Structural components of bacterial cell - 2

 

Structure of bacterial cell:

            These are prokaryotic, microscopic with simple organization of body than eukaryotes. Their cells lack nucleus, membrane bound organelles, endomembrane systems. Cell wall surrounds the body as outer membrane and makes them rigid. Capsule is the outermost layer that is present above the cell wall.

Size:

Bacteria are very small in size. They usually range from 0.2 to 2 μm in diameter, 0.5 to 5 μm in length. Example: Mycoplasmas (0.3 micrometers in diameter). Few bacteria are very large in size and are visible to the naked eye. Example: Thiomargarita namibiensis, Epullopiscium fisheloni which is rod shaped is about 600 micrometers in length and 75 micrometers in diameter.

Due to their small size cell body their surface area to volume ratio is more i.e., no internal part is far from the surface and nutrients can easily and quickly reach all the parts of the cell. It also allows rapid uptake and intracellular distribution of nutrients and excretion of wastes. At low surface area to volume ratio, the diffusion of nutrients and waste products across the cell membrane slows down the metabolic rate which makes the cell less chances to survive.

Shape:

As we previously discussed that the bacteria occur in four basic structures like cocci (spherical in shape), bacillus (rod shaped), vibrio (comma shaped), spirillum (spiral shaped) etc. Some are pleomorphic (Corynebacterium). The term bacillus has two meanings. It refers to a bacterial shape as well as genus. For Example: Bacillus anthracis, the causative agent of anthrax, Bacillus aerius, Bacillus subtilis etc.  

Cell wall:

            Usually most of the bacterial cells are bounded by a complex cell wall which is made of peptidoglycon. It is unique characteristic in bacterial cells. Peptidoglycon is also termed as murein or mucopeptide.

            Peptidoglycon is a polymer containing two sugar derivatives N-Acetyl glucosamine (NAG) and N-Acetylmuramic acid (NAM). These are joined through β-1,4 glycosidic bond. NAM is similar to NAG with the exception of presence of lactic acid connected by ether linkage. The carboxylic group in NAM is attached by a tetra peptide which is a peptide chain of four alternating D and L amino acids like L-Alanine, D-Alanine D-Glutamic acid, and either L-Lysine or meso diamino-pimetic acid (DAP).

            Few bacteria lack cell wall or may present little amount. Example: Mycoplasmas, L-form bacteria or L-Phase bacteria. L- Forms can be generated cell wall in the laboratory from species like Bacillus subtilis and Escherichia coli which have cell wall by by inhibiting peptidoglycon synthesis or by treating with lysozyme. But unlike L forms Mycoplasmas cannot derive from the bacteria having cell wall.

Bacteria can be identified by using staining techniques which uses basic dyes such as crystal violet, methylene blue. Based on the results the type of bacteria can be identified. Example: Acid fast staining technique used for identification of Mycobacterium species, Gram staining technique is used for the differentiation of type of bacteria based on their cell wall composition, flagella staining techniques etc.

Gram Positive bacteria:

            The bacteria that give positive result during staining technique are Gram Positive Bacteria. They take up the crystal violet color after the decolorization of rest of the sample due to presence of 20 to 80nm thick peptidoglycon layer outside the plasma membrane. The thinner or smaller periplasmic space is present between plasma membrane and cell wall. These bacteria have many teichoic acids attached covalently to the NAM of peptidoglycon layer or to the lipids of the plasma membrane. The teichoic acids are polyol phosphate polymers (polymer of ribitol or glycerol joined by phosphate) bearing strong negative charge. Mesosomes are more prominent.

Gram Negative bacteria:

            The bacterium that doesn’t retain the color of crystal violet after the decolorization step in staining technique is Gram Negative bacteria. This is due to absence or presence of small amount of peptidoglycon in the cell wall. These have more periplasmic space between cell wall and plasma membrane. They lack teichoic acids. Outside the cell wall layer similar to plasma membrane outer membrane is present. The constituents in outer membrane are not completely similar to plasma membrane. It contains lipopolysaccharides, lipoproteins, proteins and phospholipids. The lipids and polysaccharides are linked in the outer membrane called lipopolysaccharides. So it is named as lipopolysaccharide layer or LPS layer. LPS consists of three components, the core polysaccharide, O-polysaccharide, Lipid A. Lipid A is highly toxic, and as a result the LPS act as an endotoxin. The O-Polysaccharide functions as an antigen and is used for distinguishing Gram Negative bacteria.

