How Microbes Multiply

Under the best laboratory conditions, one E. coli cell can split into two about every 20 minutes. A cold sore can heal, then come back later in about the same spot. Both are stories about making copies. A bacterium copies itself. A virus cannot copy itself at all, yet the herpes virus that causes cold sores can stay in your body for years. How do bacteria and viruses multiply, and what stops them?

Explore: grow a population and follow a virus

The simulation has two panels. In the first, you start a bacterial population, pick a doubling time and set how much food is in the dish, then watch the count over time. In the second, you step a phage, a virus that infects bacteria, through an infection and choose the lytic or the lysogenic path.

Try this.

1. Predict how many cells one cell becomes after 2 hours with a 20-minute doubling time. Then run it.
2. Keep the doubling time the same and halve the food. Predict what changes about the curve, then test.
3. In the virus panel, choose the lysogenic path and let the host divide three times. Predict how many cells carry the viral DNA.
4. Switch a lysogenic cell to the lytic path. Predict what happens to that cell.

Explain: how bacteria make more bacteria

Asexual reproduction is reproduction with one parent, where the offspring get copies of that parent’s DNA. Most bacteria reproduce this way.

The most common method is binary fission, when one bacterial cell grows, copies its DNA and splits into two cells. Each new cell gets a complete copy of the chromosome. The result looks like mitosis, which you met in the cell cycle lesson: one cell becomes two matching cells. The process is simpler, though, because a bacterium has no nucleus.

Some bacteria use budding, when a small new cell grows out of the parent and then breaks off. In the ocean bacterium Hyphomonas neptunium, buds form at the tip of a thin stalk. When the bud breaks off, it swims away as a new cell.

The daughter cells are usually genetically identical. “Usually” matters. Copying DNA sometimes makes mistakes, and sequencing studies of lab E. coli found mutations building up over many generations.

Bacteria have one more way to change their genes. Conjugation is when one bacterial cell passes DNA directly to another through contact. The DNA is often a plasmid, a small extra circle of DNA separate from the chromosome. Conjugation does not make a new cell. Afterward there are still two cells, but one has new genes. The Virginia SOL calls conjugation a form of sexual reproduction, because genes from two cells are combined. Many biologists call it gene transfer, because no new cell is made. Conjugation is far rarer than binary fission.

Explain: how viruses make more viruses

A virus cannot split, grow or make anything on its own. It needs a host cell, the living cell a virus infects and uses to make new viruses. The living-or-not lesson explains why. A bacteriophage, or phage, is a virus that infects bacteria.

The lytic cycle is the path where a virus takes over the host, the host builds new viruses, and the cell bursts open to release them. It runs like this:

1. The phage attaches to the bacterium and injects its DNA.
2. The host’s machinery copies the viral DNA and builds viral parts.
3. New phages are assembled from those parts.
4. The cell bursts and the new phages escape. The host cell dies.

The lysogenic cycle is the path where the viral DNA joins the host’s own DNA and is copied along with it. The best-known example is phage lambda, which infects E. coli. Its DNA becomes part of the bacterial chromosome. That stored viral DNA is called a prophage. Each time the bacterium divides by binary fission, both daughter cells get a copy. The cell stays alive, and the virus spreads quietly through a growing family of cells. Later, something such as damage to the cell’s DNA can switch the prophage into the lytic cycle.

Now the cold sore. Herpes simplex virus infects people, not bacteria, but it uses a similar hiding strategy. It can stay in sensory nerve cells, the cells that carry feeling from the skin. Studies of infected human and mouse tissue suggest its DNA stays as a separate circle in the nucleus rather than joining the chromosome the way lambda does. From time to time the virus can become active again and travel back to the skin, and a new sore appears.

Explain: doubling, and why it stops

Doubling time is the time it takes a population to double in number. If the doubling time stays the same, you can predict the count:

number of cells = starting number × 2 × 2 × 2 … (one × 2 for each doubling)

Here is a worked example. A model population starts with 100 cells and doubles every 20 minutes. How many cells after 2 hours?

• 2 hours is 120 minutes.
• 120 minutes ÷ 20 minutes = 6 doublings.
• 100 → 200 → 400 → 800 → 1,600 → 3,200 → 6,400 cells.

Starting from one E. coli cell with 21 doublings (7 hours), the model gives 2,097,152 cells. That is the “millions in an afternoon” from the opening. These are calculated numbers, not counts. The model assumes no cells die, no waiting before growth starts and endless food.

Real cultures follow a curve with stages. First there is a lag, where cells are active but the count barely rises. Then comes fast growth, close to steady doubling. Last, food runs low and wastes build up, so growth levels off. The same limits apply to every population, as the carrying capacity lesson shows.

Elaborate: a real growth curve

Scientists often track a liquid culture by its cloudiness, called optical density at 600 nanometers (OD600). More cells scatter more light, so the number goes up. It is not a count of cells.

These are raw readings from one well, curve 15, of E. coli strain BW25113 grown in a defined lab medium, from the dataset of Honoka Aida and Bei-Wen Ying (deposited on figshare in 2025). The readings were taken every 30 minutes; this table shows ten of them. The raw numbers include some cloudiness from the medium itself.

1. Between which times does the reading barely change? Between which times does it rise fastest?
2. How much did the reading rise from 6 to 12 hours? How much did it change from 14 to 18 hours?
3. The 20-minute doubling time is for E. coli in the best conditions. Does this curve show that? What would you need to know?

Design problem

Baker’s yeast is a single-celled fungus, not a bacterium, but it also reproduces by budding and is safe for a classroom. Plan an investigation to test whether the amount of sugar changes how high a yeast population grows before it levels off.

Evaluate: check your thinking

One E. coli cell can become millions in hours under the best conditions, but doubling cannot go on forever; a real culture levels off. A cold sore comes back because herpes virus can stay in nerve cells and become active again, a hiding strategy like, but not the same as, a lysogenic phage.

Science practices you used: predicting and testing with a model, calculating with doublings, reading a growth table, naming the limits of one data set and planning a fair test with a control and repeated trials.