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

One way you can do it is multiplex PCR where they look for the most common and dangerous types of bacteria or viruses that can cause infections. The alternative is a bacterial or viral culture, but those results usually won’t come back in 1-2 days. They try to grow it on different medium and then look at it under the microscope or use different tests to determine what bacteria it is.