Q.83 Lethal dose curves of different microorganisms (1, 2, 3 and 4) are shown below. Which of these
microorganisms are the most lethal?
(A) 1 & 3 only (B) 1 & 2 only (C) 3 & 4 only (D) 2 & 3 only
Introduction
The lethal dose (LD) curve is a graphical representation showing the relationship between the number of microorganisms (dose) and the percentage of host death. Such curves are commonly used to compare the virulence and lethality of different microorganisms.
In competitive exams, you are often asked to identify the most lethal microorganism based on these curves. This article explains how to interpret lethal dose curves and determines the correct answer from the given options.
Understanding Lethal Dose Curves
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X-axis: Number of microorganisms
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Y-axis: Host death (%)
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A microorganism is considered more lethal if it causes a high percentage of death at a lower dose.
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In other words, the lower the LD₅₀ (lethal dose for 50% death), the higher the lethality.
Interpreting the Given Graph
From the figure:
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Curve 1 reaches high host death at the lowest number of microorganisms
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Curve 2 also shows high lethality at relatively low dose
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Curves 3 and 4 require much higher numbers of microorganisms to cause the same level of host death
👉 Therefore, curves positioned more to the left are more lethal.
Identification of Most Lethal Microorganisms
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Most lethal: Curves 1 and 2
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Less lethal: Curves 3 and 4
Correct Answer
✅ Option (B): 1 & 2 only
Option-Wise Explanation
Option (A): 1 & 3 only
❌ Incorrect
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Curve 3 requires a higher dose to cause death, indicating lower lethality.
Option (B): 1 & 2 only
✅ Correct
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Curves 1 and 2 have the lowest LD₅₀ values, making them the most lethal.
Option (C): 3 & 4 only
❌ Incorrect
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Both curves lie far to the right, indicating low lethality.
Option (D): 2 & 3 only
❌ Incorrect
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Curve 3 is less lethal than curve 1.
Key Concept Summary
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Left-shifted curve → High lethality
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Right-shifted curve → Low lethality
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LD₅₀ is inversely proportional to virulence
Conclusion
Lethal dose curves provide a powerful visual method to compare microbial virulence. The microorganisms represented by curves 1 and 2 are the most lethal because they cause significant host mortality at the lowest doses.
Final Answer
✅ Option (B): 1 & 2 only
