Microorganisms can be broadly classified based on their cellular organization:
Prokaryotes: Lack a membrane-bound nucleus and organelles. Examples: Bacteria, Archaea.
Eukaryotes: Possess a true nucleus and membrane-bound organelles. Examples: Fungi, Protozoa, Algae.
Acellular Agents: Not considered living cells; require a host to replicate. Examples: Viruses, Viroids, Prions.
1. BACTERIA
A. Structure
Bacteria are prokaryotic cells with a defined set of structures.
Essential Structures:
Cell Wall: Rigid outer layer that provides shape and prevents osmotic lysis.
Gram-Positive: Thick layer of peptidoglycan with teichoic acids.
Gram-Negative: Thin layer of peptidoglycan surrounded by an outer membrane containing Lipopolysaccharide (LPS - an endotoxin).
Cell Membrane (Plasma Membrane): Phospholipid bilayer that controls transport, contains enzymes for energy production, and is the site of the Electron Transport Chain.
Cytoplasm: Gel-like substance containing ribosomes (70S), nutrients, and the nucleoid.
Nucleoid: Region containing the single, circular chromosome of DNA. Not membrane-bound.
Ribosomes (70S): Sites of protein synthesis.
Non-Essential Structures:
Glycocalyx: A sticky, external layer.
If firmly attached: Capsule (virulence factor, protects from phagocytosis).
If loosely attached: Slime Layer (aids in adhesion).
Flagella: Long, whip-like appendages for motility.
Fimbriae: Short, hair-like structures for attachment (e.g., to host tissues).
Pili (Conjugation Pili): Longer than fimbriae; used for the transfer of genetic material between bacteria (conjugation).
Endospores: Highly durable, dormant structures formed by some genera (e.g., Bacillus, Clostridium) to survive harsh conditions. They are highly resistant to heat, desiccation, and chemicals.
B. Reproduction
Bacteria primarily reproduce asexually by Binary Fission.
The cell elongates and replicates its DNA.
The cell wall and plasma membrane begin to divide.
A septum (cross-wall) is formed, dividing the cell into two.
Two identical daughter cells are formed.
Generation Time: The time required for a population to double. It varies with species and environmental conditions.
Genetic Recombination (Horizontal Gene Transfer): This is not reproduction but a way to increase genetic diversity.
Transformation: Uptake of free DNA from the environment.
Transduction: Transfer of DNA from one bacterium to another by a bacteriophage (virus).
Conjugation: Direct transfer of DNA (often a plasmid) via a conjugation pilus.
2. FUNGI
A. Structure
Fungi are eukaryotic, heterotrophic organisms with cell walls made of chitin.
Yeasts: Unicellular fungi that reproduce by budding. Example: Saccharomyces cerevisiae, Candida albicans.
Molds: Multicellular, forming long, branching filaments called hyphae. A mass of hyphae is called a mycelium. Example: Penicillium, Rhizopus.
Dimorphic Fungi: Can switch between yeast and mold forms depending on environmental conditions (often a virulence factor in human pathogens).
B. Reproduction
Fungi exhibit both asexual and sexual reproduction.
Asexual Reproduction:
Budding (in yeasts): A small bud forms on the parent cell, the nucleus divides, and one nucleus migrates into the bud, which then pinches off.
Fragmentation: Hyphal fragments break off and grow into new mycelia.
Asexual Spores:
Conidiospores (Conidia): Unenclosed spores formed at the tips of hyphae (e.g., Penicillium, Aspergillus).
Sporangiospores: Spores contained within a sac-like sporangium (e.g., Rhizopus).
Sexual Reproduction:
Involves the fusion of gametes or hyphal elements from two compatible mating types to form a zygote.
Leads to the production of sexual spores (e.g., zygospores, ascospores, basidiospores), which are important for classification.
3. PROTOZOA
A. Structure
Protozoa are unicellular, eukaryotic organisms lacking a cell wall. They are mostly motile and heterotrophic.
Trophozoite: The active, feeding, and growing stage.
Cyst: A dormant, resistant stage formed under unfavorable conditions; allows for survival and transmission.
Locomotor Structures:
Pseudopodia ("false feet") - e.g., Amoeba.
Flagella - e.g., Giardia.
Cilia - e.g., Paramecium.
B. Reproduction
Asexual Reproduction:
Binary Fission: The most common method. The nucleus divides (mitosis), and the cell splits into two.
