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

Microbial Growth and Cultivation

Growth of Microbes in Selective and Differential Media

Microorganisms can be cultivated on various types of media, each designed for specific purposes in laboratory and clinical settings.

• Selective media: Contain ingredients that inhibit the growth of certain microbes while allowing others to grow. Example: MacConkey agar selects for Gram-negative bacteria by inhibiting Gram-positives with bile salts and crystal violet.

• Differential media: Allow multiple types of microbes to grow but contain indicators that reveal differences between organisms. Example: Blood agar differentiates bacteria based on hemolysis patterns.

• Complex media: Contain a variety of ingredients (e.g., peptones, extracts) with unknown exact composition.

• Chemically defined media: All chemical components and their concentrations are known.

Isolation Techniques

Isolation techniques are used to obtain pure cultures from mixed populations.

• Streak plate method: Involves spreading a diluted microbial sample over the surface of an agar plate to obtain isolated colonies.

• Pour plate and spread plate methods: Used for quantifying and isolating bacteria by diluting samples and spreading them on agar plates.

Classification of Microorganisms by Growth Conditions

Microbes are classified based on their optimal growth conditions:

• Temperature:

  • Psychrophiles: Grow best at 0–15°C

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

  • Thermophiles: 45–80°C

  • Hyperthermophiles: Above 80°C

• Oxygen requirements:

  • Obligate aerobes: Require oxygen

  • Obligate anaerobes: Cannot tolerate oxygen

  • Facultative anaerobes: Can grow with or without oxygen

  • Microaerophiles: Require low oxygen levels

  • Aerotolerant anaerobes: Do not use oxygen but tolerate its presence

• 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.

Bacterial Growth Curve and Binary Fission

Bacterial populations grow in a predictable pattern when cultured in a closed system, described by the bacterial growth curve:

• Lag phase: Adaptation, no increase in cell number.

• Log (exponential) phase: Rapid cell division and population growth.

• Stationary phase: Growth rate equals death rate; nutrients deplete, waste accumulates.

• Death phase: Cells die at an exponential rate.

Binary fission is the process by which bacteria reproduce, resulting in two genetically identical daughter cells.

Calculating Bacterial Growth

• Generation time (g): Time required for a population to double.

• Number of generations (n): $n = \frac{\text{total time}}{\text{generation time}}$

• Final population size: $N = N_0 \times 2^n$

• Given any two of total time, generation time, or number of generations, the third can be calculated.

Clinical Sample Collection Considerations

• Use sterile equipment and aseptic technique.

• Collect samples before starting antimicrobial therapy.

• Proper labeling and prompt transport to the laboratory are essential.

Cell Enumeration Methods

• Direct methods:

  • Microscopic cell count (using a counting chamber)

  • Viable plate count (serial dilution and plating)

• Indirect methods:

  • Measuring turbidity (optical density)

  • Measuring metabolic activity (e.g., CO2 production)

Definitions in Microbial Control

• Decontamination: Removal or reduction of microbial populations to safe levels.

• Sterilization: Destruction of all microbial life, including spores.

• Disinfection: Elimination of most pathogens (not spores) on inanimate objects.

• Antisepsis: Destruction of pathogens on living tissue.

• Bacteriostatic: Inhibits bacterial growth.

• Bactericidal: Kills bacteria.

• Disinfectant: Chemical used on inanimate objects to destroy microbes.

• Antiseptic: Chemical used on living tissue to destroy microbes.

Physical and Chemical Means of Microbial Control

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

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

• Classes of germicides:

  • High-level: Kill all organisms, including spores.

  • Intermediate-level: Kill mycobacteria, most viruses, and bacteria.

  • Low-level: Kill some viruses and bacteria.

• Selection factors: Nature of item, type of microbe, contact time, toxicity, compatibility.

Microbial Metabolism

Metabolism, Catabolism, and Anabolism

Metabolism is the sum of all chemical reactions in a cell. It includes:

• Catabolism: Breakdown of molecules to release energy.

• Anabolism: Synthesis of complex molecules from simpler ones, requiring energy.

Role of Enzymes in Metabolism

• Enzymes are biological catalysts that speed up metabolic reactions by lowering activation energy.

• Enzyme activity is influenced by temperature, pH, substrate concentration, and inhibitors.

• Competitive inhibitors bind to the active site, blocking substrate binding.

