Living or Not? Viruses and Bacteria

A cold virus on a light switch can be picked up by a fingertip many hours later, yet on the switch it does nothing on its own. Inside your nose, it gets some of your cells to build new copies of it.

A bacterium is different. In a damp place with food, it takes in nutrients, uses energy, grows and splits in two by itself. So which of these is alive, and how would you decide?

Explore: run the living-or-not checklist

In the simulation you pick one thing at a time: a bacterium, a virus outside a cell, a virus inside a cell, a human red blood cell, or a salt crystal. You switch on the characteristics of life it shows, and the simulation reports which are met and how scientists classify it, noting where they still disagree.

1. Pick one thing

2. Tick the characteristics of life it shows

3. Check my boxes Reset

Rule to keep: no single box decides it. In this sketch, each answer follows the lesson for that thing; the shapes are labelled schematics, not to scale.

Try this.

1. Predict how many characteristics a salt crystal shows. Then decide whether getting bigger is the same as growing.
2. Compare a virus outside a cell with a virus inside a cell. Predict which boxes change.
3. A mature red blood cell cannot divide. Predict whether it still counts as a living cell, then check.
4. Run the bacterium. Which characteristics does it meet on its own, without another cell?

Explain: what counts as alive

Biologists use a group of characteristics to decide whether something is alive. Living things:

• are made of one or more cells
• take in and use energy (all the chemical reactions that do this are called metabolism)
• grow and develop
• respond to their surroundings
• keep their inside conditions steady
• reproduce
• change over many generations (evolve)

No single box decides it. A mature red blood cell cannot divide, but it is still a living cell in a living body. A crystal gets bigger, but it has no cells and no metabolism.

A virus is a tiny infectious agent made of genetic material inside a protein coat. It is not a cell. Outside a cell it does not take in food, use energy or grow. A host cell is the living cell a virus infects. Inside it, the virus’s genes get the cell to build new virus parts, which come together as new viruses. Picture a factory running someone else’s blueprints. The picture breaks down because nobody in the cell chooses to follow the plan; the cell’s machinery reads whatever genes are present.

Because viruses are not cells and have no metabolism of their own, Virginia’s Biology standards (the SOL) class them as nonliving. Many scientists agree, but the question is still discussed. Viruses carry genes and evolve, and some giant viruses carry more than a thousand genes. Under a definition based on cells and metabolism, they are nonliving.

Explain: a bacterium and a virus, part by part

Bacteria are prokaryotes: living things made of one cell with no nucleus and, typically, no membrane-covered organelles. Your own cells are eukaryotes, which usually have a nucleus (mature red blood cells lose theirs). One Cell or Many compares single cells with the cells of your body.

Part	Typical bacterium	Virus	What the part does
Genetic material	DNA in the nucleoid, a region with no membrane around it	DNA or RNA, inside the capsid	Carries the instructions
Coat	Cell membrane around cytoplasm	Capsid, a protein coat	Membrane controls what enters and leaves; capsid protects the genes
Ribosomes	Yes	No	Build proteins; a virus uses the host cell’s
Cell wall	Often	No	Helps keep the cell from bursting when water rushes in
Outer layer	Some have a capsule	Some have an envelope, a fatty layer outside the capsid	Envelope helps some viruses enter cells
Flagellum (plural flagella)	Sometimes	No	A whip-like tail that pushes the cell through water

Sizes differ too. A micrometer (μm) is a thousandth of a millimeter, and a nanometer (nm) is a thousandth of a micrometer. Most viruses are about 20 to 300 nm across. Typical bacteria are about 1 to 5 μm long, roughly ten to a hundred times longer. There are exceptions on both sides, such as giant viruses and a few bacteria big enough to see without a microscope.

Viruses infect all the main kinds of living thing: bacteria, archaea, plants, animals, fungi and protists. A virus that infects bacteria is called a phage. Each virus can enter only certain cells, because proteins on its surface must fit receptors on the host cell.

Explain: how bacteria get their energy

Every living cell needs energy and carbon to build its parts. Biology sorts bacteria into three main groups by how they get them.

