Q.67 A bactericidal agent X is added after 3 hours of growth of a bacterial culture. Following the addition
of X, the bacterial growth was measured using the standard plate count method till 24 hours. Which one
of the following figures is the most accurate representation of the action of X?
(A) i
(B) ii
(C) iii
(D) iv

Bactericidal agents kill bacteria, leading to a drop in viable cell counts measured by plate counts. In GATE Life Sciences 2017 question 67, the correct figure shows growth until 3 hours, then a decline after adding agent X.

Question Overview

A bactericidal agent X is added after 3 hours of bacterial culture growth, with viable counts measured via plate count up to 24 hours. The task identifies the accurate graph among (A) i, (B) ii, (C) iii, (D) iv.​

Standard bacterial growth curves plot log viable cells (CFU/ml) vs. time, showing lag, log, stationary, and death phases.​

Correct Answer: (B) ii

Option ii depicts normal exponential growth until 3 hours, followed by a sharp decline in log CFU after adding X, potentially stabilizing at low levels or continuing slow decrease to 24 hours.​

This matches bactericidal action, where the agent kills cells, reducing viable counts immediately post-addition during log phase.​

Plate counts detect only live cells, confirming the drop reflects killing, not growth inhibition.​

Explanation of All Options

• (A) i: Likely shows growth plateauing (horizontal line) after 3 hours, typical for bacteriostatic agents halting division without killing.​

• (C) iii: Probably continuous upward growth, ignoring the agent, unrealistic for bactericidal effects.​

• (D) iv: May show instant drop to zero at 3 hours or erratic pattern; unrealistic as killing requires time and not all cells die simultaneously.​

Bacteriostatic vs. bactericidal distinction is key: static maintains counts, cidal reduces them.