Question

Which is the most resistant structure in bacteria?
A) Endospore
B) Cyst
C) Conidia
D) Gonidia

Answer 1

Hint: Resistant structures are those structures that provide resistance or protection against harsh or adverse conditions. These structures are formed by the bacteria to protect themselves from adverse environmental conditions such as heat, UV rays, nutrient deprivation, presence of toxic chemicals, etc.

Complete step-by-step answer:
Option (A) is correct. An endospore is a thick, dormant, and tough structure produced by some bacteria belonging to the phylum Firmicutes (Bacillus, Clostridium). This structure is the most resistant in bacteria as it can survive harsh environmental conditions that would otherwise kill the normal bacterium. The endospore structure consists of DNA, RNA, proteins, and dipicolinic acid which are in association with calcium ions. The protoplast of endospore is highly dehydrated making it heat resistant and also the calcium-dipicolinic acid complex present in it imparts stability to its DNA.
Option (B) is incorrect. A cyst is a dormant structure in bacteria that is formed by the contraction of the cytoplasm followed by the thickening of the cell wall. It protects against adverse environmental conditions such as nutrient deprivation, lack of moisture, presence of toxic chemicals, etc. It is a less resistant structure as compared to the endospores of bacteria because endospores can also protect against harsh UV radiations that would cause damage to the DNA of the bacterium.
Option (C) is incorrect. Conidia are produced in filamentous bacteria like Streptomyces. They are not meant for protecting the bacteria; else, they are known to grow new bacteria when they get detached from parent species and find a new substratum for growth.
Option (D) is incorrect. Gonidia are the asexual reproductive cells present in algae.

Hence, the correct option is (A).

Note: Endospore, as the name indicates, is produced inside the bacterium. It is surrounded by 4 spore layers- exosporium (outermost layer), spore coat, spore cortex, and core wall (innermost layer). The endospore is the highly resistant structure in bacteria as compared to the cyst because it contains dipicolinic acid that forms complex with calcium and provides stability to its DNA from harmful UV radiations.