Microorganisms

Overview

Microorganisms are living things too small to be seen with the naked eye, requiring a microscope for observation. This topic forms a crucial part of the Life Sciences portion in Assam TET Paper II, connecting biology fundamentals with everyday applications like food preservation, disease prevention, and environmental cycles.

For the exam, you must understand the classification of microorganisms (bacteria, viruses, fungi, protozoa, algae), their structural features, modes of reproduction, and their dual role as both beneficial and harmful agents. Questions typically test factual recall about disease-causing organisms, useful applications in medicine and agriculture, and basic distinctions between different microbial groups.

Mastery of this topic also supports understanding of related areas like nutrition, respiration, and environmental pollution—making it a high-value section for integrated learning.

Key Concepts

• **Definition**: Microorganisms (microbes) are organisms invisible to the naked eye, including bacteria, viruses, fungi, protozoa, and some algae. They exist everywhere—soil, water, air, and inside living bodies.

• **Bacteria are prokaryotes**: They lack a true nucleus and membrane-bound organelles. Cell wall contains peptidoglycan. Shapes include cocci (spherical), bacilli (rod), spirilla (spiral), and vibrio (comma).

• **Viruses are non-cellular**: They consist only of genetic material (DNA or RNA) surrounded by a protein coat (capsid). They reproduce only inside living host cells, making them obligate parasites.

• **Fungi are eukaryotes**: They have a true nucleus, cell wall made of chitin, and obtain nutrition by decomposing organic matter (saprophytes) or living on hosts (parasites). Examples: yeast, mushroom, bread mould.

• **Protozoa are unicellular eukaryotes**: They move using cilia, flagella, or pseudopodia. Examples: Amoeba, Paramecium, Plasmodium (causes malaria).

• **Microorganisms can be beneficial or harmful**: Beneficial roles include fermentation, nitrogen fixation, decomposition, and antibiotic production. Harmful roles include causing diseases in plants, animals, and humans.

• **Antibiotics kill bacteria, not viruses**: This distinction is critical—viral diseases require vaccines or antiviral drugs, while bacterial infections respond to antibiotics like penicillin.

• **Pasteurisation and sterilisation**: Pasteurisation heats liquids to kill harmful microbes without destroying nutrients. Sterilisation completely eliminates all microbes.

Key Facts

| Microorganism | Cell Type | Example | Disease Caused | |---------------|-----------|---------|----------------| | Bacteria | Prokaryotic | Vibrio cholerae | Cholera | | Bacteria | Prokaryotic | Mycobacterium tuberculosis | Tuberculosis | | Virus | Non-cellular | Influenza virus | Flu | | Virus | Non-cellular | HIV | AIDS | | Fungi | Eukaryotic | Tinea | Ringworm | | Protozoa | Eukaryotic | Plasmodium | Malaria | | Protozoa | Eukaryotic | Entamoeba | Dysentery |

**Must-Remember Facts:**

1. **Louis Pasteur** discovered pasteurisation and proved that microbes cause disease (germ theory). 2. **Alexander Fleming** discovered penicillin (first antibiotic) from the fungus Penicillium in 1928. 3. **Edward Jenner** developed the first vaccine (smallpox vaccine) in 1796. 4. **Rhizobium** bacteria in root nodules of legumes fix atmospheric nitrogen into usable form. 5. **Lactobacillus** converts milk into curd through fermentation. 6. **Yeast** (a fungus) is used in making bread, alcohol, and wine through fermentation. 7. Viruses are called **living chemicals** because they show living properties only inside a host. 8. **Communicable diseases** spread from person to person through air, water, food, or direct contact.

Worked Examples

**Example 1: Classification Question**

*Identify whether the following are bacteria, virus, or fungi: (a) Causes AIDS (b) Used in bread-making (c) Causes cholera*

**Solution:**

• (a) AIDS is caused by HIV → **Virus**
• (b) Bread-making uses yeast → **Fungus**
• (c) Cholera is caused by Vibrio cholerae → **Bacteria**

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**Example 2: Application-Based Question**

*Why do doctors not prescribe antibiotics for common cold?*

**Solution:** Common cold is caused by viruses (rhinovirus). Antibiotics work only against bacteria by destroying their cell walls or interfering with their metabolism. Since viruses lack cellular structure and reproduce inside host cells, antibiotics are ineffective against them. For viral infections, vaccines or antiviral medications are needed.

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**Example 3: Beneficial vs Harmful Role**

*Give two beneficial and two harmful effects of microorganisms.*

**Solution:**

**Beneficial:** 1. Bacteria like Lactobacillus help in curd formation from milk 2. Rhizobium bacteria fix nitrogen in soil, improving fertility for crops

**Harmful:** 1. Plasmodium (protozoa) causes malaria in humans 2. Fungi cause crop diseases like wheat rust, destroying harvests

Common Mistakes

• **Thinking viruses are living organisms** → Viruses are non-cellular and show life characteristics only inside host cells. They cannot reproduce independently, so they are considered at the borderline of living and non-living.

• **Confusing bacteria and viruses in disease treatment** → Students often forget that antibiotics work only on bacteria. Remember: Antibiotic = Anti-bacteria. Viral diseases need vaccines.

• **Mixing up cell wall composition** → Bacteria have peptidoglycan cell walls; fungi have chitin cell walls; plant cells have cellulose walls. This is a common factual trap in exams.

• **Forgetting that fungi are eukaryotes** → Unlike bacteria, fungi have a true nucleus. Yeast, mushroom, and mould are all fungi with membrane-bound organelles.

• **Assuming all microorganisms are harmful** → Most microorganisms are beneficial. Decomposers recycle nutrients, nitrogen-fixers enrich soil, and fermenters produce food items like bread, curd, and idli.

Quick Reference

• **Bacteria**: Prokaryotic, cell wall has peptidoglycan, shapes—cocci/bacilli/spirilla
• **Viruses**: Non-cellular, DNA or RNA + protein coat, reproduce only in living cells
• **Fungi**: Eukaryotic, chitin cell wall, examples—yeast, mushroom, Penicillium
• **Pasteurisation**: Heating milk to 70°C for 15-30 seconds to kill pathogens
• **Nitrogen fixation**: Rhizobium in legume roots converts N₂ to nitrates
• **First antibiotic**: Penicillin from Penicillium fungus (Fleming, 1928)