Microbiology Exam 1 Review: The Microbial World, Cell Structure, Metabolism, Growth, Molecular Information Flow, and Regulatory Systems

The Microbial World

Characteristics of Life

Living organisms share several fundamental characteristics that distinguish them from non-living matter.

• Cellular Organization: All living things are composed of cells.

• Metabolism: Ability to obtain and use energy.

• Growth: Increase in size and/or number of cells.

• Reproduction: Ability to produce offspring.

• Response to Stimuli: React to environmental changes.

• Homeostasis: Maintain internal stability.

• Evolution: Adaptation and change over generations.

Domains of Life and Their Characteristics

The three domains of life are distinguished by differences in cellular structure and genetics.

• Bacteria: Prokaryotic, cell walls with peptidoglycan, diverse metabolism.

• Archaea: Prokaryotic, cell walls without peptidoglycan, often extremophiles.

• Eukarya: Eukaryotic, includes protists, fungi, plants, and animals.

Prokaryotes vs. Eukaryotes

Prokaryotes and eukaryotes differ in cellular complexity.

• Prokaryotes: No nucleus, no membrane-bound organelles, usually smaller.

• Eukaryotes: Nucleus present, membrane-bound organelles, larger and more complex.

Microbes: Bacteria and Archaea are prokaryotic; Protozoa, Fungi, Algae, and small multicellular animals are eukaryotic.

Characteristics of Microbial Groups

• Bacteria: Prokaryotic, diverse shapes, peptidoglycan cell wall.

• Archaea: Prokaryotic, unique membrane lipids, extremophiles.

• Protozoa: Eukaryotic, unicellular, motile, no cell wall.

• Fungi: Eukaryotic, cell wall with chitin, includes yeasts and molds.

• Algae: Eukaryotic, photosynthetic, cell wall with cellulose.

• Small Multicellular Animals: Eukaryotic, includes helminths.

• Viruses: Acellular, require host for replication, not considered living.

Are viruses living? Viruses lack cellular structure and independent metabolism; they are not included in the rRNA-based tree of life because they do not possess ribosomes.

Contributions of Key Scientists

• Antoni van Leeuwenhoek: First to observe microbes with a microscope.

• Robert Hooke: Coined the term "cell"; early microscopy.

• Louis Pasteur: Disproved spontaneous generation; developed pasteurization.

• Christian Gram: Developed Gram staining technique.

• Walther Hesse: Introduced agar as a solidifying agent.

• Sergei Winogradsky: Studied microbial ecology and nitrogen fixation.

• Robert Koch: Established Koch's postulates; identified causative agents of disease.

Endosymbiotic Theory

The endosymbiotic theory explains the origin of mitochondria and chloroplasts in eukaryotic cells as a result of symbiosis between ancestral eukaryotes and prokaryotes.

• Mitochondria and chloroplasts have their own DNA and ribosomes.

• Resemble bacteria in structure and function.

Definitions

• Contamination: Introduction of unwanted microbes.

• Sterilization: Removal or destruction of all forms of microbial life.

• Disinfection: Elimination of most pathogenic microbes (not spores).

• Antiseptic: Chemical used to disinfect living tissue.

• Sanitization: Reduction of microbial population to safe levels.

• Aseptic Technique: Procedures to prevent contamination.

• Pure Culture: Population of cells derived from a single cell.

Microbial Symbioses

• Nitrogen Fixation in Plants: Microbes (e.g., Rhizobium) live in root nodules.

• Digestive Tract of Herbivores: Microbes reside in specialized compartments (e.g., rumen in cows).

Microscopy

• Light Microscope: Used for cells and larger microbes; magnification up to 1000x.

• Electron Microscope: Used for viruses and ultrastructure; magnification up to 100,000x.

• Fluorescence Microscope: Used for specific molecules; uses fluorescent dyes.

Each microscope is suited for different specimen sizes and types.

Biofilms and Quorum Sensing

• Biofilm: Community of microbes attached to a surface, embedded in extracellular matrix.

• Communication: Organisms in biofilms use quorum sensing to coordinate behavior.

• Examples: Dental plaque, medical device surfaces.

• Quorum Sensing: Cell-to-cell communication via signaling molecules.

Microbial Cell Structure and Function

Prokaryotic Structures and Functions

• Cell Wall: Provides shape and protection; composed of peptidoglycan in bacteria.

• Plasma Membrane: Selective barrier for transport.

• Flagella: Motility.

• Pili/Fimbriae: Attachment and conjugation.

• Capsule: Protection and adherence.

• Ribosomes: Protein synthesis.

• Nucleoid: Region containing DNA.

Gram Stain and Cell Wall Differences

• Gram Stain: Differentiates bacteria based on cell wall structure.

• Gram Positive: Thick peptidoglycan, stains purple.

• Gram Negative: Thin peptidoglycan, outer membrane, stains pink.

Bacterial Cell Walls: Composed of peptidoglycan (murein).

Lipid A and Endotoxin

• Lipid A: Component of lipopolysaccharide (LPS) in Gram-negative bacteria.

