Depth 1 · 57 s
Mostly harmless
Yoghurt, a cold and mouldy bread are all the work of microbes. The four kinds, how small they are, why most are harmless or helpful, and how many bacteria you carry.
About this video
A bowl of yoghurt, a week-long cold and grey fluff on old bread are all the work of living things far too small to see. A microbe is a living thing too small to see without a microscope, and there are four main kinds. Bacteria are single cells that live almost everywhere, including on your skin and in your gut. Viruses are much smaller and simpler, and can only make copies of themselves inside a living cell. Fungi are moulds, yeasts and mushrooms. Parasites live on or inside another living thing and feed on it.
How small? A typical bacterium is about a thousandth of a millimetre across, so a thousand of them in a line would cover one millimetre. That is about 1,000 nanometres; most viruses are about 20 to 300.
Most microbes do us no harm, and many help: bacteria turn milk into yoghurt and yeast makes bread rise. Most kinds of E. coli are harmless and part of a healthy gut, though a few kinds make people sick. Only a small number of microbes cause illness, and these are the ones we call germs, or pathogens.
A careful estimate in 2016 found about 38 trillion bacteria in a typical adult and about 30 trillion of the body's own cells: roughly as many of each, most of the bacteria in the gut. The old claim that bacteria outnumber your cells 10 to 1 was a rough guess, and it is wrong. This video goes with the lesson The World of Microbes in Health Level 1 on Knaph.
This video shows depth 1 of Microbes.
Learn MicrobesTranscript
- 0:00Yoghurt, a cold, mould on old bread. All three are the work of living things far too small to see.
- 0:08Microbes come in four main kinds. Bacteria, viruses, fungi and parasites. A virus can only copy itself inside a living cell.
- 0:20A thousand bacteria in a line would cover one millimetre. Most viruses are smaller still, about 20 to 300 nanometres.
- 0:31Most microbes are harmless, and many help, like the bacteria in yoghurt. Only a small number make people ill. Those are germs.
- 0:41A typical adult carries about 38 trillion bacteria and about 30 trillion of their own cells. Roughly as many of each.
- 0:50We live among trillions of microbes. Only a few are germs.