Microbial Growth and Metabolism: Study Guide (Chapters 7-8)

Microbial Growth and Cultivation

Selective and Differential Media

Microbes can be cultivated on various types of media, each designed for specific purposes in microbiology.

• Selective media: Contains ingredients that inhibit the growth of certain microbes while allowing others to grow. Example: MacConkey agar selects for Gram-negative bacteria.

• Differential media: Allows multiple microbes to grow but contains indicators that reveal differences between species. Example: Blood agar differentiates bacteria based on hemolysis patterns.

• Complex media: Contains extracts and digests of yeasts, meat, or plants; exact composition is not known.

• Chemically defined media: Exact chemical composition is known; useful for studying metabolic requirements.

Applications: Selective and differential media are essential for isolating and identifying microbes in clinical and research settings.

Isolation Techniques

Isolation techniques are used to obtain pure cultures of microorganisms.

• Streak plate method: Involves spreading a sample over the surface of an agar plate to separate individual cells.

• Pour plate method: Diluted samples are mixed with molten agar and poured into plates.

• Spread plate method: A diluted sample is spread evenly across the surface of an agar plate.

Classification of Microorganisms by Growth Conditions

Microbes are classified based on their optimal growth conditions:

• Temperature:

  • Psychrophiles: Grow best at 0–20°C

  • Mesophiles: Grow best at 20–45°C (includes most human pathogens)

  • Thermophiles: Grow best at 45–80°C

  • Hyperthermophiles: Grow above 80°C

• Oxygen requirements:

  • Obligate aerobes: Require oxygen

  • Obligate anaerobes: Killed by oxygen

  • Facultative anaerobes: Grow with or without oxygen

  • Microaerophiles: Require low oxygen levels

  • Aerotolerant anaerobes: Tolerate oxygen but do not use it

• pH:

  • Acidophiles: Grow best at low pH

  • Neutrophiles: Grow best at neutral pH (6.5–7.5)

  • Alkaliphiles: Grow best at high pH

Microbial Nutrient and Energy Sources

• Phototrophs: Obtain energy from light.

• Chemotrophs: Obtain energy from chemical compounds.

• Autotrophs: Use CO2 as a carbon source.

• Heterotrophs: Use organic compounds as a carbon source.

• Fastidious microbes: Require specific nutrients and complex growth factors; difficult to culture.

Bacterial Growth Curve and Binary Fission

Bacterial Growth Curve Phases

Bacterial populations grow in a predictable pattern when cultured in a closed system:

• Lag phase: Cells adjust to environment; little to no cell division.

• Log (exponential) phase: Rapid cell division; population doubles at a constant rate.

• Stationary phase: Growth rate slows; nutrients deplete and waste accumulates; cell death equals cell division.

• Death phase: Cells die at an exponential rate due to lack of nutrients and toxic waste buildup.

Applications: Growth curves are used in industrial microbiology to optimize production and in clinical microbiology to assess infection dynamics.

Binary Fission

Bacteria reproduce by binary fission, a process where one cell divides into two identical daughter cells.

• Steps: DNA replication → cell elongation → septum formation → cell separation.

Generation Time Calculations

Generation time is the time required for a bacterial population to double.

• Formula for number of generations (n): $n = \frac{\log N_t - \log N_0}{\log 2}$

• Formula for generation time (g): $g = \frac{t}{n}$

• Where $N_t$ = final cell number, $N_0$ = initial cell number, $t$ = total time.

Microbial Media and Sample Collection

Types of Media

• Complex media: Contains nutrient-rich substances; composition varies.

• Chemically defined media: Exact chemical composition is known.

• Selective media: Inhibits unwanted microbes, supports desired ones.

• Differential media: Distinguishes microbes based on metabolic traits.

Clinical Sample Collection

• Use sterile containers and aseptic techniques.

• Collect samples before antibiotic therapy begins.

• Label samples accurately and transport promptly to the lab.

Enumeration of Microbes

Direct Methods

• Microscopic count: Counting cells under a microscope using a counting chamber.

• Viable plate count: Counting colonies formed on agar plates after serial dilution.

Indirect Methods

• Turbidity measurement: Using a spectrophotometer to estimate cell density based on cloudiness.

• Metabolic activity: Measuring products like CO2 or ATP.

Microbial Control: Definitions and Methods

Key Terms

• Decontamination: Reducing microbial load to safe levels.