Schizogony (Multiple Fission): The nucleus divides multiple times before the cell cytoplasm splits, producing many daughter cells simultaneously. Common in parasites like Plasmodium (malaria).
Sexual Reproduction:
Conjugation: Exchange of genetic material between two cells (e.g., Paramecium).
Gametogony: Formation and fusion of gametes (e.g., Plasmodium in the mosquito).
4. ALGAE
A. Structure
Algae are photosynthetic, eukaryotic organisms with cell walls typically made of cellulose.
Can be unicellular (e.g., Chlamydomonas) or multicellular (e.g., kelp).
Contain chloroplasts with photosynthetic pigments (chlorophyll a, b, c, carotenoids, phycobilins).
B. Reproduction
Exhibit diverse life cycles involving both asexual and sexual reproduction.
Asexual: Binary fission, fragmentation, or production of motile zoospores.
Sexual: Involves the fusion of gametes (isogamous, anisogamous, or oogamous) to form a zygote. Often involves an alternation of generations between haploid (gametophyte) and diploid (sporophyte) stages.
5. VIRUSES
A. Structure (Virion)
Viruses are acellular, obligate intracellular parasites.
Genetic Material: Either DNA or RNA (single or double-stranded).
Capsid: A protein coat made of capsomeres that surrounds and protects the nucleic acid.
Nucleocapsid: The combined structure of nucleic acid + capsid.
Envelope: A lipid bilayer derived from the host cell membrane, which may contain viral glycoprotein spikes. Not all viruses have an envelope (naked viruses are more stable).
Enveloped examples: HIV, Influenza virus.
Naked examples: Adenovirus, Poliovirus.
B. Replication (The Lytic Cycle of a Bacteriophage)
Viruses cannot reproduce on their own; they hijack the host cell's machinery.
Attachment (Adsorption): Viral tail fibers attach to specific receptors on the host cell wall.
Penetration: The viral DNA is injected into the host cell. The capsid remains outside.
Biosynthesis: The viral DNA takes over the host cell. Host replication and protein synthesis are shut down. The cell is directed to synthesize viral nucleic acids and capsid proteins.
Assembly (Maturation): New viral DNA is packaged into capsids, assembling into complete virions.
Release: The host cell lyses (bursts), releasing new virions to infect adjacent cells.
Lysogenic Cycle: Some bacteriophages can integrate their DNA into the host chromosome (becoming a prophage) and replicate passively with the host cell before eventually entering the lytic cycle.
(For animal viruses, penetration occurs via endocytosis or membrane fusion, and release occurs via budding for enveloped viruses.)
Summary Table
Microorganism
Cell Type
Key Structural Feature(s)
Primary Reproductive Method(s)
Bacteria
Prokaryote
Peptidoglycan cell wall, 70S ribosomes
Asexual: Binary Fission
Fungi
Eukaryote
Chitin cell wall, hyphae/mycelium
Asexual: Budding, Fragmentation, Spores Sexual: Sexual spores
Protozoa
Eukaryote
No cell wall, motile (pseudopodia, cilia, flagella)
Disclaimer: These notes are a comprehensive guide based on standard microbiology curricula. For specific details and depth required for your BSc/BTEC course, always refer to your official course guide, lecture materials, and recommended textbooks.
Multiple Choice Questions
Test your knowledge with these questions based on the notes above.
Score: 0/0
Question 1: Bacterial Cell Structure
Which of the following is a key difference between Gram-positive and Gram-negative bacteria?
a) Gram-positive bacteria have flagella, while Gram-negative do not
b) Gram-positive bacteria have a thick peptidoglycan layer, while Gram-negative have a thin layer with an outer membrane
c) Gram-positive bacteria lack DNA, while Gram-negative have DNA
d) Gram-positive bacteria are photosynthetic, while Gram-negative are not
Question 2: Fungal Reproduction
Which of the following is NOT a method of asexual reproduction in fungi?
a) Budding
b) Formation of conidiospores
c) Formation of zygospores
d) Fragmentation
Question 3: Protozoan Structure
What is the name of the dormant, resistant stage formed by protozoa under unfavorable conditions?
a) Cyst
b) Trophozoite
c) Spore
d) Endospore
Question 4: Viral Replication
In the lytic cycle of a bacteriophage, what happens during the "biosynthesis" stage?
a) The virus attaches to the host cell
b) The viral DNA directs the host cell to produce viral components
c) New viral particles are assembled
d) The host cell bursts, releasing new viruses
Question 5: Microbial Classification
Which type of microorganism is characterized by having a cell wall made of chitin?
a) Bacteria
b) Protozoa
c) Fungi
d) Algae
Question 6: Bacterial Genetic Recombination
Which method of horizontal gene transfer in bacteria involves the direct transfer of DNA via a conjugation pilus?
a) Transformation
b) Transduction
c) Conjugation
d) Binary fission
Short Answer Questions
Answer the following questions in your own words. Use the notes for reference.
