1. Overview and historical perspective
What is a microbe?
Microorganisms (microbes) are living entities too small to be seen with the naked eye. They include bacteria, archaea, fungi, protozoa, algae and viruses. Microbes were the earliest life forms on Earth (evidence >3.5 billion years) and have shaped earth chemistry and evolution.
Where do microbes live?
Microbes inhabit virtually every environment — soil, freshwater and marine habitats, hot springs, polar ice, acidic and alkaline lakes, and inside plants and animals (including humans). Some prefer moderate conditions (mesophiles) while others are extremophiles (thermophiles, halophiles, acidophiles).
2. Broad classification based on cellular organization
Two major groups:
- Prokaryotic microbes — lack a true nucleus and membrane-bound organelles (Bacteria and Archaea).
- Eukaryotic microbes — possess a true nucleus and membrane-bound organelles (Fungi, Protozoa, Algae, some slime/water molds).
3. Prokaryotic microbes — structure, diversity and importance
General structural features
- Size typically ranges from 0.1 to 5 µm.
- Genetic material: usually one circular chromosome located in a nucleoid region; plasmids (small circular DNAs) often present.
- No membrane-bound organelles (mitochondria, ER, Golgi absent); cellular processes occur in cytoplasm or at plasma membrane.
- Cell wall common — in bacteria it contains peptidoglycan (murein); archaeal cell walls differ chemically (no peptidoglycan).
- Reproduction is primarily asexual by binary fission; genetic variation arises through mutation and horizontal gene transfer (conjugation, transformation, transduction).
Major groups of prokaryotes
Bacteria
- Shapes: cocci (spherical), bacilli (rod-shaped), spirilla (helical), vibrio (comma-shaped).
- Some form specialized structures: flagella (motility), pili (attachment/conjugation), endospores (dormant resistant forms in Bacillus, Clostridium).
- Metabolic diversity: phototrophs (cyanobacteria), chemoautotrophs, heterotrophs. Examples: Escherichia coli, Rhizobium (nitrogen-fixers), Streptomyces (antibiotic producers).
Archaea
- Often inhabit extreme environments: halophiles (high salt), thermophiles (high temperature), methanogens (produce methane under anaerobic conditions).
- Cell membranes and biochemistry are unique (ether-linked membrane lipids), and many archaeal genes resemble eukaryotic genes.
- Importance: biogeochemical cycles, extremophile enzymes with industrial potential (thermostable enzymes), models for early life.
Prokaryotic nutrition and metabolism (expanded)
Prokaryotes use a wide range of energy and carbon sources. They are classified by energy source (phototrophs, chemotrophs) and carbon source (autotrophs, heterotrophs). Examples:
- Photoautotrophs: cyanobacteria — perform oxygenic photosynthesis and contribute to primary productivity.
- Chemoautotrophs: nitrifying bacteria — oxidize inorganic compounds (e.g., Nitrosomonas) and are important in nutrient cycles.
- Heterotrophs: decompose organic matter (saprophytes) or live as parasites/pathogens.
Significance of prokaryotes
- Biogeochemical cycling: nitrogen fixation, nitrification, denitrification, sulfur and carbon cycles.
- Soil fertility and plant symbioses (e.g., Rhizobium with legumes).
- Biotechnology & industry: fermentation (yogurt, cheese), recombinant protein production, waste treatment (sewage), bioremediation.
- Pathogens: causative agents of many infectious diseases (cholera, typhoid, tuberculosis).
4. Eukaryotic microbes — structure, groups and importance
General structural features
- Cells are larger (10–100 µm) and compartmentalized.
- True nucleus with nuclear membrane and multiple linear chromosomes associated with histone proteins.
- Membrane-bound organelles such as mitochondria (energy), endoplasmic reticulum (protein/lipid synthesis), Golgi apparatus (processing/packaging), and chloroplasts in photosynthetic groups.
- Reproduction includes mitosis (asexual) and meiosis (sexual) — many microbes can switch strategies based on environment.
Major eukaryotic groups
Fungi
- Range from unicellular yeasts (Saccharomyces) to filamentous molds (Aspergillus, Penicillium) and macroscopic mushrooms.
- Cell walls composed of chitin. Nutritionally heterotrophic — absorb dissolved organic material.
- Reproduction: asexual (spores, budding) and sexual (meiosis producing spores).
- Importance: decomposers, industrial fermentation, antibiotics source (penicillin), and also plant/animal pathogens (e.g., rusts, smuts, Candida).
Protozoa
- Unicellular, mostly motile and heterotrophic. Lack rigid cell walls but may possess a pellicle for shape.
- Modes of locomotion: pseudopodia (Amoeba), cilia (Paramecium), flagella (Euglena — note: Euglena is mixotrophic/has chloroplasts).
- Significance: ecological roles in aquatic food webs, as parasites causing diseases (e.g., Plasmodium — malaria).
Algae
- Photosynthetic eukaryotes occurring in diverse forms: unicellular, colonial, filamentous and large multicellular seaweeds.
- Contain chloroplasts with pigments (chlorophylls, carotenoids, phycobilins) that determine their color (green, brown, red algae).
