Module 1 - Intro to Microorganisms and Module 2 - Microbial Growth Flashcards
What is the first postulate in Koch’s theory about pathogens in diseased animals?
The suspected pathogen must be present in all cases of the disease and absent from healthy animals. (Observed using microscopy and staining)
How is the second postulate tested in the lab?
The suspected pathogen must be grown in pure culture. This is done by streaking an agar plate with a sample from the diseased animal to form colonies of the suspected pathogen.
What does the third postulate state about pathogens and healthy animals?
Cells from a pure culture of the suspected pathogen must cause disease when introduced into a healthy animal.
How is the final postulate verified?
The suspected pathogen must be re-isolated from the newly diseased animal and shown to be the same as the original organism.
Who was Robert Hooke and what is his significance in microbiology?
Robert Hooke (1635-1703) was the first to describe microbes and illustrated the fruiting structures of molds, contributing to the early understanding of microorganisms.
What is Antoni van Leeuwenhoek known for in microbiology?
Antoni van Leeuwenhoek (1632-1723) was the first to describe bacteria, observing them in samples such as teeth scrapings and rainwater, providing the first glimpse into the microscopic world.
What are some of Louis Pasteur’s major contributions to microbiology?
- He showed that microbes are responsible for fermentation.
- Demonstrated that bacteria causing wine and food spoilage could be killed by heat without evaporating alcohol (pasteurization).
- Disproved the theory of spontaneous generation, leading to methods for controlling microorganism growth.
- Developed vaccines for diseases such as anthrax, fowl cholera, and rabies.
What are the main differences between prokaryotic and eukaryotic cells?
Prokaryotic Cells (e.g., bacteria):
- Lack a nucleus; DNA is located in a nucleoid region.
- No membrane-bound organelles.
- Have a cell wall, cytoplasmic membrane, ribosomes, and sometimes plasmids.
- Smaller in size (around 0.5 μm).
Eukaryotic Cells (e.g., animal and plant cells):
- Contain a nucleus surrounded by a nuclear membrane.
- Possess membrane-bound organelles like mitochondria, endoplasmic reticulum, Golgi apparatus, and sometimes chloroplasts.
- Larger in size (around 10 μm).
What are the common shapes of bacterial cells?
The common bacterial cell shapes are:
- Coccus (cocci)
- Rod (rods)
- Spirillum (spirilla)
- Spirochete
- Stalk
- Hypha
- Budding and appendaged
- Filamentous
What are the three fundamental characteristics of all bacterial cells?
Metabolism: Bacterial cells take up nutrients, transform them, and expel wastes. This includes:
Genetic functions (replication, transcription, translation)
Catalytic functions (energy production and biosyntheses)
Growth: Bacteria convert nutrients from the environment into new cell material, enabling cell division and reproduction.
Evolution: Bacterial cells evolve over time, displaying new properties. Phylogenetic trees can illustrate these evolutionary relationships and distinct species.
What are some additional characteristics that certain bacterial cells possess?
Differentiation: Some bacterial cells can form new structures, like spores, to survive harsh conditions.
Communication: Bacterial cells can communicate with each other using chemical messengers.
Motility: Some bacteria are capable of self-propulsion using structures like flagella.
Genetic Exchange: Bacterial cells can exchange genetic material through mechanisms like conjugation, where DNA is transferred from a donor to a recipient cell.
Why is the identification of pathogen cell surface antigens crucial for vaccine development?
Vaccines stimulate the immune system by exposing it to specific cell surface antigens unique to pathogens. This helps the immune system “learn” and recognize the pathogen. For instance, Streptococcus pneumoniae has over 90 serotypes, and vaccines like PrevenarTM13 are multivalent, targeting 13 different antigens to cover a wide range of strains while maximizing immune response.
What is the structure and main function of the bacterial cytoplasmic membrane?
The bacterial cytoplasmic membrane is a phospholipid bilayer containing embedded proteins that act as channels and receptors. In Bacteria and Eukarya, membrane lipids contain fatty acids linked to glycerol by ester bonds. In contrast, Archaea have membranes with ether-linked isoprene chains instead of fatty acids, making their membranes more stable under extreme conditions. This structure is crucial for selective permeability, allowing the controlled movement of molecules in and out of the cell without contributing to rigidity.
What is the structure of peptidoglycan, and why is it important for bacterial cell walls?
Peptidoglycan is composed of repeating disaccharide units of N-acetylglucosamine (G) and N-acetylmuramic acid (M) linked by β-1,4-glycosidic bonds. This rigid, mesh-like structure provides mechanical strength and prevents cell lysis under osmotic pressure. In Gram-positive bacteria, peptidoglycan can make up to 90% of the cell wall and is thicker than in Gram-negative bacteria. The cross-linking of peptidoglycan chains with peptide bonds strengthens the cell wall in both the X and Y directions.
How do Gram-positive and Gram-negative bacterial cell walls differ in structure and function?
