Microbiology Study Guide: Prokaryotes, Eukaryotes, Viruses, and Laboratory Techniques

Chapter 11: Prokaryotes – Reproduction, Diversity, and Extremophiles

Asexual Reproduction in Prokaryotes

Prokaryotes primarily reproduce asexually, ensuring rapid population growth and genetic consistency.

• Binary Fission: The most common method. A single cell divides into two identical daughter cells. The process involves DNA replication, elongation of the cell, and division by septum formation.

• Budding: A less common method where a new organism grows from a bud due to cell division at one particular site.

• Example: Caulobacter crescentus reproduces by budding.

Archaea vs. Bacteria: General Differences

Archaea and bacteria are both prokaryotes but differ in several fundamental ways:

• Cell Wall: Bacteria have peptidoglycan in their cell walls; archaea do not.

• Membrane Lipids: Archaea have ether-linked lipids; bacteria have ester-linked lipids.

• Genetic Machinery: Archaea's transcription and translation machinery are more similar to eukaryotes.

• Habitat: Many archaea are extremophiles, while bacteria are found in a wider range of environments.

Extremophiles

Extremophiles are organisms that thrive in extreme environmental conditions.

• Thermophiles: Prefer high temperatures (e.g., hot springs).

• Halophiles: Thrive in high salt concentrations (e.g., salt lakes).

• Acidophiles: Grow in acidic environments (low pH).

• Alkaliphiles: Prefer basic (high pH) environments.

• Example: Halobacterium (an archaeon) is a halophile.

Notable Bacteria

• Mycobacterium: Known for a waxy cell wall rich in mycolic acids; includes pathogens like M. tuberculosis.

• Cyanobacteria: Photosynthetic bacteria, sometimes called "blue-green algae"; important for oxygen production and nitrogen fixation.

• Clostridium and Bacillus: Both genera form endospores, allowing survival in harsh conditions. Clostridium includes obligate anaerobes; Bacillus includes aerobes and facultative anaerobes.

• Alphaproteobacteria (Nitrifying Bacteria): Important in the nitrogen cycle; convert ammonia to nitrite and nitrite to nitrate.

Chapter 12: Eukaryotes – Cell Division, Life Cycles, and Diversity

Mitosis and Meiosis

Eukaryotic cells divide by mitosis or meiosis, processes essential for growth, repair, and reproduction.

• Mitosis: Produces two genetically identical diploid (2n) somatic cells. Ploidy number remains the same.

• Meiosis: Produces four genetically unique haploid (n) gametes. Ploidy number is halved.

• Definitions:

  • Haploid (n): One set of chromosomes.

  • Diploid (2n): Two sets of chromosomes.

  • Ploidy Number: The number of sets of chromosomes in a cell.

Alternation of Generations in Algae

Many algae exhibit alternation of generations, alternating between multicellular haploid (gametophyte) and diploid (sporophyte) stages.

• Example: In Ulva (sea lettuce), both stages are multicellular and morphologically similar.

Binary Fission vs. Multiple Fission (Schizogony)

• Binary Fission: One cell divides into two.

• Multiple Fission (Schizogony): The nucleus divides several times before the cell splits into many daughter cells. Common in some protozoa (e.g., Plasmodium).

Protozoans and Their Morphology

Protozoans are unicellular eukaryotes with diverse morphologies and life cycles.

• Key Features: Lack cell walls, are motile (via cilia, flagella, or pseudopodia), and are heterotrophic.

• Alveolates: A group of protozoans characterized by membrane-bound sacs (alveoli) beneath the plasma membrane. Includes ciliates, dinoflagellates, and apicomplexans.

General Differences: Prokaryotes vs. Eukaryotes

Feature	Prokaryotes	Eukaryotes
Nucleus	No	Yes
Membrane-bound Organelles	No	Yes
Cell Wall	Peptidoglycan (bacteria)	Cellulose (plants), chitin (fungi), or absent
Size	Small (1-10 µm)	Larger (10-100 µm)

Chapter 13: Viruses, Viroids, and Prions

Key Characteristics of Viruses

Viruses are acellular infectious agents that require host cells for replication.

