eli5: how do medical labs find out which bacteria or viruses are in a sample if they do not know what to look for?

Question

So I do not mean a covid test where it is clear what pathogen to look for. More like:
“Hey lab, here is a sample of a patient that feels bad. Please find out what pathogen is inside of it.”

How does it work? And how does it work so fast that they usually return the results within 1 or 2 days?

In: 6

Answer 1

You can put samples on different growth media designed for bacteria, virus and fungi, stick them in suitable climates and see what grows. Cheaper than pcr but takes longer up to two weeks for some fungi if memory serves.

This is why in hospital you may be started on a broad spectrum antibiotic which is changed to targeted once the results come back.