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Here are the correct answers:

After only 4 hours, starting from a single bacterium that doubles every 20 minutes, you would already have 4096 bacteria. This is more than there are children in any school in Wales.

After 7 hours, you would have 4 million bacteria — more than there are people in Wales.

After 11 hours, you would have 8 billion (= 8000 million) bacteria — more than there are people living in the whole world.

And after 21 hours, on the next morning you would have 9,223,372,036,854,780,000 bacteria. Or fully spelled out: 9 quintillion 223 quadrillion 372 trillion 36 billion 854 million 780 thousand (= 9.223 million million million bacteria).

This is even more than the total number of grains of sand in all beaches and deserts around the world combined! 🏖️

An E. coli bacterium is 2 μm (micrometer) long, or 2 one-thousandth of a millimeter. So a chain of 500 bacterial cells would be 1 mm (millimeter) long. And 500,000 bacterial cells behind each other would be 1 meter long.

So if all the bacteria originating from a single E. coli cell were to form a perfect chain, this chain would be the size of a sand grain (0.1-0.2 mm) after a couple of hours; the height of an elephant after 7 hours; and the distance between Cardiff and London (212 km by air) after 12 hours.

And within less than a day, after around 21 hours, our chain of bacteria would reach the planet Neptune. Neptune is more than 4 billion kilometers away from us, so far that it even takes light 4 hours to travel this distance! 🚀

How many answers did you get right?

Log scales doing the trick

The reason why these numbers get so big is because bacteria numbers double every 20 minutes. They follow what we call an exponential growth, where the numbers grow faster and faster.

And this causes a problem for everyone working with bacteria (or viruses) because they have to deal with very, very small and very, very large numbers at the same time.

This is really tricky to show on a normal graph. So to visualise bacterial growth rates microbiologists and other scientists used a little mathematical trick called a logarithmic scale (or log scale in short). 😎

A log scale doesn’t use the normal linear axis that you will be familiar with. On a linear scale each tick on the y axis is separated by the same difference. For instance 1, 2, 3, 4, 5 etc. Or 10, 20, 30, 40, 50 etc.

On a log scale this changes and we don’t use the simple difference but the FOLD change of something. So the y-axis would then go 1, 10, 100, 1000, 10000 etc., with each tick separated by a 10-fold increase from each other.

But it doesn’t need to be 10-fold. It could also be something like 1, 2, 4, 8, 16 etc., that means with a 2-fold increase.

Or even 1000-fold: 1, 1000, 1 million, 1 billion etc.!