Bacteria – NEB Biology Notes

General account , structure and functions of bacterial cells

Anton Von Leeuwenhoek is regarded as “ father of bacteriology” because he first time (1683) observed these tiny ‘animalcules’ from the scum of teeth of a man. Bacteria are cosmopolitan in distribution, occur every where in water , air, soil, food stuffs etc. Bacteria are prokaryotic, achlorophyllous and u nicellular organism. Instead of true chlorophyll, bacteria may containbacterio chlorophyll and chlorobium chlorophyll. Normally, the bacteria may ranges between 1 to 5 um. It posses the foloowing forms :

  • Cocci(spherical) – Eg.Micrococcus, Diplococcus
  • Bacilli(rod shaped)- Eg. Diphobacilli, Streptobacilli
  • Spirilla (spirally coiled) – Eg. Spirochaete, Spirillum
  • Vibrio(comma shaped) – Eg. Vibrio cholera

On the basis of flagella , bacteria are :

  • Atrichous(no flagella )
  • Monotrichous(single flagellum at each end)
  • Amphitrichous(one flagellum at each end)
  • Cephalotrichous(tuft of flagella at each pole)
  • Lophotrichous(tuft of flagella at each pole)
  • Peratrichous(several flagella all over the surface)

Structure and functions

A typical bacterial cell is enveloped by 3 distinct layers: Capsule (slime layer), cell wall, and cell membrane. Capsuleis the outermost protective layer against the adverse climatic condition and toxic substances present including in host. It posses the complex cell wall composed up of proteins, carbohydrate, and chitin but cellulose is absent inner to cell wall lies cell membrane which serves as a permeability barrier and surrounds protoplast. The cytoplasm spreads uniformly throughout the cell and contains many vacuoles, food granules; ribosomes, mesosomes, and incipient nucleus(having no major cytoplasmic organelles i.e. mitochondria, lysosome, endoplasmic reticulum, nuclear membrane, etc.).

Labeled Diagram of Bacteria
Labeled Diagram of Bacteria

The mesosomes are circular bodies that help in the separation of replicated DNA during nuclear division and are a site of respiration. Bacteria posses one or more thread-like structure called flagella originating from the cytoplasm.

Such bacteria are motile in nature. Bacterial nucleus is quite primitive type, lacks nucleolus, nuclear membrane, nuclear sap etc. Some gram –ve bacteria posses fine hair like protoplasmic out growths called fimbriae, scattered over the entire surface of bacterial cell.

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Concept of autotrophic and heterotrophic life cycles

On the basis of mode of nutrition, Bacteria are of two types :

Heterotrophic bacteria

Most of the bacteria are heterotrophic in nutrition, cannot synthesis their food from simple inorganic substances. These are of three types.

Parasitic bacteria

Parasitic bacteria take their food from host which may be obligate or facultative parasites.

Saprophytic bacteria  

These types of bacteria take their food from dead organic matter and are found growing in animals dung decaying vegetables etc. It secretes and breaks the complex food into simpler and soluble forms.

Symbiotic bacteria

Some bacteria like Rhizobium species occur as symbionts on the root nodulues of leguminous plants.(Note :- Symbiotic means both the partners or the organisms are equally benefited by the symbiotic bacteria.)

Autotrophic bacteria

These type of bacteria synthesize thir food themselves from inorganic substances.The energy is either obtained chemically called chemotropic or from solar radiation called photo autotrophic forms. While manufacturing food, chemoautotrophic bacteria obtain energy from chemicals; do not require solar energy but the photoautotrophic (photosynthetic) bacteria posses chlorobium chlorophyll and bacterio chlorophyll which trap solar energy. Bacterial photosynthesis takes in light but in absence of oxygen.

Koch’s postulates

  • The specific organism should be shown to be present in all cases of animals suffering from a specific disease but should not be found in healthy animals.
  • The specific microorganism should be isolated from the diseased animal and grow on artificial laboratory media.
  • This freshly isolated microorganism when inoculated into a heated laboratory animal should cause the same disease seen in the original animal.
  • The microorganism should be reisolated in pure culture from the experimental infection.
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Gram positive bacteriaGram negative bacteria
They retain the deep blue or purple with crystal violet and subsequently with potassium iodide and alcohol.They do not retain the color when treated with potassium iodide and alcohol.
The cell wall is 100 to 200 A.The cell wall is 70 to 120A.
Peptidoglycan constitutes the 70% of cell wall.Peptidoglycan constitutes the 30 % of the cell wall.
They are generally susceptible to lysosomes or antibiotics.They are generally resistant to lysosomes or antibiotics.
Generally they do not bear fimbriae or sex pili.Generally they bear pili or fimbriae.
Cell membrane forms mesosomes.Mesosomes are absent.
Difference between Gram Positive and Gram Negative Bacteria

Economic importance of Bacteria

Beneficial activities of Bacteria

  • Some species of bacteria like Pseudomonas, Rhizobium, Clostridium, Azotobacter, fix atmospheric nitrogen in usable forms for plants.
  • Some species of clostridium are used in the manufacturing of Vitamin B(riboflavin).
  • Lactic acid bacteria(Streptococcus lactis) help in the coagulation of milk.
  • Three types of medicine – antibiotics, antisera, and vaccines are formed by bacteria.
  • Bacteria(E. coli) present in the human colon synthesize vitamin B and release it forms human use.
  • Bacteria help in digestion in herbivores animals in the digestion of cellulose. Bacillus subtilis produces the enzymes amylase and protease.
  • By the activity of clostridium, the butyl alcohol and acetone are obtained from sugar molasses.
  • It helps in fibers retting of jute, coconut, and other fibrous plants.
  • Bacteria like Bacillus subtilis, Bacillus ramosus etc. decompose complex organic compounds like proteins into ammonium compound. These are also known as ptrefying bacteria.

Harmful activities of bacteria

  • Many human diseases such as diarrhea, meningitis, pneumonia etc. are caused by pathogenic bacteria.
  • Some denitrifying bacteria reduce soil fertility by depleting the nitrogen contents of the soil.
  • Some bacteria spoils foodstuffs, leather, and wooden articles.
  • Certain bacteria destroy the activity of penicillin by producing an enzyme penicillinase.
  • Tuberculosis of cattle, anthrax of sheep is caused by Mycobacterium tuberculosis and Bacillus anthracis respectively.
  • Citrus canker by Xanthomonas citri, ring rot of potato by corynebacterium sepedonieum etc.
Ashish DhakalAshish Dhakal

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Ashish Dhakal

I am an engineering student, a blogger, and an entrepreneur. My hobbies include reading books, singing and travelling.
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