Bacteria & Structure of Bacteria Cell

 

• The bacteria are microscopic, unicellular free prokaryotes which occur singly or in aggregations to form colonies.
• In bacterial colonies, the individual cells are functionally independent. They are physiologically and biochemically diverse.
• Bacteria possess rigid cell walls. Some are motile, whereas others are not.

Distribution and Habitat

• Bacteria are cosmopolitan in distribution and are omnipresent. They occur in water, soil, air, animals, plants, in snow (upto – 190°C) and also in hot water streams.
• They are found floating in the atmosphere as “wanderers” on dust particles, up to height of several thousand feet.
• They are abundant on the surface of the human body and are present in large number in the intestinal tract and on all the mucous membranes.

Structure of Bacteria Cell

• Electron microscope is the instrument that reveals the detailed structure of a bacteria cell. The structure of the bacterial cell can be broadly divided as follows –
  • 1. Structure external to the cell wall.
    2. Structure of the cell wall.
    3. Structure internal to the cell wall.
• The structures that are present external to the cell wall are glycocalyx or capsule, flagella, fimbriae and pili. The structures internal to the cell wall include plasma membrane, mesosome, cytoplasm, etc.

Structure of Bacteria Cell External to the Cell Wall

 Glycocalyx

• • It is a sticky, gelatinous material that collects outside the cell wall forming an envelope which helps in protection as well as in adherence to the surface.
  • Glycocalyx is made of either slime layer or capsule.
  • Slime layer is made from dextran, dextrin and sugars and the main function is to protect the cell against desiccation and loss of nutrients.
  • Capsule is made up of polysaccharides and amino acids. The main function is to provide gummy or sticky character and add virulent property to the bacterial cell.
  • Glycocalyx helps in –
    1. prevention of desiccation,
    2. protection from phagocytes, toxic chemicals, drugs, viruses, etc.,
    3. attachment,
    4. immunogenicity,
    5. Virulence, etc.
  • Capsulated bacteria forming smooth colonies are called S-type bacteria. These are highly virulent. Non-capsulated bacteria forming rough colonies are called R-type bacteria.

Flagella

• • Flagella are long, thin, filamentous appendages made of globular protein protruding out of the cell through the cell wall. These are thinner than the eukaryotic flagella or cilia.
  • Flagella is made up of flagellin protein. Each flagellum is 4-5 µm in length.
  • Flagellum is the organ of motility in bacteria. Bacteria are of the following types on the basis of flagellation –
    1. 1. Monotrichous – When single flagellum occurs at one end only, e.g., Pseudomonas, Thiobacillus, Vibrio cholera.
       2. Lophotrichous – When a group of flagella is present at one end only, e.g., Spirillum volutans.
       3. Amphitrichous – When single or group of flagella is present at both the ends, e.g., Nitrosomonas.
       4. Peritrichous – When flagella are present all over the surface of bacteria, e.g., E. coli, Salmonella.
       5. Atrichous – When flagella are absent, e.g., Pasteurella pestis, Lactobacillus.
  • The flagella consists of three distinct regions namely, a basal body, the hook and filament. Flagella arises from a basal granule called blepharoplast. It is made up of one or three a-helical strands.
    1. The basal body bears ring like swellings in the region of plasma membrane and cell wall. There are two pairs of rings in Gram Negative bacteria and only a single pair of rings in Gram Positive bacteria.
    2. Hook is curved tubular structure which connects the filament with the basal body. It is the thickest part of flagellum.
    3. Filament is long tubular structure which causes turbulence in the liquid medium. These contain identical spherical subunits of a protein called as flagellin which are molecular chains that run longitudinally across each other to form a wavy helical or rope-like structure.

Pili and Fimbriae

• • Many Gram-negative bacteria possess a number of hair ¬like structures that are shorter, straighter and thinner than flagella and are used for the attachment rather than for motility. These structures, which consist of a protein called pilin, are divided into two types, fimbriae and pili.
  • Pili are longer, fewer and thicker tubular outgrowths in Gram Negative bacteria also called as sex pili as they are responsible for sexual reproduction. They help in attaching to recipient cell and forming conjugation tube.
  • Fimbriae are formed in large numbers, about 300-400 per cell. Diameter is 3-10 nm and length is 0.5 – 1.5 µm. They help in attaching bacteria to solid surfaces or host tissues.

Cell wall

• • Underlying the capsule and external to the cytoplasmic membrane is the cell wall which provides rigidity and shape to the bacterial cell.
  • The most important function of the cell wall is to protect the cell contents from external stresses and from lysis resulting from osmotic pressure.
  • Periplasmic space appears between plasma membrane and cell wall.
  • The cell walls are made up of peptidogylycan which is also called as murein or mucopeptide. Cell wall is made of polysaccharides, proteins and lipids.
  • The outer membrane consists of phospholipid bilayer structure made chiefly of phospholipids, proteins and lipopolysaccharides (LPS).
  • It gives the bacteria its main surface antigenin the cell wall. However, outer membrane is permeable to the external nutrients which is regulated by proteins called as porins. These porins form channels across the membrane through which substances are able to enter the cell.
  • Functions of Cell Wall includes –
    1. This rigidity protects the cells internal structures
    2. Provides shape to the cell.
    3. However, the main function remains to prevent the cell from expanding too much to burst. This bursting of cell may loose the cell organelles as most bacteria are in hypotonic habitat, that forces bacteria to take in much more water to eventually burst itself.
  • On the basis of differential staining technique called Gram staining technique developed by Christian Gram in 1884, bacteria can be divided into two large groups –
    1. Gram Positive bacteria and
    2. Gram negative bacteria.

