Microbiology Study Notes: Genetics, Classification, Viruses, and Eukaryotic Organisms

Microbial Genetics

Genetic Material and Processes

Microbial genetics explores the structure, function, and transmission of genetic material in microorganisms. Understanding these processes is essential for studying microbial evolution, adaptation, and biotechnology applications.

• Gene: A segment of DNA that encodes a functional product, usually a protein.

• Chromosome: A DNA molecule containing many genes; bacteria typically have a single circular chromosome.

• Genome: The complete set of genetic material in an organism.

• STRs (Short Tandem Repeats): Repeated sequences of DNA used in genetic fingerprinting.

• Purines and Pyrimidines: Nitrogenous bases in DNA and RNA. Purines: Adenine (A), Guanine (G); Pyrimidines: Cytosine (C), Thymine (T), Uracil (U).

• Complementarity: The specific pairing of bases (A-T, G-C in DNA; A-U, G-C in RNA).

Replication, Transcription, and Translation

These are the central processes by which genetic information is maintained and expressed in cells.

• Replication: The process of copying DNA before cell division. Key enzymes include DNA polymerase, helicase, ligase, and primase.

• Transcription: The synthesis of RNA from a DNA template, primarily by RNA polymerase.

• Translation: The process by which ribosomes synthesize proteins using mRNA as a template.

Key Steps in Replication:

• Initiation: Origin of replication is recognized.

• Elongation: DNA polymerase adds nucleotides.

• Termination: Replication ends at specific sequences.

Key Steps in Transcription:

• Initiation: RNA polymerase binds to promoter.

• Elongation: RNA strand is synthesized.

• Termination: RNA polymerase releases the RNA transcript.

Key Steps in Translation:

• Initiation: Ribosome assembles on mRNA.

• Elongation: tRNAs bring amino acids; peptide bonds form.

• Termination: Stop codon is reached; polypeptide is released.

Central Dogma Equation:

$\text{DNA} \xrightarrow{\text{transcription}} \text{RNA} \xrightarrow{\text{translation}} \text{Protein}$

Gene Regulation

Gene expression in microbes is tightly regulated to respond to environmental changes.

• Operon: A cluster of genes under control of a single promoter (e.g., lac operon).

• Promoter: DNA sequence where RNA polymerase binds to initiate transcription.

• Repressor/Activator: Proteins that decrease/increase gene expression.

• Inducer/Corepressor: Molecules that modulate repressor/activator activity.

• Positive/Negative Control: Activation or repression of gene expression.

Example: The lac operon in Escherichia coli is induced in the presence of lactose and repressed when glucose is available.

Genetic Exchange in Bacteria

Bacteria can exchange genetic material through several mechanisms, contributing to genetic diversity and adaptation.

• Conjugation: Direct transfer of DNA between bacteria via a pilus.

• Transformation: Uptake of free DNA from the environment.

• Transduction: Transfer of DNA by bacteriophages (viruses that infect bacteria).

Example: Plasmid-mediated antibiotic resistance can spread rapidly through conjugation.

Microbial Classification

Taxonomy and Systematics

Microbial classification organizes organisms based on shared characteristics, evolutionary relationships, and genetic information.

• Taxonomy: The science of classification, including identification, naming, and grouping.

• Hierarchy: Domain, Phylum, Class, Order, Family, Genus, Species.

• Major Groups: Bacteria, Archaea, Eukarya (including fungi, protozoa, algae).

Example Table: Microbial Classification

Domain	Representative Genera	Key Features
Bacteria	Streptococcus, Escherichia	Prokaryotic, peptidoglycan cell wall
Archaea	Halobacterium, Thermoplasma	Prokaryotic, unique membrane lipids
Eukarya	Aspergillus, Plasmodium	Eukaryotic, membrane-bound organelles

Microbial Groups and Examples

• Bacteria: Staphylococcus, Streptococcus, Escherichia, Bacillus

• Archaea: Halobacterium, Thermoplasma

• Fungi: Aspergillus, Candida

• Protozoa: Plasmodium, Giardia

Symbiosis and Microbiota

Microbes interact with hosts and each other in various symbiotic relationships.

• Mutualism: Both partners benefit (e.g., gut bacteria synthesizing vitamins).

• Commensalism: One benefits, the other is unaffected (e.g., skin microbiota).

• Parasitism: One benefits at the expense of the other (e.g., pathogenic bacteria).

Normal Microbiota: The community of microorganisms living on or in the human body, providing benefits such as protection against pathogens and aiding digestion.

Example: Lactobacillus in the vagina helps prevent infections by maintaining acidic pH.

Viruses

Structure and Replication

Viruses are acellular infectious agents that require host cells for replication. They have diverse structures and life cycles.

• Capsid: Protein shell enclosing viral genetic material.

• Envelope: Lipid membrane derived from host cell.

• Genome: DNA or RNA, single or double-stranded.

• PFU (Plaque Forming Unit): Measure of infectious virus particles.

Viral Replication Steps:

1. Adsorption: Virus attaches to host cell.

2. Penetration: Entry of viral genome into host.

3. Uncoating: Release of viral genome from capsid.

4. Replication and Transcription: Synthesis of viral components.

5. Assembly: Formation of new virions.

6. Release: Virions exit host cell (lysis or budding).

Example: Influenza virus uses hemagglutinin to bind to respiratory epithelial cells.

Eukaryotic Microorganisms

Fungi and Protozoa

Eukaryotic microbes include fungi and protozoa, which can cause a variety of diseases in humans.

• Fungi: Aspergillus, Candida (yeasts, molds, lichens).

• Protozoa: Plasmodium (malaria), Giardia (giardiasis), Trypanosoma (sleeping sickness).

Life Cycles: Many protozoa have complex life cycles involving multiple hosts.

• Definitive Host: Host in which sexual reproduction occurs.

• Intermediate Host: Host in which asexual reproduction occurs.

• Vector: Organism that transmits the pathogen (e.g., mosquito for Plasmodium).

Example Table: Protozoan Life Cycles

Protozoan	Definitive Host	Intermediate Host	Vector	Treatment
Plasmodium	Human	Mosquito	Anopheles mosquito	Antimalarial drugs (e.g., chloroquine)
Trypanosoma	Human	Tsetse fly	Tsetse fly	Antitrypanosomal drugs (e.g., suramin)

Example: Plasmodium life cycle includes sporozoite, merozoite, and gametocyte stages; treatment involves antimalarial medications.

Mycoses

Fungal infections (mycoses) are classified by the depth of infection:

• Superficial mycoses: Affect skin, hair, nails (e.g., ringworm).

• Subcutaneous mycoses: Affect deeper skin layers.

• Systemic mycoses: Affect internal organs (e.g., histoplasmosis).

Example: Candida albicans causes thrush and vaginal yeast infections; treated with antifungal agents.

Additional info:

• Some terms and examples were expanded for clarity and completeness.

• Tables were inferred and constructed to aid comparison and classification.