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

Characterizing and Classifying Prokaryotes

Asexual Reproduction in Prokaryotes

Prokaryotes primarily reproduce asexually, ensuring rapid population growth and genetic consistency. The two main methods are:

• Binary Fission: The most common form of asexual reproduction in prokaryotes. The cell duplicates its DNA, elongates, and divides into two genetically identical daughter cells.

• Budding: A process where a new organism develops from an outgrowth or bud due to cell division at one particular site. The bud eventually detaches to form a new organism.

Archaea vs. Bacteria: General Differences

• Cell Wall Composition: 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 processes are more similar to eukaryotes than bacteria.

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

Extremophiles

Extremophiles are organisms that thrive in extreme environmental conditions that are detrimental to most life forms.

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

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

• Acidophiles: Grow in acidic environments.

• Alkaliphiles: Prefer basic (alkaline) environments.

Notable Bacteria

• Mycobacterium: Known for their waxy cell walls containing mycolic acid, making them resistant to desiccation and antibiotics. Includes pathogens like M. tuberculosis.

• Cyanobacteria: Photosynthetic bacteria that produce oxygen; important for Earth's early atmosphere and as primary producers in aquatic environments.

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

• Alphaproteobacteria (Nitrifying Bacteria): Involved in nitrogen cycling, converting ammonia to nitrite and nitrite to nitrate, essential for soil fertility.

Example: Mycobacterium tuberculosis is notable for causing tuberculosis and its resistance to many antibiotics due to its unique cell wall.

Characterizing and Classifying Eukaryotes

Mitosis and Meiosis

• Haploid (n): Cells with one set of chromosomes (e.g., gametes).

• Diploid (2n): Cells with two sets of chromosomes (e.g., somatic cells).

• Ploidy Number: Refers to the number of sets of chromosomes in a cell.

• Mitosis: Produces two genetically identical diploid cells; used for growth and repair (asexual/somatic).

• Meiosis: Produces four genetically unique haploid cells; used for sexual reproduction (gametes). Ploidy number halves during meiosis.

Alternation of Generations in Algae

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

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 simultaneously (common in some protozoa).

Protozoans and Their Morphology

• Protozoans: Unicellular eukaryotes, often motile, lacking cell walls.

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

Prokaryotes vs. Eukaryotes: General Differences

• Prokaryotes: No nucleus, no membrane-bound organelles, usually smaller, single circular chromosome.

• Eukaryotes: True nucleus, membrane-bound organelles, larger, multiple linear chromosomes.

Characterizing and Classifying Viruses, Viroids, and Prions

Key Characteristics of Viruses

• Non-living: Viruses lack cellular structure, metabolism, and cannot reproduce independently.

• Obligate Intracellular Parasites: Require host cells to replicate.

Virus Morphology

• Capsid: Protein shell enclosing the viral genome.

• Nucleic Acid: Can be DNA or RNA, single- or double-stranded.

• Envelope: Some viruses have a lipid envelope derived from the host cell membrane; envelope proteins determine host specificity.

Viral Genomes

• Can be +ssDNA, -ssDNA, +ssRNA, -ssRNA, dsDNA, or dsRNA.

• +ssRNA and dsDNA genomes are most similar to host cell genetic material.

• DNA processing occurs in the nucleus; RNA processing typically occurs in the cytoplasm of eukaryotic cells.

Viroids and Prions

• Viroid: Infectious RNA molecules lacking a protein coat; cause diseases in plants.

• Prion: Infectious proteins that cause neurodegenerative diseases by inducing abnormal folding of normal proteins (e.g., Creutzfeldt-Jakob disease).

Lytic vs. Lysogenic Cycle

• Lytic Cycle Steps (in order):

  1. Attachment

  2. Entry

  3. Synthesis

  4. Assembly

  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 disrupt normal cell cycle regulation, potentially leading to cancer (oncogenic viruses).

Virus Culturing in the Laboratory

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

Laboratory Techniques in Microbiology

Blood Agar and Hemolysis

• Blood Agar: Used to test for hemolysis, the breakdown of red blood cells by bacterial enzymes.

• Types of Hemolysis:

  • Alpha (α) Hemolysis: Partial hemolysis, greenish discoloration around colonies.

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

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

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

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 separate from chromosomal DNA, often carrying beneficial genes.

• Genes on pGLO Plasmid:

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

  • Arabinose Operon: Arabinose sugar induces 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 gene).

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

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

ELISA (Enzyme-Linked Immunosorbent Assay)

• Purpose: Used to detect the presence of antibodies or antigens in a sample.

• Acronym: Enzyme-Linked Immunosorbent Assay.

Additional info: ELISA is a fundamental immunological technique, but detailed procedures are not required for this exam.