it.
In E. coli, nucleoid has 1100 µm long DNA duplex with 4.6 x 106 base pairs. Nucleoid is embedded freely in the cytoplasm. A cell can have 2 or more nucleoids but all are replicated copies of same nucleoid. It is equivalent to a single chromosome of eukaryotes because nucleoid consists of a single DNA double strand. Nucleoid may be directly attached to the plasma membrane or through the mesosome.
Bacterial Cell: Component # 4. Plasmids:
They are self-replicating, extra chromosomal segments of double stranded, circular, naked DNA. Plasmids provide unique phenotypic characters to bacteria. They are independent of main nucleoid.
Some of them contain important genes like fertility factor, nif genes, resistance factors and colicinogenic factors. Plasmids which can get associated temporarily with nucleoid are known as episomes. Plasmids are used as vectors in genetic engineering.
Bacterial Cell: Component # 5. Inclusion Bodies:
They are non-living structures present in the cytoplasm. The inclusion bodies may occur freely inside the cytoplasm (e.g., cyanophycean granules, volutin or phosphate granules, glycogen granules) or covered by 2-4 nm thick non-lipids, non-unit protein membrane (e.g., gas vacuoles, carboxysomes, sulphur granules, PHB granules).
On the basis of their nature, the inclusion bodies are of 3 types— gas vacuoles, inorganic inclusions and food reserve.
(i) Gas Vacuoles:
They are gas storing vacuoles found in cyanobacteria, purple and green bacteria and a few other planktonic forms. A gas vacuole is without any covering of its own. It consists of a variable number of hexagonal, hollow and cylindrical gas vesicles. Each gas vesicle is surrounded by a single non-unit, non-lipid protein membrane having ribs or folds.
The membrane is impermeable to water but is permeable to atmospheric gases. Gas vacuoles protect the bacteria from harmful radiations. They also constitute buoyancy regulation mechanism for their proper positioning in water during daytime for photosynthesis.
(ii) Inorganic Inclusions:
Several types of inorganic granules occur in bacteria. They include volutin granules, sulphur granules, iron granules, magnetite granules, etc. Because of the ability to pick up different colours with basic dyes, they are called metachromatic granules. Two common types of inorganic granules are volutin granules and sulphur granules.
Volutin granules are polymetaphosphates which function as storage reserve of phosphate. Sulphur granules occur in bacteria living in sulphur rich medium like the one which pick up hydrogen sulphide for obtaining reducing power in photosynthesis.
Iron granules are similarly found in those bacteria which metabolise iron compounds for obtaining energy. Aquaspirillum magnetotacticum contains magnetosomes, which are vesicles having magnetite. The granules help the bacteria to orientate themselves along geomagnetic lines.
(iii)Food Reserve:
Blue green algae have cyanophycean starch or α-granules, β- granules or lipid globules and cyanophycin or protein granules. In bacteria, starch is replaced by glycogen. Neutral fats are absent. Instead poly-beta-hydroxy-butyrate or PBH granules are present. A biodegradable plastic can be prepared from PBH. Protein granules are present. Carboxysomes occur in photosynthetic forms.
Bacterial Cell: Component # 6. Flagella (Fig. 8.10):
Bacterial flagella are unistranded, equivalent to a single micro- tubular fibre. It is about 20 nm (0.02 µm) in diameter and 1-7µm in length. Bacterial flagellum is made up of 3 parts— basal body, hook and filament. Basal body is like a rod. It is inserted in the cell envelope. The basal body bears ring-like swellings in the region of plasma membrane and cell wall.
There are two pairs of rings (L and P ring in cell wall and S and M rings embedded in cell membrane) in Gram negative bacteria and only a single pair of rings (S and M rings embedded in cell membrane) in Gram positive bacteria (Fig. 8.10 B). Hook is curved tubular structure which connects the filament with the basal body. It is the thickest part of flagellum.
Filament part is long tubular structure which causes turbulence in the liquid medium. It is made up of protein called flagellin. Protein molecules are globular. They are arranged in 3-8 spiral rows (Fig. 8.10 C). It is believed that bacterial flagella perform rotation type movement that brings about backward pushing of the water. It results in the bacterium moving forward.
Bacterial Cell: Component # 7. Pili and Fimbriae:
The two terms have been used interchangeably for bacterial appendages which are not involved in locomotion. Actually, pili (singular-pilus) are longer, fewer and thicker tubular outgrowths which develop in response to F+ or fertility factor in Gram negative bacteria.
They are made up of protein pilin. A donor bacterial cell having fertility factor develops 1-4 pili. Being long (18-20 (xm) they are helpful in attaching to recipient cell and forming conjugation tube.
Fimbriae are small bristle-like fibres sprouting from cell surface in large number. There are 300-400 of them per cell. Diameter is 3-10 nm while length is 0.5-1.5 µm. Fimbriae are involved in attaching bacteria to solid surfaces (e.g., rock in water body) or host tissues (e.g., urinary tract in Neisseria gonorrhoeae). Some fimbriae cause agglutination of RBC. They also help in mutual clinging of bacteria.