Question: How do you know what different bacteria look like and the size of them?

Samuel Ellis answered on 9 Mar 2015:

Bacteria come in lots of shapes and sizes, but can be very hard to tell apart just by looking at them. Fortunately in my experiments I have to be sterile, so I only add the bacteria I work on and no others (although I have to be very careful not to contaminate anything!). Therefore I usually don’t have to identify other types.

It is more of an issue in Medicine. Hospitals will have scientists who are experts at identifying different bad bacteria to figure out what makes a patient ill. Sometime they will do this by taking a sample (probably blood) and looking at it under a microscope to spot any bacteria mixed in with your human cells. But usually they use culture, which means growing bacteria from a sample on petri dishes. Different bacteria grow on different chemicals on the dish, or make colonies that look a particular way, letting the scientists work out what type is in the patient!

As for working out a bacteria’s size, you can do it with a microscope. If you use a lens with a known amount of magnification (how zoomed in you are basically), then you know the ratio between how large something is in the picture and how large it really is. In a lot of microscopy photos scientists will add a little line or bar at the bottom to tell you the scale of objects in the picture.

Hope that answers your question.