Congratulations! You've completed all short answer questions. You can now view the answer keys.
Question 1
Explain the key structural differences between Gram-positive and Gram-negative bacteria and how these differences affect the Gram staining process.
Word count: 0
Answer Key: Gram-positive bacteria have a thick peptidoglycan layer that retains the crystal violet-iodine complex, appearing purple. Gram-negative bacteria have a thin peptidoglycan layer surrounded by an outer membrane; the alcohol decolorizer dissolves this outer membrane, allowing the crystal violet to wash out, and they take up the safranin counterstain, appearing pink/red.
Question 2
Compare and contrast the lytic and lysogenic cycles of bacteriophages, highlighting the key differences in their replication strategies.
Word count: 0
Answer Key: In the lytic cycle, the virus immediately takes over the host cell machinery to produce new virions, leading to cell lysis and death. In the lysogenic cycle, the viral DNA integrates into the host chromosome as a prophage and replicates with the host cell without causing immediate lysis. The prophage may eventually enter the lytic cycle in response to environmental triggers.
Question 3
Describe the formation and function of bacterial endospores, naming at least two genera that produce them and explaining their medical significance.
Word count: 0
Answer Key: Endospores are highly resistant, dormant structures formed by some bacteria under unfavorable conditions. They have a thick protective coat, dehydrated cytoplasm, and can survive extreme heat, radiation, and chemicals. Genera include Bacillus and Clostridium. Medically, endospores are significant because they can survive sterilization procedures and cause diseases like tetanus (C. tetani) and anthrax (B. anthracis).
Question 4
Explain the difference between the trophozoite and cyst forms of protozoa, including the conditions that trigger transformation between these forms.
Word count: 0
Answer Key: The trophozoite is the active, feeding, and reproducing form of protozoa found in favorable environments. The cyst is a dormant, resistant form with a protective wall that allows survival during unfavorable conditions (e.g., nutrient deficiency, desiccation, extreme pH). Encystment occurs when conditions deteriorate, and excystment happens when conditions become favorable again.
Question 5
Differentiate between the three methods of horizontal gene transfer in bacteria (transformation, transduction, and conjugation), highlighting the mechanism and significance of each.
Word count: 0
Answer Key:
Transformation: Uptake of free DNA from the environment; significant for acquiring new traits from dead bacteria.
Transduction: Transfer of DNA via bacteriophages; allows gene transfer between distantly related bacteria.
Conjugation: Direct transfer of DNA (often plasmids) through a conjugation pilus; enables transfer of large DNA segments, including antibiotic resistance genes.
Essay Questions
Answer the following essay questions comprehensively. Use the notes for reference and structure your answers with clear introductions, body paragraphs, and conclusions.
Congratulations! You've completed all essay questions. You can now view the answer guides.
Essay Question 1
Discuss the structural and reproductive adaptations that enable microorganisms to survive and thrive in diverse environments. Compare and contrast at least three different types of microorganisms in your response.
Word count: 0 (Minimum: 200 words)
Answer Guide:
Introduction: Define microorganisms and their ecological significance.
Structural Adaptations:
Bacteria: Cell wall variations (Gram+ vs Gram-), capsules, endospores
Bacteria: Rapid binary fission, horizontal gene transfer
Fungi: Both sexual and asexual reproduction, spore formation
Protozoa: Complex life cycles with multiple reproductive strategies
Comparison: Highlight how each adaptation suits different environmental niches
Conclusion: Summarize how these adaptations contribute to microbial success
Essay Question 2
Analyze the clinical significance of microbial structure and reproduction patterns in the context of infectious disease diagnosis, treatment, and prevention. Provide specific examples to support your analysis.
Word count: 0 (Minimum: 200 words)
Answer Guide:
Introduction: Link microbial characteristics to clinical practice