• Endotoxin: Toxic portion of LPS; causes immune response.

• Location: Found in the outer membrane of Gram-negative bacteria.

Microbial Nutrition Types

• Chemoorganotroph: Obtain energy from organic compounds.

• Photoautotroph: Use light energy and CO2 as carbon source.

Microbial Metabolism

Enzyme Structure and Function

• Enzymes: Biological catalysts; speed up reactions.

• Structure: Protein (sometimes with cofactors).

• Production: Encoded by genes, synthesized via transcription and translation.

Redox Reactions

• Oxidation: Loss of electrons.

• Reduction: Gain of electrons.

Example Equation:

$\text{A} + \text{B} \rightarrow \text{A}^+ + \text{B}^-$

Enzyme Regulation

• Allosteric Regulation: Binding of molecules at sites other than active site.

• Feedback Inhibition: End product inhibits enzyme activity.

Lac Operon and Trp Operon

• Lac Operon: Controls lactose metabolism in Escherichia coli; induced by lactose.

• Trp Operon: Controls tryptophan synthesis; repressed by tryptophan.

• Found in: Bacteria.

Glycolysis, Cellular Respiration, and Fermentation

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

• Cellular Respiration: Complete oxidation of glucose; includes glycolysis, Krebs cycle, electron transport chain.

• Fermentation: Anaerobic process; regenerates NAD+, produces organic acids or alcohols.

Key Products: ATP, NADH, CO2, various fermentation end products.

Aerobic vs. Anaerobic Respiration: Aerobic uses O2 as terminal electron acceptor; anaerobic uses other molecules.

Microbial Growth and Its Control

Microbial Growth Curve

The microbial growth curve describes population changes over time.

• Lag Phase: Adaptation, no growth.

• Log (Exponential) Phase: Rapid cell division.

• Stationary Phase: Growth rate equals death rate.

• Death Phase: Decline in viable cells.

Generation Time: Time required for population to double.

Temperature Classes of Microbes

• Psychrophile: Grow at low temperatures (< 15°C).

• Mesophile: Grow at moderate temperatures (20–45°C).

• Thermophile: Grow at high temperatures (45–80°C).

• Hyperthermophile: Grow at very high temperatures (> 80°C).

Autoclave

• Purpose: Sterilization using steam under pressure.

• Common Temperature: 121°C.

Environmental Adaptations

• Halotolerant: Tolerate high salt.

• Osmophile: Thrive in high osmotic pressure.

• Xerophile: Grow in dry environments.

• Barophile: Grow under high pressure.

• Acidophile: Grow in acidic conditions.

• Alkalophile: Grow in alkaline conditions.

• Neutrophile: Grow at neutral pH.

Reactive Oxygen Species and Enzymes

• Catalase: Converts hydrogen peroxide to water and oxygen.

• Superoxide Dismutase: Converts superoxide radicals to hydrogen peroxide.

• Peroxidase: Reduces hydrogen peroxide.

Colony and Culture Definitions

• Colony: Visible mass of cells from a single cell.

• Culture: Growth of microbes in a medium.

• Isolated Colony: Pure colony derived from one cell.

Types of Media

• Selective Media: Favors growth of specific microbes.

• Differential Media: Distinguishes microbes by biochemical reactions.

• Enriched Media: Contains extra nutrients for fastidious organisms.

• Defined Media: Exact chemical composition known.

• Complex Media: Contains unknown components (e.g., yeast extract).

Molecular Information Flow and Protein Processing

Plasmids

• Plasmid: Small, circular DNA molecules in bacteria.

• Purpose: Carry genes for antibiotic resistance, virulence, or metabolism.

DNA Replication, RNA Transcription, and Protein Translation

• DNA Replication: Copying DNA before cell division.

• RNA Transcription: Synthesis of RNA from DNA template.

• Protein Translation: Synthesis of polypeptides from mRNA.

Central Dogma:

$\text{DNA} \rightarrow \text{RNA} \rightarrow \text{Protein}$

Microbial Regulatory Systems

Operons

• Lac Operon: Inducible system for lactose metabolism.

• Trp Operon: Repressible system for tryptophan synthesis.

Operons are found in prokaryotes and allow coordinated regulation of gene expression.

Summary Table: Microbial Groups and Characteristics

Group	Cell Type	Cell Wall	Metabolism	Example
Bacteria	Prokaryotic	Peptidoglycan	Diverse	Escherichia coli
Archaea	Prokaryotic	Varied (no peptidoglycan)	Often extremophilic	Halobacterium
Protozoa	Eukaryotic	None	Heterotrophic	Amoeba
Fungi	Eukaryotic	Chitin	Heterotrophic	Aspergillus
Algae	Eukaryotic	Cellulose	Photosynthetic	Chlorella
Viruses	Acellular	None	Obligate intracellular	Influenza virus

Example: The lac operon is a classic example of gene regulation in bacteria, allowing cells to metabolize lactose only when it is present.

Additional info: Academic context was added to expand brief points and definitions, and to clarify relationships between topics.