• Noncompetitive inhibitors bind to an allosteric site, changing enzyme shape and function.

Metabolic Pathways: Aerobic Respiration, Anaerobic Respiration, and Fermentation

• Aerobic respiration: Uses oxygen as the final electron acceptor; yields the most ATP.

• Anaerobic respiration: Uses alternative electron acceptors (e.g., nitrate, sulfate); yields less ATP than aerobic respiration.

• Fermentation: Does not use an electron transport chain; organic molecules serve as final electron acceptors; yields the least ATP.

Three phases of aerobic respiration:

• Glycolysis: Occurs in the cytoplasm (both prokaryotes and eukaryotes); breaks glucose into pyruvate; produces 2 ATP and 2 NADH.

• Krebs cycle: Occurs in the cytoplasm (prokaryotes) or mitochondrial matrix (eukaryotes); produces CO2, ATP, NADH, and FADH2.

• Electron transport chain (ETC): Occurs in the plasma membrane (prokaryotes) or inner mitochondrial membrane (eukaryotes); produces most ATP via oxidative phosphorylation.

Pyruvate is a central metabolite, linking glycolysis to the Krebs cycle and fermentation pathways.

• Lipids are broken down by lipases into fatty acids and glycerol; fatty acids enter beta-oxidation.

• Proteins are broken down by proteases into amino acids; amino acids are deaminated and enter central metabolic pathways.

Energy yield comparison:

• Aerobic respiration: ~38 ATP/glucose (prokaryotes)

• Anaerobic respiration: Variable, less than aerobic

• Fermentation: 2 ATP/glucose

ATP Structure and Production

• ATP (adenosine triphosphate) consists of adenine, ribose, and three phosphate groups.

• Cells generate ATP by:

  • Substrate-level phosphorylation

  • Oxidative phosphorylation (chemiosmosis)

  • Photophosphorylation (in phototrophs)

• Chemiosmosis drives ATP production by using a proton gradient across a membrane to power ATP synthase.

Metabolic Pathways and Microbial Identification

• Biochemical tests exploit differences in metabolic pathways to identify microbes.

• Examples:

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

  • Catalase test: Detects presence of catalase enzyme (bubbles indicate positive result).

  • Coagulase test: Detects coagulase enzyme (clot formation indicates positive result).

• Other rapid analysis techniques include molecular methods (e.g., PCR) and mass spectrometry (e.g., MALDI-TOF).

Laboratory Applications

Selective and Differential Media: Examples and Uses

• MacConkey agar: Selective for Gram-negative bacteria; differentiates lactose fermenters (pink colonies) from non-fermenters.

• Mannitol salt agar: Selective for staphylococci; differentiates mannitol fermenters (yellow) from non-fermenters.

Serial Dilutions: Process and Calculations

• Serial dilution involves stepwise dilution of a sample to reduce cell concentration for accurate counting.

• Calculation of original cell concentration:

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

Biochemical Tests: Theory and Interpretation

• Carbohydrate broth reactions: Detect fermentation of specific sugars; acid production changes pH indicator color.

• Catalase test: Detects catalase enzyme; addition of hydrogen peroxide produces bubbles if positive.

• Coagulase test: Detects coagulase enzyme; clot formation in plasma indicates positive result.

Summary Table: Types of Media

Type of Media	Main Purpose	Example
Selective	Inhibit unwanted microbes, allow target growth	MacConkey agar
Differential	Distinguish microbes by biochemical reactions	Blood agar
Complex	Support growth of many microbes; composition not fully known	Nutrient broth
Chemically defined	Exact chemical composition known	Minimal salts medium

Summary Table: Microbial Growth Classifications

Classification	Definition	Example
Psychrophile	Grows best at 0–15°C	Polaromonas vacuolata
Mesophile	Grows best at 20–45°C	Escherichia coli
Thermophile	Grows best at 45–80°C	Thermus aquaticus
Obligate aerobe	Requires oxygen	Mycobacterium tuberculosis
Obligate anaerobe	Cannot tolerate oxygen	Clostridium botulinum

Summary Table: Biochemical Tests

Test	Purpose	Positive Result
Phenol red broth	Detects sugar fermentation	Yellow color (acid)
Catalase test	Detects catalase enzyme	Bubbles (O2 release)
Coagulase test	Detects coagulase enzyme	Clot formation

Additional info: Some details, such as specific examples and equations, have been added for completeness and clarity.