Bacteriophages and phages of other protests

Phages (short for bacteriophages) are viruses that infect prokaryotic cells, mainly bacteria and archaea. The term bacteriophage literally means “bacteria eater.”

Thank you for reading. Don't forget to subscribe & share!

Similarly, phages of other protists refer to viruses that infect unicellular eukaryotes such as protozoa and algae. These are less studied but equally important in ecology and evolutionary biology. Phages play a critical role in microbial ecology, genetics, evolution, and biotechnology.

2. Bacteriophages: A bacteriophage is a virus that infects and replicates within a bacterium by injecting its genetic material into the bacterial cell.

Discovered independently by Frederick Twort (1915) and Félix d’Herelle (1917).

Herelle first demonstrated that bacteriophages could destroy bacteria and coined the term bacteriophage.

3. Structure of Bacteriophage

Bacteriophages vary in shape and size, but the T-even phages (e.g., T2, T4, T6) of E. coli are the most studied.

Main Components:

1. Head (Capsid)
   1. Icosahedral shape, contains double-stranded DNA.
   1. Protects genetic material.
2. Tail Sheath
   1. Contractile tube that injects DNA into the host cell.
3. Base Plate and Tail Fibers
   1. Help the phage attach specifically to bacterial receptors.

4. Types of Bacteriophages

1. Lytic (Virulent) Phages
   1. Immediately replicate inside the host and lyse (destroy) the bacterial cell.
   1. Example: T4 phage.
2. Lysogenic (Temperate) Phages
   1. Integrate their DNA into the host genome (forming a prophage).
   1. Host cell survives and replicates with the prophage DNA.
   1. Under stress, the prophage can switch to the lytic cycle.
   1. Example: Lambda (λ) phage.

5. Life Cycles of Bacteriophages

A. Lytic Cycle

1. Attachment (Adsorption):
   Phage attaches to bacterial cell wall receptors.
2. Penetration:
   DNA is injected into the host; capsid remains outside.
3. Biosynthesis:
   Host machinery synthesizes viral nucleic acid and proteins.
4. Maturation:
   New phage particles are assembled.
5. Release:
   Cell wall breaks (lysis), releasing new phages.

➡️ Result: Destruction of host cell.

B. Lysogenic Cycle

1. Attachment and Penetration:
   DNA enters host but does not replicate immediately.
2. Integration:
   Viral DNA integrates with host DNA → prophage.
3. Replication:
   Host divides, passing viral DNA to daughter cells.
4. Induction:
   Under stress, prophage becomes active → enters lytic cycle.

➡️ Result: Host survives until induction occurs.

6. Significance of Bacteriophages

A. In Nature

• Regulate bacterial populations in soil and water.
• Influence microbial community balance and nutrient cycling.

B. In Medicine

• Phage Therapy: Alternative to antibiotics for treating bacterial infections (effective against antibiotic-resistant bacteria).
• Diagnostics: Used in bacterial typing and detection of pathogens.

C. In Molecular Biology

• Used as vectors in recombinant DNA technology (e.g., λ phage vectors).
• Helped discover the mechanism of gene regulation and replication.

D. In Biotechnology

• Tools in genome editing and phage display technology for drug discovery.

7. Phages of Other Protists

Viruses do not only infect bacteria — they also infect other protists such as protozoa, algae, and fungi. These are known as protozoan viruses or algal viruses.

A. Viruses of Protozoa

• Infect unicellular eukaryotic organisms such as Entamoeba, Paramecium, and Acanthamoeba.
• Often cause latent infections, where the virus persists without killing the host.
• Example: Acanthamoeba polyphaga mimivirus (APMV) — one of the largest known viruses, infects amoebae.
• Importance:
  • Influence protozoan evolution.
  • Affect ecosystem balance by controlling protozoan populations.

B. Algal Viruses (Phycoviruses)

• Infect marine and freshwater algae.
• Belong mainly to the families:
  • Phycodnaviridae (infect green algae, e.g., Chlorella, Micromonas).
  • Mimiviridae (infect larger unicellular algae).
• Play a major role in:
  • Regulating algal blooms in oceans.
  • Nutrient cycling and carbon flow in aquatic ecosystems.
• Example: Chlorella virus PBCV-1 infects Chlorella species.

C. Archaeal Viruses

• Infect archaea, which live in extreme environments (hot springs, salt lakes).
• Unique morphologies: spindle-shaped, lemon-shaped, or bottle-shaped.
• Help in understanding virus evolution and early life forms.

8. Comparison: Bacteriophages vs. Protist Phages

 Applications and Importance

1. Ecological Balance:
   Phages regulate microbial populations in soil, water, and oceans.
2. Genetic Research:
   Help in understanding molecular genetics (transduction, recombination).
3. Medical Use:
   Phage therapy as an alternative to antibiotics.
4. Biotechnology:
   Used as cloning vectors, vaccine carriers, and nanomaterials.
5. Environmental Role:
   Control harmful blooms (e.g., algal blooms) and recycle nutrients.