Glycocalyx:

            It is a thick, high molecular weight secretary substance present in many bacteria. It is present outside to the cell wall composed of polysaccharide, polypeptide or both. The Glycocalyx may be thick or thin, rigid or flexible depending on their chemical nature. The terms Capsule and Slime layer are frequently used to describe Glycocalyx layers. The capsule is rigid layer organized in a tight matrix having polysaccharides. Presence of Capsule makes bacteria resistant to phagocytosis and enhances their virulence. The slime layer is loosely deformed and loosely attached to the cell wall.

Plasma membrane:

             It is present below the cell wall and also called as cell membrane. It is common in both prokaryotes and eukaryotes. It is composed of phospholipid bilayer. The only difference between them in the composition of plasma membrane is the presence of sterols and hopanoids in the eukaryotes and prokaryotes (except Mycoplasmas; contains sterols) respectively. Plasma membrane performs functions such as transport, energy transduction. The invasions in plasma membrane are Mesosomes which play role in formation of cell wall during cell division, chromosome replication. These are present both in Gram Positive and Gram Negative Bacteria.

Cytoplasm:

            It is the inner content of the cell which is prior to the plasma membrane. It contains 80% of water, 70s ribosomes, chromosomal material with nuclear envelope, Circular DNA plasmids, proteins, carbohydrates, lipids, inorganic ions, many low molecular weight compounds. It lacks unit membrane bound organelles.

            The sub units of 70s ribosomes in prokaryotes are smaller subunit 30S containing one molecule of rRNA (16S rRNA) and larger subunit 50S containing of two molecules of rRNA (23S rRNA and 5S rRNA). The cytoplasm also contains inorganic and organic substrates called inclusion bodies. They are membrane bound or membrane less. Their unit membrane varies in the composition. Some may be proteinaceous or some may be with lipids.

Inclusion bodies:

1.   Glycogen: It is a branched polymer of glucose and present in the form of granules in the cytosol. It acts as a storage unit for reserve food materials. They survive more than glycogen less bacteria. Glycogens with iodine appear in reddish brown color.

2.     Carboxysomes: These are polyhedral micro compartments that fix CO₂ by the enzyme RuBisCo (Ribulose-1, 5-Bisphosphate carboxylase) in Cyanobacteria. These are covered by proteinaceous shells. There are two types of Carboxysomes: α Carboxysomes and β Carboxysomes which appear similarly but differ in their protein composition.

3.     Polyphosphate granules: Also called volutin granules. These are reserve form of inorganic phosphate present in the cytoplasm of some bacteria. They show red or a different shade of blue when stained with blue basic dyes, methylene blue or toluidine blue. This is known as Metachromatic effect. Hence these granules are also called as Metachromatic granules. These are synthesized for both energy and phosphate storage.

4. Cyanophycin granules: it is present in cyanobacteria and heterotrophic bacteria. it is non ribosomally produced amino acid polymer.

5. Magnetosomes: these are membrane bound inclusion bodies that show movement according to magnetic field. These are tiny chain structures surrounded by lipid bilayer. Magnetosomes are present in magnetotactic bacteria. The movement that magnetotactic bacteria show is called magnetotaxis. These are motile and mostly aquatic. They usually contain crystals of magnetite (Fe₃O₃) or crystals of greigite (Fe₃S₄). It acts like a compass needle to orient magnetotactic bacteria in geomagnetic fields.

6.     Gas vesicle: These are hallow spindle shaped structure made of protein called GvpA and GvpC but absent in eukaryotes. These proteins made bacteria to float on the surface of water. This makes phototrophic bacteria able to trap light. It is impermeable to liquid water, but it is highly permeable to gases. It provides buoyancy to planktonic cells by decreasing their overall cell density.

Surface appendages:  Flagella, pili and fimbriae. These are the external appendages lies on the surface of the body. Each appendage has different function.

Flagella:

These help bacteria for motility. They differ in size in different bacteria.  It is usually present in both Gram positive and Gram negative bacteria. It has mainly three parts filament, hook and basal body.

Filament: It lies external to the cell surface. The filament is made of flagellin protein. As shown in the figure the flagellin proteins are assembled to form a cylindrical structure with hallow core. It is a rigid structure.  

Hook: It is highly curved structure present at the end of the filament as shown in the figure.

Basal body: the basal body is the part to which the hook is attached and makes flagella to propel. It has four rings MS ring (Membranous, Supra membranous), P ring (Peptidoglycon), L ring (Lipopolysaccharide), and C ring (Cytoplasm). The C ring has three proteins FliG, FliM and FliN which makes flagella to rotate. It interacts with the motor proteins. The C ring is also called as Switch complex as it can switch the direction of the flagellar motor. Based on the number and arrangement of flagella on the surface of bacteria they are of 4 types. They are

·Artichous: Bacterium without flagella.

