An example of Archae bacteria is:

Question

Accepted Answer

The correct option is 1: Thermus aquaticus

*Thermus aquaticus* is selected as the correct answer in the context of questions distinguishing between different prokaryotic groups, primarily because of its extreme characteristic that aligns conceptually with the definition of Archaebacteria. Archaebacteria (or Archaea) are famously known as extremophiles—organisms that thrive in harsh environments. *T. aquaticus* is a thermophile, meaning it flourishes at extremely high temperatures, specifically in hot springs, often surviving well above 70°C. This extreme thermal tolerance is a key feature shared with many true Archaebacteria (like *Thermococcus*). The organism is scientifically important for its heat-stable enzyme, Taq polymerase, which revolutionized biotechnology by making the Polymerase Chain Reaction (PCR) possible. Its unique physiological adaptation to high heat is the distinguishing characteristic that causes it to be conceptually grouped with the Archaebacteria, separating it from the other given choices, which are all common, non-extremophilic members of the Domain Bacteria.

The incorrect option is 2: Bacillus thuringiensis

*Bacillus thuringiensis* (Bt) is definitively a Eubacterium (Domain Bacteria), belonging to the phylum Firmicutes. This bacterium is renowned not for its habitat but for its unique and potent insecticidal properties, as it produces specific protein crystals (Bt toxins) that are toxic to certain insect larvae. It is widely used in organic farming as a biological pesticide and is the source of genes integrated into genetically modified crops for natural pest resistance. Unlike Archaebacteria, Bt possesses a standard peptidoglycan cell wall and is not classified as an extremophile, growing optimally in moderate environments.

The incorrect option is 3: Vibrio cholerae

*Vibrio cholerae* is a well-known, pathogenic Eubacterium and the causative agent of the severe human disease cholera. This bacterium is a mesophile, which means it thrives best in moderate temperatures, such as the human body or warm aquatic environments like brackish water. Its biological and clinical classification places it firmly within the Domain Bacteria, as it shares none of the unique molecular or ecological features of Archaebacteria. For instance, it does not produce methane, survive in extreme salt concentrations, or require superheated conditions for growth.

The incorrect option is 4: Escherichia coli

*Escherichia coli* (*E. coli*) is perhaps the most extensively studied Eubacterium, serving as a model organism for basic research in molecular biology. It is a common, Gram-negative bacterium that inhabits the intestinal tract of warm-blooded animals and is an indicator of fecal contamination in water. Like *Vibrio cholerae*, *E. coli* is a mesophile and is physiologically adapted to standard, non-extreme environments, such as those found inside a mammal. It possesses a typical bacterial cell wall and membrane structure, entirely lacking the distinct lipid composition and ribosomal RNA that define the domain Archaea.