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 of the cell cycle via binary fission.

• 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 species).

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 concentrations.

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

Culture Media

Types of Media

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

• Selective Media: Inhibits some microbes, allows others (e.g., MacConkey agar selects 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 Media: All chemical components are known and quantified.

Hemolysis on Blood Agar

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

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

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

Microbial Nutrition

Heterotrophs vs. Autotrophs

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

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

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, non-photosynthetic, reproduce by spores.

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

• Protozoa: Unicellular, lack cell walls, motile, heterotrophic.

Microbial Quantification

Serial Dilution

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

• Reduces concentration to countable levels.

• Colony-forming units (CFUs) are calculated by multiplying colonies by dilution factor.

Control of Microbial Growth

Disinfectants vs. Antiseptics

• 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

Sterilization is the process of destroying all forms of microbial life, including spores.

• Methods: Autoclaving, dry heat, filtration, radiation.

D Value (Decimal Reduction Time)

The D value is the time required at a certain condition to kill 90% of microorganisms.

• 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 and $N$ is the final population.

Enzymes and Metabolism

Enzymes

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

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

Coenzymes and Cofactors

• 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), Mg2+ for DNA polymerase.

Oxidation and Reduction Reactions

• Oxidation: Loss of electrons.

• Reduction: Gain of electrons.

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

Glucose Metabolism

Pathways: Glycolysis, TCA Cycle, Electron Transport Chain

• Glycolysis: Glucose (6C) → 2 Pyruvate (3C); produces ATP and NADH.

• TCA (Krebs) Cycle: Pyruvate → CO2; generates NADH, FADH2, ATP.

• Electron Transport Chain (ETC): NADH/FADH2 donate electrons; ATP produced via oxidative phosphorylation.

Inputs and Outputs of Each Step

Pathway	Input	Output
Glycolysis	1 Glucose, 2 NAD+, 2 ADP	2 Pyruvate, 2 NADH, 2 ATP (net)
TCA Cycle (per glucose)	2 Acetyl-CoA, 6 NAD+, 2 FAD, 2 ADP	4 CO2, 6 NADH, 2 FADH2, 2 ATP
ETC	10 NADH, 2 FADH2, O2	H2O, ~34 ATP

ATP Yield at Each Step

• Glycolysis: 2 ATP (net)

• TCA Cycle: 2 ATP (per glucose)

• ETC: ~34 ATP (per glucose)

• Total: ~38 ATP (prokaryotes), ~36 ATP (eukaryotes)

Aerobic vs. Anaerobic Fermentation

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

• Anaerobic Fermentation: Uses organic/inorganic molecules (not O2); lower ATP yield.

Role of Oxygen in Glucose Metabolism

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

Location of Glucose Metabolism in Prokaryotes

• Glycolysis and TCA Cycle: Occur in the cytoplasm.

• Electron Transport Chain: Located in the plasma membrane.

Additional info: In eukaryotes, the TCA cycle and ETC occur in mitochondria.