20. In an experiment bacterial cell wall was removed usingenzymatic methods. It was observed that on culturingsuch cell wall deficient bacteria, for many generations result into stable forms of bacteria permanently lacking cell wall. It is proof to
    (1) Inheritance of acquired character
    (2) Recessive mutation
    (3) Reverse mutation
    (4) Adaptive mutation

Introduction

In microbiology, experiments involving the removal of bacterial cell walls have revealed fascinating insights into bacterial adaptability and evolution. When bacterial cell walls are enzymatically removed and these bacteria are cultured over many generations, some populations develop into stable, cell wall-deficient forms that permanently lack cell walls. This phenomenon is a compelling example of adaptive mutation rather than inheritance of acquired characteristics or other types of mutations.

This article explains the biological significance of this observation, differentiates between mutation types, and clarifies why the stable loss of bacterial cell walls proves adaptive mutation.

What Happens When Bacterial Cell Walls Are Removed?

Bacterial cell walls provide structural support and protection. Removing the cell wall enzymatically creates cell wall-deficient bacteria (often called L-forms). Initially, these bacteria are fragile and may not survive well without the wall. However, under certain conditions and with proper osmotic protection, some bacteria can adapt and proliferate as stable wall-deficient forms.

These stable L-forms can reproduce indefinitely without regaining their cell walls, demonstrating a permanent genetic or epigenetic change rather than a temporary physiological state.

Understanding Mutation Types in This Context

1. Inheritance of Acquired Characters

• This outdated Lamarckian concept suggests organisms can pass on traits acquired during their lifetime.

• The stable loss of the bacterial cell wall is not due to direct inheritance of an acquired trait but rather genetic changes.

• Experimental evidence does not support this mechanism in bacteria.

2. Recessive Mutation

• A recessive mutation requires two copies (in diploids) to express a trait.

• Bacteria are typically haploid; thus, recessive mutations are less relevant here.

• The stable loss of the cell wall is not simply a recessive mutation.

3. Reverse Mutation

• Reverse mutation restores the original genotype from a mutant.

• The stable cell wall-deficient state is not a reversal but a new, stable condition.

4. Adaptive Mutation

• Adaptive mutation refers to mutations that occur in response to environmental stress, conferring a survival advantage.

• In this case, bacteria under cell wall removal stress develop mutations allowing them to survive and reproduce without a wall.

• These mutations are selected because they enable survival under the new conditions.

Why Is This Proof of Adaptive Mutation?

• The bacteria initially lose their cell walls due to enzymatic treatment (environmental stress).

• Over generations, some bacteria acquire mutations that stabilize the wall-deficient state.

• These mutations are not random but are selected because they allow survival without the wall.

• The resulting stable L-forms demonstrate how bacteria can adapt genetically to extreme environmental changes.

Biological and Medical Significance

• Antibiotic Resistance: Many antibiotics target the bacterial cell wall. L-forms can survive such treatments, contributing to persistent infections.

• Chronic Diseases: L-forms have been implicated in chronic infections due to their ability to evade immune responses and antibiotics.

• Evolutionary Insights: Studying L-forms helps understand early bacterial evolution and mechanisms of cellular adaptation.

Conclusion

The experiment where bacterial cell walls are enzymatically removed and stable wall-deficient bacteria emerge over generations is proof of adaptive mutation. This phenomenon highlights bacteria’s remarkable ability to genetically adapt to environmental stress, disproving the inheritance of acquired characters and illustrating evolutionary principles in action.

Correct answer:
(4) Adaptive mutation

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Understanding adaptive mutation in bacteria not only advances microbiology but also informs medical strategies against antibiotic resistance and persistent infections.