Introduction to bacteria-I << Back

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Introduction to bacteria-I


The purpose of this chapter is to describe the structural organisation of the prokaryotic cell i.e. Bacteria. The bacterial cell represents the simplest of all cellular organisms when seen under the microscope. Bacteria (plural word) is a prokaryotic structure. The singular for this word is “bacterium” . They do not have true nucleus. They have one chromosome of double-stranded DNA in a circular form. They reproduce by binary fission. They do not have any intracellular organelles. They belongs to kingdom Monera which  is a very diverse group. There are some photosynthetic bacteria which can do photosynthesis--they don’t have chloroplasts, but their photosynthetic pigments are their cell membranes. These organisms are called blue green algae or cyanobacteria. They are the most common and omni present microorganisms on the earth.

Statement of cell theory

  1. Cell is the structural and functional unit of life.
  2. All organisms are made up of cells.
  3. New cells arise from the pre existing cells.


Structure of bacterial cell

A variety of structures are found in prokaryotic cells.  Following image exhibits the typical structure of a cell.










All this structures are not found in all genus. Gram positive and Gram negative bacteria differs in their cell wall content. All bacterial cells were surrounded by a cell wall except mycoplasma. After cell wall there is a periplasmic space beneath which lies a plasma membrane. Bacteria does not contain the intracellular organelles so their internal structure is very simple. Genetic material consist of the circular chromosome lying in the nucleoid region. Ribosome and inclusion bodies are scattered in the cytoplasm. Many cells have flagella for locomotion. Pilli is also observed for sexual reproduction. Some bacteria also contain capsules surrounding their cell wall.

Morphology of bacteria


Generally four representative shapes of bacteria are observed.

  1. Coccus (singular)/ Cocci (plural): These bacteria are spherical in shapes.
  2. Bacillus (singular) / Bacilli (plural): They are rod-shaped
  3. Spirillum (singular.)/Spirilla (plural): They are spiral shaped.
  4. Comma shaped cells Vibrio

Apart from the above sited shapes, some typical shapes of bacteria are also observed. But these shapes are restricted to few species only.

  1. Square shaped bacteria ( A.E.Waksby, 1980. )
  2. Star shaped ( Stella gnus)
  3. Actinomycetes charecteristically form long multinucleate filaments structure.

Pleomorphism: Some bacteria are variable in shape and lack a single, characteristic form and they are called as pleomorphic.

Pattern of cell arrangement

Bacterial cells may be categorised according to arrangement or cell grouping. Every bacterial species have their characteristic arrangement pattern. Variety of arrangement pattern is observed in cocci. Most common arrangement pattrns are

  • Single
  • Diplococcus: A pair of cells
  • Streptococcus: Chains of four or more cells.
  • Tetrads: Four cocci in boxlike or square arrangements
  • Sarcinae: Cuboidal Packet
  • Staphylococci: cluster of cocci.


Bacteria range in size from about 0.2 to 5 micrometer. Smallest bacteria mycoplasma are about the same size as the largest viruses. (Pox virus). Cocci measures anywhere from 0.5 to 3.0 micrometer in diameter. Bacilli range from 0.2 to 2.0 micrometer in diameter and from 0.5 to 250 micrometer in length. Escherichia coli has the size of 1.1  to 1.5 micrometer wide by 2.0 to 6.0 micrometer long.

Habitats of bacteria

They are  found in the bottom of the deepest oceans , at the peak of mount , inside the body of animals, and even in the frozen rocks and ice of Antarctica. One feature that has enabled them to spread so far, and last so long is their ability to go dormant for an extended period. They are found in the hot springs,  frozen soils, acid drained mines, saline lakes.


Reproduction in Bacteria

Bacteria reproduce very rapidly. E.coli can reproduce in after every 20 minutes.  Bacteria reproduce by two basic methods 

1)Asexual reproduction

Asexual reproduction involves only one parent.  The offspring are exact copies of the parent.Bacteria splits into two cells using binary fission.  Each daughter cell receives the same gentic material like parent cell.

2)Sexual reproduction.

It requre two parent cell. It involves the physical contact between the two cell. It occurs with the help of the conjugation tube. Here progeny cell have a mixture of the parent cells' traits.  It occurs by the process of Conjugation.

Structures external to the cell wall

  • Flagella

Majority of motile bacteria moves with the help flagella. It is a thread like appendages extending outward from the plasma membrane and cell wall. Size of the flagella is 15-20 micrometer long. The detailed structure of a flagellum can only be seen in the electron microscope.

Flagellation (Flagellar arrangement):There are different arrangement of flagella found on bacterial cell. Different pattern of flagellation are

  1. Monotrichous: One flagella, located at one end. E.g. Pseudomonas
  2. Amphitrichous: Two flagella located at each pole. E.g. Spirillu
  3. Lophotrichous: Cluster of flagella at one end. E.g. Aquaspirillum
  4. Peritrichous: Many flagella spread evenly on whole surface. E.g. Proteus

Ultra structure of flagella:It consist of three parts.

  • Filament: longest portion. Extend from the cell surface
  • Basal body: This part is embedded in cell body.
  • Hook: curved part which connect the filaments to basal body.

Basal body is most complex structure of the flagella. In E.coli , basal body consist of four rings i.e. L,P,S,M rings.  Gram positive bacteria have only two basal rings.

