PRELIMS

Microbiology: Virus and Bacteria

Microorganisms are at the junction of life forms, exhibiting unique parasitic and metabolic characteristics.

I. Virus (Virology)

Discovered by Ivanovsky (1892) in tobacco mosaic disease.

• Nature: Connecting link between living and non-living.
• Characters: Only active inside a host cell; nucleic acids replicate rapidly; cause diseases like Bacteria/Fungi.
• Types:
  • Plant Virus: Typically contain RNA.
  • Animal Virus: Typically contain DNA (sometimes RNA).
  • Bacteriophages: Viruses that infect Bacteria (e.g., TMV).
  • Retroviruses: RNA as genetic material.
• Human Diseases: Mumps, Chicken pox, Hepatitis, Polio, AIDS, Herpes.

II. Bacteria (Bacteriology)

Discovered by Antony von Leeuwenhoek (1 Holland, 1683).

• Classification by Shape:
  • Bacillus: Rod-like/cylindrical.
  • Coccus: Round/smallest.
  • Spirillum: Spring/screw-shaped.
  • Vibrio: Comma-shaped (e.g., Vibrio cholerae).
• Nitrogen Fixation:
  • Soil: Azotobacter, Azospirillum, Clostridium (free-living).
  • Symbiotic: Rhizobium and Bradyrhizobium (in the roots of leguminous plants).
  • Cyanobacteria: Anabaena and Nostoc.

Confused Pair: Gram-Positive vs Gram-Negative Bacteria

Based on Gram staining (developed by Hans Christian Gram), which tests cell wall composition.

• Gram-Positive: Thick peptidoglycan layer; retains the crystal violet stain (appears purple/blue). E.g., Staphylococcus.
• Gram-Negative: Thin peptidoglycan layer with an outer lipid membrane; does not retain crystal violet, takes counterstain (appears pink/red). E.g., E. coli, Vibrio cholerae. Generally more resistant to antibiotics due to the outer membrane.

Pasteurization and Preservation

Louis Pasteur discovered the process of killing bacteria in milk.

1. Low Temperature Holding (LTH): 62.8°C for 30 minutes.
2. High Temperature Short Time (HTST): 71.7°C for 15 seconds.
3. Tanning: Leather industry separation of hair/fat from skin by bacteria.
4. Preservation: Pickles/syrups use salt/sugar to plasmolyse and destroy bacteria.

UPSC Relevance

Microbiology is critical for Public Health and Agriculture.

• Virus: The "Living/Non-living" debate and the role of Retroviruses (relevant for AIDS/HIV research).
• Bacteria: The nitrogen cycle is a major GS-I/III pillar; knowing which bacteria are symbiotic (Rhizobium) vs free-living (Azotobacter).
• Food Security: Technologies like pasteurization and the biological basis for food preservation (osmosis/plasmolysis).