Microbiology: Experimental Design, Nutrition, Structure, and Metabolism Study Guide

Scientific Practices & Experimental Design in Microbiology

1. Scientific Method in Microbiology

The scientific method is a systematic approach used to investigate phenomena, acquire new knowledge, or correct and integrate previous knowledge in microbiology.

• Steps: Observation → Hypothesis → Experiment → Data Analysis → Conclusion → Communication

• Hypothesis: A testable explanation that predicts results.

• Experiment: Manipulates variables to test hypotheses.

2. Variables in Microbiology Experiments

Understanding variables is essential for designing controlled experiments in microbiology.

• Manipulated variable (Independent): Changed by the researcher.

• Response variable (Dependent): Measured outcome.

• Controlled variables: Kept constant to ensure fair test.

• Positive control: Should produce expected result.

• Negative control: Should produce no result; checks for contamination or error.

3. Growth Curve Phases

Bacterial batch culture growth has four distinct phases, each reflecting changes in cell number and physiology.

• Lag phase: Adaptation; no division yet.

• Exponential phase (log): Rapid growth; balanced metabolism.

• Stationary phase: Nutrients used up; growth rate = death rate; no net change in population.

• Death phase: Cells die; viable count decreases; turbidity may stay high because dead cells remain.

4. Assays & Measurements

Microbiologists use various assays to quantify and characterize microbial growth and activity.

• Viable count methods: Only live cells (e.g., spread plate, pour plate).

• Turbidity (optical density): Measures cloudiness → live + dead cells.

• Disk diffusion (Kirby-Bauer): Tests antimicrobial susceptibility; larger clear zone = more effective drug.

• Growth curves: Measure population changes over time.

5. Experimental Design

Proper experimental design is crucial for obtaining valid and reliable results in microbiology.

• Identify variables, choose correct assay, interpret data.

• Use Claim-Evidence-Reasoning (CER):

  • Claim: Answer to question

  • Evidence: Data supporting claim

  • Reasoning: Scientific principles explaining evidence

Microbial Growth – Nutrition & Cultivation

1. Nutrients & Media

Microbes require specific nutrients for growth, which are provided by culture media.

• Macronutrients: C, N, P, S, O, H — needed in large amounts.

• Micronutrients (trace elements): Metals for enzyme cofactors (e.g., Fe, Zn).

2. Culture Media

Culture media are formulated to support microbial growth and can be classified based on their composition and purpose.

• Defined media: Exact chemical composition known.

• Complex media: Unknown composition (e.g., yeast extract).

• Rich vs minimal media: Rich = many nutrients; minimal = only essentials.

• Selective media: Suppresses unwanted microbes; allows target growth.

• Differential media: Distinguishes bacteria by metabolic traits (e.g., pH indicator).

3. Sterilization

Sterilization ensures that media and equipment are free of living organisms before use.

• Autoclave: Steam + pressure → kills all organisms, spores.

• Filtration: Removes microbes by size exclusion (heat-sensitive liquids).

• Radiation: UV or ionizing → damages DNA.

• Chemical sterilants: e.g., ethylene oxide for plastics.

4. Pure Cultures & Isolation

Isolation techniques allow microbiologists to obtain pure cultures for study.

• Streak plate method: Dilute cells on agar surface → isolated colonies.

• Agar dilution tube: Mixed into molten agar → colonies in agar depth.

• Enrichment culture: Favors growth of target species by special conditions.

Structure & Function of Bacteria

1. Cell Morphologies

Bacteria exhibit diverse shapes and arrangements, which aid in identification.

• Coccus: Sphere

• Bacillus (rod)

• Spirillum/spirochete: Spiral

• Vibrio: Comma-shaped

• Filamentous: Arrangements like diplo-, strepto-, staphylo-

2. Gram Staining

Gram staining differentiates bacteria based on cell wall structure.

• Steps: Crystal violet → iodine (mordant) → decolorizer → Safranin (counterstain)

• Gram-positive: Thick peptidoglycan → purple

• Gram-negative: Thin peptidoglycan + outer membrane → pink

3. Cell Wall

The bacterial cell wall provides structural support and protection.

• Peptidoglycan: Sugar chain cross-linked by peptides → rigid structure

• Gram-positive: Teichoic acids for stability & charge

• Gram-negative: Outer membrane with LPS (lipopolysaccharide), porins, periplasm

4. Internal Structures

Bacteria possess specialized internal structures for storage and survival.

• Nucleoid: Compact DNA; supercoiling & proteins help fit chromosome in small cell

• Storage polymers:

  • Glycogen: Carbon/energy storage

  • Polyphosphate: Phosphate storage

  • Sulfur granules: Energy from oxidation

• Gas vesicles: Buoyancy in aquatic bacteria

• Endospores: Resistant structures → survive heat, drying, chemicals

5. Surface Structures

Surface structures aid in protection, attachment, and motility.