Chemical composition of Bacteria: It is made up of protein fibers. These protein fibers are made up of flagellin protein. Molecular weight of the flagellin protein is in the range of 20 kd to 60 kd.  Flagellin protein of different bacterial species is not identical.

Mechanism of Flagellar movement:Bacteria moves when this flagellar helix rotates. Direction of flagellar rotation determines the nature of bacterial movement.Monotrichous polar flagella rotates counterclockwise during normal forward movement.Monotrichous  bacteria stop and tumble randomly by reversing the direction of flagellar rotation. Peritrichous flagella forms bundle of the flagella and moves in the similar way. It is believed that flagella moves due to interaction of the rings present in the basal body.

Swimming motility: Spirochetes moves due to the axial filament present in between the periplasmic space. It is also called endoflagellum.

Gliding Motility: This movement occurs without flagella. Gliding bacteria are able to move on solid surface.

  • Fimbriae and Pili: Fimbriae is used to denote the nonflagellar appendages other than those involved in sexual conjugation. Pili refers to appendages that are involved in the transfer of nucelic acid  during a sexual process between called conjugation. They are found mostly on Gram negative bacteria particularly on Enterobacteriaceae family.
  • Prosthecae: It is an extension of cell wall and cytoplasmic membrane.. They are observed in bacteria which are found in the freshwater and marine envrionment. E.g. Caulobacter, Stella etc. It increases the surface are of the cells for nutrient adsorption.
  • Stalks: Stalks  are non living tubular extension  that are excreted by the member of Genus Gallionella and Planctomyces. It aid to attachment of the cell to the surface.
  • Spines: Some marine Gram negative bacteria exhibits this structure. It is a rigid protein appendages. It protect bacteria against ingestion by larger microorganisms.
  • Spirae: Nitrogen fixing bacteria Azospirillum lipoferum produces sevaral spiral structures called spirae. It is single coiled thread like structure. Its function is not known.
  • Capsule and slime layers: Bacterial cells exude extracellular substance in the form of a capsule or a slime layer. A slime layer is loosely connected with the bacterial cell  and can be simply removed, whereas a capsule is attached firmly to the bacterial cell. Capsule has specific margins. Capsules can be seen under a light microscope using capsule staining india ink. Capsule does not stain by the stain and appear as a clear halos surrounding the bacterial cells. Capsules are made up of simple sugars (polysaccharides).Most capsules are hydrophilic and may help the bacterium evade dehydration by preventing water loss. Capsules can protect a bacteria from phagocytosis. It may be due more slippery nature of the capsule, which  helps the bacterium to from engulfment by WBC. Capsules are also related to the pathogenicity of the bacteria eg. Streptococcus pneumoniae. S. pneumoniae mutants which are not able to synthesize capsule do not cause disease. Such association of pathogenicity and capsule is also found in some other bacteria. It is also associated with biofilm formation.
  • Sheaths: Some species of bacteria, particularly those from fresh water and marine environments, form chains or trichomes that are enclosed by a hollow tube called sheaths. It may be sometimes become impregnated with ferric or maganese hydroxides; which stregthen them.


Question : 1 Mycoplasmas are bacterial cells that

1: Fail to reproduce on artificial media
2: Stain well with Gram’s stain
3: Are resistant to penicillin
4: Have a rigid cell wall

Question : 2 The mode of reproduction which occurs in mycoplasma is

1: Budding
2: Bursting
3: Binary fission
4: Binary fusion

Question : 3 The microbiological term describing the spherical shape of the bacterium is

1: Bacillus
2: Pleomorphic
3: Vibrio
4: Coccus

Question : 4 Which of the following is incorrect about capsules and slime layers?

1: They are required for bacteria to grow normally in culture
2: They help bacteria resist phagocytosis by macrophages
3: They consist of secreted material lying outside of the bacterial cell wall
4: They can prevent desiccation of bacteria cells

Question : 5 Fimbriae are the bacterial appendages which

1: Sense changes in the nutrient concentration
2: Cause bacteria move through fluids
3: Attach bacteria to various surfaces
4: All of the above

Question : 6 A bacillus bacterium with a single flagellum at each end is described as

1: Monotrichous
2: Amphitrichous
3: Lophotrichous
4: Peritrichous

Question : 7 The growth of flagellar filament in bacteria occurs by

1: The addition of subunits to the base of the flagellum
2: The addition of subunits to the tip of the flagellum
3: Self-insertion of subunits along the length of the filament
4: Self-assembly of subunits travelling through the hollow flagellum to the tip

Question : 8 Which of the following statement is correct regarding the ultra structure of flagella

1: Gram-positive bacteria contains four basal rings
2: E.coli contains four basal rings i.e. L, P, S, M rings
3: Gram-negative bacteria contains two basal body rings
4: Basal body is a curved segment that acts like a flagellar hook

Question : 9 Which of the following is incorrect regarding sex pili of bacteria

1: Pili often are larger than fimbriae
2: Many bacteria have about 1-10 sex pili
3: They are required for conjugation
4: None of the above

Question : 10 Which of the following is true regarding gliding motility

1: Bacteria exhibiting motility in highly viscous media
2: Bacteria are motile only when they are in contact with a solid surface
3: This kind of movement is very fast
4: Flagella is responsible for this kind of motility
Introduction to structure of bacteria