Your turn
Each lab is one step from the video. Change things and see what happens.
Each lab isolates one part of the story. The models are simplified to show the idea.
Tap the buttons and see what happens.
Tap the clock. How many 20-minute jumps until there are a million cells?
Step in 20-minute generations. How many generations to pass 106?
Tap the clock. Watch the cells!
Best-case lab conditions: E. coli can double every 20 minutes.
Drag the slider: copy the DNA loop, grow, pinch, split.
Scrub replication, segregation, elongation, the FtsZ ring and the septum.
Slide it. Pop! Two cells!
Pick a bacterium and race 12 hours. Why is one line so much steeper?
Compare E. coli (20 min) with M. tuberculosis (15–20 h) on a log axis.
Tap a germ and watch the line climb.
Change the food. What happens to the top of the curve?
Vary nutrients and watch lag, log, stationary and death phases shift.
Slide the food. Do the cells grow more?
A simplified model; real curves depend on the species and conditions.
Tap cells next to the plasmid carrier, then add the antibiotic. Who survives?
Spread an R plasmid by conjugation, then apply an antibiotic and watch selection.
Plasmids that carry resistance genes are called R plasmids.
Tap the kid, the garden or the yoghurt.
Gut, soil and food: tap each to see what the bacteria do.
Tap a thing to meet its bacteria.
Bacteria are tiny living things. They are much too small to see. Some of them help us make yoghurt from milk!
A bacterium keeps its plan on one loop, like a tiny necklace. First it copies the loop. Then it grows longer. Then it squeezes in the middle. Pop! Now there are two bacteria, just the same. This is called binary fission. It means splitting in two.
Each new bacterium can split again. So two become four, and four become eight. Soon there are more than you can count!
Bacteria need food. When the food is gone, they stop growing. Some curl up in a tough shell called a spore and wait for better times.
Want more? Find out if viruses are alive, where yeast comes from and how vaccines work.
Bacteria reproduce by binary fission, which simply means "splitting in two". One bacterium copies its single loop of DNA, grows longer, and pinches in the middle. The result is two new cells, each a genetically identical copy of the parent.12 For a living thing made of just one cell, dividing is the only way to make new individuals, and each daughter cell is a complete bacterium in its own right.2
Bacterial reproduction is always asexual: there are no males or females and no mating.1 It can be astonishingly fast. In a warm lab with plenty of food, the bacterium E. coli can double in as little as 20 minutes.1 At that rate, one cell could become 2,097,152 cells in seven hours.7
A bacterium is much simpler than one of your cells. It is tiny, usually between 0.1 and 5 micrometres across, while most plant and animal cells are 10 to 100 micrometres.6 It has no nucleus. Its genes sit on one circular chromosome in a region called the nucleoid.3 That makes splitting in two quite straightforward:
Each daughter is a clone of the parent, and each can go on to split again.7 Binary fission is the most common way bacteria divide, but not the only one: a few bud off smaller cells, and some filament-forming soil bacteria break up into many new cells at once.1
Your body cells divide by mitosis, which is more complicated. Both processes copy the DNA and make two identical cells. But your cells hold 46 separate chromosomes inside a nucleus, which must open up, while a spindle of fibres pulls the copies apart. A bacterium has no nucleus to open and no spindle, so binary fission is simpler and much quicker.2 Fun fact: FtsZ, the bacterial pinching protein, is related to tubulin, the protein your cells use to build their spindle fibres.2
| Binary fission (bacteria) | Mitosis (your body cells) | |
|---|---|---|
| Nucleus? | No | Yes, it breaks down and re-forms |
| Chromosomes | Usually one circular chromosome | 46 linear chromosomes in humans |
| Spindle? | No | Yes |
| How the cell splits | FtsZ ring and a new wall (septum) | Cleavage furrow (animals) or cell plate (plants) |
| Result | 2 identical new organisms | 2 identical body cells |
Sources: OpenStax Biology and Microbiology.12
The time it takes a population to double is called the generation time or doubling time. It varies enormously with the species and with food, temperature and acidity.1
| Bacterium | Doubling time |
|---|---|
| E. coli (ideal lab conditions) | as little as 20 minutes |
| Mycobacterium tuberculosis (causes TB) | 15 to 20 hours |
| Mycobacterium leprae (causes leprosy) | about 14 days |
| The same species in a harsh place | can be several days |
Because every new cell can divide again, the number doubles each generation. After n generations, one cell becomes:1
number of cells = 2n
