Microbiology Study Guide: Cell Structure, Microbial Growth, Metabolism, and Control

Endosymbiotic Theory

Evidence Supporting the Endosymbiotic Theory

The endosymbiotic theory explains the origin of eukaryotic organelles such as mitochondria and chloroplasts, proposing that these organelles originated from free-living prokaryotes engulfed by ancestral eukaryotic cells.

• Double Membranes: Mitochondria and chloroplasts possess double membranes, consistent with engulfment by a host cell.

• Own Genetic Material: Both organelles contain circular DNA similar to bacterial genomes.

• Ribosomes: Their ribosomes resemble those of bacteria (70S) rather than eukaryotic cytoplasmic ribosomes (80S).

• Binary Fission: Mitochondria and chloroplasts replicate independently via binary fission, like bacteria.

• Phylogenetic Evidence: Genetic analyses show close relationships between mitochondria and α-proteobacteria, and between chloroplasts and cyanobacteria.

Example: The presence of mitochondrial DNA mutations inherited maternally supports their independent genetic system.

Microbial Growth Curve

Phases of Bacterial Growth

Bacterial populations in batch culture exhibit a characteristic growth curve with distinct phases:

• Lag Phase: Cells adapt to new environment; metabolic activity without division.

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

• Stationary Phase: Nutrient depletion and waste accumulation halt growth; cell division equals cell death.

• Death Phase: Cells die at an exponential rate due to adverse conditions.

Example: In a closed flask, Escherichia coli shows all four phases over 24 hours.

Microbial Metabolism

Primary and Secondary Metabolites

• Primary Metabolites: Compounds essential for growth (e.g., amino acids, nucleotides), produced during log phase.

• Secondary Metabolites: Non-essential compounds (e.g., antibiotics), produced during stationary phase.

Example: Penicillin is a secondary metabolite produced by Penicillium species.

Temperature Classification of Microbes

• Psychrophiles: Grow at 0–20°C (e.g., Arctic bacteria).

• Mesophiles: Grow at 20–45°C (e.g., human pathogens).

• Thermophiles: Grow at 45–80°C (e.g., hot spring bacteria).

• Hyperthermophiles: Grow above 80°C (e.g., Thermococcus).

Oxygen Requirements of Bacteria

• Obligate Aerobes: Require oxygen for growth.

• Obligate Anaerobes: Oxygen is toxic; grow only without it.

• Facultative Anaerobes: Grow with or without oxygen (better with oxygen).

• Microaerophiles: Require low oxygen levels.

• Aerotolerant Anaerobes: Do not use oxygen but tolerate its presence.

Microbial Nutrition and Media

Types of Culture Media

• General (Nutrient) Media: Supports growth of many microbes (e.g., nutrient agar).

• Selective Media: Inhibits some microbes, allows others (e.g., MacConkey agar for Gram-negative bacteria).

• Differential Media: Distinguishes microbes by biochemical reactions (e.g., blood agar for hemolysis).

• Complex Media: Contains unknown exact composition (e.g., tryptic soy broth).

• Defined (Synthetic) Media: Exact chemical composition is known.

Hemolysis on Blood Agar

• Alpha (α) Hemolysis: Partial hemolysis; greenish discoloration (e.g., Streptococcus pneumoniae).

• Beta (β) Hemolysis: Complete hemolysis; clear zone (e.g., Streptococcus pyogenes).

• Gamma (γ) Hemolysis: No hemolysis; no change (e.g., Enterococcus faecalis).

Heterotrophs vs. Autotrophs

• Heterotrophs: Obtain carbon from organic compounds (e.g., most bacteria, animals).

• Autotrophs: Use CO2 as carbon source (e.g., cyanobacteria, plants).

Microbial Diversity

Major Microbial Groups

• Archaea: Prokaryotes distinct from bacteria; often extremophiles.

• Protists: Diverse eukaryotes; include protozoa and algae.

• Fungi: Eukaryotes; include yeasts, molds, and mushrooms.

Characteristics of Fungi, Algae, and Protozoa

• Fungi: Cell walls of chitin; heterotrophic; reproduce by spores.

