Bacteriophage T4: About 57 µm of DNA in a Head 0.12 µm Long

Bench card

Name

bacteriophage (phage), a virus that infects bacteria; featured here: phage T4, species Tequatrovirus T4 (formerly Escherichia virus T4)Source: ICTV Master Species List 2025 (MSL41), https://doi.org/10.5281/zenodo.19154110; GBIF Backbone Taxonomy 12252006 and 9977478, https://doi.org/10.15468/39omei

Group

tailed phages, class Caudoviricetes; GBIF 10794457

Actual size

phage T4: head 86 nm wide and 120 nm long [10⁻⁸–10⁻⁷ m]; tail sheath 24 nm wide and 92.5 nm long before it contractsSource: Rao and Black 2010, https://doi.org/10.1186/1743-422X-7-356; Leiman et al. 2010, https://doi.org/10.1186/1743-422X-7-355; bacteriophage-dimensions.csv on this page

Size in this image

100 nm scale bar printed on the opening micrograph, which shows phage ϕpp2, a T4-like phage of Vibrio, not T4; its authors measured its head at about 90–95 nm by 150–160 nmSource: Lin and Lin 2012, https://doi.org/10.1186/1471-2164-13-224

Lives in

seawater and other natural waters, alongside the bacteria it infectsSource: Mann 2005, https://doi.org/10.1371/journal.pbio.0030182; Bergh et al. 1989 (abstract), https://doi.org/10.1038/340467a0

Seen with

electron microscope for shape and size; in seawater stained with a fluorescent dye, a standard light microscope shows virus particles only as points of lightSource: Mann 2005, https://doi.org/10.1371/journal.pbio.0030182; Valli et al. 2021, https://doi.org/10.1016/j.jbc.2021.100791; figure captions on this page

Color in images

native in the electron micrographs (no color); fluorescence (SYBR Green) in the seawater imageSource: figure captions on this page

Key source

Rao and Black 2010, Virology Journal 7:356 (structure and assembly of the T4 head)

bacteriophage-bench.csv (CSV) · where each value comes from, and its license (JSON)

Data credits for this card

Data: International Committee on Taxonomy of Viruses, ICTV Master Species List 2025 (MSL41), version 1, CC BY 4.0, https://doi.org/10.5281/zenodo.19154110 (Tequatrovirus T4; retrieved 2026-10-25); GBIF Secretariat 2023, GBIF Backbone Taxonomy, Caudoviricetes (10794457), Tequatrovirus T4 (12252006), Escherichia virus T4 (9977478), checklist dataset https://doi.org/10.15468/39omei, CC BY 4.0, retrieved 2026-10-25; Rao VB, Black LW 2010, Virology Journal 7:356, CC BY 2.0, https://doi.org/10.1186/1743-422X-7-356; Leiman PG et al. 2010, Virology Journal 7:355, CC BY 2.0, https://doi.org/10.1186/1743-422X-7-355 (T4 head and sheath; retrieved 2026-10-25); Lin YR, Lin CS 2012, BMC Genomics 13:224, CC BY 2.0, https://doi.org/10.1186/1471-2164-13-224 (phipp2 head size; retrieved 2026-10-25); Mann NH 2005, PLoS Biology 3(5):e182, CC BY 4.0, https://doi.org/10.1371/journal.pbio.0030182; Bergh Ø et al. 1989, Nature 340:467–468, https://doi.org/10.1038/340467a0 (abstract); Valli J et al. 2021, Journal of Biological Chemistry 297(1):100791, CC BY 4.0, https://doi.org/10.1016/j.jbc.2021.100791 (habitat and seeing; retrieved 2026-10-25)

How big is a bacteriophage?

Sizes vary from one phage to another, so this page follows one of them, phage T4. Rao and Black's 2010 review describes the phage's head as an elongated, or prolate, icosahedron, a many-faced shell drawn out along one axis, 120 nm long and 86 nm wide. The tail parts below come from Leiman and colleagues' 2010 review. Neither review gives the neck's length, so no total length appears here.

The micrograph above shows not T4 but ϕpp2, a T4-like phage that infects Vibrio bacteria. One of its hosts in Lin and Lin's 2012 study is a Vibrio species widely distributed in coastal waters, and they measured the phage's heads at 150–160 nm long, longer than T4's.