• Not Living: Cannot carry out metabolism, do not grow, and cannot reproduce independently.

• Structure: Consist of a nucleic acid genome (DNA or RNA), a protein capsid, and sometimes a lipid envelope.

Virus Morphology

• Capsid: Protein shell that encases the viral genome.

• Envelope: Lipid membrane derived from the host cell; contains viral proteins that determine host specificity.

• Genome Types: Can be single-stranded or double-stranded DNA or RNA (+/-ssDNA, +/-ssRNA, dsDNA, dsRNA).

• Host Cell Specificity: Determined by proteins on the viral envelope or capsid.

Viral Genome Processing

• +ssRNA and dsDNA: Most similar to host cell processes; +ssRNA can be directly translated, dsDNA can be transcribed like host DNA.

• Genome Replication Sites: DNA viruses often replicate in the nucleus; RNA viruses usually replicate in the cytoplasm.

Viroids and Prions

• Viroid: Infectious RNA molecules without a protein coat; cause plant diseases.

• Prion: Infectious proteins that cause neurodegenerative diseases (e.g., Creutzfeldt-Jakob disease).

Lytic vs. Lysogenic Cycle

• Lytic Cycle Steps (in order):

  1. Attachment

  2. Penetration

  3. Biosynthesis

  4. Maturation

  5. Release

  Each step involves specific interactions between the virus and host cell, leading to the production and release of new virions.

• Lysogenic Cycle: Viral genome integrates into the host genome, becoming a prophage and remaining latent until induced to enter the lytic cycle.

Viruses and Cancer

• Some viruses can cause cancer by integrating their genome into host DNA, disrupting normal gene regulation (e.g., human papillomavirus and cervical cancer).

Virus Culturing in the Laboratory

• Viruses are cultured in living cells, such as bacterial cultures (bacteriophages), animal cell lines, or embryonated eggs.

Laboratory Study Guide: Microbiology Techniques

Blood Agar and Hemolysis

• Purpose: To test for hemolysis, the breakdown of red blood cells by bacterial enzymes.

• Types of Hemolysis:

  • Alpha (α): Partial hemolysis; greenish discoloration.

  • Beta (β): Complete hemolysis; clear zone around colonies.

  • Gamma (γ): No hemolysis; no change in the medium.

• Streak-Stab Technique: Used to test for anaerobic hemolysis by creating an anaerobic environment beneath the agar surface.

Catalase and Oxidase Tests

• Catalase Test: Detects the enzyme catalase, which breaks down hydrogen peroxide into water and oxygen. Positive result: bubbling upon addition of hydrogen peroxide.

• Oxidase Test: Detects cytochrome c oxidase using test strips. Positive result: color change (usually purple or blue).

pGLO Plasmid Experiment

• Plasmids: Small, circular DNA molecules in bacteria, often carrying beneficial genes.

• pGLO Genes:

  • GFP: Green fluorescent protein gene, glows under UV light when expressed.

  • Arabinose Operon: Arabinose sugar turns on GFP expression.

  • Ampicillin Resistance: Allows growth in the presence of ampicillin.

• Plate Results:

  • LB (no plasmid): Growth, no fluorescence.

  • LB/AMP (no plasmid): No growth (no resistance).

  • LB/AMP (+pGLO): Growth (ampicillin resistance), no fluorescence (no arabinose).

  • LB/AMP/ARA (+pGLO): Growth and fluorescence (arabinose induces GFP).

ELISA

• Purpose: Enzyme-Linked Immunosorbent Assay (ELISA) is used to detect the presence of antigens or antibodies in a sample.

• Principle: Uses enzyme-linked antibodies and a color change to indicate a positive result.