UPSC Prelims 2021 Q69: Culturing Bacteria, Fungi and Viruses Explained
Consider the following:
1. Bacteria
2. Fungi
3. Virus
Which of the above can be cultured in artificial/synthetic medium?
- 1 and 2 only
- 2 and 3 only
- 1 and 3 only
- 1, 2 and 3
Answer: (a) 1 and 2 only
Statement-wise explanation
- 1: correct — Bacteria are complete cells with their own ribosomes, enzymes and energy systems, so they grow and divide outside a host if given nutrients. Microbiology labs routinely grow them on non-living media such as nutrient agar, nutrient broth or MacConkey agar. Fully chemically defined (synthetic) media are also used for many species, such as E. coli grown on glucose and mineral salts.
- 2: correct — Fungi, including yeasts and moulds, are free-living eukaryotic cells that take in nutrients by absorption. They grow readily on artificial media such as Sabouraud dextrose agar and potato dextrose agar. Penicillium, for example, is grown in fermentation tanks to make penicillin, and yeast is grown on sugar-based media for baking and brewing.
- 3: incorrect — Viruses are obligate intracellular parasites. A virus is nucleic acid (DNA or RNA) in a protein coat, and it has no ribosomes or metabolism of its own. It can multiply only inside a living host cell, using that cell's machinery. So viruses are grown in living systems: cell or tissue cultures (for example Vero cells), embryonated hen's eggs, laboratory animals, or bacterial cultures in the case of bacteriophages. They cannot be grown on a cell-free artificial medium.
How to eliminate
The question can be solved from one basic fact: a virus is not a cell and replicates only inside a living host cell. With that fact, statement 3 is false, and every option that contains 3 drops out. That leaves only the option with 1 and 2. You do not need culture-medium details for bacteria or fungi, because no option tests them separately from statement 3. A trap to avoid: virologists do 'culture' viruses, but always in living cells such as cell lines or eggs, never in a non-living medium. Remember the phrase 'obligate intracellular parasite' and connect it to 'needs living cells to grow'.
Know more
Why viruses need a living host
A culture medium is a nutrient preparation that supports microbial growth. It can be natural (milk, potato extract), semi-synthetic (nutrient agar with peptone and beef extract), or synthetic/chemically defined, where every ingredient and its amount is known. Organisms that have their own metabolism can use such media. Viruses cannot.
Viruses sit at the border of the living and the non-living. Outside a cell, a virus particle (virion) is inert and can even be crystallised. Wendell Stanley crystallised tobacco mosaic virus in 1935. Inside a host cell, the virus takes over the cell's ribosomes, enzymes and ATP to copy itself.
| Feature | Bacteria | Fungi | Viruses |
|---|---|---|---|
| Cell type | Prokaryotic cell | Eukaryotic cell | Not a cell |
| Own ribosomes and metabolism | Yes | Yes | No |
| Genetic material | DNA | DNA | DNA or RNA |
| Grown on artificial media | Yes (nutrient agar, broth) | Yes (Sabouraud, potato dextrose agar) | No |
| How they are grown in the lab | Plates, broth, fermenters | Plates, fermenters | Cell lines, embryonated eggs, animals, bacterial lawns (phages) |
| Affected by antibiotics | Yes | No (antifungals used) | No (antivirals used) |
Ways to grow viruses
- Cell or tissue culture: monkey kidney (Vero) cells, HeLa cells. Many inactivated vaccines are made this way.
- Embryonated hen's eggs: the traditional method for influenza vaccines.
- Laboratory animals: mice and other animals, used historically and for some research.
- Bacteriophages: grown on living bacterial cultures, where they form clear 'plaques'.
Related exam points: Rickettsiae and chlamydiae are bacteria, but they are also obligate intracellular organisms and are grown in cells or eggs. Mycoplasma has no cell wall but still grows on special artificial media. Mycobacterium leprae still cannot be grown on artificial media.