Ten doublings give 1,024 cells. With a 20-minute doubling time, that is three generations an hour, so 21 generations (seven hours) give 2,097,152 cells, and 24 generations (eight hours) give 16,777,216.7 Numbers like these quickly get too big for an ordinary chart, so microbiologists plot them on a logarithmic scale, where each step up the axis means ten times more. On that scale, steady doubling makes a straight line.1
If E. coli doubled every 20 minutes for a whole day, the numbers would be unimaginable. In real life that never happens, because the food runs out. When bacteria are put into a flask of fresh broth and counted over time, their numbers follow four phases:1
Sources: OpenStax Microbiology.1 Growing bacteria are also the easiest to stop. Cells in the log phase are the most sensitive to disinfectants and common antibiotics, while cells in the stationary phase are much harder to kill.1
Some bacteria have a survival trick. When food runs short, they can form an endospore, a tough, dormant capsule that protects a copy of the cell's DNA. Endospores survive long periods without food or water, and they can resist chemicals, extreme temperatures and even radiation. Some have stayed dormant for thousands of years. When conditions improve, the spore germinates and becomes an active, dividing cell again.3 A spore is not reproduction, though: one cell makes one spore.3
Bacteria never mate, but they have three other ways to get new genes from each other. Scientists call this horizontal gene transfer:4
Plasmids are small extra loops of DNA, separate from the main chromosome. A single cell can have hundreds of them.3 They often carry useful genes, such as ones that protect against an antibiotic.7 Plasmids that carry resistance genes (R plasmids) can move between bacteria of the same species and even between different species.4 None of this makes new cells. It changes which genes the existing cells carry.
Put the two tricks together, fast doubling and gene sharing, and you can see why antibiotic resistance is a problem. An antibiotic kills or stops the bacteria that aren't protected against it, but any that carry a resistance gene survive. They then multiply, and they can pass their resistance genes on to other germs, even ones that were never exposed to the drug.8 In the US, more than 2.8 million antibiotic-resistant infections happen each year, and more than 35,000 people die from them.9 The CDC estimates that about 1 in 3 antibiotic prescriptions at US doctors' offices is unnecessary.10 That is why antibiotics should only be used when they are really needed (see how antibiotics work). Your immune system has its own defences too, which vaccines train (see how vaccines work).
Only a few kinds of bacteria cause disease.5 A careful count estimated that an adult carries about 38 trillion bacteria, mostly in the gut. That is roughly the same as the number of human cells (about 30 trillion), not ten times more, as an old claim said.11 Our gut bacteria help us digest food, make some vitamins, and protect us from harmful germs.512 Researchers have counted more than 10,000 kinds of microbes living in and on the human body, and nearly everyone carries some disease-causing ones without being ill.12
Outside our bodies, bacteria are just as important. Only prokaryotes (bacteria and their cousins, archaea) can turn nitrogen gas from the air into a form plants can use. Some of these bacteria live in the roots of beans and peas and act as natural fertiliser.5 And people have used bacteria to make cultured milk foods like yoghurt for at least 4,000 years.5
Yes. Bacteria always reproduce asexually. One cell copies its DNA and splits into two identical daughter cells (binary fission). There are no males or females, and no mating is needed.
Yes. A bacterium is a complete living cell. Given food and the right conditions, such as a suitable temperature, it grows, copies its DNA and divides by itself. That is why bacteria can live almost everywhere, from soil and oceans to arctic snow.
Strictly, bacteria reproduce in one main way: binary fission. A few divide by budding or fragmentation. The 'three types' people often list (transformation, transduction and conjugation) are ways of swapping genes, not of making new cells.
It depends on the species and conditions. In a warm lab with plenty of food, E. coli can double in as little as 20 minutes. The bacterium that causes tuberculosis takes 15 to 20 hours, and in harsh places bacteria may take days.
No. Both make two identical cells, but binary fission is simpler: bacteria have one circular chromosome and no nucleus, so there is no nucleus to open and no spindle. The cell copies its DNA, grows, and a protein ring pinches it in two.
Because food runs out and waste builds up. A growing population goes through a lag phase, a fast log phase, a stationary phase when growth stops, and a death phase. Real doubling only lasts while conditions are good.
No. Some bacteria form endospores, tough, dormant capsules that protect their DNA from heat, drying and chemicals. One cell makes one spore, so the number doesn't go up. The spore wakes up and grows again when conditions improve.