Microbiology Exam 1 Study Guide: Foundations, Cell Structure, and Metabolism

Chapter 1: The Microbial World

Louis Pasteur and the Disproof of Spontaneous Generation

• Spontaneous generation was the belief that living organisms could arise from nonliving matter.

• Louis Pasteur disproved this theory through his famous swan-neck flask experiment:

  • He boiled broth in flasks with long, curved necks, which allowed air in but prevented the entry of airborne microorganisms.

  • No microbial growth occurred until the neck was broken, demonstrating that microbes come from other microbes, not spontaneously.

• Importance of Sterility: Pasteur's work established the necessity of sterile techniques in microbiology to prevent contamination by unwanted microorganisms.

• Example: Modern autoclaving and aseptic techniques in laboratories are based on these principles.

Koch’s Postulates

• Koch’s postulates are a set of criteria to establish a causative relationship between a microbe and a disease:

  1. The microorganism must be found in all organisms suffering from the disease, but not in healthy organisms.

  2. The microorganism must be isolated from a diseased organism and grown in pure culture.

  3. The cultured microorganism should cause disease when introduced into a healthy organism.

  4. The microorganism must be re-isolated from the inoculated, diseased experimental host and identified as being identical to the original specific causative agent.

• Limitations: Not all diseases fit these criteria (e.g., viruses require host cells to grow, some pathogens only cause disease in humans, and some diseases are caused by multiple organisms).

Genetic Sequencing in Microbiology

• Genetic sequencing allows scientists to determine the order of nucleotides in DNA or RNA.

• Importance:

  • Enables identification and classification of microorganisms, including those that cannot be cultured.

  • Helps track outbreaks, study microbial evolution, and understand gene function.

• Example: 16S rRNA gene sequencing is widely used for bacterial identification.

Chapter 2: Microbial Cell Structure and Function

Bacterial Morphologies and Their Advantages

• Bacteria exhibit various morphologies (shapes):

  • Coccus (spherical)

  • Bacillus (rod-shaped)

  • Spirillum (spiral)

  • Other forms: vibrio, filamentous, spirochete

• Advantages of Shape:

  • Shape can affect motility (e.g., spiral forms move efficiently in viscous environments).

  • Surface area-to-volume ratio influences nutrient uptake and growth rate.

  • Some shapes help evade predation or adhere to surfaces.

Gram-Positive vs. Gram-Negative Bacterial Cell Envelopes

• Bacteria are classified by the Gram stain into two main groups based on cell envelope structure:

Feature	Gram-Positive	Gram-Negative
Peptidoglycan Layer	Thick (multiple layers)	Thin (single layer)
Teichoic Acids	Present	Absent
Outer Membrane	Absent	Present (contains LPS)
Lipopolysaccharide (LPS)	Absent	Present
Membrane Proteins	Present (cytoplasmic membrane)	Present (cytoplasmic and outer membranes)

• Peptidoglycan: A mesh-like polymer of sugars and amino acids providing structural support.

• Teichoic acids: Found in Gram-positive cell walls; contribute to rigidity and cell wall maintenance.

• Lipopolysaccharide (LPS): Found in the outer membrane of Gram-negative bacteria; important for immune recognition and barrier function.

Intracellular and Extracellular Bacterial Structures

• Capsule & Slime Layer: Polysaccharide layers outside the cell wall; protect against desiccation and phagocytosis, aid in attachment.

• Fimbriae & Pili: Hair-like appendages for attachment (fimbriae) and genetic exchange (pili).

• Inclusions: Storage granules for nutrients (e.g., polyphosphate, sulfur, glycogen).

• Gas Vesicles: Protein-bound structures that provide buoyancy in aquatic bacteria.

• Flagella: Long, whip-like structures for motility; powered by a rotary motor.

• Gliding Proteins: Enable movement along surfaces in some bacteria without flagella.

Endospore Formation and Its Importance

• Endospores are highly resistant, dormant structures formed by some Gram-positive bacteria (e.g., Bacillus, Clostridium).

• Process: Involves asymmetric cell division, engulfment, cortex and coat formation, and maturation.

• Importance: Endospores allow survival in extreme conditions (heat, desiccation, chemicals, radiation).

• Example: Endospores can remain viable for decades and are a concern in food safety and sterilization.

Chapter 3: Microbial Metabolism

Nutrients, Cofactors, and Transport Mechanisms

• Bacteria require nutrients (carbon, nitrogen, phosphorus, etc.) and cofactors (e.g., metal ions, vitamins) for growth and metabolism.

• Transport Mechanisms:

  • Simple Transport: Driven by the proton motive force; includes symporters (move two substances in the same direction) and antiporters (opposite directions).

  • Group Translocation: Substance is chemically modified during transport (e.g., phosphotransferase system).

  • ABC Transporters: ATP-binding cassette transporters use ATP hydrolysis to transport substances; involve periplasmic binding proteins and membrane-spanning proteins.

Energy Classes of Microorganisms

• Microorganisms are classified based on their energy, electron, and carbon sources:

Type	Energy Source	Electron Source	Carbon Source
Phototroph	Light	Varies	CO2 (autotroph) or organic (heterotroph)
Chemotroph	Chemicals	Organic (organotroph) or inorganic (lithotroph)	CO2 or organic

• Example: Escherichia coli is a chemoorganoheterotroph (uses organic compounds for energy, electrons, and carbon).

Redox Reactions and Electron Carriers

• Redox reactions involve the transfer of electrons from a donor to an acceptor.

• Electron donors are oxidized; electron acceptors are reduced.

• Electron carriers: Molecules like NAD+ and FAD accept and donate electrons during metabolism.

• Importance: Redox reactions drive energy generation in cells.

• Key Equations:

  • Reduction of NAD+: $\mathrm{NAD}^+ + 2\,\mathrm{H}^+ + 2\,e^- \rightarrow \mathrm{NADH} + \mathrm{H}^+$

  • Reduction of FAD: $\mathrm{FAD} + 2\,\mathrm{H}^+ + 2\,e^- \rightarrow \mathrm{FADH}_2$

Respiration and Fermentation Pathways

• Respiration: Involves glycolysis, the citric acid cycle, and the electron transport chain (ETC) to generate ATP.

• Fermentation: Anaerobic process where organic molecules serve as both electron donors and acceptors; less ATP produced.

• Key Pathways:

  • Glycolysis: Glucose → 2 pyruvate + 2 ATP + 2 NADH

  • Citric Acid Cycle: Oxidizes acetyl-CoA to CO2, generating NADH and FADH2

  • Electron Transport Chain & ATP Synthase: Uses NADH/FADH2 to generate a proton gradient, driving ATP synthesis. $\mathrm{ADP} + \mathrm{P}_i \rightarrow \mathrm{ATP}$

  • Alternative Fermentation Pathways: Produce various end products (e.g., lactic acid, ethanol, butyrate) depending on the organism.

• Electron Carrier Recycling: NADH produced in glycolysis must be re-oxidized to NAD+ for glycolysis to continue; this occurs via the ETC in respiration or by reducing fermentation products.

Alternate Energy Conservation Strategies

• Bacteria exhibit metabolic diversity by utilizing different substrates and pathways for energy conservation, depending on their environment.

• Examples:

  • Some bacteria use inorganic compounds (e.g., H2, Fe2+, S0) as energy sources (chemolithotrophy).

  • Others perform photosynthesis (phototrophy) or anaerobic respiration using alternative electron acceptors (e.g., nitrate, sulfate).

• This diversity allows bacteria to colonize a wide range of ecological niches.