Module 103. Microbiology – Virus, Bacteria, Algae, Fungi, Protozoa

Microbiology is the branch of biological science that deals with the study of microscopic organisms invisible to the naked eye. These include viruses, bacteria, algae, fungi, and protozoa. Despite their minute size, microorganisms play an enormous role in ecosystems, health, and industry. They are responsible for essential processes such as nutrient cycling, fermentation, disease causation, and even biotechnology applications. Understanding microbiology is crucial for medicine, agriculture, and environmental science, as it reveals the intricate balance between beneficial and harmful microorganisms in nature.

Scope and Importance of Microbiology

Microorganisms exist in every conceivable environment — from deep ocean vents to human intestines. Their diversity and adaptability make them fundamental to life on Earth. Microbiology contributes to:

• Medicine: Development of vaccines, antibiotics, and diagnostic techniques.
• Agriculture: Nitrogen fixation, pest control, and soil fertility management.
• Industry: Fermentation processes in food and beverages, production of enzymes, vitamins, and biofuels.
• Environment: Decomposition, bioremediation, and recycling of organic matter.

Microbiology thus combines fundamental biological principles with practical applications, offering insights into both the beneficial and pathogenic aspects of microorganisms.

Viruses: Acellular Infectious Agents

Viruses are microscopic entities that occupy the boundary between living and non-living matter. They consist of genetic material — DNA or RNA — enclosed in a protein coat called a capsid, and sometimes an additional lipid envelope. Viruses lack cellular structures and metabolic machinery, hence they depend entirely on host cells for replication.
Key characteristics of viruses include:

• Acellular structure: No cytoplasm or organelles.
• Obligate parasitism: Can multiply only within living cells.
• Host specificity: Infect specific organisms or even specific cells within an organism.

Examples include:

• Animal viruses: Influenza virus, HIV (AIDS), SARS-CoV-2 (COVID-19).
• Plant viruses: Tobacco mosaic virus (TMV).
• Bacteriophages: Viruses that infect bacteria, used in genetic research.

Viruses can cause diseases such as measles, rabies, and hepatitis, yet they also have scientific and medical applications, including use in gene therapy and vaccine development.

Bacteria: Unicellular Prokaryotic Organisms

Bacteria are single-celled prokaryotic microorganisms without a well-defined nucleus or membrane-bound organelles. Their genetic material lies in a circular DNA molecule within the cytoplasm. Bacteria exhibit tremendous diversity in shape, size, and metabolism.
Common bacterial shapes include:

• Cocci: Spherical (e.g. Streptococcus).
• Bacilli: Rod-shaped (e.g. Escherichia coli).
• Spirilla: Spiral-shaped (e.g. Spirillum volutans).

Bacteria reproduce asexually through binary fission, allowing rapid population growth. They can survive extreme conditions through spore formation.
Classification based on oxygen requirement:

• Aerobic bacteria: Require oxygen for growth (Mycobacterium tuberculosis).
• Anaerobic bacteria: Grow in absence of oxygen (Clostridium botulinum).
• Facultative anaerobes: Can survive with or without oxygen (E. coli).

Beneficial roles:

• Nitrogen fixation (Rhizobium in leguminous roots).
• Decomposition of organic matter.
• Fermentation in yoghurt and cheese production (Lactobacillus).
• Production of antibiotics (Streptomyces).

Harmful roles:

• Disease causation such as cholera, typhoid, and tuberculosis.
• Food spoilage and contamination.

Bacteria thus act as both life-sustaining and disease-causing agents, reflecting their dual nature in biological systems.

Algae: Photosynthetic Aquatic Organisms

Algae are autotrophic, photosynthetic organisms found mostly in aquatic environments. They may be unicellular (e.g. Chlamydomonas) or multicellular (e.g. Laminaria). Algae contain chlorophyll and other pigments, enabling them to perform photosynthesis and produce oxygen, which supports aquatic life.
Classification based on pigment and storage material:

• Green algae (Chlorophyceae): Contain chlorophyll a and b (e.g. Spirogyra).
• Brown algae (Phaeophyceae): Contain fucoxanthin pigment (e.g. Sargassum).
• Red algae (Rhodophyceae): Contain phycoerythrin pigment (e.g. Polysiphonia).

Economic importance:

• Source of agar, alginates, and carrageenan used in food and pharmaceuticals.
• Biofuel production and wastewater treatment.
• Provide food for marine organisms and contribute to carbon fixation.

Though beneficial, excessive algal growth (algal bloom) in water bodies can lead to oxygen depletion and aquatic life destruction, a phenomenon known as eutrophication.

Fungi: Non-photosynthetic Eukaryotes

Fungi are eukaryotic, heterotrophic organisms that absorb nutrients from organic matter. They include moulds, yeasts, and mushrooms. Fungi possess chitin in their cell walls and reproduce through spores.
Structural features:

• Composed of filaments called hyphae, forming a network known as mycelium.
• Reproduce both sexually and asexually by spore formation.

Types and examples:

• Yeasts: Unicellular fungi (e.g. Saccharomyces cerevisiae used in baking and brewing).
• Moulds: Multicellular fungi (e.g. Penicillium used for antibiotic production).
• Mushrooms: Large fruiting bodies (e.g. Agaricus).

Beneficial roles:

• Decomposition and nutrient recycling.
• Production of alcohol, bread, and cheese.
• Source of medicines such as penicillin.

Harmful roles:

• Cause plant diseases like rusts and smuts.
• Human diseases such as athlete’s foot and ringworm.
• Spoilage of food and stored grains.

Fungi thus play vital ecological roles as decomposers and industrial resources while also including parasitic and pathogenic varieties.

Protozoa: Unicellular Eukaryotic Animals

Protozoa are single-celled eukaryotic organisms that exhibit animal-like behaviour, such as movement and ingestion of food. They live in aquatic habitats, moist soil, or as parasites within other organisms.
Modes of movement:

• Cilia: Tiny hair-like structures (e.g. Paramecium).
• Flagella: Whip-like structures (e.g. Euglena).
• Pseudopodia: Temporary cytoplasmic projections (e.g. Amoeba).

Nutritional types:

• Holozoic: Ingest solid food (e.g. Amoeba).
• Parasitic: Depend on host organisms (e.g. Plasmodium causing malaria).

Examples and importance:

• Entamoeba histolytica causes amoebic dysentery.
• Trypanosoma causes sleeping sickness.
• Euglena exhibits both plant-like and animal-like characteristics, performing photosynthesis while being motile.

Protozoa are crucial for ecological balance, forming part of aquatic food chains and aiding in nutrient cycling, though many species are parasitic and pathogenic.

Role of Microorganisms in Human Welfare

Microorganisms, though often associated with diseases, have immense positive roles in human welfare and natural processes:

• Agriculture: Nitrogen-fixing and decomposing bacteria enrich soil fertility.
• Industry: Fermentation for production of alcohol, vinegar, and antibiotics.
• Medicine: Production of vaccines, insulin, and recombinant drugs through biotechnology.
• Environment: Waste degradation, sewage treatment, and pollution control.

Microorganisms also form the basis of microbial genetics and biotechnology, helping scientists understand heredity, gene regulation, and evolution.

Significance of Microbiology in Modern Science

Microbiology has evolved into a cornerstone of biological and medical research. It provides insights into the causes of infectious diseases, mechanisms of immunity, and molecular interactions between hosts and pathogens. With the rise of antibiotic resistance, microbial ecology, and genetic engineering, the study of microbes continues to have profound implications for health, agriculture, and environmental sustainability.