Names have shifted too. In a 2022 update reported by Turner and colleagues, the International Committee on Taxonomy of Viruses renamed T4's species Tequatrovirus T4 (formerly Escherichia virus T4), the name in its 2025 list, and dropped families based on tail shape, such as Myoviridae. Tailed phages with double-stranded DNA now sit in the class Caudoviricetes. As of its 2023 records, GBIF's backbone still files both names under Myoviridae.

Next to its host, T4 is small. Cultured Escherichia coli cells measure 1.0–1.5 µm wide across 22 growth conditions (Volkmer and Heinemann 2011), about 8–13 T4 head lengths, computed for this page. Even the smallest cells on the How Big Is a Cell? page are larger: the marine SAR11 bacteria, which Young puts at 0.2 µm, and ultra-small groundwater bacteria, 0.24–0.28 µm as spheres of equal volume, computed there from Luef and colleagues' 2015 volumes (abstract). Those are about 1.7 and 2.0–2.3 head lengths, computed for this page.

Slide the diagram sideways to read its labels, or open it full size.

Can a light microscope see a phage?

Not its shape, at least through standard optics. The wave nature of light limits the finest detail a microscope can separate, and Abbe showed that the limit grows with the wavelength. For a standard fluorescence microscope detecting light of 470–700 nm, Valli and colleagues' 2021 review puts it at around 170–250 nm; two points closer than that blur into one spot. That is 2.0–2.9 times the width of T4's 86 nm head, computed for this page, and longer than the head's 120 nm length.

Super-resolution light microscopes do get below it, the same review notes, from about 120 nm with pixel-reassignment methods to a reported 1 nm with a technique called MINFLUX.

A standard microscope can still show that phages are there. In seawater stained with the dye SYBR Green, an epifluorescence microscope shows bacterial cells as larger dots and virus particles as smaller ones, in an image by Jed Fuhrman printed in Mann's review. That green is borrowed color, added by the dye rather than coming from the particles. Nor do the dots give sizes: light from even a single glowing molecule still spreads to the diffraction limit, Valli and colleagues explain.

Shapes came with the electron microscope. The first phage electron micrographs were published in 1940 in Germany, Ackermann recounts in a 2011 history: H. Ruska showed phages mixed with bacteria at 14,000 times, and Pfankuch and Kausche, of the same Siemens & Halske laboratory, saw round particles 60 nm across at 22–25,000 times. Negative staining followed in 1959, introduced by Brenner and Horne: particles show pale against a darker stain, uranyl acetate in both electron micrographs here. By Ackermann's 2007 count, at least 5,568 phages had been examined by electron microscope since then, 96% of them with tails (abstract).

How does 57 µm of DNA fit in a 120 nm head?

Rao and Black give T4's genome as 171 kb, about 171,000 base pairs. At 0.335 nm per base pair, the spacing the How Long Is DNA? page computes from a B-DNA crystal (Protein Data Bank entry 1BNA), that is about 57.3 µm, computed for this page.

Other counts land close: the sequenced genome, 168,903 bp in GenBank as Lin and Lin cite it, gives 56.6 µm, and the 172 kbp in Yap and Rossmann's 2014 abstract gives 57.6 µm. All three use the same spacing. So they differ only in base pairs. A full head holds about 1.02 genome lengths, Rao and Black write: about 58 µm, computed for this page.

That genome runs about 480 head lengths, computed for this page, a ratio of lengths rather than a packing density. Phage φ29 packs 6.6 µm of DNA into a capsid, or head shell, 42 × 54 nm, Smith and colleagues reported in 2001 (abstract). That is about 120 times the capsid's longer side, computed for this page. By the How Long Is DNA? page's figures, a nucleated human cell keeps about 2 m in a nucleus about 10 µm across, some 200,000 times its width.

Slide the diagram sideways to read its labels, or open it full size.

Inside T4 and other tailed phages with double-stranded DNA, the strands lie 2.5 nm apart at about 500 mg/ml, Rao and Black write. Packing DNA to near-crystalline density means overcoming its entropy, electrostatic repulsion and stiffness against bending, Smith and colleagues wrote of φ29 (abstract). In a 2026 review, Mateu notes that DNA's persistence length, a measure of that stiffness, is comparable to a capsid's diameter.