Difference Between Gram Negative Bacteria and Gram Positive Bacteria

 

Plasma Membrane

• • Bacterial plasma membrane or plasmalemma is made of a phospholipid bilayer with proteins of various types (extrinsic, integral, transmembrane). However, typical sterols (e.g., cholesterol) are absent. Instead penta-cyclic sterols are found in some bacteria which stabilize the membrane.
  • It is the living part semipermeable in nature that controls the movements of many aqueous substances in and out of the cells.
  • Bacterial membrane is metabolically active as it takes part in respiration, synthesis of lipids and cell wall components.

Mesosome

• • The cell membrane gets folded inwards to form a structure called mesosome. It is a circular to villiform membranous structure which develops as an ingrowth from the plasma membrane.
  • It is less prominent in Gram Negative bacteria but is quite visible in Gram Positive bacteria.
  • Mesosome is of two types, septal and lateral.
  • Septal mesosome connects nucleoid with plasma membrane. It takes part in the replication of nucleoid.
  • Lateral mesosome is not connected with nucleoid. It contains respiratory enzymes and is therefore, often called chondrioid.
  • These mesosome also increase the surface area of absorption by the cell and help in septa formation during binary fission.

Cytoplasm

• • It is crystallo-colloidal complex that forms the protoplasm excluding its nucleoid. It is jelly like homogeneous mass containing carbohydrates, fats, proteins, lipid, nucleic acids, minerals and water.
  • Cytoplasm is granular due to presence of a large number of ribosomes.
  • Membrane bound cell organelles are absent. However, all biochemical pathways are found in prokaryotic cells.
  • Cytoplasmic streaming is absent.
  • Sap vacuoles are absent. Instead gas vacuoles are present.

Cytoplasmic Structure

• • Various structures present in cytoplasm are as follows-

1. 1. 1. Ribosomes

         • The ribosomes are 70S in nature. Each ribosome has two subunits, larger 50S and smaller 30S.
         • Ribosomes take part in protein synthesis. Free or matrix ribosomes synthesise proteins for intracellular use while fixed ribosomes synthesise proteins for transport to outside.

      2. Chromatophores

         • In certain photosynthetic bacteria, most of the cytoplasm is occupied by single-membraned, vesicular structures which are the extensions of the cytoplasmic membrane called Chromatophores.
         • They contain photosynthetic pigments along with the enzymes and electron carriers for the photosynthetic phosphorylation.

      3. Inclusion bodies

         • They are non-living structures present in the cytoplasm.
         • The inclusion bodies may occur freely inside the cytoplasm (e.g., phosphate granules, glycogen granules) or covered by 2-4 nm thick non-lipid, non-unit protein membrane (e.g., gas vacuoles, carboxysomes etc.).
         • On the basis of their nature, the inclusion bodies are of 3 types –
           • Gas vacuoles
             • They are gas storing vacuoles found in cyanobacteria, purple and green bacteria and a few other planktonic forms.
             • Gas vacuoles protect the bacteria from harmful radiations.
             • They also constitute buoyancy regulation mechanism for their proper positioning in water during day time.
           • Inorganic inclusions
             • Several types of inorganic granules occur in bacteria.
             • They include sulphur granules, iron granules, magnetite granules, etc. Because of the ability to pick up different colors with basic dyes, they are called metachromatic granules. 
           • Food reserves
             • Cyanobacteria have cyanophycean starch or granules of lipid globules and cyanophycin or protein granules. In bacteria, starch is replaced by glycogen.

Nucleoid

• • (Pro chromosome, Genophore, incipient nucleus)
  • Bacteria lack nuclei and do not possess the complex chromosomes characteristic of eukaryotes.
  • Their genes are encoded within a single double-stranded ring of DNA which is present in the centre of cell not surrounded by any membrane (so DNA is considered naked).
  • This primitive type of nucleus is known as nucleoid.
  • DNA does not has free ends and not associated histone proteins which is termed as bacterial chromosome.

Plasmids

• • Many bacterial cells also possess small, independently replicating circles of DNA called plasmids. Plasmids contain only a few genes, which are not essential for the cell’s existence.
  • Plasmids which can get associated temporarily with nucleoid are known as episomes. Plasmids are used as vectors in genetic engineering.
  • Plasmids carry non vital genes which may or may not be useful to bacteria. Plasmids which do not confer any useful trait to the cells are called cryptic plasmids.
  • Three types of useful plasmids are –
    1. F-Plasmids – They are plasmids which contain genes for conjugation or fertility. An F-plasmid is, therefore, also called fertility factor or F-factor. A bacterium having F-plasmid is called male or donor bacterium. It develops sex pili for conjugating with female or recipient bacterium which is devoid of F-factor.
    2. R-Plasmids – They are plasmids which carry genes for resistance against common antibiotics like chloramphenicol, tetracycline, streptomycin and sulphonamide. Pathogenic bacteria having R-plasmids are very difficult to treat.
    3. Col-Plasmids – They are also called colicinogenic factors. They produce toxins called colicins or bacteriocin which are lethal to other enterobacteria. Col-plasmid can also get transferred from one bacterium to another just like F-plasmid or R-plasmid.

So, this was all about the Bacteria and Structure of Bacteria Cell. In the Next Post (Click Here), we will learn about the Types of Bacteria on the Basis of Shape, Nutrition etc. You can also read about Structure of the Cell here.

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