- Ecological importance as primary producers and oxygen producers; economic importance (agar, carrageenan, alginates).
Slime molds and water molds
- Often treated historically with fungi but now placed in distinct groups. Slime molds show motile plasmodial stages and are important decomposers; water molds include plant pathogens like Phytophthora species.
Eukaryotic nutrition and life cycles (expanded)
Many eukaryotic microbes have complex life cycles with alternating sexual and asexual stages (e.g., many fungi and some protozoa). Algae perform photosynthesis as primary producers; protozoa and many fungi are heterotrophs. Some groups are mixotrophic (both photosynthesis and heterotrophy) depending on conditions.
5. Detailed comparison: Prokaryotes vs Eukaryotes
| Feature | Prokaryotic Microbes | Eukaryotic Microbes |
|---|---|---|
| Nucleus | Absent; DNA in nucleoid region (circular chromosome). | Present; true nucleus with nuclear membrane and multiple linear chromosomes. |
| Chromosomes | Usually single, circular; histones absent in bacteria (some archaea have histone-like proteins). | Multiple, linear; associated with histone proteins. |
| Organelles | Membrane-bound organelles absent. | Membrane-bound organelles present (mitochondria, ER, Golgi, chloroplasts in photosynthetic groups). |
| Cell size | Generally small (0.1–5 µm). | Larger (10–100 µm). |
| Cell wall composition | Bacteria: peptidoglycan; Archaea: varied (no peptidoglycan). | Fungi: chitin; algae: cellulose or other polysaccharides; protozoa: usually absent. |
| Reproduction | Asexual by binary fission; horizontal gene transfer for variation. | Asexual (mitosis) and sexual (meiosis); complex life cycles possible. |
| Examples | Bacteria (E. coli, Bacillus), Archaea (Halobacterium, Methanobrevibacter). | Fungi (Aspergillus, Saccharomyces), Protozoa (Amoeba, Paramecium), Algae (Chlamydomonas, Spirogyra). |
6. Roles and applications of microbes (expanded)
Ecological roles
- Decomposition: Fungi and bacteria decompose dead organic matter, recycling nutrients into ecosystems.
- Primary production: Photosynthetic microbes (algae, cyanobacteria) produce organic matter and oxygen in aquatic ecosystems.
- Biogeochemical cycles: Microbes drive nitrogen, sulfur, carbon and phosphorus cycles.
Agricultural importance
- Symbiotic nitrogen fixation by Rhizobium in legume root nodules improves soil fertility.
- Mycorrhizal fungi enhance plant nutrient uptake and drought tolerance.
- Microbial biofertilizers and biopesticides are sustainable alternatives to chemical inputs.
Industrial and biotechnological applications
- Food and beverage fermentation — yeasts and bacteria (bread, curd, yogurt, cheese, alcoholic beverages).
- Antibiotic production — many antibiotics originate from microbes (e.g., penicillin from Penicillium, streptomycin from Streptomyces).
- Enzymes and metabolites — microbial enzymes used in detergents, textiles, paper and pharmaceuticals.
- Recombinant DNA technology — microbes as hosts for producing insulin, vaccines and industrial proteins.
Medical importance
- Pathogens: Bacteria (cholera), viruses (influenza), protozoa (malaria), fungi (aspergillosis) cause diseases.
- Beneficial microbes: gut microbiota aid digestion, produce vitamins and protect against pathogens; probiotics support health.
- Vaccines and antibiotics: Microbes and their components are central to developing preventive and therapeutic agents.
Environmental biotechnology
- Bioremediation: microbes degrade pollutants (oil, pesticides) and remove heavy metals.
- Waste treatment: microbial communities break down organic matter in sewage treatment plants.
- Bioenergy: microbes produce methane (biogas) and research on algae for biofuels is ongoing.
7. Practical tips for B.Sc. students
- Understand structures by sketching — cell envelope of Gram-positive and Gram-negative bacteria, fungal hypha and yeast cell, protozoan cell types.
- Learn key examples and link them to functions (e.g., Rhizobium ▶ nitrogen fixation; Streptomyces ▶ antibiotics).
- Practice classification using morphological/physiological traits and molecular evidence (16S rRNA for prokaryotes, ITS regions for fungi).
- Relate microbial processes to real-world applications (fermentation, biodegradation, plant symbioses).
8. Glossary (select terms)
- Nucleoid: region in prokaryotic cells containing DNA.
- Plasmid: small circular DNA molecule separate from chromosomal DNA.
- Endospore: dormant, resistant structure formed by some bacteria.
- Pellicle: a protective layer found in some protozoa.
- Mixotroph: organism that can use both autotrophic and heterotrophic modes of nutrition.
9. Suggested reading & references
- Madigan, M.T., Bender, K.S., Buckley, D.H., Sattley, W.M. & Stahl, D.A. (2018). Brock Biology of Microorganisms.
- Pelczar, M.J., Chan, E.C.S. & Krieg, N.R. (2001). Microbiology.
- Prescott, L.M., Harley, J.P. & Klein, D.A. (2008). Microbiology.
