How big is Bacteriophage? | Scale of the Universe

A bacteriophage, or phage, is a virus that infects bacteria, and its name comes from the Greek word for "devour". The explorer's picture shows a phage shaped like T4, a virus that infects the bacterium Escherichia coli. T4 is about 260 nanometers long from the top of its head to the bottom of its tail. A nanometer is a billionth of a meter.

Size

Viruses come in several shapes, and phages such as T4 have what biologists call a complex shape. They have a head that holds the virus's DNA, a tail, and fibers that attach to a bacterium. T4 has long been used as a model for studying how viruses and cells work, and it has been measured in fine detail.

T4's head is a stretched 20-sided shape, 120 nanometers long and 86 nanometers wide. It holds one DNA molecule about 171,000 base pairs long, a base pair being one rung of the DNA ladder. The tail below the head is about 140 nanometers long. Its outer sheath is 92.5 nanometers long and 24 nanometers wide, and it ends in a six-sided baseplate about 52 nanometers across.

Six long tail fibers, each bent at a joint, spread out from the baseplate like legs. Each fiber is about 144–160 nanometers long, a little longer than the tail itself.

Size comparisons

The E. coli bacterium that T4 infects is about 2 micrometers long and 1 micrometer wide, so it is about eight times as long as T4. A micrometer is a thousandth of a millimeter.
If T4 were enlarged to the height of a person 1.7 meters (5 feet 7 inches) tall, an E. coli cell would be about 13 meters (43 feet) long and 6.5 meters (21 feet) wide.
About 300–400 T4 phages, laid end to end, would span the width of a human hair, which is 80–100 micrometers wide.
T4's head is less than half as wide as the finest detail an ordinary light microscope can show, about 200 nanometers, the limit of optical microscopes. Phages are photographed with electron microscopes instead.
T4 is about 1.8 times as long as an HIV particle is wide. A Mimivirus particle, about 750 nanometers across, is nearly three times T4's length.

How a phage infects a bacterium

A phage first attaches to particular molecules on the surface of a bacterium. Most phages can infect only one species of bacteria, or even one strain. T4's tail sheath then contracts and acts like a hypodermic needle, injecting the virus's DNA through the cell wall. The head and the rest of the phage stay outside.

Inside, the phage's genes take over the cell, which starts making the parts of new phages. The new phages are assembled, and the cell bursts open, releasing them to infect other cells. One infected cell releases about 100–300 phages like T4.

Phages as medicine

Because each phage attacks only certain bacteria, doctors can use phages to treat bacterial infections. This is called phage therapy. Phages were discovered early in the 20th century. The later discovery of penicillin and other antibiotics led most countries to abandon phage therapy, but the former Soviet Union and a few countries in Eastern Europe kept using it.

Phages have one advantage over antibiotics: they kill only the bacteria they target, while antibiotics also kill helpful bacteria in the body. A 2016 microbiology textbook describes interest in phage therapy as re-emerging, because more and more bacteria resist antibiotics.