• Sterilization: Destroying all forms of microbial life, including spores.

• Disinfection: Eliminating most pathogens (not spores) from inanimate objects.

• Antisepsis: Reducing microbes on living tissue.

• Bacteriostatic: Inhibits bacterial growth.

• Bactericidal: Kills bacteria.

• Disinfectant: Chemical used on inanimate objects.

• Antiseptic: Chemical used on living tissue.

Physical and Chemical Control Methods

• Physical methods: Heat (autoclaving, pasteurization), filtration, radiation (UV, ionizing).

• Chemical methods: Alcohols, phenolics, halogens, aldehydes, quaternary ammonium compounds.

Classes of Germicides

Class	Example	Use
High-level	Glutaraldehyde	Sterilize medical equipment
Intermediate-level	Alcohols	Disinfect surfaces
Low-level	Quaternary ammonium compounds	General cleaning

Factors in selection: Microbial target, surface type, toxicity, cost, and contact time.

Microbial Metabolism

Metabolism, Catabolism, and Anabolism

• Metabolism: All chemical reactions in a cell.

• Catabolism: Breakdown of molecules to release energy.

• Anabolism: Synthesis of complex molecules from simpler ones; requires energy.

Enzymes in Metabolism

• Enzymes: Biological catalysts that speed up reactions by lowering activation energy.

• Factors affecting enzyme activity: Temperature, pH, substrate concentration, inhibitors.

• Competitive inhibitors: Bind to the active site, blocking substrate.

• Noncompetitive inhibitors: Bind elsewhere, changing enzyme shape.

ATP: Structure and Production

• ATP (adenosine triphosphate): Main energy currency of the cell.

• Composed of adenine, ribose, and three phosphate groups.

• ATP generation methods:

  • Substrate-level phosphorylation

  • Oxidative phosphorylation

  • Photophosphorylation

• Chemiosmosis: Movement of protons across a membrane drives ATP synthase to produce ATP.

Respiration and Fermentation Pathways

Aerobic Respiration

• Three phases: Glycolysis, Krebs cycle, Electron Transport Chain (ETC).

• Glycolysis: Occurs in cytoplasm (both prokaryotes and eukaryotes); glucose → pyruvate.

• Krebs cycle: Cytoplasm (prokaryotes), mitochondrial matrix (eukaryotes).

• ETC: Plasma membrane (prokaryotes), inner mitochondrial membrane (eukaryotes).

• ETC produces the most ATP.

Anaerobic Respiration

• Similar to aerobic, but uses inorganic molecules other than O2 as final electron acceptor (e.g., nitrate, sulfate).

• Less energy produced than aerobic respiration.

Fermentation

• Occurs when oxygen is absent; organic molecules are final electron acceptors.

• Produces less ATP than respiration.

Central Role of Pyruvate

• Pyruvate is a key intermediate; can be used in fermentation or enter the Krebs cycle.

• Lipids and proteins are broken down into intermediates that enter glycolysis or the Krebs cycle.

Energy Yield Comparison

Pathway	ATP per Glucose
Aerobic Respiration	~38 (prokaryotes), ~36 (eukaryotes)
Anaerobic Respiration	Varies (<38)
Fermentation	2

Microbial Identification Techniques

Biochemical Tests

• Used to identify bacteria based on metabolic properties.

• Examples:

  • Phenol red carbohydrate broth: Detects fermentation of sugars (color change indicates acid production).

  • Catalase test: Detects presence of catalase enzyme (bubbles with H2O2).

  • Coagulase test: Detects ability to clot plasma (important for identifying Staphylococcus aureus).

Other Identification Tools

• Molecular techniques: PCR, DNA sequencing.

• Rapid analysis: MALDI-TOF mass spectrometry for quick identification of bacteria.

Laboratory Techniques

Serial Dilutions

Used to reduce microbial concentration for accurate colony counting.

• Process: Sequentially dilute a sample, plate aliquots, and count colonies.

• Calculation: $\text{CFU/mL} = \frac{\text{Number of colonies} \times \text{dilution factor}}{\text{volume plated (mL)}}$

Interpretation of Biochemical Tests

• Carbohydrate broth: Yellow color indicates acid production (fermentation).

• Catalase test: Bubble formation indicates positive result.

• Coagulase test: Clot formation indicates positive result.

Additional info: These topics align with Chapters 7 and 8, covering microbial growth, metabolism, and laboratory identification methods essential for microbiology students.