Eubacteria

Eubacteria, also known as true bacteria, constitute a vast and diverse domain of single-celled prokaryotic organisms. They are the most abundant and widespread life forms on Earth, inhabiting nearly every environment imaginable—from the human gut to the deepest oceans, and from polar ice to volcanic hot springs. Eubacteria were the first life forms to appear on Earth and have since evolved to occupy a wide range of ecological niches. While most are microscopic, their collective biomass is immense.

1.0What is Eubacteria

• Eubacteria, also known as “true bacteria,” are a large group of prokaryotic microorganisms characterized by the absence of a true nucleus and membrane-bound organelles. 
• They are unicellular organisms with a cell wall made of peptidoglycan, which provides structural support and shape.
• Eubacteria are incredibly diverse and can be found in a wide range of habitats, including soil, water, air, extreme environments, and living organisms. 
• They exhibit varied modes of nutrition—some are autotrophic (like cyanobacteria that perform photosynthesis), while others are heterotrophic (feeding on organic matter or living hosts).

Characteristics of Eubacteria

Eubacteria are defined by a set of fundamental characteristics that separate them from other life forms:

• Prokaryotic Organization: They lack a true, membrane-bound nucleus and other membrane-bound organelles.
• Unicellular Nature: Typically, each bacterium is a single, independent cell.
• Cell Wall: Possess a rigid cell wall primarily composed of peptidoglycan (a polymer of sugars and amino acids), which is a definitive characteristic.
• Cellular Size: Generally microscopic, ranging from 0.5 to 5.0 micrometers in length.
• Genetic Material: A single, circular chromosome of DNA concentrated in a region called the nucleoid.
• Reproduction: Primarily reproduce asexually through binary fission, a process that results in two genetically identical daughter cells.
• Metabolic Diversity: Exhibit an incredible range of metabolic pathways, allowing them to be autotrophic or heterotrophic, and aerobic or anaerobic.
• Locomotion: Many are motile, using appendages like flagella.

2.0Structure of Eubacterium

A typical bacterial cell is a relatively simple structure compared to a eukaryotic cell. However, each component is highly specialized to ensure the cell's survival and function.

Cell Wall

• The cell wall is a rigid, protective outer layer that maintains the cell's shape and prevents it from bursting in a hypotonic (low solute) environment. It's made of peptidoglycan (also known as murein), a polymer of sugars and amino acids. 
• The difference in the cell wall structure is the basis for the Gram stain. 
• Gram-positive bacteria have a thick peptidoglycan layer, while Gram-negative bacteria have a thinner layer and an additional outer membrane.

Plasma Membrane

• A semi-permeable phospholipid bilayer that regulates the movement of substances in and out of the cell. It contains proteins involved in transport, energy production, and signal transduction

Capsule/Glycocalyx: 

• Some bacteria have an additional outer layer made of polysaccharides or polypeptides. 
• This capsule protects the cell from desiccation and phagocytosis and aids in surface attachment and biofilm formation

Cytoplasm and Ribosomes

• The cytoplasm is a jelly-like substance that fills the cell and contains various molecules and structures. 
• The ribosomes in bacteria are smaller than those in eukaryotes (70S vs. 80S) and are responsible for protein synthesis. 
• This difference in size is exploited by some antibiotics, like streptomycin, which specifically target bacterial ribosomes without affecting eukaryotic ones.

Nucleoid 

• The region within the cytoplasm where the single, circular chromosome (DNA) is found. 
• It is not enclosed by a membrane.

Plasmids 

• Small, circular, extrachromosomal DNA molecules that can carry genes for antibiotic resistance, virulence, or metabolic functions

Flagella and Pili

• Flagella are long, filamentous appendages that help in motility. 
• Their structure is different from the flagella of eukaryotes. 
• Pili (or fimbriae) are shorter, hair-like appendages that help bacteria adhere to surfaces (e.g., host tissues) and to other bacterial cells during conjugation.

Mesosomes

• Infoldings of the plasma membrane that are involved in cellular respiration, DNA replication, and septum formation during cell division

3.0Types of Eubacteria

Classification Based on Shape (Morphology)

Coccus (Spherical)

• Spherical cells that may occur singly (Micrococcus), in pairs (Diplococcus – Neisseria gonorrhoeae), chains (Streptococcus pyogenes), or clusters (Staphylococcus aureus).