·Monotrichous: Single flagellum on one side of bacterial surface.

· Amphitrichous: Single flagellum on both sides of bacterial surface.

· Lophotrichous: Cluster of flagella on one side of bacterial surface.

·Amphilotrichous: Cluster of flagella on both sides on the surface of bacteria.

· Peritrichous: Flagella present all over the body of bacteria. 

Pili:

      It is also called as Sex pili as it plays an important role during conjugation. It is a thin, hair like appendage present on the surface of the bacteria. it is usually present in Gram negative bacteria and rarely in Gram positive bacteria.

Sex pili

Function of Pili in conjugation

Fimbriae:

      These are smaller hair like projections spread all over the body. It is present mostly in Gram negative bacteria and some in Gram positive bacteria. It helps the bacteria to adhere to the surface. It is also known as “attachment pili”.

 

Endospores:

These are the heat resistant spores that can withstand harsh conditions such as heat, UV radiation, γ radiation, chemical disinfectants and desiccation, produced by some Gram positive bacteria. They are Bacillus, Clostridium and Sporosarcina. It develops within the cell. Endospores were discovered by Tyndall in his process called Tyndallization.

      The ability to produce spores also is of ecological advantage to the organism as it enables it to survive under adverse conditions. Thus formation of spores is generally occurs under conditions of nutrient depletion.  

      The endospore structure is very complex and has many layers that are absent from the vegetative cell. Exosporium is a thin delicate protein covering, Spore coat lies beneath the exosporium composed of several layers of protein, Cortex is present beneath the spore coat, made up of a peptidoglycon. Spore cell wall is present beneath the cortex and surrounds the protoplast. It contains ribosomes and nucleoid which is non functional.

    The endospores contain dipicolinic acid complexed with calcium which makes spore heat resistant. Staining method is used for the identification of morphology of spore in different bacteria by using Schaeffer-Fulton endospore stain.

 Differences between Gram positive and Gram Negative bacteria:

Property	Gram negative bacteria	Gram positive bacteria
Cell wall	2 to 7 nm thick	20 to 80 nm thick
Teichoic acids	Absent	Present
Periplasmic space	More than Gram positive bacteria	Negligible
Outer membrane	Present	Absent
Motility	Usually motile	Mostly non motile
Appendages	Usually present	Rarely present
Flagellar structure	4 rings in basal body	2 rings in basal body
Endospores	Cannot produce Endospores	Some genera produce endospores.
Gram reaction	Retains purple color	Does not retain color.

Genetic material:

       The bacterial genome is called nucleoid or genophore which contains one chromosome. It lacks nuclear membrane. It comprises a single circular, covalently closed, double stranded helical, negatively super coiled molecule of DNA. Bacterial chromosome contains DNA and proteins. The bacterial chromosome forms domains which made fit into the nucleoid. This domain organization enables the chromosomal DNA to undergo structural changes during different cellular processes like replication, transcription, segregation that occur simultaneously in a bacterial cell. The nucleoid associated proteins that help in compaction of DNA are HU (Heat Unstable nucleoid protein), HNS (Histone like Nucleoid Structuring), IHF (Integration Host Factor), and SMC (Structural Maintenance of Chromosomes). These proteins also involve in cellular processes. The Archaebacteria contain histones which is the common feature in both Archaea and Eukarya domains. The genome in bacteria is circular due to lack of telomeres which are present in Eukaryotes.

BACTERIA	CHROMOSOME (Number, Shape and Size)
Agrobacterium tumifaciens	One linear (~2.1 Mb) + one circular (~3.0Mb)
Bacillus subtilis	One circular (~4.2Mb)
Brucella melitensis	One circular (~2.1 + ~1.2 Mb)
Vibrio cholerae	Two circular (~2.9 + ~1.1 Mb)
Rhodobacter sphaeroides	Two circular(~3.0 + ~ 0.3 Mb)
E.coli K-12	One circular (~4.6 Mb)

 

Plasmids:

      The plasmids are usually circular, negatively supercoiled self replicating double stranded DNA molecule that are maintained as discrete, extrachromosomal genetic elements in bacteria. Plasmids are usually smaller than chromosomes in bacteria. They usually appear varying from less than 5 kbp to more than several hundred kbp. Some bacteria also contain about 2Mbp plasmids. Linear plasmids also appear in some genera Borrelia and Streptomyces and Clostridium. Plasmids are considered as replicons, units of DNA capable of replicating autonomously within a suitable host. They contribute to about 0.5 to 5 % of total DNA.

Joshua Lederberg