Group	Energy comes from	Carbon comes from	Example
Heterotroph	Carbon compounds it takes in	The same compounds	E. coli in your gut; bacteria that decompose dead matter
Photoautotroph	Light	Carbon dioxide	Cyanobacteria in ponds and oceans
Chemoautotroph	Chemicals such as sulfur, iron or nitrogen compounds	Carbon dioxide	Bacteria near deep-sea vents and in soil

A few bacteria mix these patterns, so the three groups are a useful simplification.

Most bacteria carry out every life process themselves when conditions suit them. Conditions matter: fully dried bacteria do not grow. A few bacteria, such as most of the Rickettsia that cause diseases like Rocky Mountain spotted fever, can grow only inside other cells. They are still living cells, with their own ribosomes and metabolism. How Microbes Multiply shows how bacteria divide.

Elaborate: compare real sizes

Object and what was measured	Reported size	Size in nanometers (nm)	Kind of sample
SARS-CoV-2 (COVID-19 virus), spike on the surface	about 9 to 12 nm	9 to 12	Virus grown from patient samples, electron microscope
SARS-CoV-2, particle diameter	about 60 to 140 nm	60 to 140	Virus grown from patient samples, electron microscope
Influenza virus, diameter of round particles	80 to 120 nm	80 to 120	Range from a review of studies
T4 phage head, after drying	about 75 × 60 nm	75 × 60	Atomic force microscope
T4 phage head, in liquid	about 100 × 85 nm	100 × 85	Atomic force microscope
T4 phage, whole particle length	about 200 nm	200	Earlier electron microscope work
Tupanvirus (a giant virus), average length	about 1.2 μm	1,200	Electron microscope
E. coli bacterium, length × width	about 2 × 0.5 μm	2,000 × 500	Reference size for normal cells
Human red blood cell, mean diameter	about 7.5 μm	7,500	Medical reference for normal cells

These values come from different studies, not one experiment: SARS-CoV-2 from Zhu and colleagues (2020), virus grown from samples of patients admitted to a Wuhan hospital in December 2019; influenza from a 2006 review by Machała and Brydak; T4 from Kuznetsov, Chang and McPherson (2011); Tupanvirus from Abrahão and colleagues (2018); E. coli from Maki and colleagues (2000); red blood cells from a 1990 medical textbook chapter by Lynch. The nanometer column was converted here (1 μm = 1,000 nm).

1. About how many times longer is an E. coli cell than the largest influenza particle in the table?
2. How much shorter was the T4 head after drying than in liquid? Why should a scientist always report how a sample was prepared?
3. The Tupanvirus is longer than an E. coli cell is wide. Does that make it alive? What evidence would you need instead?

Design problem

A sealed jar of pond mud slowly grows dark specks of a mineral. Your partner says living bacteria are making the specks. Plan a test of that claim. Name your groups, what you keep the same, what you measure, how many jars you need, and how you stay safe.

One way to do it

• Groups: jars of fresh mud (living microbes present) and jars of the same mud heated in a pressure cooker by your teacher to kill the microbes. A third group has only the water drained from the same mud, with no mud added at all.
• Independent variable: whether living microbes are present.
• Dependent variable: the area covered by dark specks, measured from photos each week.
• Constants: same amount of mud and water, same temperature, same light, same jar size and same number of weeks.
• Repeated trials: at least three jars in each group, so one odd jar does not decide the result.
• Safety: keep jars closed with a loose lid so gas can escape, wear gloves, never smell or taste the contents, and let your teacher dispose of them. Never grow microbes taken from people.
• What would mean what: specks only in jars with living microbes support the claim. Specks in the heated jars too would point to a chemical reaction that needs no life. Even then, the specks do not prove the bacteria use that chemical for energy. A 2020 study of manganese-forming bacteria answered that by tracing whether carbon from carbon dioxide ended up in new cells.

Evaluate: check your thinking

The cold virus on the light switch is not alive by the usual definition. It is not a cell, it does nothing on its own, and only living host cells, such as the cells lining your nose, can build new copies of it. The bacterium is a living cell that grows and divides by itself when conditions allow. Scientists still discuss where viruses fit, but the SOL answer is clear: bacteria are living, viruses are not.

Science practices you used: testing predictions in a model, converting units and comparing sizes in a data table, making a claim from evidence and naming its limits, and planning a fair test with a control group and repeated trials.