🦠 Introduction to Kingdom Monera

Kingdom Monera includes all prokaryotic organisms, with bacteria as its sole members. Bacteria are the most abundant microorganisms on Earth, found in nearly every habitat, from a handful of soil to extreme environments like hot springs, deserts, and deep oceans. Many also live as parasites in or on other organisms.

🔬 General Characteristics of Bacteria

  • Structure & Shapes: While their cellular structure is simple, bacteria are very complex in their behavior. They are grouped into four categories based on shape:
  1. Coccus (spherical)
  2. Bacillus (rod-shaped)
  3. Vibrium (comma-shaped)
  4. Spirillum (spiral)
  • Nutrition: Bacteria exhibit the most extensive metabolic diversity among all organisms.
  • Autotrophic: They synthesize their own food from inorganic substrates. This can be photosynthetic (like cyanobacteria) or chemosynthetic (oxidizing inorganic substances).
  • Heterotrophic: The vast majority of bacteria are heterotrophs, depending on other organisms or dead organic matter for food (saprophytic).

🧫 Sub-groups of Monera

1️⃣. Archaebacteria

These bacteria are unique because they live in some of the most harsh habitats known.

  • Halophiles: In extreme salty areas.
  • Thermoacidophiles: In hot springs.
  • Methanogens: In marshy areas and the guts of ruminant animals like cows. Archaebacteria possess a different cell wall structure compared to other bacteria, which is the key feature responsible for their survival in these extreme conditions.

2️⃣. Eubacteria ('True Bacteria')

Eubacteria are characterized by the presence of a rigid cell wall and, if motile, a flagellum.

  • Cyanobacteria (Blue-green Algae): These are photosynthetic autotrophs containing chlorophyll 'a', similar to green plants. They can be unicellular, colonial, or filamentous. Their colonies are often surrounded by a gelatinous sheath. Some can fix atmospheric nitrogen in specialized cells called heterocysts, such as in Nostoc and Anabaena.
  • Chemosynthetic Autotrophs: These bacteria gain energy for ATP production by oxidizing various inorganic substances like nitrates, nitrites, and ammonia. They play a crucial role in recycling nutrients like nitrogen, phosphorus, iron, and sulphur.
  • Heterotrophic Bacteria: These are the most abundant bacteria in nature and are important decomposers. They are vital for making curd from milk, producing antibiotics, and fixing nitrogen in legume roots. Some are pathogens that cause diseases like cholera, typhoid, and tetanus.

3️⃣. Cyanobacteria (Blue-Green Algae)

  • Photosynthetic prokaryotes with chlorophyll a like plants.
  • Contain heterocysts for nitrogen fixation (e.g., Anabaena, Nostoc).
  • Found in moist soils, water, or as symbionts (e.g., Azolla–Anabaena).

4️⃣ Mycoplasma (PPLO – Pleuro Pneumonia Like Organisms)

Mycoplasma are unique organisms that completely lack a cell wall. They are the smallest living cells known and are capable of surviving without oxygen. Many forms of mycoplasma are pathogenic in animals and plants.

🧬 Reproduction in Bacteria

  • Asexual: The main method of reproduction is fission. Under unfavorable conditions, they can produce spores.
  • Sexual: They also reproduce through a primitive type of DNA transfer from one bacterium to another.

⚖️ Economic Importance of Bacteria

Beneficial Roles Harmful Roles
Nitrogen fixation (Rhizobium) Disease (e.g., Mycobacterium tuberculosis)
Curd and cheese production (Lactobacillus) Food spoilage
Antibiotics (Streptomyces) Crop damage (Xanthomonas)
Sewage decomposition —

💡 Solved Examples

Q1. Name the three main groups of Archaebacteria.
A1. Methanogens, Halophiles, and Thermoacidophiles.

Q2. What is the main difference between Eubacteria and Archaebacteria?
A2. Archaebacteria have cell walls without peptidoglycan, whereas Eubacteria possess peptidoglycan cell walls.

Q3. Give one example each of beneficial and harmful bacteria.
A3. Beneficial: Rhizobium (nitrogen fixation); Harmful: Mycobacterium tuberculosis (disease-causing).

Q4. What are heterocysts?
A4. Specialized cells in cyanobacteria for nitrogen fixation.

Q5. Why are Mycoplasmas unique among Monerans?
A5. They lack a cell wall and can survive in extreme conditions.