• Algae: Photosynthetic; cell walls of cellulose; aquatic.

• Protozoa: Unicellular; lack cell walls; motile by cilia, flagella, or pseudopodia.

Microbial Quantification and Control

Serial Dilution

Serial dilution is a method to estimate microbial concentration by stepwise dilution and plating.

• Reduces cell density to countable levels.

• Used to calculate colony-forming units (CFU) per mL.

Disinfectants, Antiseptics, and Sterilization

• Disinfectant: Chemical used on inanimate objects to destroy microbes (e.g., bleach).

• Antiseptic: Chemical used on living tissue to reduce infection risk (e.g., iodine).

• Sterilization: Complete destruction/removal of all forms of microbial life, including spores (e.g., autoclaving).

D Value (Decimal Reduction Time)

The D value is the time required at a specific temperature to reduce a microbial population by 90% (one log).

• Used to assess effectiveness of sterilization methods.

Equation:

$D = \frac{t}{\log_{10} N_0 - \log_{10} N}$

where $N_0$ is the initial population, $N$ is the final population, and $t$ is time.

Enzymes and Metabolism

Enzymes, Active Sites, Coenzymes, and Cofactors

• Enzymes: Biological catalysts that speed up chemical reactions without being consumed.

• Active Site: Region on enzyme where substrate binds and reaction occurs.

• Coenzymes: Organic molecules (often vitamins) that assist enzymes (e.g., NAD+, FAD).

• Cofactors: Inorganic ions (e.g., Mg2+, Fe2+) required for enzyme activity.

Examples: NAD+ (nicotinamide adenine dinucleotide) is a coenzyme; Mg2+ is a cofactor for DNA polymerase.

Oxidation and Reduction Reactions

• Oxidation: Loss of electrons from a molecule.

• Reduction: Gain of electrons by a molecule.

Example: In cellular respiration, glucose is oxidized and oxygen is reduced.

Glucose Metabolism

Pathways: Glycolysis, TCA Cycle, Electron Transport Chain

• Glycolysis: Converts glucose to pyruvate; produces ATP and NADH.

• TCA (Krebs) Cycle: Oxidizes acetyl-CoA to CO2; generates NADH, FADH2, and ATP/GTP.

• Electron Transport Chain (ETC): Transfers electrons from NADH/FADH2 to oxygen (aerobic) or other acceptors (anaerobic); produces ATP via oxidative phosphorylation.

Inputs and Outputs of Glucose Metabolism

Pathway	Inputs	Outputs
Glycolysis	Glucose, 2 NAD+, 2 ADP, 2 Pi	2 Pyruvate, 2 NADH, 2 ATP (net)
TCA Cycle (per glucose)	2 Acetyl-CoA, 6 NAD+, 2 FAD, 2 GDP/ADP, 2 Pi	4 CO2, 6 NADH, 2 FADH2, 2 ATP/GTP
ETC	10 NADH, 2 FADH2, O2 (aerobic)	H2O, ~34 ATP

ATP Yield at Each Step

• Glycolysis: 2 ATP (net)

• TCA Cycle: 2 ATP (as GTP)

• ETC: ~34 ATP (aerobic respiration)

• Total: Up to 38 ATP per glucose in prokaryotes

Aerobic vs. Anaerobic Fermentation

• Aerobic Respiration: Uses O2 as final electron acceptor; high ATP yield.

• Anaerobic Fermentation: Uses organic molecules as electron acceptors; lower ATP yield.

Role of Oxygen in Glucose Metabolism

• Oxygen acts as the terminal electron acceptor in aerobic respiration, allowing efficient ATP production.

• Absence of oxygen leads to fermentation or anaerobic respiration.

Location of Glucose Metabolism in Prokaryotes

• Glycolysis and TCA Cycle: Occur in the cytoplasm.

• Electron Transport Chain: Located in the plasma (cell) membrane.

Additional info: In eukaryotes, glycolysis occurs in the cytoplasm, TCA cycle in the mitochondrial matrix, and ETC in the inner mitochondrial membrane.