Gram-Positive: The cell wall is thick, with up to 90% peptidoglycan, which provides rigidity and shape. Teichoic acids (acidic glycerol phosphate or ribitol phosphate polymers) are embedded, providing structural support and contributing to surface charge. Lipoteichoic acids anchor the wall to the cell membrane.
Gram-Negative: The cell wall has a thinner peptidoglycan layer (around 10%) located between two membranes. The outer membrane includes lipopolysaccharides (LPS), which contain Lipid A (an endotoxin) and the O-polysaccharide. The outer membrane serves as an additional barrier, contributing to antibiotic resistance and immune evasion.
What is the Gram stain, and how does it differentiate between Gram-positive and Gram-negative bacteria?
Steps:
1. Crystal violet stain is applied, staining all cells purple.
2. Iodine is used as a mordant, binding the dye to peptidoglycan.
3. Alcohol or acetone is applied to decolorize; Gram-negative cells lose the purple stain due to the thin peptidoglycan layer, while Gram-positive cells retain it.
4. Safranin is added as a counterstain, making Gram-negative cells appear pink and Gram-positive cells remain purple.
Principle: Gram-positive cells retain the crystal violet stain due to their thick peptidoglycan layer, while Gram-negative cells’ thin peptidoglycan layer and outer membrane result in a loss of the dye during decolorization.
What makes acid-fast bacteria unique, and how does the Ziehl-Neelson stain identify them?
Acid-fast bacteria, like Mycobacterium species, contain mycolic acids, which are waxy lipids making up about 60% of the cell wall. This composition prevents the uptake of Gram stain. The Ziehl-Neelson stain uses heat to penetrate the waxy layer, coloring acid-fast bacteria red, while other cells counterstain blue. These bacteria are resistant to antibiotics and immune responses due to the impermeable cell wall.
How do Archaeal cell walls differ from those of Bacteria, and what are the functions of the S layer and pseudomurein?
Archaeal cell walls lack peptidoglycan and sometimes have an S layer, a paracrystalline protein or glycoprotein structure that provides structural integrity and acts as a molecular sieve. Some methanogenic Archaea possess pseudomurein, a peptidoglycan-like polymer composed of N-acetylglucosamine and N-acetylalosaminuronic acid. This unique structure stabilizes Archaea in extreme environments.
What are bacterial capsules, and what functions do they serve?
Capsules are polysaccharide layers surrounding some bacterial cells, aiding in adherence to surfaces, biofilm formation, and immune evasion by preventing phagocytosis. They also protect bacteria from desiccation. Capsules are found in both Gram-positive and Gram-negative bacteria, such as Streptococcus pneumoniae, Staphylococcus epidermidis, and Pseudomonas aeruginosa. S. pneumoniae’s capsule, for instance, helps it avoid immune detection, making it a critical factor in its pathogenicity.
How does the structure of cell walls and capsules contribute to bacterial survival?
Cell walls provide shape, rigidity, and protection against osmotic stress. Capsules enable attachment, biofilm formation, and immune evasion. In Gram-positive bacteria, thick peptidoglycan provides rigidity, while Gram-negative bacteria have an outer LPS membrane for additional defense and antibiotic resistance.
What molecular differences distinguish Gram-positive and Gram-negative cell walls?
Gram-positive bacteria have a thick peptidoglycan layer and teichoic acids for stability. Gram-negative bacteria have a thin peptidoglycan layer, an outer membrane with LPS (containing endotoxin Lipid A), and a periplasmic space. This structure aids in immune evasion and enhances resistance to external agents.
How do bacterial, archaeal, and eukaryotic cell walls differ at the molecular level?
Bacteria have peptidoglycan-based walls, which differ in thickness between Gram-positive and Gram-negative types. Archaea lack peptidoglycan, sometimes having an S layer or pseudomurein instead. Eukaryotic cells generally lack cell walls (except for plants and fungi), which instead have cellulose or chitin, respectively.
What properties define microbial populations growing in batch culture?
Microbial growth in batch culture proceeds through distinct growth phases, each with characteristic properties: lag phase (adaptation), exponential phase (active division), stationary phase (nutrient limitation), and death phase (decline due to waste accumulation).
How is population growth calculated during the exponential phase?
Growth during this phase follows the formula N=N0×2n, where N is the final cell count, N0 is the initial cell count, and n is the number of generations. This phase is characterized by a constant specific growth rate.
Why is gene expression change crucial for bacterial adaptation in the stationary phase?
Bacteria in stationary phase often activate genes for survival mechanisms, such as stress response proteins, to adapt to nutrient scarcity, build-up of waste, and overall starvation conditions.
What are the main methods to measure bacterial growth?
- Total cell count (microscopy or flow cytometry).
- Viable count (plate counts).
- Turbidimetric measurement (optical density). Each method offers insights into cell density and viability.
Calculate the number of generations (n), generation time (g), and specific growth rate (k) for a cell population that g rows from 2×10^5 cells/mL to 3.2×10^6cells/mL in 8 hours.
