Microbiology Exam Study Guide: Key Concepts and Processes

History and Foundations of Microbiology

Key Historical Developments

Understanding the history of microbiology provides context for modern discoveries and techniques.

• Spontaneous Generation: The disproven theory that life can arise from non-living matter.

• Germ Theory of Disease: The concept that microorganisms are the cause of many diseases.

• Koch's Postulates: A set of criteria used to establish a causative relationship between a microbe and a disease.

Example: Robert Koch used his postulates to identify the causative agent of tuberculosis.

Classification and Diversity of Microorganisms

Prokaryotes vs. Eukaryotes

Microorganisms are classified based on cellular structure and genetic differences.

• Prokaryotes: Organisms without a nucleus (e.g., Bacteria, Archaea).

• Eukaryotes: Organisms with a nucleus (e.g., Fungi, Protozoa, Algae).

Viruses

Viruses are acellular entities that require host cells for replication.

• Contain either DNA or RNA, not both.

• Lack cellular structure.

Evolutionary Relationships

Carl Woese proposed three domains of life based on rRNA sequences:

• Bacteria

• Archaea

• Eukarya

Microbial Metabolism and Energy

Microbial Energy Types

Microorganisms obtain energy and carbon in various ways:

• Chemotrophs: Obtain energy from chemical compounds.

• Phototrophs: Obtain energy from light.

• Chemoorganotrophs: Use organic compounds for energy.

• Chemolithotrophs: Use inorganic compounds for energy.

Example: Nitrosomonas species are chemolithotrophs that oxidize ammonia.

Microscopy and Cell Morphology

Types of Microscopes

Microscopes are essential for visualizing microorganisms. Types include:

• Light microscopes

• Electron microscopes

Bacterial Cell Morphology

Bacteria exhibit various shapes:

• Cocci: Spherical

• Rods (Bacilli): Cylindrical

• Spirochetes: Spiral-shaped

Cell Structure and Function

Cell Membrane

The cell membrane is a selectively permeable barrier composed of phospholipids and proteins.

• Functions: Transport, energy generation, signal transduction.

• Contains membrane-strengthening agents (e.g., hopanoids in bacteria, sterols in eukaryotes).

Archaeal Membranes

Archaeal membranes differ from bacterial membranes in lipid composition and linkage type.

• Ether-linked lipids (archaea) vs. ester-linked lipids (bacteria).

Transport Systems

Microbes use various systems to transport substances across membranes:

• Simple Transport: Driven by energy (e.g., proton motive force).

• Group Translocation: Substance is chemically modified during transport.

• ABC Transporters: Use ATP to transport substances.

Cell Wall Structure

Bacterial cell walls provide shape and protection. Major differences exist between Gram-positive and Gram-negative bacteria.

• Gram-positive: Thick peptidoglycan layer, teichoic acids.

• Gram-negative: Thin peptidoglycan, outer membrane with lipopolysaccharide (LPS).

Example: Staphylococcus aureus is Gram-positive; Escherichia coli is Gram-negative.

Periplasm

The periplasm is the space between the inner and outer membranes in Gram-negative bacteria, containing enzymes and transport proteins.

Surface Structures

• Capsule: Polysaccharide layer for protection and adhesion.

• Pili: Hair-like structures for attachment and conjugation.

• Inclusion Bodies: Storage granules for nutrients.

Endospores

Endospores are highly resistant, dormant structures formed by some bacteria (e.g., Bacillus species) for survival under harsh conditions.

• Vegetative cell: Active, growing form.

• Spore: Dormant, resistant form.

Bacterial Motility and Chemotaxis

Flagella

Flagella are whip-like structures used for motility. Arrangements include:

• Polar: At one or both ends.

• Peritrichous: All over the cell surface.

• Lophotrichous: Tuft at one end.

• Amphitrichous: At both ends.

Flagella rotate like a propeller, powered by the proton motive force.

Flagellar Motor

The flagellar motor is a complex protein structure that converts ion flow into mechanical rotation.

• Fli proteins act as a clutch to regulate rotation.

Chemotaxis

Chemotaxis is the movement of bacteria in response to chemical gradients (attractants or repellents).

Bioenergetics and Catabolism

Bioenergetics

Microbial metabolism involves energy transformations:

• Aerobic Respiration: Uses oxygen as the terminal electron acceptor.

• Anaerobic Respiration: Uses other molecules (e.g., nitrate, sulfate) as electron acceptors.

• Fermentation: Organic molecules serve as both electron donors and acceptors.

• Chemolithotrophy: Inorganic compounds are oxidized for energy.

Key Concepts:

• Endergonic vs. exergonic reactions

• Oxidation-reduction (redox) reactions

• Reduction potential and the redox tower

• High-energy bonds (e.g., ATP)

Equation Example:

$\text{Glucose} + 6\ O_2 \rightarrow 6\ CO_2 + 6\ H_2O + \text{energy (ATP)}$

Catabolism of Glucose

Glucose catabolism occurs via respiration and fermentation.

• Coenzymes: Molecules like NAD+ shuttle electrons during metabolism.

• Each step occurs in a specific location in the bacterial cell.

• ATP synthase uses the proton gradient to generate ATP.

• Alternative pathways (e.g., Entner-Doudoroff, pentose phosphate) exist.

• Fermentation produces end products like lactic acid or ethanol.

Equation Example:

$\text{ADP} + P_i + \text{proton motive force} \rightarrow \text{ATP}$

Laboratory Techniques

Aseptic Technique

Practices that prevent contamination of cultures and the environment.

Staining Methods

• Simple Stain: Uses a single dye to visualize cells.

• Negative Stain: Stains the background, not the cells.

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

Diluting Bacteria (Streak Plate)

Streaking is used to isolate pure colonies from a mixed culture.

Table: Comparison of Bacterial Cell Wall Structures

Feature	Gram-Positive	Gram-Negative
Peptidoglycan Layer	Thick	Thin
Outer Membrane	Absent	Present
Lipopolysaccharide (LPS)	Absent	Present
Teichoic Acids	Present	Absent

Additional info:

• Some details (e.g., specific examples, alternative catabolic pathways) were inferred for completeness.

• For exam preparation, review all laboratory techniques and be familiar with the structure and function of all major cell components.