I notice that in a lot of bacteria testing procedures the sample has to be cultured. You place the sample in agar or another growth medium and wait for the bacteria to proliferate before examining them.
Why is this necessary? If the bacteria are present in the sample, can't you just examine the sample directly with a microscope to find them? Is it a needle in a haystack problem?
For example, a listeria bacteria is about 1 micrometer in size and has a positive gram stain. So, at 1000x magnification the organism would be 1mm in size in the objective which is highly visible. Even at 100x magnification you should still be able to see a tiny dot and be able to zoom in on the dot.