GATE 2026 BT – Question 33
To determine the viable cell count of a bacterial culture, you have plated 50 $\mu$L of a 100-fold diluted sample of the culture on a nutrient agar plate and obtained 20 colonies after overnight incubation. The viable cell count of the culture is ______ CFU mL$^{-1}$. (answer in integer)
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Correct answer: 40000
Explanation
The 50 $\mu$L plated is 0.05 mL, so the diluted sample has $\frac{20}{0.05} = 400$ CFU/mL. The culture was diluted 100 times, so it has $400 \times 100 = 40000$ CFU/mL.