Microbiology

Bacteriophage

Viruses that infect bacteria — most numerous biological entities on Earth

Bacteriophages (or phages) are viruses that infect bacteria. Most abundant biological entities on Earth — ~10³¹ phages, outnumbering bacteria 10:1. Two main lifestyles: lytic (replicate, lyse host cell, release progeny — kills bacteria) and lysogenic (integrate into host genome as prophage, replicate with host, may later switch to lytic). Important for: bacterial population control, evolution (transferring genes between bacteria via transduction), molecular biology research, phage therapy (alternative to antibiotics). T4, T7, lambda are classic phages. Discovered 1915.

  • DefinitionViruses that infect bacteria
  • Population~10³¹ globally; outnumber bacteria 10:1
  • Lytic cycleReplicate, lyse host, release; kills bacteria
  • Lysogenic cycleIntegrate as prophage; latent
  • Famous phagesT4, T7, lambda, M13, P22
  • Phage therapyUsed in some countries; alternative to antibiotics

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Why phages matter

  • Bacterial control. Largest bacterial predator.
  • Phage therapy. Alternative to antibiotics.
  • Research tool. Phage display, cloning vectors.
  • Evolution. Drive bacterial gene transfer.
  • CRISPR origin. Bacterial defense against phages.
  • Biotechnology. Industrial applications.
  • Ecology. Marine and soil ecosystem cycling.

Common misconceptions

  • Phages affect humans. Specific to bacteria; safe for humans.
  • Phages rare. Most abundant biological entities.
  • Phages always destructive. Lysogenic phages persist passively.
  • One phage type for all bacteria. Highly host-specific.
  • Phage therapy unproven. Used clinically in some countries; growing evidence.
  • Phages kill bacteria immediately. Lysogenic doesn't kill.

Frequently asked questions

How do phages infect bacteria?

Steps. (1) Attachment — phage binds specific receptor on bacterial surface. (2) Penetration — DNA injected into bacterium (capsid stays outside). (3) Replication — phage genes replicate; phage proteins made (using host machinery). (4) Assembly — new phage particles formed. (5) Release — bacterial cell lysed (lytic) or phage DNA integrated (lysogenic). Lytic cycle: ~30 min for E. coli phage to make 100 progeny.

What's the difference between lytic and lysogenic?

Lytic: phage replicates immediately; bacterial cell killed. Lysogenic: phage DNA integrates into bacterial chromosome (as prophage); replicates with host; latent until switched to lytic by triggers (stress, UV, etc.). Some phages: only lytic (T4); only lysogenic (rare); switch (lambda). Lambda phage: classic system for studying gene regulation.

What's transduction?

Phage-mediated gene transfer between bacteria. During replication, phage occasionally packages bacterial DNA instead of phage DNA. New phage delivers bacterial DNA to next host. Mechanisms: generalized (any bacterial gene can transfer) and specialized (specific genes near integration site). Important: spreads genes (including antibiotic resistance) between bacteria; sources virulence factors; lab tool.

What's phage therapy?

Using phages to treat bacterial infections. Predates antibiotics (1920s). Largely replaced in West by antibiotics; continued in Eastern Europe (Georgia, Russia). Current: increasing interest due to antibiotic resistance. Advantages: specific (target specific bacteria; no harm to good bacteria); evolves with resistance (phages evolve too). Disadvantages: bacteria evolve phage resistance; immune response may neutralize phage.

How are phages used in research?

Many uses. (1) Phage display: peptides displayed on phage surface; screen for binding partners. Used for: drug discovery, antibody engineering. (2) Cloning vectors: lambda for large DNA fragments. (3) M13: ssDNA for sequencing, mutagenesis. (4) CRISPR origin: bacterial defense against phages provided basis for gene editing tool. (5) Model organisms for molecular biology.

What's CRISPR's connection to phages?

CRISPR evolved as bacterial immune system against phages. Bacteria store snippets of phage DNA from past infections. When phage attacks again: stored sequences guide Cas proteins to recognize and destroy phage DNA. Co-opted for genome editing — programmable using guide RNAs matching desired target. Won 2020 Nobel.

What's the abundance?

~10³¹ phages on Earth. Outnumber bacteria 10:1 in most environments. Largest cause of bacterial death globally — ~10²³ bacteria killed per second by phages. Critical for: biogeochemical cycles (kill bacteria → release nutrients), bacterial evolution. Most diverse biological entity. Most uncharacterized.