GATE 2019 BT Q.40: solution and answer

Official answer: DSource: Official answer key, GATE 2019 BT (IIT Madras)

1. What the lag phase is. It is the period right after inoculation when the cell number barely rises, because the cells are adjusting to the new environment. They are making the enzymes, ribosomes and cofactors the new medium needs. Anything that changes how much adjusting the cells must do changes how long the lag lasts.

2. P, inoculum size. A small inoculum gives a long lag. Essential metabolites and cofactors that leak out of the cells (and dissolved ${\text{CO}}_2$, which some cultures need) are diluted into a large volume, so the cells have to rebuild them before they can grow. A large inoculum (typically 5–10% by volume) keeps these at a useful level and shortens the lag. So P matters.

3. Q, inoculum age. Cells taken from the mid-exponential phase of the seed culture are already metabolically active and start growing almost at once. Cells taken from the stationary or death phase must repair damage and rebuild their machinery first, so their lag is longer. So Q matters.

4. R, medium composition. If the fermentor medium differs from the seed medium (a different carbon source, missing growth factors, a different pH or osmolarity), the cells must switch on new enzymes, so the lag is longer. A seed grown in the same medium shows almost no lag. A medium holding two carbon sources can even give a second lag when the preferred one runs out and the cells switch to the other (diauxic growth). So R matters.

5. All three matter, so the answer is P, Q and R, option (D).