Why can't viruses be cultured on nutrient agar like bacteria? And what does agar actually do?
Zoya Khan12th Sample
15 Sept 2026 · 1 min read
I was reading the opening part of Microbes in Human Welfare slowly this morning, because I realised I have been skipping the "easy" intro parts and just memorising the lists. The text says microbes are present everywhere, in soil, water, air, inside our bodies and also in places like geysers and deep snow. Bacteria, fungi, protozoa, viruses, viroids and prions all come under the microbial world.
Then it says microbes can be grown on nutrient media in the lab to get pure cultures. That's where I started wondering.
- Agar: is it a food for the microbes? In my head it is "jelly with nutrients", but I read somewhere that it is not digested by most bacteria. So why add it?
- If viruses are microbes, why can't they be grown on nutrient agar? I think because they need a host cell, but what is the correct way to say it?
- How is a pure culture obtained from a mixed sample? I remember the term "streak" but not the logic behind it.
I'm not asking for a ton of memorisation, I want the idea behind these things to make sense so that when a weird statement-based question shows up I can reason it out.
7 answers & replies
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[+][–]Rahul DubeyDropperSample· 1mo ago
Agar is a gelling agent. It comes from seaweed, and most bacteria cannot digest it. So it doesn't feed them, it just turns the liquid broth into a solid surface. The nutrients (peptone, beef extract and so on) are mixed in separately. Solid surface = individual cells stay where they land and each one multiplies into its own visible colony.
[+][–]Zoya Khan12thSample· 1mo agoAuthor
That makes sense, thank you. The "agar is a gelling agent, not food" line is the one I will keep.
[+][–]Mehak AroraDropperSample· 1mo ago
Streak plate logic: you touch a sterile loop to the mixed sample and drag it across one part of the plate, then flame the loop, drag it from that area into the next part, and again. Each time you dilute the number of cells. By the last streak, individual cells are far enough apart that each makes a single separate colony. Pick one colony and you have a pure culture, since every cell in it came from one parent cell.
This whole aseptic thing is a bit of a thing for me, I get anxious about contamination 😅 but the logic is nice.
[+][–]Ishita GoyalDropperSample· 1mo ago
Hmm, I think the virus answer is simply that they're not capable of any metabolism outside a host. Nutrient agar has lots of nutrients so I think they would reproduce there if given enough time? Like how bacteria do? Not sure.
[+][–]Siddharth RaoDropperSample· 1mo ago
Ishita, no. Viruses have no metabolic machinery of their own, they can't make proteins without the host's ribosomes and enzymes, so nutrient agar alone does nothing for them. Nutrient agar has food but a virus doesn't eat. You need living host cells, which in the lab means cell cultures, or in older methods embryonated eggs, and for plant viruses, infected plants, or bacteriophages with bacteria. That's why they are called obligate parasites.
[+][–]Ishita GoyalDropperSample· 1mo ago
ohh makes sense, I mixed up "nutrients" with "machinery". thanks Siddharth!
[+][–]Kavya NaikDropperSample· 1mo ago
Small extra: some microbes live in extreme places like hot springs and are not easy to culture because we cannot always copy their conditions in a lab. NCERT tells us only a small number of microbes can be grown in the lab on artificial media. That's one reason the soil and sea are still full of unknown microbes. Also useful: curd is a good everyday example of microbes at work. Lactic acid bacteria convert milk to curd.
