Bacteriophages: Viruses That Infect Bacteria Explained

A bacteriophage (phage) is a virus that infects bacteria. The name means bacteria-eater, because the first phages discovered appeared to eat away bacterial cultures on laboratory plates. Like all viruses, a phage is acellular and reproduces only inside a bacterial cell.

Bacteriophages matter for two reasons. They are the most abundant biological entities on Earth, controlling bacteria in oceans and soil. In the laboratory they became the key tools with which scientists discovered the nature of genes, and today they are being developed as medicines against antibiotic-resistant bacteria.

Key Concepts

The classic phage, such as T4 which infects E. coli, has a complex structure unlike simpler viruses. It has an icosahedral head containing double-stranded DNA, a tail with a contractile sheath, a base plate and tail fibres that recognise the bacterial surface. Phages were discovered by Frederick Twort in 1915 and named by Felix d'Herelle in 1917.

• Virulent phages: always follow the lytic cycle and kill the host cell.
• Temperate phages: can follow the lysogenic cycle, hiding inside the host instead of killing it at once.
• Prophage: the phage DNA integrated into the bacterial chromosome during lysogeny.
• Burst size: the number of new phage particles released when one infected bacterium bursts, typically around 100 to 200 for T4.

How It Works

The lytic cycle runs in five steps. First, attachment: tail fibres bind to receptors on the bacterial wall. Second, penetration: the tail sheath contracts and injects the phage DNA into the cell, leaving the protein coat outside. Third, biosynthesis: bacterial machinery copies phage DNA and makes phage proteins. Fourth, maturation: new heads, tails and fibres assemble into complete phages. Fifth, lysis: the enzyme lysozyme breaks the bacterial wall and the new phages burst out. In the lysogenic cycle, the phage DNA instead integrates into the bacterial chromosome as a prophage and is copied silently with it; stress such as UV light can trigger induction, switching it back to the lytic cycle.

Worked Example

One E. coli cell is infected by a single T4 phage with a burst size of 100. Assuming every released phage finds a new bacterium, how many phages exist after two complete lytic cycles?

1. After the first cycle: 1 infected cell releases 100 phages.
2. Each of those 100 phages infects a new cell, and each releases 100 more: 100 x 100 = 10,000.

Answer: after just two cycles there are 10,000 phages. This explosive multiplication is why phages can clear a bacterial culture so fast.

Common Misconceptions

• Misconception: Bacteriophages can infect humans. Correction: Phages are highly specific; a phage that infects E. coli cannot attach to human cells, which lack the right receptors.
• Misconception: All phages kill bacteria immediately. Correction: Temperate phages can live quietly as prophages for many generations before switching to the lytic cycle.
• Misconception: The whole phage enters the bacterium. Correction: Only the DNA is injected; the empty protein head and tail remain outside the cell.

Real-World Applications

• Phage therapy: used in countries like Georgia to treat bacterial infections, and increasingly studied worldwide against antibiotic-resistant superbugs.
• Food safety: phage sprays approved in several countries kill Listeria on cheese and deli meats.
• Molecular biology: phage lambda is a classic cloning vector, and studying phages led to the discovery of restriction enzymes.

Bacteriophages are bacteria-infecting viruses with a head-tail structure that inject their DNA into the host. Virulent phages run the five-step lytic cycle ending in lysis, while temperate phages can hide as prophages in the lysogenic cycle. Their specificity and rapid multiplication make them powerful tools in medicine, food safety and genetics.