Bacillus (Rod-shaped)

• Cylindrical or rod-like bacteria.
• It can appear singly, in chains, or in filaments.
• Examples: Bacillus subtilis, Bacillus anthracis.

Spirillum (Spiral-shaped)

• Rigid spiral-shaped bacteria are usually motile with flagella.
• Examples: Spirillum volutans, Campylobacter jejuni.

Vibrio (Comma-shaped)

• Curved or comma-shaped rods, often aquatic.
• Example: Vibrio cholerae.

Classification Based on Gram Staining

Gram-positive Bacteria

• Thick peptidoglycan cell wall.
• Stains purple.
• Examples: Staphylococcus, Streptococcus, Bacillus.

Gram-negative Bacteria

• Thin peptidoglycan layer with an outer membrane.
• Stains pink/red.
• Examples: Escherichia coli, Salmonella, Vibrio.

Classification Based on Nutrition and Metabolism

Autotrophic Bacteria

• Produce their own food via photosynthesis or chemosynthesis.
• Examples: Cyanobacteria such as Anabaena and Nostoc.

Heterotrophic Bacteria

• Obtain food from organic matter.
• • Saprophytic: Decomposes dead matter (Bacillus subtilis).
  • Parasitic: Live on host organisms (Mycobacterium tuberculosis).
  • Symbiotic: Live in mutual association (Rhizobium in legumes).

Chemoautotrophic Bacteria

• Obtain energy by oxidising inorganic compounds (ammonia, hydrogen, or sulfur).
• Examples: Nitrosomonas and Nitrobacter.

Special Types

Cyanobacteria (Blue-Green Algae)

• Photosynthetic and nitrogen-fixing bacteria.
• Form filaments, colonies, or mats in aquatic and terrestrial habitats.
• Examples: Anabaena, Nostoc.

Endospore-forming Bacteria

• Gram-positive bacilli form endospores to survive extreme conditions.
• Examples: Bacillus, Clostridium.

4.0Reproduction in Eubacteria

Bacteria primarily reproduce asexually by binary fission.

Binary Fission

• This is a simple and rapid process of cell division. 
• The single circular chromosome replicates, and the two resulting copies move to opposite ends of the cell. 
• The cell then elongates, and a new cell wall and membrane form, dividing the cell into two genetically identical daughter cells. Under favorable conditions, some bacteria can divide every 20 minutes, leading to exponential population growth.

Genetic Recombination

While binary fission produces clones, bacteria can exchange genetic material through several mechanisms of genetic recombination, which increases genetic diversity.

• Conjugation: A "male" bacterium with a plasmid (F-factor) forms a temporary bridge called a pilus to a "female" bacterium. The plasmid DNA is then transferred, allowing the recipient to acquire new genes.
• Transformation: A bacterium takes up fragments of "naked" DNA from its surroundings.
• Transduction: A virus (bacteriophage) transfers DNA from one bacterium to another.

5.0Ecological and Economic Significance

Bacteria are indispensable for life on Earth.

Ecological Roles:

• Decomposition: Saprophytic bacteria are the primary decomposers in most ecosystems, breaking down dead organic matter and returning nutrients to the soil.
• Nitrogen Fixation: Nitrogen-fixing bacteria, like Rhizobium in the root nodules of legumes, convert atmospheric nitrogen (N2​) into a usable form for plants.
• Symbiosis: Gut bacteria aid in digestion and produce essential vitamins.
• Oxygen Production: Cyanobacteria were instrumental in the Great Oxidation Event, which created the oxygen-rich atmosphere we breathe today.

Economic Importance:

• Food Industry: Bacteria are used in the production of yogurt, cheese, vinegar, and other fermented foods.
• Biotechnology: Genetically engineered bacteria are used to produce valuable products like human insulin, hormones, and antibiotics.
• Wastewater Treatment: Bacteria are used in sewage treatment plants to break down organic waste.
• Bioremediation: Bacteria can be used to clean up oil spills and other environmental pollutants.
• Medicine: They are the source of many antibiotics.