T4's motor, powered by ATP, pushes with up to about 60 pN (piconewtons), close to the loads of up to 57 pN on average that φ29's motor worked against (abstract). It feeds in up to about 2,000 base pairs a second, the fastest packaging motor reported as of 2010. Rao and Black put the whole genome at about 5 minutes, an average of about 570 base pairs a second, computed for this page.

How does a phage get its DNA into a bacterium?

Packed that tightly, the particle is under pressure. When double-stranded DNA is forced into a ready-made shell this way, the pressure inside may reach tens of atmospheres, Mateu's review says. The review adds that experiments show the pressure greatly helps inject the genome into the bacterial cell: lowering phage λ's pressure by shortening its DNA made release take more energy. In λ capsids opened in a test tube, Evilevitch and colleagues found that an osmotic pressure like that in the host's cytoplasm halved DNA ejection, and 20 atmospheres stopped it (abstract).

The bacteria these double-stranded DNA phages infect have a thick wall that is hard to get through, Mateu notes. By the same account, the phages inject their genome without entering the cell. Leiman and colleagues describe T4's long tail fibres as extended in solution, probably jiggling with thermal motion; once three or more bind receptors, they hold the phage perpendicular to the surface and, acting as levers, move the baseplate up to 100 nm toward it. Imaging T4 on E. coli minicells in 2015, Hu and colleagues instead found most long fibres folded back along the sheath until the phage attached for good (abstract).

Then the tail contracts, irreversibly, starting at the baseplate, which turns from a hexagon about 52 nm across into a star 61 nm across. The sheath shortens from 92.5 to 42 nm, to about 45% of its length (computed for this page), and widens from 24 to 33 nm. That drives the tube through the cell envelope, rotating almost a full turn, so in Leiman's account it drills rather than punctures the outer membrane. An enzyme in the tip protein gp5 digests the wall's peptidoglycan.

Slide the diagram sideways to read its labels, or open it full size.

Next, Hu and colleagues report, the tube pauses in the periplasm, between the outer and inner membranes, while it breaks down the peptidoglycan layer, and the inner membrane bulges outward and fuses with the tail tip (abstract). The DNA then passes through the tube into the cell.

How many phages are there?

Back on the beach, the hosts drift with the plankton. In the ocean's surface layers, bacteria may number 10⁶–10⁷ cells per millilitre, and phages are typically ten times as many, Mann wrote. Bergh and colleagues found up to 2.5 × 10⁸ virus particles per millilitre in natural waters in 1989 (abstract). Upper estimates for the oceans exceed 10⁸ (Wigington and colleagues 2016, abstract).

That tenfold ratio does not hold everywhere. Across 5,671 estimates from 25 marine surveys, Wigington and colleagues found that a 10:1 model has limited or no explanatory power: virus counts typically climb more slowly than cell counts, so the ratio falls where cells are dense (abstract). A 2017 correction removed 163 records from three freshwater datasets, leaving 5,508. The key results were qualitatively unchanged.

On average about 20% of marine heterotrophic bacteria are infected by viruses, and viruses break open 10–20% of the bacterial community each day, Suttle estimated in 1994, noting considerable variation around both figures (abstract).

Slide the diagram sideways to read its labels, or open it full size.

Totals depend on what is counted. Suttle put the ocean's viruses at about 10³⁰ in 2007, with about 10²³ infections there every second (abstract); Mann put phages in the whole biosphere at 10³⁰ or more. The 10³¹ frequently quoted for virus particles on Earth often comes without a source, Mushegian wrote in 2020. He traces it to Hendrix and colleagues in 1999 and concludes this best estimate remains close to 10³¹, unlikely to be much lower or higher (abstract).

Suttle offered a picture for the ocean's share: stretched end to end, those 10³⁰ viruses would reach farther than the nearest 60 galaxies (abstract).

Sources

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3. Valli J, Garcia-Burgos A, Rooney LM, Vale de Melo e Oliveira B, Duncan RR, Rickman C. "Seeing beyond the limit: A guide to choosing the right super-resolution microscopy technique." Journal of Biological Chemistry 297(1):100791 (2021). https://doi.org/10.1016/j.jbc.2021.100791 Read: full text. Text license: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/).
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Cite this page

Ants, Cells & Atoms. “Bacteriophage T4: About 57 µm of DNA in a Head 0.12 µm Long.” Published 2026-10-25. https://antscellsandatoms.com/microscopic-life/bacteriophage/. Accessed (add the date you read it).

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