• Capsules & slime layers: Protection, attachment, biofilms

• Fimbriae & pili: Attachment; pili also for conjugation

• Flagella: Motility powered by proton motive force (PMF)

  • Arrangements: monotrichous, lophotrichous, amphitrichous, peritrichous

6. Cell Division

Bacteria reproduce primarily by binary fission, a process involving cell elongation and division.

• Binary fission: Cell elongates → FtsZ ring forms → septum → daughter cells

• Merodiploid control: Regulates chromosome copy number

• Peptidoglycan synthesis:

  • Bactoprenol: Transports precursors

  • Transglycosylase: Links sugars

  • Transpeptidase: Cross-links peptides

• Autolysin: Cuts existing wall for growth

Metabolism – Bioenergetics, Fermentation, and Respiration

1. Bioenergetic Basis

Metabolism encompasses all biochemical reactions in the cell, divided into catabolism and anabolism.

• Catabolism: Breakdown of molecules → releases energy, makes ATP

• Anabolism: Building macromolecules → uses energy, consumes ATP

• ATP (adenosine triphosphate):

  • Energy stored in phosphoanhydride bonds (between phosphates)

  • Hydrolysis of ATP → ADP + Pi releases energy for cellular work

  • Energy coupling: Catabolism produces ATP → ATP powers anabolism & cellular processes

2. Electron Carriers & Redox Reactions

Electron carriers and redox reactions are central to energy metabolism in microbes.

• Redox reaction: Oxidation + reduction

• Oxidation: Loss of electrons/hydrogen (becomes more positive)

• Reduction: Gain of electrons/hydrogen (becomes more negative)

• Electron carriers: NAD+/NADH, FAD/FADH2, quinones, cytochromes, Fe-S proteins

• Redox tower: Compounds arranged from most negative (best donors) → most positive (best acceptors)

Equation:

$\text{Redox reaction:} \, \text{A}_{\text{red}} + \text{B}_{\text{ox}} \rightarrow \text{A}_{\text{ox}} + \text{B}_{\text{red}}$

3. Glycolysis

Glycolysis is a central metabolic pathway converting glucose to pyruvate.

• Definition: Metabolic pathway converting glucose → 2 pyruvate

• Key outcomes:

  • 2 ATP (net) via substrate-level phosphorylation (direct transfer of phosphate to ADP)

4. Fermentation

Fermentation is an anaerobic process that generates ATP and regenerates electron carriers.

• Occurs when no external electron acceptor (like O2) available.

• Products: lactate, ethanol, or other organic acids/alcohols (depending on microbe).

• Energy yield is low — only 2 ATP/glucose.

• Electron transport chain: only substrate-level phosphorylation.

5. Respiration

Respiration uses an external electron acceptor for more energy.

• Aerobic respiration: O2 = terminal e- acceptor → highest energy yield (≈38 ATP/glucose)

• Anaerobic respiration: NO3-, SO42-, or others = terminal acceptor → less ATP

• ATP synthesis via oxidative phosphorylation: ETC → proton gradient (PMF) → ATP synthase → ATP

6. Electron Transport Chain (ETC)

The ETC is located in the bacterial plasma membrane or mitochondrial inner membrane and is essential for ATP synthesis.

• NADH dehydrogenase: First complex, takes e- from NADH

• Cytochromes: Iron-containing proteins transfer e- in the membrane

• Quinones: Lipid-soluble carriers; transfer e- and H+

• Proton Motive Force (PMF): H+ pumped out → electrochemical gradient → drives ATP synthesis, transport, and flagellar rotation

7. Energy Yield Comparisons

Energy yield depends on the electron acceptor and pathway used.

Pathway	Terminal e- Acceptor	ATP Yield	Oxygen?
Aerobic Respiration	O2	Highest (≈38)	Yes
Anaerobic Respiration	NO3-, SO42-, etc.	Moderate	No
Fermentation	None (internal)	Lowest (2 ATP)	No

8. Environmental Effects & Predictions

Environmental conditions affect microbial energy metabolism and growth.

• No oxygen → cells use fermentation or anaerobic respiration → less energy → slower growth

• Block ETC component → reduces PMF → less ATP → slower cell growth

9. Experimental Applications

Experimental approaches can be used to study microbial metabolism and energy yield.

• Growth assays measure ATP yield under different conditions: O2 vs NO3- vs no external acceptor

• Redox tower + experimental data → predict which pathway a bacterium is using

• Can design experiments to test energy metabolism by measuring growth rates, ATP levels, or fermentation products