I explained this in an earlier article. However, it requires some further clarification. So, the following is based on another analogy.
Assume that people in a city noticed that many have eye infections, reflected by redness and watering of eyes (symptoms). People tried all possible treatments, including the medical one, but to no avail.
So, it is considered a new illness caused by something (“thing/agent”) in the environment, such as air or water (from a pond, lake, or river). This becomes the first observation that something in water/air is causing it. Let us find out. It means extracting/isolating the thing from the air, water, or both. Now, we are starting an isolation step.
Let’s say water (pond, lake, or river) causes more intense illness, so the water may contain a higher quantity of the thing. Therefore, water is chosen for further evaluation or isolation. Water looks quite clear, meaning whatever the thing is in there is mostly of very small size (particles or microbe, we do not know). Most likely, filtration would not help, meaning it would not isolate the thing.
In that case, people usually use adsorption technology. Assuming these are particles, let’s try some adsorbent, such as silica (a refined or specific version of sand). Put some quantity of silica in (very large) quantity of water and leave it there so that the thing may get adsorbed onto silica. Let us assume we are lucky, and that thing got absorbed onto silica. Put an extremely small amount in the eyes (humans, often). Oops, the eyes got flared up right away. So, it means we got the stuff out of water on silica. However, we do not know what it is.
Next, there are ways to get the stuff out of silica. Let us try using a small amount of alcohol. For example, shake the silica with alcohol. Filter again, remove the silica, and evaporate the alcohol part to dryness. Most likely, we will get a small amount of white powder. Reconstitute it with water and test again with eyes (extremely small amount). Wow, eyes reacted again very strongly. Let us assume we are so lucky that the powder after ethanol contains only that compound (extremely rare chance). However, let us assume it is one single component, just particles.
We have isolated the agent of interest.
Send it to the chemical lab to see if it is a chemical in nature. Let us assume it is chemical in nature. Now, the chemistry lab will analyze the chemical composition based on (for example) carbon, hydrogen, oxygen, nitrogen, etc., and then their arrangement (joining) to establish the molecule’s structure.
If it is a commercially available compound, the chemist will get/buy it, dissolve it in water, and test it on the eyes. If it reproduces the effect, it will confirm that it is the caustic agent or agent of interest. Otherwise, it has to be synthesized (chemically or biologically) and tested with the eyes. If it reproduces the eye effect. Then, it became a new and novel toxin – label it a “virus”).
We have isolated the new and novel toxin – virus. Label, store, and use it as a reference (gold) standard for future testing or studying.
Do you see any culturing step anywhere? No, culturing is not required for isolation. It’s a gross misunderstanding of the subject of isolation.
However, if it is microbial instead of chemical particles, the sample will be sent to the biological lab to see if they can identify it or if it is new. It will become a microbial standard.
Note it is an extreme example. In practice, a vast number of steps are usually involved (often takes years to complete), but conceptually, this is exactly how the isolation is done or should be done. Also, the chemist/biologist will have a physical sample of the thing at the end, the must requirement of the isolation step.