What are the Two Kingdoms of Bacteria?


Bacteria are single-celled microorganisms that play an essential role in various ecosystems. They are classified into different kingdoms based on their characteristics and evolutionary relationships. In this article, we will explore the two main kingdoms of bacteria and delve into their distinct features, functions, and examples. Let’s dive in!

The Prokaryotic World of Bacteria

Before we discuss the two kingdoms of bacteria, let’s first understand the basic structure of these microorganisms. Bacteria belong to the prokaryote domain, which means they lack a nucleus and membrane-bound organelles. Instead, their DNA floats freely in the cytoplasm. This unique characteristic distinguishes them from eukaryotic cells found in plants, animals, and fungi.

Bacteria Kingdoms: Overview

There are two primary kingdoms of bacteria: Eubacteria and Archaebacteria. Each kingdom encompasses diverse groups of bacteria that exhibit distinct characteristics and occupy different ecological niches. Let’s explore each kingdom in detail:

Eubacteria: The True Bacteria

Eubacteria, also known as true bacteria, is the most well-known and abundant kingdom of bacteria. They are found in various habitats, including soil, water, and the bodies of plants and animals. Eubacteria can be further classified into multiple phyla, each containing numerous genera and species. Some important characteristics of eubacteria include:

  • Cellular Structure: Eubacteria have a diverse range of cell shapes, including cocci (spherical), bacilli (rod-shaped), and spirilla (spiral-shaped).
  • Cell Walls: Most eubacteria possess cell walls composed of peptidoglycan, a unique molecule absent in the cell walls of archaea.
  • Metabolism: Eubacteria exhibit various metabolic pathways, including aerobic respiration, anaerobic respiration, and fermentation.
  • Habitats and Roles: Eubacteria can be found in virtually every environment on Earth. They play crucial roles in nutrient cycling, symbiotic relationships, and some cause diseases in humans and other organisms.
  • Examples: Common examples of eubacteria include Escherichia coli (E. coli), Streptococcus pneumoniae, and Salmonella enterica.

Archaebacteria: The Ancient Bacteria

Archaebacteria, also known as archaea, were initially classified as bacteria due to their prokaryotic nature. However, further research revealed significant differences in their genetic makeup and cellular processes. Archaebacteria are known for their ability to thrive in extreme environments. Here are some key features of archaebacteria:

  • Cellular Structure: Archaebacteria have diverse cell shapes similar to eubacteria, but their cell walls lack peptidoglycan.
  • Metabolism: Archaebacteria employ unique metabolic pathways different from both eubacteria and eukaryotes.
  • Habitats and Roles: Archaebacteria are commonly found in extreme environments such as hot springs, salt lakes, and deep-sea hydrothermal vents. They play crucial roles in biogeochemical cycling and are believed to resemble some of the earliest life forms on Earth.
  • Examples: Well-known examples of archaebacteria include Thermus aquaticus, Methanocaldococcus jannaschii, and Halobacterium salinarum.

Comparing Eubacteria and Archaebacteria

Now that we have explored the main features of eubacteria and archaebacteria individually, let’s compare them side by side:

Characteristics Eubacteria Archaebacteria
Cellular Structure Variety of shapes Variety of shapes
Cell Walls Contain peptidoglycan Lack peptidoglycan
Metabolic Pathways Aerobic respiration, anaerobic respiration, fermentation Unique metabolic pathways
Habitats Wide range of environments Extreme environments
Examples E. coli, Streptococcus pneumoniae, Salmonella enterica Thermus aquaticus, Methanocaldococcus jannaschii, Halobacterium salinarum


Bacteria are incredibly diverse microorganisms that populate our planet in various forms. The two kingdoms, Eubacteria and Archaebacteria, represent different branches of the bacterial tree of life. Eubacteria are the true bacteria, widely distributed and involved in numerous ecological processes, including some diseases. Archaebacteria, on the other hand, are ancient bacteria that thrive in extreme environments, offering insights into the early stages of life on Earth. Understanding the characteristics and distinctions between these two kingdoms enhances our knowledge of the vast microbial world and its significance in our ecosystems.

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